You sure that you do not have a broken wire going to the bias transistor or bias network on this channel?
It could be one of the driver transistors base junctions breaking down as well, but the fact both polarities are being loaded seemingly the same would imply a bias network issue in my opinion.
It could be one of the driver transistors base junctions breaking down as well, but the fact both polarities are being loaded seemingly the same would imply a bias network issue in my opinion.
The wire to the bias transistor PCB has been repaired. The right channel is now biasing up correctly, at about 40mV! It's really odd, after all this time, to feel both heat sinks getting warm.
I suppose the next thing to try while things are not fully assembled would be to hook up a source and play some music through it.
It bothers me that I have mismatching bias pots, so I'd like to go back and replace the left channel one. But I should verify I'm actually getting sound out of it before I order parts.
I suppose the next thing to try while things are not fully assembled would be to hook up a source and play some music through it.
It bothers me that I have mismatching bias pots, so I'd like to go back and replace the left channel one. But I should verify I'm actually getting sound out of it before I order parts.
Something else you should check while you've got it still apart.
Once its fully warmed up check the voltage drop across all of the 1ohm emitter resistors R130 - R143. This is to identify any transistors that might have abnormally high or low conduction. These voltages should all fall within a range of +/- 10%. The resistance can play a part in this too if one of the 1ohm resistors is off by 10% then the voltage drop will be off as well. The main thing is to look for big differences in case you've still got a borderline transistor in there that might fail on you.
Once its fully warmed up check the voltage drop across all of the 1ohm emitter resistors R130 - R143. This is to identify any transistors that might have abnormally high or low conduction. These voltages should all fall within a range of +/- 10%. The resistance can play a part in this too if one of the 1ohm resistors is off by 10% then the voltage drop will be off as well. The main thing is to look for big differences in case you've still got a borderline transistor in there that might fail on you.
So I hooked up my matching Nak preamp and I got it to make some music out of the left channel. It seems that the right channel of my preamp is not working, so I swapped plugs to verify the right channel of the amp works too. There's definitely some 60 cycle hum on both channels. I'm not sure if that's an issue with not being fully enclosed in a chassis nor not.
I tried another preamp, but the Nak amp doesn't seem to like it. The overload LEDs stay on and the amp relays never click on. I can try another preamp I have tomorrow. At the same time I can check the DC offset and look at those voltage drops.
I tried another preamp, but the Nak amp doesn't seem to like it. The overload LEDs stay on and the amp relays never click on. I can try another preamp I have tomorrow. At the same time I can check the DC offset and look at those voltage drops.
Do you have the ground connections fully hooked up as shown in the manual? Is the RCA ground attached to chassis ground and then to the C-GND connection on the Main PS board? If not then once you get all of them fully connected the hum will likely clear up.
I'm not a big fan of the Nak grounding scheme in these amps, but my PA-5 seems quiet. I've thought about installing a diode ground break between the star and chassis grounds to see how it affects residual noise and distortion but I've never gotten around to it. That's another one of those rainy day projects that I seem to have way too many of.
I'm not a big fan of the Nak grounding scheme in these amps, but my PA-5 seems quiet. I've thought about installing a diode ground break between the star and chassis grounds to see how it affects residual noise and distortion but I've never gotten around to it. That's another one of those rainy day projects that I seem to have way too many of.
I had the two grounds to each of the PCBs hooked up, but I missed the RCA ground to the chassis. Using alligators clips to ground that, I'm still getting some hum, but it's a bit less. It's present on both channels, about equally, and even with the RCA + and - both grounded to chassis (using alligator clips, again), I get hum. However I know that using alligator clips is not ideal so I should probably solder the input wires to the RCA on the back rather than using clips. And I bet having it fully enclosed will make a huge difference.
FYI I tried powering off fluorescent lights and other items nearby, but it didn't seem to make a difference.
DC offset is 63.4mV in the left channel and -21.7mV in the left channel. I also noticed that when I probed the right channel offset, the hum increased. I will check the R130-R143 voltage drops tomorrow or this weekend, depending on how busy things are around here.
FYI I tried powering off fluorescent lights and other items nearby, but it didn't seem to make a difference.
DC offset is 63.4mV in the left channel and -21.7mV in the left channel. I also noticed that when I probed the right channel offset, the hum increased. I will check the R130-R143 voltage drops tomorrow or this weekend, depending on how busy things are around here.
The extended length ground wires are probably creating some odd ground loops which is not helping you. Do you also have the black ground wire to the CRC caps connected? This is the one that is soldered to terminal C on the channel boards at the c114/c115 ground pad.
