ST-120 low output left channel - fixed !

Hi folks,

My St-120 has very low (but not zero) output in one channel, and I'm hoping you can help me troubleshoot the amp.  There are other threads on this topic, and I've tried most of the suggestions there. Just wondering what you would try next.

* I've tried known good tubes and the problem remains, so it's not the tubes.
* The voltages at the tube pins are all fine.
* I've checked continuity from the inputs through the attenuator to the board; that's all fine.  
* The resistors on the board are all fine: same in both channels.  
* I've poked around with a chopstick; can't find a problem with the solder joints.

I've ordered a new LM334, since that turned out to be the problem for someone else who had this symptom.  Will replace that when the part arrives.  Is there something else I should consider at this point?  The amp has been flawless until recently, when I had a snap and a spark in one KT-120 that took out a 10 Ohm resistor in the now-bad channel.  I replaced the resistor (and the tubes on that side) and things seemed OK, but it's possible that that did some damage elsewhere in the circuit.

Thanks in advance,

Gideon

Hello Gideon,

Try to get your hands on an oscilloscope so that you can measure the signal strength at different points inside your amp. When I built my monoblocks one of them had a week signal but every measurement that I took with my meter looked good. When I got my hands on a scope I was able to see the bad channel was good from the output transformer but weak after the binding post. I went overboard and upgraded to some WBT post and have been happy ever since. A scope will show you things that a meter just cannot. Just sharing a story from my past to pass along some wisdom...

Good luck. Mike

Thanks, Mike.  I can borrow a scope, but I might also try to buy one ... to learn how to use it and get a better sense of how these amps work.  Is there some not-too-expensive scope that you can recommend?

If you need some information on selection and use of an oscilloscope, take a look at the collection of videos that blueglow electronics has on youtube.
He did a series of videos a few months ago aimed at hobbyists who are new to audio equipment repair.

Replaced the LM334 and I'm back in business.  Thanks!