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THE AM BULLETIN BOARD => Technical Forum => Topic started by: WZ8J on December 09, 2017, 10:11:52 PM



Title: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 09, 2017, 10:11:52 PM
I have a kit built Valiant I that has pots to adjust both amp bias and mod bias on the right side of the chassis. I have been having trouble setting the static mod current. The reading on the meter jumps up right at the point that it approaches 50 mils and I cannot get it to behave.
I have tried contact cleaner sprayed into the pot and working it back and forth, but that hasn't helped. I may have a replacement in my parts box.
Anyone know what the proper resistance and wattage is for this pot?

Thanks!


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: MikeKE0ZUinkcmo on December 09, 2017, 11:25:39 PM
Should be in the parts list section of the manual.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: w8khk on December 10, 2017, 12:44:58 AM
The Valiant I modulator and final RF amplifier bias settings are adjusted by taps on R22, a wirewound 5000 ohm, 25 watt resistor.  This thread will provide a link to the manual and the adjustment procedure...

http://amfone.net/Amforum/index.php?topic=42250.0

Hope this helps.  GL, OM.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 10, 2017, 07:32:23 AM
R21 (Modulator Bias) and R22 (RF Bias) are both 5k Ohm, 25 Watt wirewound potentiometers.

In some manuals, R21 and R22 are listed as R61 and R62 but the values are the same.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WA5VGO on December 10, 2017, 09:52:52 AM
Try swapping the two outer leads. That should move the wiper to a different place on the element.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: w8khk on December 10, 2017, 10:00:09 AM
R21 (Modulator Bias) and R22 (RF Bias) are both 5k Ohm, 25 Watt wirewound potentiometers.

In some manuals, R21 and R22 are listed as R61 and R62 but the values are the same.

Thanks for the correction, Clark.  It seems my valiant, and manual, are an older version.  My apologies for the mis-information.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 10, 2017, 11:23:02 AM
Thanks for the correction, Clark.  It seems my valiant, and manual, are an older version.  My apologies for the mis-information.

No, no, no... it's not a correction. We're both right and it depends on the version of Valiant. I struggled with some Valiant issues for a month because the original builder followed the construction steps exactly. As it turned out, the engineers at E.F.J. had a change of heart (several times) and a friend's factory built unit featured some important updates.

The only reason I suggested the pots version is from the title of the post, and I should clarify that the resistor number references suggest the earlier schematic version and not just a component number change. Technically, R22 became R62, a potentiometer for RF Bias only and R61 and R63 were added as a separate Mod Bias circuit.

Here's an example of "We should start doing it like this from now on...":


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 10, 2017, 11:45:24 AM
Thanks Clark,
That's the info I needed. I should have pointed out that the build manual I have is for the earlier version which had the taps vs. the pots. Either way, the reader will learn from the exchanges. 8)


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 10, 2017, 11:51:24 AM
Try swapping the two outer leads. That should move the wiper to a different place on the element.

I like this idea. Will give it a try and let you know if that worked.

Thanks!


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 10, 2017, 11:56:02 AM
I should have pointed out that the build manual I have is for the earlier version which had the taps vs. the pots. Either way, the reader will learn from the exchanges. 8)

Here's the later schematic showing the bias adjustment pots:


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 10, 2017, 12:01:29 PM
Suffering from low drive on 20M?

While we're on the topic of "Keeping up with the Johnsons", here's another before & after schematic for an E.F. Johnson update to L6A in the driver tank circuit. It seems as though the stock multi-tap tank coil didn't provide enough inductance on 20M so the 15M turns were dragged in to help and a coil added (L6C: 12T, 5/8" dia) to the bandswitch to take on the 15M tank role.

Band label colors indicate lead colors from L6A...


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 16, 2017, 02:38:16 PM
Okay back to report on the bias pot issue with the Val-Aint.
I switched the outside leads on the pot and this seem to help...for a short while, but now it is again acting as before. I believe the "pot is shot" to make a rhyme.

I am able after careful fiddling to adjust the mod bias current to about 45 ma, but if I try to go higher it jumps way up and pegs the meter! That can't be good for the modulator transformer.

There are 25 watt 5K pots available from Mouser for $2 per watt - ouch! These look awfully large. I am wondering if the builder or  a previous owner added smaller ones for mod and PA bias at some point to replace the resistors with the taps? Maybe the pots that are in there now are lower power rated and that's why at least one is failing.

I suppose I would be asking for trouble if I pulled a 5K pot out of an old PA amp I have laying around and put it in for the bias mod adjustment? I doubt that pot is good for more than 2 watts.

