The AM Forum

I have been using a restored Viking Valiant for some months here. I decided that I might like to perform a few modifications to open up the audio. I searched the net for the modifications and selected what seemed to be a simple mod and one that was posted in numerous places.
The mod that I selected was “Remove C54, C55, C56 from the accessory plug” OK seemed simple enough. I pulled out the schematic and turned the radio on its side. I had a bit of a hard time figuring out if the schematic was showing the front or the back of the plug. Finally I realized that pin 9 goes to ground only and pin 9 is right next to the socket keyway. So, all I had to do was jump over the keyway and the caps to be removed would be three in a row.
Well I did all that and removed the caps. When I fired up the radio all was well. But when I hit the PTT switch a few seconds later I saw a flash at the base (as in under the chassis) of the rear 866A (V19). After that all went dark. Today I pulled the fuses in the wall plug and both fuses were blown.
I went back and checked my work on the mod and it still seems like I removed the correct caps. ANY thoughts would be appreciated.
I am a mechanical guy trying to be an electrical guy here.
Question # 2 I would be interested in any plans out there that would allow me to solid state this rectifier circuit. Where would I find that? Short of that were does one buy 866A tubes?
I am very sorry to have to bother you folks about this and the way it is going it might not be the last.
All assistance is appreciated!

I doubt that removing those caps had much to do with a flash near the 866's.  They are just bypass caps around the accessory socket, unless you left a dangling lead or solder blob or something?

Are you sure the flash was from under? I just ask because if you had the thing sideways with the 866s in it one of them might have flashed over.  They like to be kept upright, and if you put them sideways it's good to let them warm up for a good while before hitting the HV.  You've got to allow the mercury time to evaporate from all the internal structure and start to condense around the base of the bulb.  I find that when I first turn on the filaments, the whole bulb turns silver, after a bit it begins to clear. Once most of the bulb is clear, the mercury is where it's supposed to be (my theory, but I haven't lost one yet!) takes about 15 minutes, a bit longer won't hurt and if in doubt, let them cook.

Check the wiring around the rectifier sockets, it's been found that the filament wiring on some valiants 866s was not up to the job.  Also check the solder joints etc around the HV section.   If it was something related to that, it should have left evidence, like scorch marks.

866's can be found on ebay or post a request here (better).  You can also get 3B28 tubes and just plug them right in, since they are a direct replacement. No mercury issues.  Though I used 866s for quite a while with no troubles (warm them up 90 seconds min). I do like them enough to use them in my HB amp in a full wave bridge.

As far as going SS in the HV rectifier,  just string together two sets of stout diodes and replace the 866s with them.  I can't remember which ones I used, but they were ~1000V PIV and 3 amp jobs. I think I used 5 in each leg... more is better, and diodes are cheap.   If you pull the sockets and filament wiring for the 866's you can fit in a terminal strip easy to support the diodes.   You can also use the 5V winding to buck the voltages down a bit.  

If you haven't, it would be a really good time to replace the filter caps with modern ones of higher ratings too.  I think I used 100uf 450v caps in there.

check this out



http://www.amwindow.org/tech/htm/valiant/ps.htm


I likes  1N5408's    3A and 1KV

ya can get 100 of 'em from mouser for $11 plus shipping ($8.00 USPS??)
http://www.mouser.com/Search/ProductDetail.aspx?R=1N5408virtualkey53310000virtualkey821-1N5408

If your going to do any lectroniks work, you might just as well byte the bullette now and have the stuff on hand fer the future.
klc

I think Ed nailed it.  When a MV rectifier is moved from its normal vertical position it should be run filament only for at least 10 minutes.  This is just one of the drawbacks of MV rectifiers.   Unless you must watch the flashing light show of your rectifier I would strongly suggest replacing it with a solid state unit (or if you want to stay with tubes go with a 3B series Xenon filled).

While you are modifying, make sure that the 5 volt wiring to the 866 sockets is in good shape.  Early Valiants were built with low voltage wire and it will arc over taking out transformers in the process.  If you switch to SS you can just disconnect and insulate the 5 volt wiring from the LV transformer.  Otherwise sleeve the wiring or replace it with HV rated.

