Nakamichi PA7II serious problems, need ideas, please

My Nakamichi PA7II USA version was in beautiful aesthetic condition and functioning beautifully in our home. I check bias about every 180 days, and set it to 40mA. It has been used about 6-8 hours a week with no troubles at all.

A few weeks ago I lent it to a good friend whose amp started making noise. He took that amp to his tech, and I installed my amp into his system to make sure it wouldn't be damaged.

He used it every day for over a week, then decided he wanted to change speakers and preamps. He noticed after that the amp dropped volume in the left channel, and the right became distorted. He indicated that he shut it down immediately, and not wanting to return it to me damaged, and knowing I was in the middle of rebuilding two APT amplifiers and working on other projects, he took it to his tech.

A few days ago the tech was diagnosed with cancer, and returned both amps to my friend, but unfortunately, my beautiful Nakamichi is now a basket case.

The output transistors were removed, and a few caps were removed from each amp board. The front panel limiter PCB was opened up for testing. The tech gave my friend some minimal notes of his observations, but I'm really unsure of the best way to proceed.

The tech reported that he put the amp onto a dim bulb tester which lit up but then seemed to draw less current, so he checked bias on the left, and it could not be adjusted. The red bias LED was brightly lit at low bias current, then flickered out when the bias pot was raised. The right amp board then started smoking the 5R1 Zobel resistor, so he shut it down. He also noted the VAS transistor area of the board was running hot, so he installed some small heat sinks.

What I've done so far is to replace the missing electrolytic caps as well as the remainder on each board. I've also tested the output transistors with a Peak DCA75 just to find out if they were shorted, and get a rough idea of condition. The photos show the hFE reported by the Peak for each output transistor in their original positions (they were numbered when removed). They don't look great to me - many are very low, and the variance is pretty wide. Unfortunately, new replacements aren't available - the ones for sale are fakes.

Additionally, I have...

1: Checked all of the diodes using my DMM in diode check mode. No shorts or opens, but some (labeled B2 types in manual) read similar voltages in each direction.
2: Checked all small transistors using DMM in diode check mode. No obvious shorts or opens.
3: Checked all resistors. Most check very close to values specified, with a few that are difficult to measure because they were in circuit.
4: Replaced the 5R1 Zobel resistor and the poly cap on both boards.
5: Checked the power supply - no obvious problems.
6: Replaced the electrolytic caps in the limiter PCB. The 2R2 20W resistor is not burned nor are there any other bad components.
7: Re-installed the output transistors.

I then installed only the left channel (mounted to heat sink) to the amp and connected to the limiter board, and then tested on the DBT.

Here is what I observed.

The limiter circuit switches on properly, but the left channel would not bias properly. Adjusting the pot would result in a momentary increase from a few mA to about 16mA, then it would fall back to a few mA.

I removed the left channel and installed the right channel. It would not bias - stuck at about 4mA no matter which way the bias pot was adjusted. I then found 76V at the DC offset test points, and shut it down.

I've since removed the boards from the heat sinks and the output transistors from the boards again, and have tried to re-measure everything on the board, but can't find anything that leads me in the right direction. I don't truly understand the Stasis topology, and can't figure out why there would be 76VDC on the offset test points without having either shorted output transistors, or some other short on the amp boards, but I can't find any evidence.

This is a beautiful amp with a clean, straight chassis and otherwise great condition, and it has real value to me, but I am not sure of my next steps. I've rebuilt or repaired 11 other amplifiers and learned from each one, and don't want this one to be the one I can't bring back to life.
 

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What I would like to know is the exact differences between your system and that of your friend, it can help to determine if it is a coincidence or a major technical variation which led to a breakdown.

And did you check ? L and R
c106
R122
R123
R163
VRII
Q139
 
Hi Kramtweeter,
Thank you. I have checked all transistors, and only the outputs were removed. I have the service manual and schematic, and have verified correct parts, locations and orientation. Also, the tech did not report removing any small transistors. He noticed the Zobel resistor smoking on his bench, but I'm not sure if that happened only at that time or previously at my friend's house. It is difficult to get any more useful information.
 
