After months of replacing stuff and trying to trace the problem with my oldest Mark III it went to a tech last week. Since I asked many questions and got much advice here on the board I feel obliged to report back so others can benefit from my problem and final solution. $60 bucks and an hour of tech time later he found a pesky grounding problem and now we're up and running, steady bias and stable amp.
This amp was bought from a sound company in the early '70's and was probably 15 years old even then. It had a mod where the input was converted to an unisolated 1/4" phone jack, and for the output they drilled and added a 1/4" phone jack with the tip to the 8 ohm terminal but no ground to the C terminal, just the chassis ground. This worked well or so I thought and since we used them for PA and live sound the convenience of the phone plugs was great. I also did a similar mod to my other Mark III and an ST70.
Fast forward 30 years, bring them out of the basement and get new caps, diode swap, big new multicap, tubes checked OK and it keeps blowing fuses won't start up where I can even check the voltages. Seems a lot of the issues I've had with all three amps come from that input and output mod where they are grounded to the chassis. They are isolated on the original amp of course but I never dreamed it could cause so much havoc and difficulty in diagnosing problems. And for a novice like me it just seems logical that the chassis would provide a common ground for everything and that would be OK.
Unfortunately this is my final Dyna fix since I have no cores left and moving on to some guitar amps and old Grundig radios. Do appreciate the help, never imagined the grounding of these amps could be so weird and be the source of so many other issues. Guess they knew what they were doing should have put them back stock as a first step. Thanks to all.
This amp was bought from a sound company in the early '70's and was probably 15 years old even then. It had a mod where the input was converted to an unisolated 1/4" phone jack, and for the output they drilled and added a 1/4" phone jack with the tip to the 8 ohm terminal but no ground to the C terminal, just the chassis ground. This worked well or so I thought and since we used them for PA and live sound the convenience of the phone plugs was great. I also did a similar mod to my other Mark III and an ST70.
Fast forward 30 years, bring them out of the basement and get new caps, diode swap, big new multicap, tubes checked OK and it keeps blowing fuses won't start up where I can even check the voltages. Seems a lot of the issues I've had with all three amps come from that input and output mod where they are grounded to the chassis. They are isolated on the original amp of course but I never dreamed it could cause so much havoc and difficulty in diagnosing problems. And for a novice like me it just seems logical that the chassis would provide a common ground for everything and that would be OK.
Unfortunately this is my final Dyna fix since I have no cores left and moving on to some guitar amps and old Grundig radios. Do appreciate the help, never imagined the grounding of these amps could be so weird and be the source of so many other issues. Guess they knew what they were doing should have put them back stock as a first step. Thanks to all.
