Dynaco MK III - pulling my hair out - need expert opinions...
- Thread starter Blueglow
- Start date
Yeah ,
I'm a little lost by your description too
I'm a little lost by your description too
Thought I'd update the group. Owner has had the unit back about 3 weeks now and it hasn't cut out yet. Since I took a shotgun approach and replaced the thermistor, power cord, switch, fuse holder, and power xfmr, I thought I'd do some testing to see what was the culprit. I ended up taking the power transformer apart after testing the other components successfully. Boy was that a job, one laquer coated E & I section after another (had to be 100 of them) and finally got to the bobbin. Started unwrapping it. Found a break in one of the larger wires on the outer wrapping not far into it. I'm guessing whoever said 'it was a primary winding that would open up with heat and then close back down once it cooled off', nailed it on the head.
Thanks for all your input and advice on this one. I've worked on hundreds of tube amps and receivers in my time but this one was kicking my A#$...
Case closed
Mark
Thanks for all your input and advice on this one. I've worked on hundreds of tube amps and receivers in my time but this one was kicking my A#$...
Case closed
Mark
I've had a bad power cap do that (internal arcing inside the cap). That's definitely one possibility. It would explain the smell, too.
Just because the caps are new, doesn't categorically mean one couldn't be defective...
One other possibility (and I really would hope this is not the case), is a fault in the output transformer. That would explain the noise coming from up top. Since the output transformer is right next to the power transformer, it would be easy to mistake the noise coming from one rather than the other.
I would, before doing anything else, look carefully and comprehensively inside the amp, for any connections that are too close together (solder blob, etc), or for any sort of foreign object that could be causing an intermittent short between connections. Make sure everything is visually perfect in terms of all the connections. I've had more than once, that this was an issue, over the years.
Regards,
Gordon.
triode17
Super Member
Did anyone think to take any resistance readings between the primary and secondary's? Or between the secondary's? Two things alarm me here:1. The extra heat the trans. is producing. 2. The fact that it blew the fuse. More than once.
It may be partially shorted windings. By that I mean AC leakage between them, causing extra heating. Possibly pulling down the heater windings (?) Did the fuse blow after the tubes went dark?
It may be partially shorted windings. By that I mean AC leakage between them, causing extra heating. Possibly pulling down the heater windings (?) Did the fuse blow after the tubes went dark?
triode17
Super Member
My mistake, sorry. Duh!
soundmotor
super modified
Thanks, I'll take one of the bows. I am glad you were able to fix it.
You did more than I'd ever do taking it down to the bobbin. You didn't say but did you repair the break on the primary or install a new PT?
Kudos,
Honestly i thought that was a long shot.
Still odd you couldn't get it to rear it's ugly head at your house. Conditions being the same i might've done what you did (shipping being the killer). There were only a few things that could come into play. I'm glad you got it beat. Thanks for sharing the experience
Honestly i thought that was a long shot.
Still odd you couldn't get it to rear it's ugly head at your house. Conditions being the same i might've done what you did (shipping being the killer). There were only a few things that could come into play. I'm glad you got it beat. Thanks for sharing the experience
Same. Transformers are so rarely the problem, especially when talking intermittent that I'd have dismissed it unless I could have gotten it to act up in front of me.