Hi Huggygood,

Thank you for posting the schematic in the thread.

My friend has a couple of preamps and two sets of speakers, nothing special. His cables are not great - either speaker or interconnects. That is why I hooked up my amp at his house originally. It's not clear what my friend did or how he did it, but he has admitted it happened after he swapped components and that's about all I'll be able to determine, most likely.

C106 was one of the caps removed from each board, which I replaced with new. I did get a bag of the parts removed and will check everything including those caps.

I pulled both trimmers and they seemed to track cleanly but I preemptively replaced them with new 4-turn trimmers.

Resistors will be re-checked, and compared between each board again. There are a couple I need to lift a leg to get clean readings.

I pulled and checked Q139 and it was good, but I replaced it with a new old stock identical part.

Thank you
 
Starting with the left channel board:

C106 - original was good, replacement is good - 10uF/35V. EDIT: the solder pad was partially gone - apparently when the tech removed it he damaged it, but there was a connection to the circuit. I rebuilt the pad with wire.
R122 - Specified 750R, actual 725R
R123 - Spec. 1k, actual 1.006k after lifting leg
R163 - Spec. 750R, actual 726R
VRII - New 4 turn 1K
Q139 - original tested good, replaced with NOS C3423

So no clues on left board with these components. I'll start on the right channel board next.
 
Right channel readings are:

C106 - original was good, replacement good.
R122 - Spec. 750R, measured 727R
R123 - Spec. 1k, measured 1k
R163 - Spec. 750R, measured 727R
VRII - New 4 turn 1k
Q139 - original tested good, but replaced with NOS C3423.
I also verified both were installed correctly.
 
One thing that really puzzles me is how I can measure 79VDC across the offset test points, yet the DBT is not glowing and there is very little current being drawn. Is that the protector circuit doing its job? And, why, if the outputs and drivers are not shorted, would that even occur in the first place? When I measure across the rails on each board, there is no indication of a dead short.
 
Here is a photo of the board and section of the schematic showing TPI2. The PA7II doesn't have offset adjustment, just test points.

Yesterday I verified proper operation of the transformer and power supply. Hopefully today I will have time to properly test the Zener diodes (none are open or have shorts, but I want to check their voltages).
 

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I tested the output transistors on my Tektronix 577 and found two C3856 and one A1492 with early onset breakdown. Linearity was quite varied across all transistors, as was gain. In fact, hFE under higher V/I test conditions than the Peak DCA75 could deliver showed a pretty wide variance in gain, with many measuring worse on the 577. I'm starting to search for suitable replacements. Not many in TO3P with similar characteristics. There is a Toshiba pair and an OnSemi pair that are close, but 30mHz devices, not 20 as original, and I don't know if that will cause problems (assuming I can get the front end working properly).

I have yet to pull any of the input transistors to check out of circuit, or the protector circuit transistors. Not sure what I'll do if the C3333 or A1320 are bad since they are NLA and as yet I haven't found very close matches in available devices, though it seems there should be something.

All Zeners on the left board are reading the same in circuit as those on the right board, which could perhaps mean they are good.
 
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You only need one output pair in circuit to retest the output offset but should you do this make sure its Q126 and Q127 as the protection circuit also monitors the current across Q126 's emitter resistor . I think you are fast approaching the point when you need to start measuring voltages around the circuit and comparing them to those shown on the right hand channel schematic .
 
@epicyclic - thank you for your comments. Pwdiya12 made the same recommendation to me offline, but I've been sidetracked with dealing with repair contractors for the house. I hope to get back to this later today or tomorrow.

@northpaw - unfortunately, their website is incorrect and they have no stock at all. Also unfortunately, they are going out of business after something like 40 years! I've been buying up a bunch of parts for future repairs, but the in-demand stock is disappearing fast.