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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 4774 times)
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KD6VXI
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« Reply #75 on: February 04, 2018, 07:07:38 PM »

Measure resistance from.one end of the mic cable to the other yet.

--Shane
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WZ8J
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« Reply #76 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.
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WZ8J
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« Reply #77 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)
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N1BCG
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« Reply #78 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.
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WZ8J
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« Reply #79 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.




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KD6VXI
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« Reply #80 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
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WZ8J
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« Reply #81 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.
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KA2DZT
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« Reply #82 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
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N1BCG
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« Reply #83 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.


* IMG_7251.GIF (8.75 KB, 350x224 - viewed 53 times.)
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« Reply #84 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
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WZ8J
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« Reply #85 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.
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N1BCG
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« Reply #86 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.
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WZ8J
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« Reply #87 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.
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KA2DZT
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« Reply #88 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
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KA2DZT
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« Reply #89 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.
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KA2DZT
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« Reply #90 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.
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WZ8J
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« Reply #91 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?
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N1BCG
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« Reply #92 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.


* Valiant_Input.jpg (134.16 KB, 528x302 - viewed 35 times.)
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WZ8J
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« Reply #93 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.

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« Reply #94 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

* Viking Valiant Upgrade Schematics for AMFONE.pdf (90.77 KB - downloaded 20 times.)
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« Reply #95 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.
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N1BCG
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« Reply #96 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.
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« Reply #97 on: February 19, 2018, 06:35:30 PM »

Killed some of these myself. The important thing is to find a high wattage pot. Adjust the value of the fixed resistor if you can't get it within range. Some mods have put a regulator tube in that circuit where the rectifier used to live. That assumes you have solid stated the PS. Those screens need a decent stable voltage for the best audio.
Keith
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