Finally - I determined that the reason the mod current kept rising during transmissions was due to a weak 6146 mod tube. I swapped in two good matched Taylor tubes and at least the mod current doesn't constantly climb now.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on December 16, 2017, 03:08:41 PM
Either get two 25W 5K pots from Ohmite or two 5K 25W resistors with the adjustable slider.  Look on ebay for these.

Fred


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 16, 2017, 03:58:33 PM
For convenience, spring for the potentiometer solution. It's a hassle to have to test, turn off, adjust, turn on, test...etc.

Some possibilities:

https://www.ebay.com/p/NOS-IRC-5000-Ohm-25-Watt-Potentiometer-Rheostat-Wirewound/1541302449

http://www.surplussales.com/potentiometers/rheostats/PotsRheost-4.html

$9.95 + $6.50 shipping :-)  Just gotta cut the shaft a bit:

https://www.ebay.com/itm/MEMCOR-5000-OHM-25-WATT-RHEOSTAT-2-SHAFT-/400682271238


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on December 16, 2017, 06:34:10 PM
The Memcor 5K 25W pots on ebay are perfect and cheap enough.  Get two of them before they sell out.  I have some Memcor power pots and the quality is just as good as Ohmites.

Fred


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: K3FEF on December 16, 2017, 07:03:27 PM
Following this too. :-)  It's a 2W pot! LOL


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 16, 2017, 07:20:08 PM
It's a 2W pot! LOL

To me, the only thing worse than being wrong is to have it go unchecked.

DANG IT!

Yes, R61 (Mod Bias) is 2W. R62 (RF Bias) is 4W. Both are 5k.

OK, Mike is right. There, I said it.

I'll never respond to a post before the first cup of coffee. Look for that resolution on January 1.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 17, 2017, 10:41:24 AM
He is, after all, human  ;D

So I was a bit premature in saying that the newer tubes solved the problem of the static mod current upward drift. It still rises after a while and several key down sequences.
I am thinking there is a resistor somewhere that is changing value.

And Clark, full disclosure, I thew caution to the wind as I couldn't resist installing the 5K pot I pulled from an old PA amp. Now it is possible to adjust the mod current without it jumping all over. The trick now, is getting it to stop drifting upward.
I will, of course order a new 5K pot with known 2 watt capability from Mouser along with some high precision resistors to replace the meter shunts.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on December 17, 2017, 11:23:48 AM
He is, after all, human  ;D

You're right. I forgive Mike for pointing out the error...

What value does the modulator current rise to? Put a voltmeter on the wiper of the bias pot to see if it's the bias voltage that's changing. It could also be the bias supply in general in which case both the Mod and RF bias would change. Could be a resistor heating and changing value or the 6BY5 bias rectifier (V21) output dropping.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 17, 2017, 12:58:50 PM
V-21 6BY5 has been solid stated (not by me)
I unplugged the mic and measured voltage at the wiper of the mod bias pot:
Set the Mod resting current to 60ma
The voltage prior to HV is showing -48V
After flipping on the HV, the mod current shows 75ma, bias drops to -45V
Another sequence or two of flipping on the HV and mod current rises to 100ma and bias drops to -41V


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on December 17, 2017, 01:17:59 PM
Try regulating the bias voltage after the bias supply filter.  Use a string of zeners.

Looking at the schematic,  that negative supply is -285V at the filters.  The supply is also for the keying circuit.  I would measure the static voltage at the 15K and 9K resistors where they connect to each of the pots.  Add zeners across each pot to drop the voltage by about 15 volts.  Do this for each pot. The 15K and 9K resistors will serve as the dropping resistors for each of the zener strings.  You may need to increase the wattage of the 15K and 9K resistors do to the increased current.

Fred

I did some quick calculations,  looks like dropping 15 volts through the 15K resistor draws only 1ma. and 1.7ma for the 9K, no big deal.  Current through the 9K is 20ma and 14ma for the 15K, so another 1 or 2 ma is not going to matter.  The wattage for the two resistors is probably good as is.

Looks like there would be about 100 volts at the junction of the 9K resistor and pot,  about 70 volts for the 15K.  So, you would need something like a 85V zener string and maybe about 55V zener string.  Anything close to these numbers should work.

Wattage for the zeners would be less than 1/2W with a safety margin for each string.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: DMOD on December 17, 2017, 03:20:11 PM


"...It could also be the bias supply in general in which case both the Mod and RF bias would change. Could be a resistor heating and changing value or the 6BY5 bias rectifier (V21) output dropping."

Could be but if you get tired chasing down old resistors that have changed value, one could use these regulators (pdf file below) for rock solid control.

