Nad 705 repair

[Eastham]

One other thing I forgot to mention, I'm getting 32v on the collectors of Q409 and Q410 and I'm getting 17v on the emitter of Q401.

 

[NAD80]

Sorry about the delay. Are the voltages at Q401 on the base and collector close to the schematic. What are Q403,Q404,Q402, Q405, Q409, Q410 base, collector, emitter voltages?. And what are the voltages on IC 403? Been trying to understand how the circuit works.

 

[Eastham]

Voltages for the transistors. Going E,C,B

Q401: 6.86v 120mv (fluctuates) 6.86v
Q402: -15.86v 7v -15.4v
Q403: 5.10v 11.86v 5.8v
Q404: 0v 8v 7.6v
Q405: 5.01v 11.6v 5.5v
Q409: 0v 31.5v 0v
Q410: 6mv 31.5v 10mv

Going to meaure all the voltages on IC403 now.

 

[Eastham]

Voltages for IC403

1: 1.737v
2: 0
3: 0
4: 0
5: 0
6: 5v
7: 0
8: 0
9: 10.77v
10: 0
11: 66.7mv
12: 71.8mv
13: 71.9mv
14: 2.79v
15: 5.04v
16: 5.05v
17: 0
18: 0
19: 0
20: 1.37v

 

[NAD80]

Check the R452,R453,R454, R456 resistors, Caps C445,C446,C450. Hope I got the Parts right. The schematic is not too clear when magnified. Wondering if the 32vdc is coming from the 32vdc power supply line. 7.6vdc on base of Q404. The collectors of Q409,Q410 at 31vdc.

 

[Eastham]

Check the R452,R453,R454, R456 resistors, Caps C445,C446,C450. Hope I got the Parts right. The schematic is not too clear when magnified. Wondering if the 32vdc is coming from the 32vdc power supply line. 7.6vdc on base of Q404. The collectors of Q409,Q410 at 31vdc.

All the resistors check fine and I've replaced all three of those caps. I think that is where the 32v is coming from.

 

[NAD80]

Desolder one side of R452 from the PCB. If the 32v goes away then we know the source.

 

[Eastham]

Desolder one side of R452 from the PCB. If the 32v goes away then we know the source.

Yep, it's gone. But interestingly I'm reading 590mV on the collector of Q409 now.

 

[NAD80]

Wonder if there is a solder wick/ bridge getting the 32 v to the collectors of 409,410. R453 fails under load? Since the 10k should drop the voltage from 32v.

 

[Eastham]

I tried replacing R453 with a part from the other Nad with no luck sadly, checking for a short between the connector and the collector of Q409 shows roughly 9.8k. it seems impossible that I could have 32v on these components but somehow I do..

 

[NAD80]

It will be something unexpected at this rate, you already done the obvious fixes.

 

[Eastham]

It will be something unexpected at this rate, you already done the obvious fixes.

It's certainly a puzzle.

 

[NAD80]

You have replaced the 10K resistor with an identical resistor. Have seen resistors fail under load and measure good with a DMM.

 

[Eastham]

You have replaced the 10K resistor with an identical resistor. Have seen resistors fail under load and measure good with a DMM.

I used R308 from the main PCB of the other receiver which I assume should be good.

 

[NAD80]

1/4W vs a 1/6 W part better wattage part.

 

[Eastham]

1/4W vs a 1/6 W part better wattage part.

I figured it would suffice and being from a known working part of the receiver would help me troubleshoot.

 

[NAD80]

The 1/4 R308 watt is better than the 1/6W part. Thinking about running this by a EE at work tomorrow.

 

[Eastham]

The 1/4 R308 watt is better than the 1/6W part. Thinking about running this by a EE at work tomorrow.

It's a really curious problem isn't it?

 

[NAD80]

Yes it is. Will let you know if I get further ideas on the cause of this. And the fix.

 

[Eastham]

Yes it is. Will let you know if I get further ideas on the cause of this. And the fix.

Thanks I really appreciate it.