I've been running a pair of Dynaco 70s with the VTA boards in a vertical bi-amp configuration for about 15 months now, and have loved the result. Today, I heard a slight fizzing sound from the left side for a second or two, then it seemed to settle down and play ok. Within a few minutes, the left side was notably distorting, and eventually stopped making sound.
I've narrowed the issue to one channel of the left side amp that was driving the mids/tweeters, and am wondering if I've lost an output transformer. I've switched tubes around, and the issue stays with that same channel. I've removed the tubes from the bad channel, and reconnected the mids/tweeters with the woofer channel of the problematic amp, and am getting good sound from it, but that side is no longer bi-amped.
Any suggestions for what might be wrong, or logical next steps to help trace the problem?
I've narrowed the issue to one channel of the left side amp that was driving the mids/tweeters, and am wondering if I've lost an output transformer. I've switched tubes around, and the issue stays with that same channel. I've removed the tubes from the bad channel, and reconnected the mids/tweeters with the woofer channel of the problematic amp, and am getting good sound from it, but that side is no longer bi-amped.
Any suggestions for what might be wrong, or logical next steps to help trace the problem?
Not necessarily, might be many other things. Bad (solder) joint anywhere, bad resistor, bad fuse, bad resistor, capacitor.
1. Did you check PSU DC voltages?
2. Do you have a scope? Did you trace signal integrity from input to output through every stage?
Best regards,
Alexander.
1. Did you check PSU DC voltages?
2. Do you have a scope? Did you trace signal integrity from input to output through every stage?
Best regards,
Alexander.
Take the short path and bring it to a Tech.
Unless you have equipment and skills to troubleshoot it yourself that is.
Thanks for the reply.
I don't have a scope, and haven't pulled the amp out of service to trace anything yet, but will do so soon.
Ironically, the output tubes on the bad side still hold a bias when I reinstalled them. It'd be great if the OT isn't fried.
.
I don't have a scope, and haven't pulled the amp out of service to trace anything yet, but will do so soon.
Ironically, the output tubes on the bad side still hold a bias when I reinstalled them. It'd be great if the OT isn't fried.
.
Do you have a multimeter? Check the resistance of the primary winding. Compare it to the good transformer.
It isn't impossible, but I've never seen a bad Dynaco output transformer,
and I've seen a lot of them. They were very well made, so look elsewhere first.
and I've seen a lot of them. They were very well made, so look elsewhere first.
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Reassuring to know. it'll likely be a few days before I can get to it, but will definitely pull it out of hte system and give it a good look through, and check some things with the VM.
me too, i tore down one and indeed, adjacent turns shorts are what i found...
i use this tester to find out adjacent turns shorts...https://www.amazon.com/Transistor-Tester-Digital-Mega328-Capacitance/dp/B08BYFF5MH
an opt with good primary winding will show both inductance and resistance, a bad one will show resistance readings only...
Bad output transformer?
I experienced a similar situation with a Dyna 70 once. Turned out one of the nine pin socket pins needed tightining. I purchased it cheap because the owner was told by the repair shop it had a bad output.
I took test leads and jumped the working channel to the suspected bad output and found out it was good. I have had several of these....power transformer are more likely to go and do!🙂
I experienced a similar situation with a Dyna 70 once. Turned out one of the nine pin socket pins needed tightining. I purchased it cheap because the owner was told by the repair shop it had a bad output.
I took test leads and jumped the working channel to the suspected bad output and found out it was good. I have had several of these....power transformer are more likely to go and do!🙂