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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 6387 times)
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WZ8J
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« Reply #50 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.
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N1BCG
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« Reply #51 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.
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WZ8J
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« Reply #52 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.
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N1BCG
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« Reply #53 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.
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WZ8J
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« Reply #54 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.
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N1BCG
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« Reply #55 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!
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WZ8J
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« Reply #56 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.
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N1BCG
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« Reply #57 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.
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WZ8J
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« Reply #58 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.
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KA2DZT
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« Reply #59 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
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N1BCG
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« Reply #60 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.
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WZ8J
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« Reply #61 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.
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WZ8J
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« Reply #62 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.
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N1BCG
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« Reply #63 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.
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WZ8J
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« Reply #64 on: February 04, 2018, 03:15:29 PM »

Thanks Clark
I have 330 pf ceramic 1K caps. Any problem with putting this across C77?
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N1BCG
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« Reply #65 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!
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WZ8J
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« Reply #66 on: February 04, 2018, 03:32:43 PM »

Okay, guess we can rule out c77. No change to report.  Huh
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N1BCG
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« Reply #67 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.
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WZ8J
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« Reply #68 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  Roll Eyes

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.  Shocked
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N1BCG
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« Reply #69 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.
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WZ8J
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« Reply #70 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?
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N1BCG
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« Reply #71 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.
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WZ8J
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« Reply #72 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.  Wink
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N1BCG
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« Reply #73 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.  Wink

The official acronym is "Number 1 Boys Catch Girls"
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WZ8J
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« Reply #74 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! Angry
Think I'll go watch Brady do it again.
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