Nakamichi PA-7II won't power on, limiter resistor blown

[tenelson]

Never mind, I found some nice sealed Bourns 4 turn trimpots on Digikey.

 

[Chamberman]

You might want to measure the hole pattern to verify anything you're buying will fit. Someone recently did this change on Audiokarma, here.

I wouldn't do it the way he did it though. I've done this before with the Bourns 3296P pot he mentions and bent the legs such that it will flip the pot to where the adjust screw is facing up. There's generally enough lead length to allow this.

 

[tenelson]

I wound up going with these: 3339W-1-102LF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey.

And I’m planning on bending the pins front to back and back to front such that the adjuster is facing up, like you mention.

 

[tenelson]

I’ve received my replacement transistors and have started removing the old ones. Should I use a little thermal paste between the transistors and the pad mounted to the heat sink, or is it ok to just mount them dry? The old transistors seem to be a little stuck to the pad, but I don’t see any obvious paste.

 

[Chamberman]

You should use a new silpad or equivalent pad dry. Or use mica & grease. It's messier but works great. Don't reuse the old thermal pad.

 

[tenelson]

I have removed and replaced all of the main transistors on the right channel. It was pretty each once I got going.

Now, when I power on the amp through the DBT, the bulb is lit for a few seconds then dims. With both channels connected to the power supply, I see that both have an LED lit on the amplifier boards indicating overload. Adjusting the trimpots for bias has no effect, on both channels it doesn't budge from around 4mV DC. What are the reason the power amp protector circuit can indicate overload? The service manual doesn't seem to say anything about this circuit.

 

[Chamberman]

The overload circuit is probably seeing high DC or the DBT may be dropping too much voltage and messing with it. What wattage bulb are you using?

Whats your DC offset measure across pins 1 & 3 on TP12R & TP12L?

If the DC offset is high then adjust it down, try to get it < 100mV anyway.

If the DC offset is low as measured across those test points and the DBT is dimming appropriately a few seconds after turning the amp on then try removing the amp from the DBT and see if the overload on both channels goes away.

 

[tenelson]

With the DBT removed, I powered on the amp, measured, the offset, and got -81.1v on both channels! This must be implying another part of the circuit is not working, but I'm surprised that both channels would have the same problem. Ideas of what to check next?

 

[Chamberman]

That's very odd that both channels are doing the same thing. Have you checked to be certain the positive DC rail voltage is present?

 

[tenelson]

Yes, I get +82.3v for the positive rail and -82.2v for the negative rail, as measured on the large resistors between the rail and the main transistors.

 

[Chamberman]

So the DC offset measured <100mV with the DBT in the circuit and then after removing the DBT the offset jumped to max rail V?

 

[tenelson]

No, sorry if I wasn't clear, but the DC offset was the same with and without the DBT.

 

[Chamberman]

I wouldn't have taken the DBT out of circuit with the offset so high. The DBT was dimming though correct?

I'd put the DBT back into the circuit. Then measure the DCV at the base of Q116 & Q117 for each channel.

Are you sure the DC rail polarity is correct you didn't get wires swapped while checking the PS voltages while troubleshooting the amp prior?

 

[tenelson]

Correct, the DBT is still dimming in a few seconds. It's back in-circuit.

Good thought on the potential for swapping the wires. I think I have it correct because I get a positive voltage on R130 and a negative voltage on R131.

Base measurements:
Q116L: -77.6v

Q117L: -77.6v

Q116R: -77.6v

Q117R: -77.6v

Are those measurements supposed to be closer to 0v?

 

[Chamberman]

Yes they should be close to 0v.

Did you check the resistors R124 & R125 (3r/2W) while you had it apart?

 

[tenelson]

R124L: 3.1 ohms
R125L: 3.1 ohms

R124R: 3.1 ohms
R125R: 3.1 ohms

 

[tenelson]

Also, I just measured Q114 and Q115 R&L in-circuit pin-to-pin and I see measurements in the megs or ks. Which doesn't mean they're not blown of course.

 

[Chamberman]

Did you remove Q114 & 115 from both channels when you measured them previously?

Also check the 10r resistors R160 & R161.

 

[Chamberman]

The reason I asked about Q114 & 115 is if they were pulled earlier then verify they were not switched when reinstalled.

With the amp off you should be able to measure resistance from Pin 1 on TP12 (offset measurement) to the positive rail and then pin 1 to the neg rail and you should have a relatively high resistance unless one of the transistors is shorted. This is no guarantee that the transistors are working properly but it will eliminate shorts as an issue.

The fact you have full neg rail on both the P & N driver bases seems to indicate your problem is in the front end of the amp.

 

[tenelson]

Good idea, but no, Q114 and Q115 have never been removed.

R160L: 10.3 ohms
R161L: 10.4 ohms

R160R: 10.3 ohms
R161R: 10.1 ohms

TP12L pin 1 to blue (-rail): 1.894M
TP12L pin 1 to red (+rail): 3.221M

TP12R pin 1 to blue (-rail): 1.836M
TP12R pin 1 to red (+rail): 3.073M

Seems like reasonable resistance.

I guess I should start probing all of the small transistors near the input jack connector?
I'm definitely going to owe you a drink sometime for all this help! My father in law lives in Dallas.