ST-120 no left channel - fixed !

Bob Latino wrote:My question to Mr. C is > "Did you at any time ever use any driver tube OTHER THAN a 12AU7/12BH7/5963/5814/6189 on this board ?" If you did, it is possible that you have taken out the LM334 IC current regulator for that channel. This will cause a dead channel. You can't use 12AT7, 12AX7, 12AV7, 12AY7, 5751 tubes on the VTA driver board. Although these tubes are plug compatible with a 12AU7 type tube, they have other characteristics that are incompatible with the board. I have had a couple of customers plug the wrong driver tube into the board and as a consequence found themselves with a dead channel. Both times we found that the LM334 IC for than channel was now bad. I can't say for sure that this is your problem but it could be ?

Bob

anbitet66 wrote:Mr. C,

A quick way to provide the (possibly) missing bias voltage to your 12AU7 tubes would be to use a 2.2k to 2.7k resistor and bridge from the V+ terminal to chassis ground (the V- terminal) across the LM334 IC to provide an approximation of the bias current.  This is just a quick test to see if sound is restored.  If you can try this, you'll either have your answer, or need to look elsewhere.  If Bob is right, this would be the quick way to prove it.

Tony

Mr C wrote:
Hi Bob.  The only tubes I have are the RCA 5963/12AU7s supplied with the kit.  Other tubes I've had in the amp are KT-120s, A Weber WS1-T and a Mullard GZ37/CV378.  

Bob Latino wrote:

Mr C wrote:
Hi Bob.  The only tubes I have are the RCA 5963/12AU7s supplied with the kit.  Other tubes I've had in the amp are KT-120s, A Weber WS1-T and a Mullard GZ37/CV378.  

Try this with an INSULATED plastic pen > Play the amp in the upside down position on a doubled up towel on a table at a lower volume. Tap on every connection on the left (dead) channel with your insulated pen. This also includes all the left channel driver board connections. If you hear a "crackle" noise and/or the left channel comes back on even for a split second, you may have found a bad solder connection. Shut the amp off and resolder that connection. Try the amp again after the connection has been resoldered.

Bob

Mr C wrote:I found a 2.2k 1/4 watt resistor.  Red, red, red?  Right?  I haven't tried tacking it in yet.  I want to verify if that's okay to do.  It seems like its pointing at the LM334 as the offending component.  Mouser has TI LM334Z/NOPD  TO92-3 in stock.  Is this the correct part?