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

[tenelson]

R109 L&R replaced . I powered up and the relays clicked as normal.

Q116L base: 0.697v
Q116R base: 0.747v

Q117L base: -0.627v
Q117R base: -0.710v

So, very close to the +/-0.8v they should be! Should I check anything else before soldering Q116-Q119 back in?

 

[Chamberman]

Excellent, that looks great!

You can adjust those base voltages down with the VR11 bias adjustment pot. I'd bring them down to their minimum voltages and then dial them back up once you've installed some of the outputs and are ready to set the output stage bias.

You may want to check all of the output stage low value resistors once again to make sure none of them is out of spec or changed during this process.

Other than that I think you're good to go here. Hopefully you're passing a signal through the amp soon.

 

[Chamberman]

Also when you get a chance check the voltage drop across R109. I'm interested see how much current is flowing through there.

 

[tenelson]

R109L rail side: 72.7v
R109L transistor side: 72.1

R109R rail side: 72.6v
R109R transistor side: 72.1v

Both bias pots are adjusted to their minimum. I'm going to check the 2W resistors on the output state, and if they're still ok, will proceed with installing Q116-Q119.

 

[Chamberman]

The current through the second stage is right where its supposed to be. I was a bit worried about it because much earlier you had a higher voltage drop there at R109, but now that you've got everything in the front end squared away it looks like everything is measuring normally.

Sounds like you're all set to install and bias up the output stage of the amp.

Let us know how its goes!

 

[tenelson]

I reinstalled q116-q119, powered up, still through the DBT, and the DBT stays (fairly dimly) lit. The relays never click on. Does this mean one or some of those transistors are blown, or that I have a short? After I installed them, I checked pin-to-pin for shorts and everything was in the M-ohm regime.

 

[Chamberman]

Put your meter on TP12 and see what DCV you measure there.

Do you still have about the same voltages at the base of Q116 - 119?

Its possible that one of the transistors is bad. According to the pictures you posted of the boards last week many of the output transistors were still installed in the boards. So those should be okay as they weren't causing a problem before. So this means one or more of the Q116 - 119 driver transistors may be bad.

 

[tenelson]

TP12L: 0.290v, varies a bit +/-0.25v
TP12R: 0.3758v, varies a bit +/-0.25v

Q116L base: 0.86v
Q116R base: 0v

Q117L base: -0.24v
Q117R base: 0v

The left channel voltages are definitely a bit different, but the right channel makes me think there might be a short there. Should I disconnect the right channel and power up the left channel only?

 

[Chamberman]

Yes try pulling those right channel driver transistors q116 - 119 and power it back up.

The DCV measured at TP12 is low enough on both channels that it should allow the protection relay to pull in. I wouldn't think it should stay in protection until you get to a couple of DCV of offset or higher.

 

[tenelson]

Ok, on the right channel I removed Q116-Q119 and reinstalled the PCB. The DBT still remains lit and the relays don't click on. Q116R and Q117R bases are still 0v. So that's a bit frustrating. Remove the right channel completely?

 

[Chamberman]

Try verifying that you have +/- voltage on the rails on the front end of the amp for the right channel (to the left of R160/161).

Thats odd that you have absolute zero on the bases of Q116 - 119 on the right channel.

 

[Chamberman]

BTW are you reattaching the large power transistors to the heatsink with mica & grease or silpads between them and the heatsink?

 

[tenelson]

The power transistors are attached to the heat sink using silpads.

R160: 28.43v
R161: -28.29v

So something is pulling down the supply hugely.

 

[Chamberman]

Did you measure the resistance of those 2 resistors? They're fusible if I remember correctly.

 

[Chamberman]

You know I think that one of the output transistors has shorted in the right channel.

There's really nothing in the front end of that circuit that can dump that much current and not have smoke rolling out of it. I think that the low voltage that you're seeing on the front end is due to the loading of the DBT which is dropping the mains voltage. So most likely the voltage is low coming into R160/161.

 

[tenelson]

Both R160 and R161 are about 10 ohms, so they appear intact.

What do you think could have cause the output transistor shorting? It was all intact until I put Q116-119 back in, which is odd. I’m pretty sure I didn’t make any mistakes in probing or anything.

 

[Chamberman]

My guess would be that one or more of the outputs was compromised when you had the output transistor failure prior to removing Q116 - 119.

It could also be that one of the Q116 - 119 drivers is bad and conducting when you put them into the circuit which may have put an unbalanced voltage onto the output stage.

Did you replace all of the drivers & outputs when you put the new ones in early in the process? Also where did you source your output transistors from?

 

[tenelson]

Yes, I replaced all transistors mounted to the heat sink early in the process. I got them from Digikey. I think I have one spare of each type, though some of the original transistors I removed may still be good, and now that I have a transistor tester I can check them.

Since there have been so many issues with this thing, I suppose it makes sense that there could be more blown transistors. I will remove the right channel tomorrow and probe around a bit.

 

[Chamberman]

The problem could exist in the left channel as well so don't rule that out. The thing is because both channels are sharing the same incoming low AC power due to the DBT dropping the voltage. It may be that the very low rail voltage is keeping the R ch front end from coming up but not the L ch. The thing that implies there's not a shorted output transistor is that your TP12 measurements for both channels were fairly low. I would've expected one of those to be several volts or more if an output was shorted.

For the left channel,

I would pull the power leads to the right channel then power up.
If the DBT is still not dimming then measure the DC rail voltage for the L ch is it > 60V? If not then something has it loaded way down. Then measure across the 1r resistors R126 - 129 to see if any of them has an abnormally high DCV across them when powered up. This would point to a bad driver transistor. Then measure across R130 - 143 to see if any of those are abnormally high. This will allow you to essentially test all of those transistors with high rail voltage.

 

[tenelson]

I removed the right channel and powered up only the left channel.
The DBT is still not dimming completely. Rail voltage is 29.08v.

R126L top side: 0.32v
R126L transistor side: 0.41v
R127L top side: 0.27v
R127L transistor side: 0.31v
R128L top side: 0.31v
R128L transistor side: 0.4v
R129L top side: 0.34v
R129L transistor side: 0.09v

I double checked the R129 transistor side and it seems different than the rest at nearly 0v.

R130L top side: 27.7v
R130L transistor side: 27.65v
R131L top side: -27.7v
R131L transistor side: -27.57v
R132L top side: 27.58v
R132L transistor side: 27.55v
R133L top side: -27.59v
R133L transistor side: -27.67v
R134L top side: 27.57v
R134L transistor side: 27.56v
R135L top side: -27.58v
R135L transistor side: -27.58v
R136L top side: 27.57v
R136L transistor side: 27.53v
R137L top side: -27.57v
R137L transistor side: -27.57v
R138L top side: 27.59v
R138L transistor side: 27.56v
R139L top side: -27.58v
R139L transistor side: -27.63v
R140L top side: 27.59v
R140L transistor side: 27.56v
R141L top side: -27.66v
R141L transistor side: -27.59v
R142L top side: 27.63v
R142L transistor side: 27.52v
R143L top side: -27.57v
R143L transistor side: -27.55v

It doesn't look like the voltage across any of these resistors is abnormally high.