ST-70 channel cutting out

My ST-70, original condition except as noted below, has an issue I'm trying to figure out. At low input levels most of the sound comes from R channel, then as input is increased the amp's overall volume suddenly decreases, and L & R are balanced. Usually this happens when the amp is still warming up, but sometimes after it is well warmed also.

I reviewed the Sticky post "Quick troubleshooting guide for Dynaco tube amps" and 13 (below) seems to relate, but in an opposite manner.

13. Problem - One channel cuts out completely or drops in volume after time (as the amp heats up)
Areas to check - Bad solder connection, bad solder trace on driver board, bad coupling capacitor

When I first got the amp about a year ago, the driver tubes were so loose in the sockets that I got some loud honking, so I would manually adjust the tubes until contact was consistent. Therefore, I thought the sockets were causing the channel cut out problem. I replaced the driver sockets last night, but the issue remains.

While I had it open I took the mono switch off line, replaced the RCAs, the 8 ohm speaker terminals and I padded the inputs with series resistors to coordinate with my preamp (see: http://www.bottlehead.com/smf/index.php/topic,626.15.html post #25 for details on the padding) and checked around for loose or poorly soldered connections, re-soldering several randomly. All the traces on the board remained attached.

Thanks, David

Bob,
Thank you very much for the reply and troubleshooting suggestion! I do understand the board and components have seen better days. It sounds great when it's working, so I'm putting off the upgrade$ and enjoying the hunt. I'll poke around awhile and see what I can find out.

Any hints toward resistors vs. capacitors vs. tracings vs. something else I might recognize?

You write:

"My ST-70, original condition except as noted below, has an issue I'm trying to figure out. At low input levels most of the sound comes from R channel, then as input is increased the amp's overall volume suddenly decreases, and L & R are balanced. Usually this happens when the amp is still warming up, but sometimes after it is well warmed also."

So, this happens as you turn the volume UP? When doing so the volume on the right channel drops?

How "good" does it sound BEFORE it drops? And after it does so, both channels sound good, correct?

Incidentally, I have had several DIFFERENT symptoms arise from problems with a bad 1.5M resistor in the input stage of the 7199 tube. On many units this resistor is off value or has become unstable. Whether or not it's the cause of your problem, I'd recommend replacing it with a new one.

Thanks for chiming in on my issue. Yes, as I turn up the preamp it sounds like the signal becomes strong enough to fully jump across some bad component and get things balanced between L & R. Before that it sounds good, but overall louder, and about 4:1 louder in the Right channel than the Left. I figured since it gets balanced between L & R that that is the state which is "correct." I'm not sure either sounds worse than the other, but the balance is off center at low listening levels.

I'll order some new 1.5 M resistors...metal film I suppose? I'll look to the original specs unless I hear from anyone otherwise.

I would use metal film. Carbon resistors in the megohm range seem to be less reliable than in the hundreds or kilohm range. They usually drift high.

"Poking" around I found this:
1) tapping with a pencil on individual components did not produce a change;
2) pushing gently on the PC board gets to the point and produces MORE volume and the right balance and sonics. Generally, pushing on the right-middle portion of the board works the best.

I know this means the PC board is generally suspect, but shouldn't re-soldering the connections, releading or wiring the leads get it back in shape? I'm not ready to make a new driver board purchase, so I'm looking for continuing advice on troubleshooting.

Should I start with the owner's manual tests? First I think I'm going to check for continuity between points across leads, etc.

Thanks, David

Thanks for the direction and "I know." I'm going to take a look under the hood in the light of day and start saving / shopping.

GP49 wrote:How "good" does it sound BEFORE it drops? And after it does so, both channels sound good, correct?

Incidentally, I have had several DIFFERENT symptoms arise from problems with a bad 1.5M resistor in the input stage of the 7199 tube. On many units this resistor is off value or has become unstable. Whether or not it's the cause of your problem, I'd recommend replacing it with a new one.

Thank you GP49 for a) the question about "good" sound, and b) the suggestion about the 1.5MOhm resistor!

The good sound I was getting when flexing the PCBoard was louder, with bass and dynamics. The previous symptom description is no longer valid as things are different now. RE: the 1.5 MOhm resistors - They are both reading high: L=2.36 MOhm, R=2.04 MOhm. After re-soldering most of the board the "not good" sound is consistent - missing bass and dynamics. Looks like I finished off the parts with the heat from the iron. All the traces are good, however.

Switching pairs of tubes from L to R produces no change. L channel is now consistently lower volume, and I'm guessing this correlates with the "1.5 MOhm" resistor values I'm reading.

I found additional advice on the web touting rebuilding the stock board, and since the board is still in good shape, that's my plan. I will post again in about a week when I've had a chance to do some soldering.