VTA ST120 - Lost right channel, help needed

I recently attempted to change the power tubes in my 120, with apparently at least one bad 6550 on the front right, which redplated. After shutting down and reinstalling four known good KT120s (and then swapping those for four new 6550s from a reputable supplier), there is no sound from the right channel. Left channel plays fine. Switching the positions of the three driver tubes or swapping them out with three others has no effect. I have the yellow sheet mod with IN4007 diodes installed, but can't seem to get a consistent ohm reading on them.

I made the following measurements following Bob's diagnostic checklist:

All four 10 ohm resistors at the bias points measure 10 ohms.

B+ on driver board: 412 volts DC (slightly high)

GZ33/5AR4s (three different rectifiers measure similarly)
Pin 4: 218 volts AC (very low)
Pin 6: 220 volts AC (very low)
Pin 2: 517 volts DC
Pin 8: 518 volts DC

Quad cap
#1 (square):     515 volts DC
#2 (half circle): 510 volts DC
#3 (circle):       510 volts DC
#4 (triangle):    428 volts DC (a bit high)

Sovtek 6550s (all four measure similarly)
Pin 1: .500 volts DC
Pin 3: 509 volts DC
Pin 4: 508 volts DC
Pin 5: -62 volts DC
Pin 6: -62 volts DC
Pin 8: .500 volts DC
Across pins 2 and 7: 6.9 volts AC (slightly high)

Any advice would be much appreciated!

Thank you for your quick reply, Bob.

I tried switching the interconnects from channel to channel. Then I swapped everything out completely with different RCAs, speaker wire, preamp, source component and speakers, and the problem remains the same: no signal output from the 120's right channel. The issue started after I lost control of the right front power tube.

Bill

Perhaps when you removed the tube from the socket something came loose. Clean the socket and make sure the socket pins are fine. Also make sure the tube is going in the slot and plugging in correctly - the tube sockets can allow the tubes to go in wrong.

Thank you Bob and rebellovw for your sugestions. I found a loose connection at pin 3 of V7. After resoldering it I got faint sound coming from the right channel for a short time, then back to silent. I haven't been able to find any other loose connections that create static or sound in the right channel.

Forum reading suggests that replacing the LM334Z IC on the right channel would be the next step?

You can buy genuine TI LM334 @ Digikey for $1.34

There is so much fake garbage out there it is not worth saving a few cents. Manufacturers like TI & AD will not accept failed parts unless you can provide provenance.

Pin 3 or 8 on v1 & v3 should both have about 10.5 volts according to the schematics. If they do, the LM334 is OK.

Check the speaker terminal connections are well soldered.

Unplug the amp and disconnect the speakers. Remove the power tubes. Ensure there is no voltage across the SCM. Now measure the resistance from B+ to pins 3 & 4 on each power tube and note the values. They should be the very close on each channel. If not, OPT maybe toast.

Next measure resistance on the output side from GND to 4Ω, 8Ω and NFB. Values should be very close. If not, the OPT is toast.

If you have a voltmeter and a computer you can generate a steady state signal follow the A/C signal through the amp.
Online Tone Generator


USE CLIP LEADS AND KEEP ONE HAND IN YOUR POCKET

Thank you Seamus for the link. After checking Mouser and the TI site and finding that both currently have the LM334 on backorder for many months, I bought some on Amazon (which may indeed be fake). But I installed one an hour ago and my 120 is back in business. Thanks again to everyone who chimed in, especially Bob.

That is great news!