Recommend you replace the C93A,B capacitors with 22 uf@350V and the R50 1k resistor if you haven't done so already.

Phil - AC0OB

 




Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 24, 2017, 12:08:14 PM
So oddly, the Valiant seems for have fixed itself  :o
The static mod current now stays put at 60ma. Not sure why its okay now but been that way for several days.
The old rigs are sure mysterious.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: Steve - K4HX on December 24, 2017, 03:01:40 PM
You sounded good on 40 meters today.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KD6VXI on December 24, 2017, 03:17:47 PM
So oddly, the Valiant seems for have fixed itself  :o
The static mod current now stays put at 60ma. Not sure why its okay now but been that way for several days.
The old rigs are sure mysterious.

Bad connection / cold solder joint  is what usually gets this symptom for me.  Moving things around, probing, and you put enough tension on it to cause a good connection.

Give it time hahahaha....  😀

--Shane
KD6VXI


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on December 25, 2017, 10:34:03 AM
You sounded good on 40 meters today.

Well, you may have heard me after I switched to the Kenwood TS590


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 03, 2018, 11:01:17 AM
Back on 75M with the Valiant today. Got my new 5K pot in the mail yesterday. Still waiting for the D104.
Found some suspect wiring and replaced that too.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 06, 2018, 02:17:51 PM
I installed my D104 and it has made a very noticeable improvement in my reported audio. It is an amplified version, and I have not yet attempted to bypass the amplifier in the mic base.
All the success was on 75M band. When I tried to used the Valiant on 40M, the mod current runs wild when the audio level on the rig is adjusted. It is like the rig is going into oscillation.
Seems to be okay on CW on 40 except there is a chirp in the signal that doesn't happen on 80M.
I tried it on 20M using AM and again the mod current just pins as I increase the audio level slightly on the rig. This is not quite as abrupt as happens on 40M.
Could I be getting some type of RF feedback through the amp on the mic that would cause this on the higher bands?
Again - it works fine on 75M just does the runaway modulation current on 40 & 20.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 06, 2018, 03:13:17 PM
...and again the mod current just pins as I increase the audio level slightly on the rig.

That sounds like either an anti-ultrasonic feedback capacitor, or combination of them, were removed or you're getting RF into your audio, possibly at the mic preamp.

Try a different mic as well as shorting the mic input while turning up the audio gain. That will help determine where the unwanted signal in originating.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 09:49:54 AM
I went through the tube sockets checking resistance to ground per the manual. Everything looks in range except I only get 1K ohms at Pin #5 of XV21 (6by5G) where the manual says it should be 12K ohms. Now some previous owner solid stated this rectifier using two diodes. I tried putting a tube in the socket but the resistance is still 1K at Pin 5.

Any ideas why this would be?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 19, 2018, 09:57:59 AM
1k is also the value of the series resistor in the bias power supply. You might see 1k between pin 5 of the rectifier (plates) to the chassis if the load is shorted.

For reference, I get about 8.7k from pin 5 top chassis which is primarily the voltage dividers (AF, RF) and the 1k series resistor. What the resistance across the filter caps?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 10:07:47 AM
1K ohms across C93 A&B


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 19, 2018, 10:22:30 AM
That's odd because there's a 1k resistor between them, or at least there should be. Also, measuring resistance across caps can result in erroneous readings because of the charging effect. Do you still get 1k if you reverse the Ohmmeter leads?

It may be necessary to disconnect the bias supply from the rest of the circuit to locate where that 1k resistance to chassis is coming from.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 10:56:06 AM
Hey Clark,
I re-checked after poking around and now I am seeing 8.4K ohms at pin 5 and across the two caps ---weird. I must have moved a wire that was shorting and now is clear?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 19, 2018, 11:02:14 AM
I lie awake at night thinking about problems that resolve themselves.

Wiggle stuff. It would be good to be sure.

Also, do you get different readings depending on which Ohmmeter lead is on the chassis? Caps can cause funny things like that. Losing bias suddenly... not as funny.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 11:18:19 AM
Yes looks like values for resistance are same even when leads are switched. Lot of things soldered together on the tube pins. I am not a great fan of using tube sockets as terminal strips. Should get some strips and fix that...yeah. Wish I knew where the short was, don't want that to happen again.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 19, 2018, 11:25:25 AM
Are the solid state rectifiers still connected? One possibility is that you're seeing the value of dropping resistors, if used, since the transformer center tap is grounded. Even then, the value would change if you reversed the Ohmmeter leads.

