First, I would consider myself somewhat of a dynaco expert, or at least an RKI but this one is baffling me. I rebuilt a pair of MK IIIs for a guy a few months back. They turned out beautiful.
Here is what I did to them:
New chrome chassis from dynapartkit
Powder coated xfmr end bells & cages
SDS power supply boards
SDS driver boards
All new tube sockets, pot, input & output jacks, power switch.
Used original power xfmrs, chokes, & OPTs. Pretty much everything else is new.
One of his original units was older than the other, it has cloth wired power & OPT xfmrs, the other more modern plastic/pvc coating.
The issue:
Both sound amazing and biased out correctly. However one of them (the one with older cloth wire xfmrs) randomly cuts out as in all of the tube filaments go out (12v, 6v, 5v), all of them, and a minute or less later, they all come back on again. It has done this dozens of times he says, maybe as often as 2-3 times an hour. He made me a video of them all going out so I believe it to be true. Out of those dozens of times, twice it has blown the 3A fuse, but only twice. He has tried different power outlets in his house, tried both this and the other amp in the same outlet and it still does it but only on this amp. Leads me to believe its not power, but I'm not 100% sure as I wasn't there to measure (he lives many states away). I had him swap the GZ34s between amps and it still did it on this amp. After a month of this, we agreed it would be best for me to get it back on my bench.
What I'm finding:
I've had several DMMs monitoring it for 2 hours now and it's yet to cut out on me...
The power xfrm seems pretty hot to me (145 degrees with laser thermometer).
Wish I could get it to fail, hard to diagnose it with it working
My hunch:
If all the filaments are going out, it has to either be power from wall, power cord, power switch, thermistor, fuse/fuse holder, wiring, power transformer... I'm placing bets on power transformer breaking down with heat but the off and then back on doesn't seem to make sense...
Looking for:
Anyone got insight on DCR that these power transformers should display for the various inputs & outputs?
Any ideas or tips on how to chase down?
Thanks,
Mark
Here is what I did to them:
New chrome chassis from dynapartkit
Powder coated xfmr end bells & cages
SDS power supply boards
SDS driver boards
All new tube sockets, pot, input & output jacks, power switch.
Used original power xfmrs, chokes, & OPTs. Pretty much everything else is new.
One of his original units was older than the other, it has cloth wired power & OPT xfmrs, the other more modern plastic/pvc coating.
The issue:
Both sound amazing and biased out correctly. However one of them (the one with older cloth wire xfmrs) randomly cuts out as in all of the tube filaments go out (12v, 6v, 5v), all of them, and a minute or less later, they all come back on again. It has done this dozens of times he says, maybe as often as 2-3 times an hour. He made me a video of them all going out so I believe it to be true. Out of those dozens of times, twice it has blown the 3A fuse, but only twice. He has tried different power outlets in his house, tried both this and the other amp in the same outlet and it still does it but only on this amp. Leads me to believe its not power, but I'm not 100% sure as I wasn't there to measure (he lives many states away). I had him swap the GZ34s between amps and it still did it on this amp. After a month of this, we agreed it would be best for me to get it back on my bench.
What I'm finding:
I've had several DMMs monitoring it for 2 hours now and it's yet to cut out on me...
The power xfrm seems pretty hot to me (145 degrees with laser thermometer).
Wish I could get it to fail, hard to diagnose it with it working
My hunch:
If all the filaments are going out, it has to either be power from wall, power cord, power switch, thermistor, fuse/fuse holder, wiring, power transformer... I'm placing bets on power transformer breaking down with heat but the off and then back on doesn't seem to make sense...
Looking for:
Anyone got insight on DCR that these power transformers should display for the various inputs & outputs?
Any ideas or tips on how to chase down?
Thanks,
Mark
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