The DC offset looks okay. It would be nice to get the 63mV channel offset down a bit, but its acceptable. Definitely check those output resistor voltage drops.
The DC offset looks okay. It would be nice to get the 63mV channel offset down a bit, but its acceptable. Definitely check those output resistor voltage drops.
Yes, I have had the solder-in ground wire to the CRC caps connected during all of my testing.
Left channel voltage drops:
R130: 59.5mV
R131: 52.5mV
R132: 63.8mV
R133: 46.5mV
R134: 65.3mV
R135: 52.6mV
R136: 65.4mV
R137: 50.6mV
R138: 68.5mV
R139: 56.3mV
R140: 76.5mV
R141: 51.0mV
R142: 72.3mV
R143: 57.9mV
Right channel voltage drops:
R130: 6.5mV
R131: 53.2mV
R132: 0.6mV
R133: 52.6mV
R134: 1.1mV
R135: 50.3mV
R136: 2.6mV
R137: 65.9mv
R138: 0.6mV
R139: 55.0mV
R140: 0.6mV
R141: 49.5mV
R142: 0.5mV
R143: 49.4mV
So, obviously the output transistors on the right channel don't see to be drawing any current. I've only played this through tiny speakers, so it's perhaps not surprising that I didn't notice anything weird other than the hum.
I measured the even resistors on the right channel and they're all about 1 ohm, so, intact. R124R is 3 ohms. R160R is 11 ohms. What else should I look at?
Left channel voltage drops:
R130: 59.5mV
R131: 52.5mV
R132: 63.8mV
R133: 46.5mV
R134: 65.3mV
R135: 52.6mV
R136: 65.4mV
R137: 50.6mV
R138: 68.5mV
R139: 56.3mV
R140: 76.5mV
R141: 51.0mV
R142: 72.3mV
R143: 57.9mV
Right channel voltage drops:
R130: 6.5mV
R131: 53.2mV
R132: 0.6mV
R133: 52.6mV
R134: 1.1mV
R135: 50.3mV
R136: 2.6mV
R137: 65.9mv
R138: 0.6mV
R139: 55.0mV
R140: 0.6mV
R141: 49.5mV
R142: 0.5mV
R143: 49.4mV
So, obviously the output transistors on the right channel don't see to be drawing any current. I've only played this through tiny speakers, so it's perhaps not surprising that I didn't notice anything weird other than the hum.
I measured the even resistors on the right channel and they're all about 1 ohm, so, intact. R124R is 3 ohms. R160R is 11 ohms. What else should I look at?
Well that should be more than enough Vbe voltage to turn on those outputs. Can you measure the voltage at the base of one of the positive polarity output transistors?
So the only two things I can see that would cause all of them to be off on one polarity aside from some odd continuity issue to the positive rail or speaker out.
1st they're all bad, which seems a stretch.
or
2nd the 646mV drop is not being seen at the bases of the pos polarity output transistors for some reason.
So the only two things I can see that would cause all of them to be off on one polarity aside from some odd continuity issue to the positive rail or speaker out.
1st they're all bad, which seems a stretch.
or
2nd the 646mV drop is not being seen at the bases of the pos polarity output transistors for some reason.
Looks like around 500mV Vbe so it should be working. Yeah its probably time to start looking at those output transistors.
Did you try to turn up the bias towards max to see if the positive bank began to conduct at some point? I don't know why there would be that large of a disparity between the conduction points, but its worth checking.
Did you try to turn up the bias towards max to see if the positive bank began to conduct at some point? I don't know why there would be that large of a disparity between the conduction points, but its worth checking.
I connected alligator clips to R142R and powered the amp on. With no changes or adjustments to bias, where I was getting no voltage drop before, I saw it rise up to 130mV, then it started to drop down, eventually settling at 60mV. I then did a bias adjustment and got it to settle around 40mV after being powered on for a number of minutes.
I then remeasured the even resistors:
R130: 45.4mV
R132: 41.4mV
R134: 41.3mV
R136: 40.6mV
R138: 38.5mV
R140: 38.3mV
R142: 35.5mV
It seems like maybe I have a marginal connection somewhere. I checked all of my power and ground connections and them seem good. I guess I'll leave it powered on for a while as a sort of smoke test and see what happens.
I then remeasured the even resistors:
R130: 45.4mV
R132: 41.4mV
R134: 41.3mV
R136: 40.6mV
R138: 38.5mV
R140: 38.3mV
R142: 35.5mV
It seems like maybe I have a marginal connection somewhere. I checked all of my power and ground connections and them seem good. I guess I'll leave it powered on for a while as a sort of smoke test and see what happens.
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