Ampeg svp1600

[schraedertr]

Hi all,

I've got an Ampeg SVP1600 that had no output on either channel. I traced the signal through to IC101 (a NE5532 dual op amp) and lost it. Pulled the board out and put in a socket. As I didn't have a 5532 in hand, I put in a TL072 to test.

Now I have some output, but not much. TP4, the pin 1 output of the opamp. has 12VDC on it with no signal and it should read +/- 1VDC.

I'm guessing that the problem won't go away when I install the proper op amp, and I should dig deeper. Any suggestions?

I attached a partial schematic, I apologize for the quality of the scan. I've had the amp and the hard copy schematic for a couple of years and don't know what I did with the electronic version of the schematic. Now someone needs a big bass amp and wants to buy it, so I've dusted off this project.

Thanks, Tom

[PRNDL]

What are TP6 and 7 voltages?
Did you test the transistors and diodes?
If one of the Q102-5 or diodes are bad, it will mess with the bias voltage.

IMHO, if one is bad, I'd recommend replacing all of them.

That's the main problem with DC coupled transistor amps -- one bad component will throw all the bias voltages off, and one burning out will take out its neighbors usually in a sequential puff of smoke. Definitely use a current limiter while testing.

[Enzo]

The TL072 should work just fine in place of the 5532. The 5532 is a low noise op amp, so it might be less hissy, but the DC won't care. SO more is wrong.

What's happening at pin 7, the other half?

I would start thinking the DC might be coming in from elsewhere, or the IC is trying co drive the power stages that direction to compensate for either a missing rail there or a slammed over output.

You have 12v on TP4, so what is on the other end of R111? Higher? Lower? Note the R112 comes directly from the output bus of that power amp.

Can I assume the speaker relays DO NOT click in after powr up? Or do they? And if they don't, is there DC on the output busses of the two channels? Not the output jacks, the output busses - the amplifier side of the relays. A convenient spot to look is the large inductor or R173 or maybe any one of those .47 ohm 5w ballast resistors. Got DC there?

Check the power supply. Supplies. The power amps run on the main HV rails, TP21,22, which are about 80V - those are DC, not AC, the chart is goofy. But note there are auxiliary HV rails for the drivers +/-VD, TP 20,23. Those are also about 80v and also DC.

You lose one of those driver rails and the amp won't work and there will be large DC offsets. Lose a main HV rail and same will occur.

[schraedertr]

TP 6 and TP 7 are +5 and -5 Volts. The whole Q102-105 circuitry is replicated for the B channel with Q202-205. Both channels work the same, so it (probably) can't be a fault in that part unless both channels are blown in the same manner.

The relays do click in, I get some output to the speakers. I use a sacrificial Behringer speaker for testing, plenty of those available from amps that I'll never get parts for.

Started looking at V+ & V- and would like to kill the guy who put silicone seal in a caulking gun. Parts and labels can't even be seen let alone touched with a test probe.

Anyway, I have no V-. I'll look into that tomorrow.

Thanks,

[Enzo]

Silicone sealer keeps the parts from shaking off the board.

If there is DC on the speaker outs, don't waste shop speakers, slap a voltmeter there and just keep an eye on it. The amp doesn;t need loads.

If V- is missing, that explains much. No circuit works right without proper power supply. I'd check the main bridge and the solder to the negative leg.

[schraedertr]

Fixed the negative power supply, bad solder joint at the bridge diode. Looks like a lot of current at some point, minor charring of the board.

The + and - power supplies read correctly now as do the auxiliary power supplies, and I have output from channel B. I've got +26VDC on the A output bus at the input to the A output relay, but it's not steady. It varies about 3 volts and doesn't settle down.

I've now got 14VDC at TP4, 9VDC at the other side of R111 and 26VDC where R112 ties to A out.

So the next step is checking output transistors?

I don't mind a little silicone sealer to eliminate vibration on caps, but I'm having trouble locating components buried under 3/4" blobs. I've had to shine a light through them and look for shadows on the back side of the board to see what might be there.

Thanks for the help,