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    ST-70 channel cutting out

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    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    ST-70 channel cutting out

    Post by Aljaheejus on Sun May 09, 2010 12:00 am

    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 Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 channel cutting out

    Post by Bob Latino on Sun May 09, 2010 2:56 am

    Hi David,

    You probably have some component(s) on the driver board that is(are) "going thermal". When this happens this (these) component(s) go way out of spec when heated. To try and find the component, try tapping everything on the driver board with the eraser end of a pencil and see if one of the hits causes the sound level to come back to normal. If tapping one of the components causes the sound to come back then try replacing that component. If that doesn't work, IMHO, you should just replace the entire driver board. Replacing the entire driver board will almost assuredly cure the problem. Those original ST-70 boards are now 40 - 50 years old and they were not made to last 40 - 50 years.

    Bob

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    ST-70 channel cutting out

    Post by Aljaheejus on Sun May 09, 2010 9:44 pm

    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?

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST-70 channel cutting out

    Post by GP49 on Mon May 10, 2010 1:26 am

    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.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70 channel cutting out

    Post by Aljaheejus on Mon May 10, 2010 12:29 pm

    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.

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST-70 channel cutting out

    Post by GP49 on Mon May 10, 2010 5:27 pm

    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.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70 channel cutting out

    Post by Aljaheejus on Fri May 14, 2010 10:12 pm

    "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

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 channel cutting out

    Post by Bob Latino on Fri May 14, 2010 10:29 pm

    David,

    You have either a bad connection on the board or a bad solder trace. Those boards on the original ST-70 were cheaply made. Try resoldering all the connections on the channel with the lowered volume. If that doesn't do it then check all the solder traces on the bottom of the board with a magnifying glass. Touch a soldering pencil to any area that shows a hint of a break in the solder trace.

    You really need a new board and parts set for your amp if you intend to play the amp on a regular basis.

    Bob

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Thanks...

    Post by Aljaheejus on Fri May 14, 2010 11:50 pm

    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.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70 channel cutting out

    Post by Aljaheejus on Sun May 16, 2010 1:23 pm

    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.

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 channel cutting out

    Post by Bob Latino on Sun May 16, 2010 4:04 pm

    "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."

    Aljaheejus,

    It is your amp to do with what you want but I don't recommend a complete rebuild of an older Dynaco ST-70 "brown" board. The newer boards are made of epoxy/fiberglass and are much more heat resistant than the older original boards. You may still run into the issue of an open solder trace some time in the future. A new board is only $25 (or less). If you are going to go through the expense of buying all new board parts you should also get a new board. That way you will completely eliminate the possibility that an open solder trace is the cause of your problems.

    Bob

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