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Modulation Bias Adj Pot value on Valiant




 
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Author Topic: Modulation Bias Adj Pot value on Valiant  (Read 7352 times)
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WZ8J
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« 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!
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MikeKE0ZUinkcmo
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« Reply #1 on: December 09, 2017, 11:25:39 PM »

Should be in the parts list section of the manual.
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Mike KE0ZU

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w8khk
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« Reply #2 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.
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Rick / W8KHK  ex WB2HKX, WB4GNR
N1BCG
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« Reply #3 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.
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WA5VGO
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« Reply #4 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.
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w8khk
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« Reply #5 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.
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Rick / W8KHK  ex WB2HKX, WB4GNR
N1BCG
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« Reply #6 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...":


* Bias_Earlier.jpg (51.72 KB, 333x293 - viewed 118 times.)

* Bias_Later.jpg (71.22 KB, 304x263 - viewed 124 times.)
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WZ8J
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« Reply #7 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. Cool
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WZ8J
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« Reply #8 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!
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N1BCG
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« Reply #9 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. Cool

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


* Johnson_Viking_Valiant_schematic.jpg (735.03 KB, 1944x1476 - viewed 229 times.)
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N1BCG
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« Reply #10 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...


* L6C_Original.jpg (195.64 KB, 520x614 - viewed 101 times.)

* L6C_New.jpg (253.04 KB, 644x785 - viewed 109 times.)

* Valiant_L6C_2.jpg (1422.96 KB, 3264x1836 - viewed 192 times.)
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WZ8J
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« Reply #11 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.
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KA2DZT
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« Reply #12 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
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N1BCG
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« Reply #13 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
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KA2DZT
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« Reply #14 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
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K3FEF
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« Reply #15 on: December 16, 2017, 07:03:27 PM »

Following this too. :-)  It's a 2W pot! LOL
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N1BCG
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« Reply #16 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.
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WZ8J
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« Reply #17 on: December 17, 2017, 10:41:24 AM »

He is, after all, human  Grin

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.
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N1BCG
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« Reply #18 on: December 17, 2017, 11:23:48 AM »

He is, after all, human  Grin

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.
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WZ8J
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« Reply #19 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
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KA2DZT
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« Reply #20 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.
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DMOD
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« Reply #21 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

 



* Viking Valiant Regulator Upgrade Schematics.pdf (68.24 KB - downloaded 50 times.)
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"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
WZ8J
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« Reply #22 on: December 24, 2017, 12:08:14 PM »

So oddly, the Valiant seems for have fixed itself  Shocked
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.
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Steve - K4HX
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« Reply #23 on: December 24, 2017, 03:01:40 PM »

You sounded good on 40 meters today.
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KD6VXI
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« Reply #24 on: December 24, 2017, 03:17:47 PM »

So oddly, the Valiant seems for have fixed itself  Shocked
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
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