Nakamichi PA-7 Help?

[Mercury3]

I did a dumb thing. After reading about others checking and adjusting the bias I accidently shorted the left channel test points which caused a component right under where the test point is to smoke and then the amp shut off.

Does anyone know what I blew or if I did very much damage? I'm pretty sure it was just one component because it was just a quick very small puff of smoke.

I'm an idiot I tried to fit to big of alligator clips to those tiny tp points instead of finding a proper connector.

Another quick question is: before this happened I was going back and forth and adjusting it ok, but when I started the bias was pretty much zero. I'm wondering if those pots can drift towards zero or if they usually drift upwards.

I've been using the AMP as a subwoofer amp so it may have been hard to easily detect any performance degradation.

Anyway lesson learned.

[jacco vermeulen]

MK-I or MK-II ?

Picture would be smarter.

[Mercury3]

thanks, it's the regular PA-7 not the later version.

[jacco vermeulen]

The component part just below the trimmmer pot is the transistor for the bias regulator (Vbe multiplier).
Means, if you just fried that one, you're in luck.

The amp board still has to come out to fix it.

(trimmer pot resistance normally goes up in time. Higher pot resistance translates to lower bias voltage across the drivers, means less quiescent current in the output stage)

[Mercury3]

Thanks so much! I've decided I'm going to go ahead and send it to Jon at Vintageamprepair so I can have the whole thing gone through. Caps & pots replaced etc. Should be worth while I think.

Thanks!
Mark

[jam]

Looking at the schematic , shorting the bias test points should not cause any problems when setting the bias nor should it cause the bias transistor to blow.
I fear something else was shorted out.

Jam

[Mercury3]

Originally Posted by jam

Looking at the schematic , shorting the bias test points should not cause any problems when setting the bias nor should it cause the bias transistor to blow.
I fear something else was shorted out.

Jam

Seems like that's what had to happen because shorting those pins would pull it down to zero mv right? then it's when I turned the pot up to try to increase the voltage
is when it blew. Again just a small puff of smoke from right under the test point.

Anyway I don't know but I'm pretty sure Vintageamprepair can fix it for me. I already spoke with Jon yesterday. I think it could benefit from the upgrade's and hopefully my bias circuit won't be a huge issue for him. I hope.

Thanks, Mark

[Mercury3]

Update to this:

I had blown a couple of output transistors. (I donno how but I suspect by turning the bias all the way up) Anyway it's fixed.

Jon Soderberg of vintageamprepair.net repaired it for me as well as changed out all the signal path caps and up graded the bias circuit.

I set the bias on my second PA-7 but was more careful this time.

Both amps running good!

Thanks, Mark

[mirolab]

Hello Mark,

I have a PA-7A mkII, and I have been contemplating upgrading the driver board caps (either myself or sending it to Jon). Being that you have 2 PA-7's.... one upgraded.... what differences do you notice between the two??

Thanks...... Miro.

[Mercury3]

Miro,

Much to my surprise the upgraded PA-7 absolutely sounds better than my non-upgraged one.

Another thing about it is prior to the upgrade in was the OTHER PA-7 that sounded best.

If it's an amp you plan to keep I'd do it.

Mark