Is the solid state rectifier hard wired in or does it plug into the tube socket?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 11:34:34 AM
Are you referring to the diodes soldered on the 6by5 socket or the HV rectifiers?
The HV rectifiers are the plug in type.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 19, 2018, 11:37:39 AM
Soldered on the rectifier socket. Are there also series resistors?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 11:38:54 AM
I checked once more. I guess the values are slightly different when the leads are reversed. One cap show 7.4K ohms, reversed it is 7.5K. The other cap reads 8.5K, reversed its 8.4K


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on January 19, 2018, 11:41:15 AM
Soldered on the rectifier socket. Are there also series resistors?

No series resistors are used. They are wired directly between the plate pins and the cathode pins. Is this problematic?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on January 19, 2018, 11:50:54 AM
No, not necessarily. The 6BY5 has some voltage drop, but in the case of a bias supply with adjustable outputs, you'd just need a slightly lower setting since more voltage is available.

Given that the rectifiers are soldered to the socket, the resistance from pin 5 to chassis would be very low in one direction (rectifier + transformer winding) and 8k or so in the other (voltage divider resistors).

That's the theory at least...


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 02, 2018, 03:00:55 PM
...and again the mod current just pins as I increase the audio level slightly on the rig.

That sounds like either an anti-ultrasonic feedback capacitor, or combination of them, were removed or you're getting RF into your audio, possibly at the mic preamp.

Try a different mic as well as shorting the mic input while turning up the audio gain. That will help determine where the unwanted signal in originating.

Finally getting around to diagnose this issue with not being able to use AM on 40M while 80 works okay after switching to the D104 from the Electrovoice EV719. I went back to the Electrovoice mic and sure enough, no problem with AM modulation on 40M. So it looks like I am getting some type of feedback with the D104 on 40M and some of the other high bands.
Are the anti-ultrasonic feedback caps mentioned above supposed to be in the mic or in the rig?
The mic pre-amp has been bypassed, could it still be the point of entry for RF?
Any suggestions about how to solve this problem either way?

Thanks again for the great responses I anticipate receiving!


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 02, 2018, 03:36:18 PM
It may be something as simple as a bad ground/shield connection in the mic. What caps, if any, were removed?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: DMOD on February 02, 2018, 04:43:12 PM

Finally getting around to diagnose this issue with not being able to use AM on 40M while 80 works okay after switching to the D104 from the Electrovoice EV719. I went back to the Electrovoice mic and sure enough, no problem with AM modulation on 40M.

...So it looks like I am getting some type of feedback with the D104 on 40M and some of the other high bands...

The mic pre-amp has been bypassed, could it still be the point of entry for RF?
Any suggestions about how to solve this problem either way?


Make sure the Preamp has been by-passed correctly. I suspect it has not.

If that doesn't work, take a look at this schematic using a Ferrite bead and an extra capacitor.

Phil


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 02, 2018, 04:47:16 PM
It may be something as simple as a bad ground/shield connection in the mic. What caps, if any, were removed?
??? no caps removed


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 02, 2018, 04:50:17 PM
If I unplug the mic and turn up the AUDIO pot, The mod meter will peg. Sumpin' tells me there's that hum being modulated somehow.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 02, 2018, 05:18:22 PM
See if there's a difference if the Valiant is in a dummy load. The mic input is incredibly high impedance and sensitive to RF. If it happens on a load, it's either hum or feedback. Also try shorting the mic input to chassis to see if the loop begins there or further in the modulator.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 02, 2018, 05:27:41 PM
So I "unbypassed" the internal amp circuit in the D104 and put the battery back in and things are much improved, at least on 75M. Think I will use it like this for the AM forum.
With the "amp" (impedance transformer) in circuit I only need to turn the audio gain up to 9:00 position to get decent level of modulation. Do we have an big impedance mismatch with the D104? Maybe that's the culprit?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 02, 2018, 05:33:16 PM
See if there's a difference if the Valiant is in a dummy load. The mic input is incredibly high impedance and sensitive to RF. If it happens on a load, it's either hum or feedback. Also try shorting the mic input to chassis to see if the loop begins there or further in the modulator.

I test it on the dummy load, so no real difference there that I can notice. I will try shoring the input to the chassis. I am thinking not having to have the audio gain on the rig all the way up makes it less likely to go haywire.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 02, 2018, 11:16:35 PM
Still get RF or something on 40M. I can modulate the rig but poorly if I turn the mic amp up high and keep the AUDIO pot on the rig at a low level, but there is distortion.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 02, 2018, 11:38:40 PM
The Valiant's mic level control comes after the first stage of amplification, so setting it low and turning up the D-104 output can overload the first stage.

Two things to check... that V1 is a 12AX7 and that C77, 78, and 80 disk bypass caps are still in the circuit. Some ops remove them for better highs but high frequency feedback can result.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 03, 2018, 09:12:44 PM
The Varmint's mic level control comes after the first stage of amplification, so setting it low and turning up the D-104 output can overload the first stage.

