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Modulation Bias Adj Pot value on Valiant




 
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January 21, 2018, 07:58:24 AM *
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Author Topic: Modulation Bias Adj Pot value on Valiant  (Read 1308 times)
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WZ8J
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« Reply #25 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
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WZ8J
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« Reply #26 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.
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WZ8J
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« Reply #27 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.
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N1BCG
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« Reply #28 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.
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WZ8J
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« Reply #29 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?
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N1BCG
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« Reply #30 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?
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WZ8J
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« Reply #31 on: January 19, 2018, 10:07:47 AM »

1K ohms across C93 A&B
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N1BCG
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« Reply #32 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.
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WZ8J
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« Reply #33 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?
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N1BCG
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« Reply #34 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.
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WZ8J
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« Reply #35 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.
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N1BCG
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« Reply #36 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?
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WZ8J
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« Reply #37 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.
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N1BCG
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« Reply #38 on: January 19, 2018, 11:37:39 AM »

Soldered on the rectifier socket. Are there also series resistors?
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WZ8J
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« Reply #39 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
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WZ8J
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« Reply #40 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?
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N1BCG
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« Reply #41 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...
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