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Nakamichi PA-7II won't power on, limiter resistor blown
Nakamichi PA-7II won't power on, limiter resistor blown
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Old 25th March 2020, 11:54 AM   #41
Chamberman is offline Chamberman  United States
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Location: USA, Dallas/Ft Worth, Tx
Those measurements seem normal.

The fact you have full neg rail showing up on both channels bothers me. It still leads me to think that something is universally affecting both channels. Usually when I see channel failure its rare that both channels exhibit the same symptom unless there is a short but your measurements show there is not a shorted transistor on the output.

With the amp off and fully de-energized measure the resistance between the base of Q116 & Q117. This should be about 20k ohms or so.

I would find some where on the front end of the circuit to verify that you have full +/- rail voltage on the front end side of the 10r resistors R160/R161

Also with the amp powered on measure the DCV at the base of Q114 & Q115.

BTW I'm not sure what you're using to probe the voltages in the active circuit with but if you're using a standard set of DMM probes with the long bare tips I'd insulate them by wrapping a small bit of electrical tape around them leaving just the very tip of the probe exposed. This will save you a lot of heartache if the probe slips. Take it from someone who has experienced that heartache firsthand.

I'm in the mid-cities area between Dal & Ft Worth.
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Old 25th March 2020, 10:38 PM   #42
tenelson is offline tenelson  United States
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I'm using a Fluke 115 with the standard probes. Great idea on the electrical tape. I've added some heat shrink material to prevent sparks from flying when the probes slip.

with the amp powered off:
Q116L to Q117L base measurement: 2.227k
Q116R to Q117R base measurement: 2.578k
So, 10x lower than what your were expecting.

With the amp powered on, I can measure the +/- rail voltages at the Q114 and Q115 collectors. I probed all the pins on these transistors for completeness.

Q114L
C: +77.3v
B: -77.2v
E: -76.7v

Q115L
C: -77.3v
B: -77.2v
E: -76.7v

Q114R
C: +78.1v
B: -77.2v
E: -77.4v

Q115R
C: -78.1v
B: -78.1v
E: -77.6v

So it looks like the rails are ok as measured on these transistors too. Should I probe the transistors between Q114/115 and the input connector next?
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Old 26th March 2020, 12:06 AM   #43
Chamberman is offline Chamberman  United States
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Okay looking again at the bias network shows 2 - 3k ohms is about right for the Q116/117 resistance.

I'm interested now in what DCV you measure at the base of Q112?
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Old 26th March 2020, 03:00 AM   #44
tenelson is offline tenelson  United States
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Q112L base: -77.5v
Q112R base: -77.8v
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Old 26th March 2020, 11:51 AM   #45
Chamberman is offline Chamberman  United States
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The combination of R113/114/115 & D105 is a current regulated voltage divider network which should give you around -60V (in ref to gnd) on the base of Q112. But you are also measuring full rail voltage on the collector of Q112 from your earlier measurement so I would power down and check the resistance from C - E on Q112 to see if its shorted. This short would also explain the full rail voltage on the base of Q112.

It's possible the voltage divider I just mentioned is not conducting but this wouldn't explain the full neg DC rail on the collector of Q112. To me only a shorted Q112 or Q115 explains this. Even if q112 is shorted I'd still check that divider to make sure its dropping voltage across R115. The DCV across R115 (100k ohm) should measure well over 100V if that network is working properly. If its not then I'd suspect D105 which is a 1mA current regulator diode.
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Old 26th March 2020, 02:04 PM   #46
tenelson is offline tenelson  United States
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Q112L C to E: 5.56M
Q112R C to E: 3.579M
Hmm, definitely not shorted, but perhaps something else in circuit is making it seem as if it's not shorted?

Before powered on, I did a diode check on D105:
D105L: 0.700v
D105R: 0.640v

With the amp powered on, measuring across R115:
R115L: 95.0v
R115R: 111.6v
the 95v seems out of place.

voltage measured on +rail side of R115L: 78.5v
voltage measured on +rail side of R115R: 78.6v

voltage measured on diode side of R115L: -16.05v
voltage measured on diode side of R115R: -31.9v

Seems like at least the left channel diode might be shot despite the reasonable diode check voltage?
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Old 26th March 2020, 02:32 PM   #47
Chamberman is offline Chamberman  United States
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So the R115 voltages sound within reason. That works out to .95mA on the L and 1.1mA on the right which are prob within spec for that diode even if it's going to give some differences between channels. Anyway that's not your problem.

I should've told you before but check Q112 for C to B and B to E for shorts.
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Old 26th March 2020, 02:42 PM   #48
tenelson is offline tenelson  United States
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At the same time I did the C to E measurements, I measured Q112 C to B and B to E but didn't note them. They were in the 2-5Mohm span, no obvious shorts to me. But I've definitely had issues in the past where shorted transistors didn't measure as shorted until I removed them from the circuit.
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Old 26th March 2020, 02:45 PM   #49
Chamberman is offline Chamberman  United States
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I was going to suggest that you may need to remove these from the board and put them on a transistor tester. Or at least perform a diode test with your DMM on the junctions out of circuit.
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Old 26th March 2020, 06:00 PM   #50
tenelson is offline tenelson  United States
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I don't have a transistor tester, but I can check for shorts and do the diode test. Are there any parts other than Q112 I should pull while I have the desoldering iron out?

And, when working on these beasts, do you normally bend the PCB back to allow access to the back side of the board? That seems fine once or twice, but eventually that could stress the leads on the transistors. On the other hand, if I remove the transistors from the heat sinks, I could damage the sil-pads or the transistors themselves.
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