Two things to check... that V1 is a 12AX7 and that C77, 78, and 80 disk bypass caps are still in the circuit. Some ops remove them for better highs but high frequency feedback can result.

I think you meant V12 not V1. And V12 is indeed a 12AX7. C77, 78 and 80 are still in the circuit.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 03, 2018, 09:33:30 PM
I think you meant V12 not V1.

Yes, exactly.

How about the shorted mic connector test? Even then there will be some noise pickup at higher "Audio" control settings, but it should be significantly less.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 11:32:35 AM
I think you meant V12 not V1.

Yes, exactly.

How about the shorted mic connector test? Even then there will be some noise pickup at higher "Audio" control settings, but it should be significantly less.

Performed the test above shorting the mic at the input. Still squeals when the audio is advanced.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 11:53:57 AM
Still squeals when the audio is advanced.

Is it a squeal or hum? A squeal suggests some form of feedback, possibly due to a bypass cap having deteriorated if the rig is unmodified. Either way, the input is *very* sensitive to all kinds of pickup due to the high gain and impedance. Tubes can also become microphonic where minute movements of the elements are picked up like a transducer.

Here's a odd but easy thing to try... swap the modulator plate leads. Doing this will invert the phase of the audio at the output of the modulator, which is the highest level point in the transmitter. The downside is that this will also invert the modulating waveform so if your positive peaks were higher than your negative peaks (a good thing) then this will reverse that. Swapping the leads on the back of the D-104 cartridge will fix this. Of course, see of this idea fixes the feedback issue first!


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 12:16:30 PM
Swapped plate cap leads on the mod tubes but still have the squeal when advancing the audio gain - so this didn't work.
A little modulation on the mic and off she goes squealing and pinning the mod current meter.
I can modulate on 40M only by advancing the clipping to the point where the meter doesn't pin, but I can still hear the squeal on the monitor receiver and the audio sounds pretty lousy. At low levels of audio, the squeal comes in on voice peaks.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 12:19:49 PM
This is really sounding like a failed cap, or a few, which can be tricky to locate, but at least it seems to be centered in the circuitry before the level control.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 01:26:45 PM
Thanks for the clues Clark. So, it looks like the area to focus on is the circuit around the first audio stage of V12?
I have already replaced all of the disc and paper caps in the circuit for V12 except for the micas. Can I use modern 1000V disc caps to replace the micas?
Just a reminder that this problem doesn't occur on 75M. And it also doesn't manifest itself using the Electrovoice mic on 40M.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on February 04, 2018, 01:42:07 PM
Check the decoupling caps C-78 C-82 and C-85.  C-85 is in the clipper circuit.  They use .1ufd caps but they should be larger caps like 10ufd

Fred


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 01:43:04 PM
Just a reminder that this problem doesn't occur on 75M. And it also doesn't manifest itself using the Electrovoice mic on 40M.

That suggests it's RF getting back in, particularly since it's more pronounced on higher bands, right? The lower impedance EV mic should help, except that it would require a higher audio level setting and you mentioned that the squeal exists even when the mic input is shorted.

Yes, you can use the new disc caps, at least for testing, to see of they make a difference. Also check for physical connection issues. Is the shielded mic input wire connected to ground lugs at V12 and at the mic input jack?

Another interesting test would be to ground the grid of V12A (pin 2) with a short jumper. This would remove the mic input lead from the equation.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 01:50:24 PM
Also check for physical connection issues. Is the shielded mic input wire connected to ground lugs at V12 and at the mic input jack?

Verified and checked for continuity to ground.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 02:50:35 PM
Another interesting test would be to ground the grid of V12A (pin 2) with a short jumper. This would remove the mic input lead from the equation

Performed this test and can advance the audio with out it squealing or the mod meter pinning, but a load hum is heard when the audio is advanced to high levels.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 03:03:13 PM
Performed this test and can advance the audio with out it squealing or the mod meter pinning, but a load hum is heard when the audio is advanced to high levels.

OK then... That hum is present in all Valiants, primarily due to the use of unbalance heater wiring in the audio circuitry. You can reduce it significantly by using twisted leads to V12 from the transformer. The grid shunting result means that the circuit between the mic jack and Pin 2 of V12A is picking up the RF.  C77 (300pF) should present a low impedance to RF that could come in on the mic input lead. That cap could be faulty.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 03:15:29 PM
Thanks Clark
I have 330 pf ceramic 1K caps. Any problem with putting this across C77?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 03:17:24 PM
I have 330 pf ceramic 1K caps. Any problem with putting this across C77?

