SHP3 rebuild testing

[robnyc]

"DC voltages with record playing at junction of R90 & Q22/23 and junction of R67 RQ12/13 is -1.9 instead of -.7 per schematics. And others not correct either....."

I have to ignore that because if a record is playing those voltages will rise due to signal. Based on what you've now said, I suspect that part of the amp is working...up to the point where it gets connected to the output transformers. This would explain why there is no thump at power-on. The hum is another matter though, Can you tell if it is 60Hz (heavy bass) or 120 Hz (sort of nasal).

60Hz indicates a shield or ground problem. 120 Hz indicates that power supply is either overloaded (probably not the case here) or that the output capacitors are somehow connected directly to the pre-filtered power source.

If as you say, the volume control has no effect, it indicated the problem is likely from that control onward.

For reference here is a wav file of the two types of hum. First 60 then 120: www.fileconvoy.com/dfl.php?id=g814a7faa16d029991000138192163c4a9fe9a0b238

RobNYC

[paradisecs]

I played the sound file and I feel like it's the second sound, the 120 hz. I'll have to hook it again and check to be certain. How do you check the transformers? I did notice that the large one gets warm like you would expect but the little two stayed stone cold.

Stephen

[robnyc]

Ste[hen, the two smaller trans are outputs. They have no DC and should remain cold under all but the heaviest driven conditions.

Measure from the case of each output transistor at the top of the amp to chassis. These are the two closest to the test sockets. You should see approx 31 vdc with no drive.

Back to basics; play a record and test for varying low level ac at these points on the volume control:
www.flickr.com/photos/90641375@N06/25828588007/ Measure from the red to green terminals there. Lift the tonearm and make sure the voltage drop to at/near nothing. If so it means you have input to the volume control from the preamp/AGC card. Y

You mentioned that adjusting the volume control doesn't affect the hum -?

As for the hum, if it is 120 Hz but no fuse blowing nor any bright glow from the limiter lamp, there is likely a wiring error.

RobNYC