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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 Yesterday, 04:48 PM   #231
Chamberman is offline Chamberman  United States
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Location: USA, Dallas/Ft Worth, Tx
I think you're still battling bad output transistors.

The only thing that I can think of is that one or more of the + polarity outputs is coming on and conducting which is dropping the rail volts. There could be a faulty transistor in the output positive polarity that once it gets a signal of any type its turning on for some odd reason. Once you touch your probe to the C114+ there may be a small spike of noise that goes through the circuit and it could be enough to possibly cause that faulty output to come on partially.

There should'nt be enough current draw in the front end of the circuit to cause a high wattage DBT to glow. The R160 10r 1/2w CRC resistor or another resistor in the front end would go up in smoke in a hurry if that were the case. Only the drivers or output transistors could cause that DBT to glow.

Do you see any kind of shift in the Q116/117 base voltage after measuring the C114+ pin?
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Old Yesterday, 05:01 PM   #232
Chamberman is offline Chamberman  United States
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You know now that I think about it a bit more. If the neg rail isn't dropping off in voltage and only the positive rail is then this drop is not due to the DBT. A drop due to a glowing DBT should show up on both rails.

You should measure the DC voltage at the primary caps to see if it's showing the drop there too. C001/C002 that is. Also measure AC volts across the primary caps with the problem present.

I would also verify that you have good solid connections all the way back to those primary filtering capacitors. It could also be that possibly one of the power supply diodes feeding those large caps has opened up or that the main filtering cap itself is going bad and has developed a high current draw across it. Normally those diodes and capacitors fail shorted, but there's always a possibility that one of the diodes is open or a cap has partially shorted.
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Old Yesterday, 06:16 PM   #233
tenelson is offline tenelson  United States
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It's a bit hard to measure at the primary caps due to the way the left channel is connected to the central chassis. But the big cables supplied the rail voltages are easy enough to probe.

-rail: -76.0vdc after about 30s. When I remove my probe, the DBT starts to glow. Then when I remeasure, I get -55vdc and it starts to charge up again to the full rail and the DBT dims as it does this. AC measurements starts at about 2v and drops down to 0v.

+rail: +76.0vdc after about 30s, same effect as above, but I see it get down to about +50vdc, then it charges back up again to the full rail. AC measurement starts at about 1v and drops down to 0v.

To do the diode measurements and voltage measurements on the caps themselves, I'd need to disconnect the left channel and drain the charge. I should be able to do that a bit later.
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Old Today, 02:06 AM   #234
tenelson is offline tenelson  United States
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Measuring the diodes on the power supply PCB is pretty difficult. In part because the PCB is not removable without unscrewing a bunch of wires and removing the cross-straps on the - side of the big caps.

But I can get my probes in there and make some measurements which I think are accurate. Nothing that the 401 and 402 parts are different polarities, and using the diode setting on my Fluke, I see the following

D401L: both diodes measure about 1v
D401R: both diodes measure about 1v
D402L: both diodes measure about 1v
D402R: both diodes measure about 1v

Powering on an measuring the caps themselves, I see:

L+ 81.8v
L- -81.8v
R+ 81.8v
R- -81.8v

The DBT never dims when I measure them.
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Old Today, 12:39 PM   #235
Chamberman is offline Chamberman  United States
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If both DC rails are dropping and rising similarly then it is probably the DBT that is dropping the voltage.

I was under the impression earlier that only one polarity was dropping.

Its odd that touching the probe to the front end DC rail is having that effect but it may be an artifact of the amp being powered through DBT and not only that the fact that the output stage is still having issues. I think you need to just get four good devices in the back end of the amp, two drivers and two output transistors and get it biased and idling properly and remove the DBT to see what happens.

Once the amp is properly biased and the rails are loaded and the DBT is out of circuit, then its liable to act totally different.
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