Go for it!


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 03:32:43 PM
Okay, guess we can rule out c77. No change to report.  ???


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 03:38:55 PM
Freakin' heck.

So when you short the mic input at the jack to ground, no difference, but when you short the other end of the mic input wire to ground, the squeal stops.

The only other component between those points is a 4.7k resistor embedded within the shield of that wire. I'm not entirely sure what to suggest although Fred's suggestion of changing the bypass caps has a lot of merit, but it should be OK if those caps are of proper value (mine are).

Do you have a tube shield? It shouldn't need it, but perhaps V12 needs to have a shield around it in this case. Granted, that doesn't address the issue directly but it would add a clue.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 03:57:17 PM
Freakin' heck.

So when you short the mic input at the jack to ground, no difference, but when you short the other end of the mic input wire to ground, the squeal stops.

The only other component between those points is a 4.7k resistor embedded within the shield of that wire. I'm not entirely sure what to suggest although Fred's suggestion of changing the bypass caps has a lot of merit, but it should be OK if those caps are of proper value (mine are).

Do you have a tube shield? It shouldn't need it, but perhaps V12 needs to have a shield around it in this case. Granted, that doesn't address the issue directly but it would add a clue.

Yep, have a shield, installed it over v12. No joy  ::)

I have to say that I don't see any indication of a resistor embedded in the mic input lead shield. would that be near the mic input jack?
I don't like the way they designed this rig with the mic jack in the back and the 18" lead all the way to the first audio. Looks suspiciously like an antenna to me.  :o


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 04:16:23 PM
Also be sure that the shielded mic cable is grounded securely every few inches. RF eddy currents can build on the shield otherwise.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 05:19:24 PM
Also be sure that the shielded mic cable is grounded securely every few inches. RF eddy currents can build on the shield otherwise.

I don't see any grounding of the mic shield along its length. What does this look like in your Valiant?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 05:29:57 PM
Short lengths of bare wire wrapped a couple of turns around the shield and soldered to lugs on nearby screws. I'd bet a single grounding halfway along its length should make a difference.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 05:37:07 PM
Well if you're right, we'll say that the "G" in your call sign is for genius.  ;)


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 04, 2018, 05:46:47 PM
Well if you're right, we'll say that the "G" in your call sign is for genius.  ;)

The official acronym is "Number 1 Boys Catch Girls"


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 06:27:19 PM
Yeah, well, gotta revoke the "genius" moniker. I grounded the cable in two places -- no difference urghh! >:(
Think I'll go watch Brady do it again.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KD6VXI on February 04, 2018, 07:07:38 PM
Measure resistance from.one end of the mic cable to the other yet.

--Shane
KD6VXI


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 04, 2018, 08:41:25 PM
Measure resistance from.one end of the mic cable to the other yet.

--Shane
KD6VXI

Thank you Shane,
Assuming "mic cable" means from the termination inside the base of the mic to the connector, it looks like 0.6 ohms.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 05, 2018, 02:06:53 PM
Here's a few more things I've tried to find clues to the 40M D104 "feedback" problem.

Checked resistance/continuity from shield in mic base to pins on top of mic stand - OK
Checked resistance/continuity from shield in mic base to shield on mic cable connector - OK

Replaced 12AX7 (V12) with 12AU7 - Problem still there - doesn't seem to be that the 12AX7 is bad.
Inserted tube shield alternately over 6CL6, 12AU7, 6C4, 6AL5, 5763 (no effect)


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 05, 2018, 04:15:17 PM
...and you're seeing about 4.7k between C63 (by mic input) and pin 2 of V12? There's a resistor in that shielded cable. Is C63 good, btw? If the value changes, that could affect things.

The only reason I can think of that shorting the hot lead of the mic connector has no effect, but shorting pin 2 (grid) does, is that the latter kills the gain of V12A.

Some DEMON is getting into that audio input lead.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 05, 2018, 04:37:27 PM
I think I've made a huge bit of progress. I took a good look at the mic and noticed something...corrosion on the metal bracket that connects the PTT bar to the TU8G shaft --bad connection.
Well, I tried sliding the PTT bar clamp in place, then turned up the audio gain up and guess what???

wait for it...

no feedback...it's modulating...so I cleaned the bracket and reinstalled it.

Still wants to feedback whenever I touch any part of the mic. I could send code with it, its that reliable.

So I tried adding a jumper from the mic spring (connected to base ground) and the spring (shield on the connector to the rig and eureka - no feedback!
Now need to figure out how to fix this proper like.






Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KD6VXI on February 05, 2018, 08:03:52 PM
I've found D104s can be super temperamental like that at times.

I usually disassemble them and clean all metal to metal connections before reassembly.  Also, noalox can be used between the cheaper G stand base and the chrome(d) upright tube.

Earlier I was speaking of the 4.7k resistor in the shielded wire. Kind of a //mute// point now lol.

--Shane
KD6VXI


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 05, 2018, 09:53:08 PM
I've found D104s can be super temperamental like that at times.

I usually disassemble them and clean all metal to metal connections before reassembly.  Also, noalox can be used between the cheaper G stand base and the chrome(d) upright tube.

Earlier I was speaking of the 4.7k resistor in the shielded wire. Kind of a //mute// point now lol.

--Shane
KD6VXI

Hi Shane,
Well I tried to locate that 1 meg ohm resistor inside the shield until I realized it is soldered directly to the tube socket pins without any shield around it. I wonder if that is creating any issues?
Still trying to get the mic properly grounded to the rig.
I'm getting about 2-4 ohms between the mic body and the jack housing Cleaned the connection threads on the Amphenol jack on the rig. Took the mic connector apart to make sure the shield was making contact with the spring but nothing thus far has made much difference. As soon as I touch the mike with my hand it howls. Slide the PTT lock up and keep hands off, its okay, mostly.
I may need to disassemble the D104 like you said.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on February 05, 2018, 11:45:20 PM
Most mikes have a ground wire and a shield.  Usually the ground wire connects to the shield at the connector end and not at the mike end.  Maybe someone reverse the phase in the mike head.  This would make the hot lead ground and the wire that should be grounded the hot lead.

I'm not familiar with D-104s so I'm not sure how they're wired.  Maybe this is something you should check.

Fred


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 06, 2018, 01:30:33 AM
Valiants use around 180 VDC for PTT switching, derived from the R19/R20 voltage divider on the LVB+ supply. That's used to operate a relay and it runs through the mic cord so a good chassis/ground path is important for a few reasons.

The amplified D-104 schematic is shown below. The blue wire should be attached to the shield at the mic connector and the red wire to pin 2 of the mic connector. It sounds like that's the case but be sure the shield is making a good contact to the microphone enclosure.

Try running a lead from the shield to a lug on of the the amplifier mounting screws to be sure.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: DMOD on February 06, 2018, 02:16:46 PM
I believe you said you bypassed the amplifier?

Does the Green wire (high side audio) now go directly to the white Wire without any connection to the amplifier board?

I would connect the Yellow wire (low side audio) directly to the shield.

Suggest you ohm out the current wiring to determine what you actually have wrt schematic. However, do not place an ohmmeter across the element. 

Phil - AC0OB


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 09, 2018, 05:28:37 PM
I believe you said you bypassed the amplifier?

Does the Green wire (high side audio) now go directly to the white Wire without any connection to the amplifier board?

I would connect the Yellow wire (low side audio) directly to the shield.

Suggest you ohm out the current wiring to determine what you actually have wrt schematic. However, do not place an ohmmeter across the element. 

Phil - AC0OB

Thanks Phil,
The Green wire from the mic does connect to the white wire without any connection to the amp board (i have now completely removed the amp from the mic leaving only the terminal strip in the base)
The Yellow wire is connected to the shield.
There are 8 different wires coming from the mic element and switching mechanisms into the base:
RED
RED/WHITE
GREEN
GREEN/WHITE
BLUE
BLACK
YELLOW
WHITE

I have the wires above connected to the mic cable wires as follow:

Mic      Cable
Yellow - Shield
Green/White - Shield
Red - Red
White - White (element)
Green - White (element)
Blue - Black
Red/White - Black
Black - Not connected (wasn't sure about that one)

The mic works on 75M but it has a distinct hum. I would say it sounded much better with the amp in the circuit. I must have something screwed up.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 09, 2018, 06:11:37 PM
Black - Not connected (wasn't sure about that one.

Connecting that black wire to the shield/chassis might solve the issue.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 09, 2018, 06:51:37 PM
black now to shield. No change. Still loud hum. The hum puts the mod current to about 100ma from idle.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on February 09, 2018, 09:35:03 PM
I located an original Astatic hook up manual for those D-104s.  There are a number of different diagrams.  I need to know if you have a 3 conductor cable or a 5 conductor cable.

Let me know.

In the meantime I'm studying the diagrams.  I'll try to figure something out.  The diagram that is posted above is not your mike.  I found a diagram that has the white/red wire and the white/green wire.

Fred


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on February 09, 2018, 09:58:06 PM
OK I can tell you have a 3 conductor cable from the number of wires you have coming from the mike head and PTT switch.

