Dynaco MK III - pulling my hair out - need expert opinions...

Blueglow

Blueglow

Eternal Tinkerer
Subscriber
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

IMG_2080.JPG
 
Last edited:
IMO, if you can't get it to fail, it will be impossible to fix. That said, I'd start by looking at two things- AC in at the primary wires, and LV out, again right at the wire connections. Maybe wiggle the transformer wires slightly with a plastic rod. If it fails, you'll at least know whether it's the transformer, or some other very weird thing. I had a pair of these many years ago (built from new kits) and they're one of the few things I wish I had kept.
 
Swap the power transformers, it's relatively easy and will quickly answer at least one question. You may have to put a signal through them to get the transformer to heat up enough internally to cause the problem, or whatever else is the problem.
 
If there's a cracked winding in the PT, it could do what's described.

And blowing the fuse isn't unexpected, if it's suddenly going off and on- the transient inductive spikes can cause the fuse to blow.

Regards,
Gordon.
 
6DZ7 said:
Is it still using the original power cord? And what is the 12v heater source you mentioned? Is it derived from something? It's not an original PT winding.
Click to expand...
No, new 3 prong cord. I should have said the 6v going to both filaments in the 12AU7 tubes.
 
So it only loses the filament voltage and not the B+? If the B+ too it's likely an open on the primary side of the PT.

The PT heats up and opens, cools down and it closes. If the B+ stays on though I have no idea except changing out the PT for a new one.
 
I have no idea on B+, I can't get it to fail here on my bench with meters hooked up to determine all that. All I know is he made me a video of it with all the filaments gone out on the GZ34, 12AU7s, & KT88s.
Mark
 
Video he sent me. He took this after it has been playing for a while and then cut out.
 
I have several wall outlets which do not grab the plug securely. I know where they are and in some cases what caused them to not work so well (way too much current being drawn for too long) before I had the customer ship me the amp I would have them try it in a different wall socket, just to rule that out. I am chasing an inaudible hum right now that is the same way. Can't verify it exists, can't fix it... frustrating, I have one purpose in life, fix stuff.
 
6DZ7 said:
Maybe try a itty bitty blanket over the thermistor, too. If it's opening instead of just limiting current, you need to find that out. Is it rated correctly?
Click to expand...

Standard CL-80 thermistor I use in all tube gear (120v 3a).
However, it has had my suspicion as well...
mark
 
It's 2 filament windings IIRC
A 5v and a 6v
It seems strange both would fail at xfmr at same time (unless you have a wonky primary winding issue.
You said line cord is new. I'd check switch, CL (should be CL70 IIRC on this one), fuse holder for cold solder joints, arcing,....
 
Just thinking outside the box.
If it has no issue at your house and it does at your friends, maybe issue is local to what he has it plugged into. Latching GFI (?), utility outlet in back of other piece, surge strip, outlet...
 
Yeah with the 5v and 6v filaments going out, I doubt its wiring. I've used a bamboo chopstick to push on every wire & connection in this thing, flexed the boards, flexed the power cord, etc. It doesn't seem to be physical in nature.
BTW almost 4 hours playing here non-stop and hasn't failed. Frustrating...
 
Couple thoughts,,, is the new 3 prong plug tight in the wall outlet... I've spread the the individual brass prongs by slipping a thin blade between the folded metal that forms each prong... I've also found new power switches to be intermittent occasionally,,, if it goes out again on your watch, you can easily jump the switch and eliminate it from the circuit...
Seems most other situation was mentioned... I assume you dont have both amps on your bench to compare DCR between PTs...
Best of luck,,, intermittents are hard to find...
 
I think we're all pretty much on the same page. Between the plug and the primary winding of the power transformer. It still troubles me that it doesn't do it on your bench though:idea:
 
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