modulation bias on rca bta1r

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w5tmg:
well i finally have  producing rf but i now have a bias issue on the modulators. there is -140 volts on them but i cant balance them and the left tube glows red. ive switch out the tubes, but its in that left socket... i replaced the bias potinceometers, and that didnt do anything either. and the tubes are good any help would be appreciated. my email is w5tmg0@gmail.com or my no. is 972 74 30541 leave a voice mail and ill call ya right back... thnxs... oh ya ive checked all the res. and i cant find an issue with them. the simlpest circuit in the whole dang thing, and i just cant figure it out...frustrated!!!!!!!!!!

KL7OF:
What does the bias measure at the tube socket(s) using a voltmeter?...Key down, key up?

w5tmg:
-140

KL7OF:
If the bias is -140 at both tube sockets, with the key up or down, it would seem to be a problem with the tube...Since you have switched the tubes, and the problem stays on the same socket, that would seem to eliminate the tubes as a problem....Are you sure the bias is constant and not changing with the key down/up? Are there 2 separate bias supplies for the modulators or a double secondary on the bias transformer? Balanced how?.. Perhaps a bad ground in the bias circuit...What is the bias return path? Bypass caps in the bias circuit?   Check the connections.....Good Luck

n2bc:
I would try swapping the wires at 34F and 35F (they feed the bias to the individual tubes. See if it stays on the socket.

I had a lot of charring on the 'sockets', they are simply individual pin sockets mounted on phenolic.  The filament pins were the worst, but big hunks of phenolic turned to charcoal & the 'socket' in general was unstable.  Was not hard to repair.  The overheated pins were useless... I scavenged replacement pins from some EFJ sockets & repaired the phenolic with hi-temp epoxy after scraping away all the charcoal.

 Also "just because they're ancient" I would replace 1C503 and 1C504, the .1uF blocking caps that feed the audio from the 2E26s.  And make sure the 100K resistors, 1R512 and 1R513, and the 100 ohm Rs, 1R301 and 1R302,  are all within spec.

If you still don't get anywhere, try disconnecting the feedback ladders from the mod plates & see if things stabilize.

Be careful in there!   73, Bill N2BC

A pix of my BTA1R1 after a small remodel:

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