Quantum QSA 1500D

[mike49504]

Im working on this amp. The amp powers up and produces output but very low.

I turned the gain pot up and the amp will stop producing audio. So i sprayed some contact cleaner in it and turned it back and fourth 100 times.

Still have the same problem the amp will work until i turn the gain up and it will stop producing audio. When i turn it back and fourth when the amp is up and running the test speaker will flutter back and fourth .

So to me im thinking i have a bad pot. Would this be correct?

 

[Perry Babin]

It would seem that you have a bad pot but there are other possible problems. I'd suggest checking the output voltage of the op-amps to see if any have more than a fraction of a volt of DC.

 

[mike49504]

Perry,

Want me to post the voltages on all 4 of the op-amps?

The problem is still there with a differnt pot in circuit

 

[Perry Babin]

Yes. All of the op-amps.

 

[mike49504]

All of the op-amps are 4558's

1:12.56
2:12.56
3:0
4:-14.66
5:-.01
6:0
7:0
8:1.84


The other 3 read the same.
1:0
2:0
3:0
4:-14.66
5:0
6:0
7:0
8:1.84

 

[Perry Babin]

There's definitely a problem. One op-amp has positive voltage on its outputs but has no positive supply voltage. I would suspect that something is shorting to the output of the amp (solder bridge, etc.) if the voltages you posted are correct.

None of the others have positive supply voltage either. You need to trace this problem back to the 15v regulator to determine where the problem is.

 

[mike49504]

Ok if i have have theese right they are using C2073 and A940 as the regulators?

C2073
1:1.00
2:49.0
3:0.40

A940
1:-0.94
2:-49.0
3:-0.34

I dont see any solder bridges at all on the amp. Myabye a shorted output?

 

[Perry Babin]

Those appear to be the outputs.

It's not likely to be a shorted output.

If you can't find the regulators, post a photo of the inside of the amp.

 

[mike49504]

Pin 1 and 2 connect to a C2383 and thoose voltages are

1:14.73
2:5.38
3:2.46


I will get the camera out right now and take a pic of the amp

 

[mike49504]

Heres the pics of the board.

 

[Perry Babin]

The regulators are likely the small transistors connected to the large resistors near the last output transistor (nearest the RCA end of the board). Look for bad solder connections on the resistors. Post the voltages on those transistors if you don't find any bad solder connections.

 

[mike49504]

If theese are the transistors you are talking about 1 is a 2383 and the other is an a1013

2383
1:14.7
2:17.9
3:15.3

A1013
1:-0.1
2:-18.3
3:-15.2


I re did the solder connections on the resistors and the transistors still have 12 volts on pins 1 and 2 of the op-amp

 

[Perry Babin]

Do you still have -15 on the op-amps (pin 4)?

Do you now have +15 on the op-amps (pin 8)?

 

[mike49504]

I have both + and- 15 volts on all op-amps now

 

[mike49504]

Ok i did some more checking and cant figure this out i have + - 15 volts on all op-amps but the 1 op amp still has 12 volts on pins 1 and 2 of it

 

[Perry Babin]

If you still have 12v on pins 1 and 2 of the op-amp, remove it and see what you have on pins 1 and 2 after removing the op-amp.

 

[mike49504]

Ok i removed the op-amp and on pins 1 and 2 read 0 volts

 

[mike49504]

I replaced the op-amp and amp is now working like it should Thanks again Perry

 

[mike49504]

Well im getting this amp back again later today I hooked it up for the guy and showed him it worked 100% flawless. He got home last night with it and thought he could make it sound better then what it did and touched the speaker wires together while they were still in the amp dont know how he did it but he did . So now its going back to the bench once again.