Nakamichi PA-7II: Thermofuse in the potted transformer

[electrafixion71]

Hi Everyone,

I purchased a Nakamichi PA-7aII power amplifier this weekend, which is a model I've coveted for many years...unfortunately the amp's first owner shorted the speaker outputs on this amp a few years ago, and it hasn't turned on since.

A local Tech diagnosed the problem with amp for the previous owner as being a burnt out thermo fuse; this would be an easy fix, except that device is located INSIDE the huge potted proprietary transformer. The Tech told the old owner that it would be necessary to physically chip away at the potting material inside the transformer to replace the thermo fuse, so he declined to perform the repair.

I did some basic testing and reviewed the service manual schematic with a Technically minded friend, and confirmed the thermo fuse is, and is located inside the transformer. Does anyone have any solutions on how to fix this problem? I'm assuming I can't just safely bypass the connection where the thermo fuse is wired into limiter PCB. Are there solutions that might help to dissolve the potting compound so I could get inside the transformer?

Any suggestions would be welcome!

Thanks,
Trevor

[firechief]

Can you show the part of the schematic for the power transformer and assoicated circutry? Also some pictures of the power supply section. There is a chance someone will come up with an idea.

[electrafixion71]

Thanks for the quick reply. Here's the schematic for the transformer and how it connects to the limiter PCB.

[anatech]

Hi Trevor,
Have you confirmed that the transformer primary is in fact open?

There is a soft start circuit that will prevent the amp from turning on. This is commonly damaged if anyone actually is successful in blowing one of these up. There was only ever one blown under warranty (for real) and I had to fix it. I was a copper staple from the box that fell inside. The copper staple did not survive.

-Chris

[electrafixion71]

Hi Chris,

Thanks for the reply. I think the circuit you're referring to is located on the The "limiter PCB" which has both a fuse and a thermofuse on it; both are OK.

The primary winding is open, so we're operating on the assumption it's the thermofuse rather than the winding itself. My hope was that if I chiseled enough material out of the transformer I might get to the thermofuse and just bypass it to restore the main winding. Have you ever seen a transformer setup that way?

Trevor

[anatech]

Hi Trevor,
Good lord!

Do you have any idea how much it takes to open the thermal fuse on one of those?? I'm impressed with the obvious stupidity involved with this (by the original owner). Is it possible that some moron cranked the bias up "to improve the sound quality"?

I have never once heard of an open thermal fuse on any of these series of amplifiers. I didn't think it was possible to do under normal circumstances - even under "professional" use (no professional would use one of these, they are mismatched to the job at hand).

I can think of two reasonable options for you. One (cheapest I bet), find a destroyed PA-7II and buy it for parts. Two, contact Plitron and see how much they can build one for, offer to bring in the dead thing. With the original, they will be able to make an exact match and they can also estimate how much since they will have size and weight, along with the specs from the manual and also loaded secondary voltages.

Chiseling into the potting will not end nicely. You'll know you've cut the copper once you see copper in the cut. It's probably a better bet that someone who does this for a living can maybe repair it for you. Who knows, Plitron might be able to do a repair. I've never asked them. Thing is, once you destroy the transformer, no one can measure the inductances or capacitive coupling. Useful information I've been told.

Of all your options, working on the core yourself is the least likely to succeed. It will also probably end up as wasted effort and a destroyed set of ex-clues. Please don't do that until you have at least a new transformer mounted inside the Nakamichi and running first.

-Chris

[anatech]

Hi Trevor,
I forgot to answer your other question ...

My hope was that if I chiseled enough material out of the transformer I might get to the thermofuse and just bypass it to restore the main winding. Have you ever seen a transformer setup that way?

yes, but they were all EI core types, and a few "R" core types. Any sealed toroid type transformers (as opposed to just wound and varnished) are really inaccessible without trashing the entire transformer. The thermal fuse is indeed in series with the primary. If you could easily access it, you could replace the thermal fuse. I even bought some from Nakamichi for some older amps (620 I think). They were glass with black ends and normal axial leads. Keep in mind that in the 80's, manufacturer's adhering to the new CSA temperature rating (only 10 °C lower than US standards - nice going guys!), don't get me started on this, elected to replace the entire transformer than replace the fuses. These were EI transformers that you could very easily access the thermal fuse. Over the years, I have properly repaired the thermal fuse problem in some power transformers. Never short the fuse though. That's just too silly in my book. If all you can get is a US transformer, go for it! To heck with the morons at CSA (who are not protecting the public in any way).

Another thought. If you can find another amplifier that uses a similar secondary voltage and current (these should be easy to find actually), install that instead. I'll bet there are some toasted professional amps that use similar toroid transformers. So what if the mounting is different at this point. If this was my amp, that's what I would be doing.

-Chris

[electrafixion71]

I would certainly love to find a dead PA-7II, but I expect the cost of even a dead one, plus shipping, would be really high. Getting a replacement transformer made is a reasonable option, since I didn't pay much for this unit and they do still fetch $700-800 in original shape.

I've actually been chipping away at the potting material already, and I've got to the bottom of the transformer now (see the attached photo). I have nicked a couple wire jackets, but no damage other than one thin lead that is covered in a white cloth over at about 11 oclock (easily repairable). The cut wire seems to be part of the main winding as one end meters to the brown wire, the other side is dead. Another thin lead covered in white cloth is to the right of the wire I cut, and it meters to the other end of the main winding (black lead). I'm wondering of the two white cloth covered wires aren't the leads for the thermofuse, in which case I can just connect them and I should be good to go.

[Old'n'Cranky]

if the t.fuse is gone, id be worried about what caused it to fail, and what other damage has been done to the amp...
could be more to repair than its worth.......

[electrafixion71]

Originally Posted by Old'n'Cranky

if the t.fuse is gone, id be worried about what caused it to fail, and what other damage has been done to the amp...
could be more to repair than its worth.......

The original owner shorted one of the speaker outputs. I guess there could be other problems to uncover still. The output transistors etc OK visually, there's no blown fuses, and there's no dreaded "fried electronics" smell either.