modulation bias on rca bta1r

[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:

[w5tmg]

thnxs steve and bill, fyi, ive change coupling caps, reostats, all res. are in tolerance, and it still acting up. kina leaning towards a bad tube socket. my bias is -140 on both tubes now, and i found a disconnectected wire, thats why i wasnt getting any reading on the meter. i hooked it up and it pegged the %meter and it broke. i have a spare, luckily... my next move is to unhook the ladders and see what happens... wow never had this much trouble with an audio circuit. gettin to know this old bucket of bolts intemently... guess if it was easy everybody would be doin it... again thnxs for the suggestions...

[n2bc]

Before you kill another meter....   what wire and what position on the % meter?  Rather than thinking that the meter died, I would suspect something KILLED the meter.  You would be best to shoot the bug on paper first - or cobble in a VOM in place of the  % meter and set the VOM to read a couple of amps - I suspect the meter is a 1MA.

And again, don't rule out the socket itself.  Yank the tubes, climb up in there and shine a flashlight into the plenum to inspect the pins.  From underneath the plenum, wiggle the connections a bit, if the screws move then the bottom of the plenum in that area is burned.
If the area is burned then the socket pins are toast too (lost all contact pressure).

[w5tmg]

LOL, nope dont wana kill another meter, im gona hook up the vom to it instead of my spare. hope i didnt sound like i was gona do that. and thnxs for the info on what to look for on that tube socket...its almost got to be that, ive checked everything else...

[w5tmg]

oh ya and the posision was 11 the last pos. 1kl 2nd af

[w5tmg]

well i found another issue that ive fixed, the cathode res. were all open except for one, but before i found it i found a wire dissconnected , rung it out and put it back, so keyed it up and promptly took out my % meter. the wire was in corralation with one of those open res. luckily i have a spare meter, and res back in place. threw it on the air last nite and signal was good but low audio, and the left tube is still getting red. heres whats gonna get me cursed out, i did this with out replacing the % meter... i think im very close but im just not seeing it. bias is till good on each tube, -140...

[w5tmg]

i located the problem with the bias. yea, but now i have 35 to 40% audio. its clear without distortion, just low. ive checked all of the caps, and there good, the % meter is readin 100% on each tube. im using the same audio chain as i use with my other bc tx, threw an audio distribution amp. im just wondering if the phasing on the mod transformer might wired wrong. any thoughts on this would be appreciated. again thnxs for all the help ive gotten on this adventure, and all the patience yall all have with me...

[KB5MD]

Tracy, do you have a scope to put on the output.  If that tx is anything like my BTA 1R1, putting a scope on the output is about the only way to tell if you're getting enough modulation to be close to 100%.  The percentage meter is sort of a guesstimate.

[w5tmg]

well i just got it on the air with 100% modulation, very happy!!!!!!!