Nakamichi PA-7II: Thermofuse in the potted transformer

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

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anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
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
 
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

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
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

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
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
 
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.
 

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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.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Trevor,
Shorted outputs will not blow that thermal fuse. That kind of damage will only take out the soft start resistor and possibly the thermal fuse on it. The only possible explanation for the thermal opening in that amp is a high core temperature. That means silly high dissipation for hours until the core managed to heat up to that point.

One of the tests for that amplifier involves running it just before clipping into a 2 ohm load. The only thing I've ever seen is the breaker opening on that dedicated circuit to my bench. No fuses ever went, nothing. Therefore, to succeed in opening the thermal, I can only imagine that the amp was run hot at a party (probably) for a few hours without proper ventilation. The interior chassis temperature would have been very close to the heat sink temperature with restricted air flow and the transformer would not have been able to lose the heat into ambient. Then the thermal fuse opens without any other catastrophic failures. The reliability of the amp may have suffered simply due to M.T.T.F. (mean time to failure) being accelerated due to high heat. Once the fuse opened, no other damage is possible if the amp is connected as it would be for it's intended use. Watch this, someone was using it to run a shaker table.

Even if the AC mains fuse was blown and someone tried the cigarette paper trick to short the mains fuse, the soft start resistor and thermal would have both died next. The only other way to damage anything electrically would be to introduce power through the speaker terminals or input jacks. I doubt that happened.

Test the output transistors for shorts, speaker output line to each power supply rail. You should read either open or a diode drop (then reverse the leads).

To open the thermal fuse in anything, you must get the power transformer core very hot, and that normally will take an hour to three hours of abuse. There is no excuse for this fault, and it took hours to fail. This shows intent or stupidity, take your pick. We used to see many New Year's Eve party victims with open power transformers. No smell. The way most of these people accomplished this was to direct air flow over the output heat sink(s). The thermal cutout normally does not operate because the heat sink temperature never rises to that critical level. Unfortunately, the power transformer has no additional cooling and it's internal temperature reaches the open temperature of the thermal fuse. Then, it's game over.

So you see, if the outputs or rectifiers failed, the primary fuse would open. Failing that, the soft start circuit would fail. After this happens, no other damage can occur until those circuits are repaired. I guess someone could have shorted that soft start circuit, but that implies they had access to the service manual. That means they know better. I know you can open these thermal fuses with greatly excessive current flow, but that takes some time and those other parts will fail first. The soft start resistor is the thing that normally opens in that case.

As for the power transformer, you're venturing in a direction that only has negative outcomes. There is no rush, it's only an amplifier. The time you spend would have been better served looking for a replacement, consider Hammond along with Plitron to see if they have any options for you. Check the cost for new while you're at it. You could have been calling around to PA companies, music stores or even service shops. A close replacement is very likely out there somewhere for you.

Stop messing about with that thing and use your time productively. You never know how close a dead 250 wpc amplifier may be. Sorry if I sound a bit harsh, I don't intend that. However, the work you are doing now is not useful in my view. I've seen too many amps (all brand names) come in for service where the owner has had the same idea as you do. More than once they had taken a repairable situation and made it into a replace only job. You are very close to that now.

Good luck with this, I really do mean that! I am wishing you success.

-Chris
 
Well sometimes ignorance and persistence pay off:D

I connected the two exposed leads which were covered by soft white cloth insulation, since I was pretty sure they were what put the thermofuse in the main winding, and quick test with a multimeter suggested I was right. Next I installed the transformer in the amp and tested the output windings with AC connected, and all seemed good. Then I hooked up all the connections, powered up, and after a few seconds the protection circuit clicked off, confirming operation.

I've now had tunes playing for about an hour, and all seems fine. The heat sinks are warm at about 38 degrees C, and it's sounding great:D

When time permits I'll re-cap, upgrade the adjustment pots, reset bias & DC offset, and do some critical listening.
 
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Ohh! It worked! That's great!!

I have been following this thread from the beginning and am very happy that you have found success!

Please post more photos as your project progresses!

Thanks. Attached is how the transformer looked right before I connected the two wires to bypass the thermofuse. I basically connected points A & B by joining the wires in the white cloth insulation.
 

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Are you going to re-pot the case now or just leave it as-is?

I'm going to see if I can buy a small amount of potting compound just to "finish it" but I'm not sure if it's really necessary or not.

Attached is the picture of the amp and matching components running in my basement right now.
 

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