Sorry to not make it clear that when I powered up the rig after the mod it was resting in its normal operating position. Howver I might have powered up the rasdio and gave a very short time before I hit the PTT switch. Maybe i should replace the fuses and try the power up again.

You're allowed only one screw-up per month.

It's 4AM and now I have to answer you post.

My first thought when I read it,  866's laying on their side, big no no.  Ed  KB1HYS already gave you a more detailed answer, so I can go back to bed.

But before I go, don't let that happen again or we'll have to come over and take away all the tubes from your set.

Fred

OK I put everying thing back together this morning and it appears that I was a bit quick to flip the ptt switch after the main power up. The radio powered up just fine this morning and after letting it warm up for 10 minutes I flipped the PPT switch and it appears that all is well. I will try to TX tonight.

As Always thanks to everyone!!!!

Happy Holidays!!

I am glad your Valiant survived the flashover episode.  The fuses saved the plate transformer this time but it experiences a very high current surge when a rectifier flashes over and next time could easily be when the transformer goes out. 

I repaired a Valiant a few months ago that lost its plate iron from a MV tube flashing over.  The operator had multiple rigs on his desk and pressed the ptt lock on the Valiant mic when he was using another transmitter.  The next time it was powered up the plate relay activated well before the 866 cathodes were up to temperature and although his fuses blew one side of the HV secondary failed.  Although this was operator error it could have just as easily happened with a sticking plate relay or problem in the microphone wiring.  SS rectifiers are a plug and play modification and you don't need a dropping resistor when replaceing MV types since the forward voltage drop is very low with these tubes.

With only 650+v on them, 866s usually wont flash over under any circumstances. I have run my Valiant quite a few times on it's side.

I think everyone is missing the biggest possible suspect here.............. The accessory socket itself!! That rats nest of small chokes and coils, and the shield cap with those little pieces of wire and spaghetti tubing going through them. With TVI no longer being much of a problem, I would clean out that whole corner of the chassis and neaten up that wiring, and lose that cover over the socket, just for reliability!!

                         

Yep, I did that exact operation to my valiant. removed the ACCY socket and all associated baloney, and never looked back.

Muntz Valiant ;D

When you reffer to "filter caps" Am I correct in thinking that you are reffering to the two large caps (mine are 80 mf, 450v) attached to the 866 sockets?

Would these be a suitable replacement for these?

http://www.mouser.com/ProductDetail/Nichicon/UPT2W121MRD/?qs=sGAEpiMZZMtZ1n0r9vR22W5LieB67ml26e%252bPvyMo47E%3d
 (http://www.mouser.com/ProductDetail/Nichicon/UPT2W121MRD/?qs=sGAEpiMZZMtZ1n0r9vR22W5LieB67ml26e%252bPvyMo47E%3d)

http://www.mouser.com/ProductDetail/Nichicon/TVX2W101MDD/?qs=sGAEpiMZZMtZ1n0r9vR22SmzhV1iNnMtgMXo9YUPeBg%3d (http://www.mouser.com/ProductDetail/Nichicon/TVX2W101MDD/?qs=sGAEpiMZZMtZ1n0r9vR22SmzhV1iNnMtgMXo9YUPeBg%3d)

I see that under the mod tuber there are another pair of large caps (mine at 15 mf, 450v) should these be replaced as well? Should we always try to increase the f rating and the voltage rating in these cases?

The guys laid it out for you.  Glad your rig is not damaged.  Make sure the acc socket wiring is 100%.  Its very common for the Fil to short for the 866s.  I have a valiant that way so we used diodes. You can get the plug in type on feabay or on the net. No need to hack anything up.

Most of those caps will do nothing for the audio of the valiant. In my opinion three things should be done. The Coupling caps from stages should be changed and the neg feedback needs to be added.  The final thing is to use a 10Meg ohm resistor for the Grid for a D104.  After those mods, The Valiant sounds pretty decent.

Make sure the HV filter caps are up to the task. Otherwise, The SS diodes will add up to 100 volts extra over those old 866s. If your caps are old, Dont try it until they are replaced.

The 866a from cold should have at least 1 min warm up time and if you care for the radio 5 full minutes.  If you cant do that, Then use 3b28s where are near instant on tubes.


C

That's why I use pole pigs. Then the only loss is the copper vaporized out of the primary relay.