First thing I see is that you may need a different cable which was my first thought.  You need a cable that has two shielded conductors and two unshielded.

I'll add more stuff as I study the diagrams.  I'm looking at the differences between the amplified D-104s and the one without the amp.

Fred

Your mike may also have a SPDT switch somewhere in the base.  That switch is used to change the function of the PTT switching.  One way it grounds the PTT line (red) when engaged and the other way it makes connection to the PTT return line (black) when engaged.

The blue, red and white/red wires are only for the PTT circuit.  The red wire from the PTT switch connects to the red wire in the cable.  The blue wire from the PTT switch connects to the black wire in the cable.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: KA2DZT on February 09, 2018, 11:41:40 PM
Studying the schematics,  there are a few differences between the amplified and non amplified D-104s.  The 3 pin head connector is wired different.  Your mike has pin1 connected to pin2 at the head end.  The non-amp mike has pin1  coming down on a ninth wire (brown).  The brown wire then connects to the yellow at the terminal strip ground.

So in your mike yellow connects to the shield AND the terminal strip ground (terminal #1 ground).

You removed the amp, so you should have removed the brown, green and white wires from the terminal strip that when to the amp.  This will leave only the green wire from the mike head on terminal #3 (end of terminal strip).  Now connect a jumper from terminal #3 to terminal #8 (end to end).  This connects the green wire from the head to the white wire in the cable.  It also leaves in place the switch terminals that short the mike when on receive.

The black wire on terminal #7 was for the battery negative return.  Should be no connection to terminal #7

That should do it.  The mike should work.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 10, 2018, 09:42:02 AM
Thanks very much for the input on the D104 wiring.
The mic indeed does work now. Clark and I were working on this for a looong time last night. I still have hum in the audio.
I installed a 15UF electrolytic cap parallel with C79 which is on the plate of the first audio stage (V12) to chassis ground.
Still have hummmm....maybe 15UF not enough?


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 10, 2018, 10:34:08 AM
Still have hummmm....maybe 15UF not enough?

If the hum were coming in on the LVB+ then that would have absolutely resolved it. I posted the input schematic for all to see. Clean with Audio Input pot R28 down, hum with it up.

One thing you could try for simpler monitoring is to connect a small amplifier to R44 in the cathode circuit of V14 (not shown) via a D.C. blocking cap.  This would allow you to hear the audio using only the LV supply and without keying up.

The main question is how much is beyond the normal Valiant hum. Part of the design reason for using low value coupling caps was to roll off the low frequencies.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 10, 2018, 10:44:53 AM
I think I might have solved the hum.
I put two 15UF caps in parallel with the C96A & B on the LV rectified and reports on 75 meters tell me no more hum in my audio!
I guess I had a bad cap or two there (both are new!)
Haven't tried it on 40M yet, but I am fully expecting it to work.

 ;D ;D ;D ;D


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: DMOD on February 10, 2018, 06:24:05 PM
The EFJ Vikings and HK Apaches of that era were woefully underfiltered and if you scope the power supplies you would see high ripple voltages.


Extra filtering at every stage does help reduce the hum .


Phil - AC0OB


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: WZ8J on February 14, 2018, 07:58:59 AM
I think it's time to wrap up this long thread on the Valiant's latest issues with both hum and RF feedback on 40M.
The RF feedback was resolved by completely removing the the amplification components from the D104 and finally getting the mic properly wired.
The hum returned after I thought I had it licked. It turns out the source of the hum after trying many different tests, was that I had installed the aluminum shield on around the mic jac wrong side up! Duh. I only discovered this when I looked at pictures in the manual.
Very happy that the rig now gets good audio reports, no hum and works fine now on 40M. It also appears to have resolved the chirp I was getting on 40M CW.

Thanks to all who helped, especially N1BCG who has spent hours helping me tame this little beast. I hope others will find this thread helpful in solving some of their own Valiant issues.


Title: Re: Modulation Bias Adj Pot value on Valiant
Post by: N1BCG on February 14, 2018, 11:26:42 AM
Sounded great on the air!

One thing to check for lingering chirp is that the modulator is disabled in the CW mode. The circuit is designed so that the mod screen grids are brought to 0 Volts and the xformer secondary is shorted. See if you can hear your voice when in CW and that the modulator current is reduced below the 50-70 resting range.

Glad you hung in there and remained patient throughout the long process. This thread will certainly show others that troubleshooting classic gear can be challenging, but in the end, rewarding and educational. Think of how familiar you are with the Valiant now!

Unfortunately, discontinued microprocessors and surface mount technology in newer rigs make such hands-on repair nearly impossible.
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands