Viking Valiant modulator destruct

[WB5IRI]

Yakking on 3890 when the modulator current pegged the meter, smoke appeared, and then mod current dropped to zero. The 22 ohm resistor in the plate lead of one 6146 modulator tube cracked in half, and that tube shattered its glass. Was fun removing it without cut fingers. The other 6146 in the modulator circuit is fine, as is the 22 ohm plate resistor. So, what happened? Where should I start looking?

Doug, WB5IRI

[steve_qix]

Wow, I hate it when that happens!!!!  Seriously, it could have been a parasitic, a bad tube (particularly a grid to grid short) or even some type of audio oscillation (like a parasitic, but different).

[WB5IRI]

Yeah, me too. And I was just about ready to slide it back into the cabinet, too. Painted it 2 months ago and still have not got it back in. Every time, something like this happens. But, hey, boat anchor fun, right? These radios were not meant to last this long, and certainly not designed to ever be put back into their cabinets!

I'm going to replace the plate resistor and tube, then check voltages. Maybe it was just a catastrophic tube failure and not the total loss of the modulation transformer.

Doug, WB5IRI

[WQ9E]

If the modulation transformer was lost it was a result and not the cause.  Check the bias voltage at both sockets but it sounds like an internal tube short causing it to go to full conduction.  The resulting heat from extreme plate dissipation cracked the envelope.  Does the tube plate show signs of heat damage?

[AB2EZ]

"... and then mod current dropped to zero."

One working tube would draw half the normal modulator resting current if everything else were okay. Zero current suggests that something else (besides one destroyed modulator tube) is bad/not working properly.

Check to see if both halves of the mod transformer's primary are still okay (low DC resistance from either end to the center tap).

Check to see if both halves of the audio driver transformer's secondary winding are still okay (low DC resistance from either end to the center tap).

Check to see if the negative bias voltage measured between each modulator tube's grid socket pin and ground is correct. You can check this with both modulator tubes removed: to prevent destruction of another tube.

Possible causes:

A) Grid bias voltage goes to 0V on one tube's grid, due to a cold solder joint at the point where one side of the secondary of the driver transformer connects to the socket. But there is still a resistive path (e.g. a 10k ohm resistor) from that tube's grid to ground. The tube is destroyed, and the mod transformer's primary winding opens up on the side that feeds B+ to the other mod tube. (But why would that side of the winding open up?)

B) Grid bias voltage goes to 0V on both tubes' grids, due to a failure in the bias supply circuitry. One tube is destroyed, and the mod transformer's primary winding opens up on the side that feeds B+ to the other tube.

C) One tube shorts out internally... destroying itself and causing the mod transformer's primary winding to open up on the side that feeds B+ to the other tube. (But why would that side of the winding open up?)

D) Same as B) above, but an open circuit develops in the path between the B+ supply and the center tap of the primary of the mod transformer (saving the transformer).

Stu

[AB2EZ]

Doug

You wrote as part of a post in a different thread, earlier this month that:

Your Valiant's modulator current was 150mA when you first "fired it up", and then dropped to 50-70mA  after 15 minutes of normal operation.

You also said, in that post, that you had the bias adjustment pot "backed all the way down"

If the bias adjustment pot is still backed all the way down, that might explain why the modulator's resting current fell to zero after the first tube failed.

The tube that failed was possibly defective... drawing essentially all of the measured cathode current... and eventually dying from excessive plate dissipation.

Stu

[WB5IRI]

Yes, it has been acting strange of late. When I get home from work tonight, I will check the mod transformer (keeping my fingers crossed here) and hope it hasn't opened up. Your post gives me new hope that all might not be lost. I'll check the voltages and component values associated with the modulator circuit, then, if there are no obvious faults, replace the 22 ohm plate resistor and tube and see what happens. I hadn't considered how one tube being bad might be causing the high initial static modulator current. Couldn't figure out why it would settle down after awhile, either. But your thoughts on the matter shed some light. If all seems to be working again, I may just go ahead and do the Timtron audio mods while I am in there. Got a 12BH7 on order to replace the 12AU7.

Doug, WB5IRI

[W7SOE]

The modulator tube plate resistors on my Valiant are bad as well.  What power rating should those resistors be?  What are the value and power ratings of the final tube plate resistors?  My schematic does not list their value......

(Parasitic Suppressor resistors)

Thanks

Rich

[KB2WIG]

R48 and R 49 are listed as 22 ohm, 1/2 watt.


http://bama.edebris.com/manuals/johnson/valiant/

The construction manual,

http://bama.edebris.com/manuals/johnson/valiantconst/

[W7SOE]

There they are, Thanks

Rich

[AB2EZ]

I guess this makes sense.

If either tube draws 150mA, the corresponding 22 ohm, 0.5W resistor will be right at its rated dissipation.

300mA (2W of dissipation) should fuse the resistor fairly quickly.

Stu

[W7SOE]

What are the values of the suppressor resistors on the 6146 finals?  Not in the schematic and mine are too burnt to read.

Thanks

Rich

[AB2EZ]

Doug

Any update?

Stu

[WB5IRI]

Yes, although work has been keeping me so busy I haven't had much time to spend on it. I replaced the 22-ohm plate resistor and the 6146. Preliminary tests show the modulator still draws in excess of 150 mA static modulator current and the audio has a really loud hum, so I suspect a capacitor has failed. I am waiting on parts now, and when they arrive I am going to completely redo the modulator section, replacing all bad or out of tolerance parts and all the old capacitors (someone did most of them before I got the rig, but there are still some old paper caps in there), and I will do the WA1HLR mods at the same time. The modulator transformer seems OK, so I dodged a bullet on that one. Now, if only I could find a way to work on my rigs more and work to be able afford them less, life would be more satisfactory!

Doug

[WQ9E]

Doug,

Make sure you replace the bias caps and I would NOT modify anything until you get the transmitter working as intended then feel free to molest it.

[KA0HCP]

No mods until it is working perfectly!!!

[KB2WIG]

"  Make sure you replace the bias caps and I would NOT modify anything until you get the transmitter working as intended "

                                                                               C.

Ask me how I know.


klc

[Pete, WA2CWA]

"  Make sure you replace the bias caps and I would NOT modify anything until you get the transmitter working as intended "

                                                                               C.

Ask me how I know.


klc

Ask anyone with a Valiant 

[kb3ouk]

"  Make sure you replace the bias caps and I would NOT modify anything until you get the transmitter working as intended "

                                                                               C.

Ask me how I know.


klc

Ask anyone with a Valiant 

Where's Philip? I know he has his share of "Valiant go BOOM" stories. 

Shelby

[AB2EZ]

Doug

I suggest that there is an easier and more methodical way to troubleshoot the problem

Step 1: let's see if there is a low frequency or high frequency oscillation, and let's see whether the loud hum is being caused by hum on the bias supply's output voltage.

A) Remove the 12AU7 driver tube (V15).

B) Let us know if the bias current behaves normally, including the operation of the bias adjustment pot. Let us know if the loud hum is still there.

Naturally, there will be no normal modulation of the RF output with the audio driver tube removed; but removing the audio driver tube should kill any low or high frequency oscillations, and should eliminate hum on the RF envelope that is originating in the lower level stages of the audio chain. At the same time, hum originating in the bias supply will still result in modulation of the RF, and the associated hum on the RF envelope, even with the audio driver tube removed.

If removing the audio driver tube fixes the strange biasing behavior, and also eliminates the hum, then we can proceed to step 2. If removing the audio driver tube does not fix the strange biasing behavior, and/or does not eliminate the hum, we can proceed to step 2A.

I agree that you should defer any mods until you get the Valiant working properly in stock configuration.


Stu

[ka4koe]

Valiant go boom about 3 weeks ago. Mom is in the hospital with a fractured femur right below the ball part, so just got home. This is the only radio play time I'm gonna have for awhile.

I've got a bunch of doorknobs waiting to be installed. I blew my modulator tubes early in my excruciatingly painful learning process by having the drive set too high. Also blew the resistors. Hope your mod iron is okay. I made about every dumb mistake one can with one of these but managed to not blow up any of the iron. Do not install any screws on the cabinet after repair (a Valiant is never truly "fixed" and one utters this VERBOTEN word in its vicinity only at extreme peril).

[WB5IRI]

OK, OK, I get the message. I was feeling really disgusted and was just going to rip into her and make all the mods at once, but you guys are right -- get it working first, then do the mods. I will pull the 12AU7 and check things next time I get some working time on the rig. Was going to do some voltage checks tonight, but over tightened the microphone connector on my VTVM (what genius thought a mic connector was a good idea on a VTVM!?!?) and the chassis connector turned and obviously broke the internal connection. Calls for removing all the knobs and a bunch of teeny tiny nuts and a couple of boards to get to it, and I am just too tired after work tonight. Plus, no AM action on 7295 tonight. Couple sigs on 7290 but too weak to try to work. Also thinking about buying a Viking 500 but then I keep asking myself if I REALLY want to spend that much money on a rig that will go belly up every time I look cross eyed at it and is too darn heavy for me to move and work on anyway. Think I'll stick to the Valiant while I can still move it.

Point well taken about the cabinet screws. I've learned my lesson there.

Doug

[AB2EZ]

As an addendum to my earlier post:

If there is hum on the modulator grid bias, it will be applied to both modulator tube grids in equal amounts. If there is no other grid signal (for example, if the audio driver tube is removed), plate currents from the modulator tubes into the modulation transformer will cancel each other, in terms of the hum on the secondary of the modulation transformer.

We can explore that further (by removing one of the modulator tubes) after you complete step 1 in my earlier post.

As an aside, although it is a good idea to supplement the bias supply capacitors by placing a suitable electrolytic capacitor in parallel with each existing capacitor... some of the symptoms suggest that hum on the output of the bias supply is not the problem here.

Stu

[WB5IRI]

OK, I think it is time for step 2A. Removed the 12AU7, static modulator current remained at the same high level, nearly 180 mA, no difference in hum. R61, bias control, backed all the way down. Plugged the 12AU7 back in just to verify, same results. Then removed one modulator tube as well as the 12AU7, hum reduced but still there, static current at about 90 mA, as you would expect. Replaced the driver tube but still on one modulator tube, no difference. Replacing R63, 15k resistor on one leg of the bias pot, with 5.6k, brought static current down to normal levels (this is with both modulator tubes and the driver tube in place), but hum greatly increased. R63 goes from the bias pot through a 1k resistor to the bias rectifier tube, V21, a 6BY5GA. Returned R63 to its correct value, hum reduced but still there, current back to excessive value.

The bias caps are C93 A&B and C96 A&B, 15uF, correct? I haven't replaced them, but I will.

STEP 2?

Doug

[WQ9E]

The bias caps are C93 A&B and C96 A&B, 15uF, correct? I haven't replaced them, but I will.

STEP 2?

Doug

Doug,

The bias caps are C93 A&B; C96 A&B are the filter caps for the +300 volt supply.  As I stated earlier the bias caps should be replaced and in general the bias filter caps should always be replaced if there is any question since a bias supply problem or failure can cause many big and expensive problems.  Be sure to connect the positive leads to ground for the bias caps since it is a negative voltage supply.

If C96 A and B are original then it would be a good idea to replace them also along with the HV filter caps to improve the odds of reliable operation.  Replace C99 (10uf cathode bypass for the audio driver) while you are at it since this is usually degraded in my experience with MANY Valiant transmitters.