Peavey MX 6

[Danlampton]

I have an MX 6 powered mixer in for repair. The customer complained that it does not power up. One board has the power supply and output amplifier circuits. This board connects to a board for the master section of the mixer.

The output from the power supply measures fine (just under +15Vdc and -15Vdc) when it is not connected to the next circuit board. When I connect the master section board the power drops to less than +2 and -2.

I'm new to troubleshooting so I am looking for a little direction. Is the problem in the PS or the master circuit (or none of the above)? I would appreciate a push in the right direction. Thanks.

[Enzo]

The +/-15v rails are generally used by any op amps in the circuit, plus other things. If an op amp IC shorts internally, the two rails cab become shorted together, which drags them both down.

But it is possible both 15v supplies failed in the same way at the same time. You could test that. A 15 ohm resistor across a 15v supply ought to draw one amp. SO if you have a 15-20 ohm or thereabouts power resistor you could briefly connecto across each 15v supply, that would test its ability to provide current.



If you power up the mixer, and kleave it on a little with those rails dragged down that far, touch each IC that runs off that and see if any are burning hot.

And visually inspect for small caps that look bulged out or burnt looking.

[Danlampton]

Thanks Enzo. I powered it up for a while and nothing got hot or even warm. I put a 15 ohm resister on each 15v rail and they stayed good - not sure if I did that right. I just slid the lead into that part of the connecter and tested the other end with my meter.

I checked the electrolytic caps with an ESR meter and they look fine. There is a few poly caps that I'll pull and check.

Am I right to assume that the problem is likely in the second board (master section of the mixer)?

[Danlampton]

I just got some schematics and layouts from Peavey. There are three opamps on the master section board. On the schematic they are all 4558 but the board has two 4560 opamps and one 4558. Is the 4560 a reasonable substitution or could that be my problem? Thanks for any help.

XM 6 and XM 4 schematic[1].pdf

[Techknowman]

I think they are reasonable for one or two replacments. I believe I was told that the 4560 pulls more current than the 4558.
My guess would be that 2 of the original 4558's went out, were replaced and now the last 4558 went out. If they are socketed you can pull them out and see if your power comes back correct. the bad one may even get hot as a tell tale, While still in the circuit that is.

[Enzo]

Those op amps are fine, just about ANY dual op amp will work there. They may be bad ones in your amp, but the problem is NOT that they are a wrong type.

So the power amp alone works OK, but connecting the master board drops your 15s. Can I assume the input channel board is not connected at this point? If not, please disconnect the input channel board to see if it is involved or not.

You have three op amps on the board. Are all three facing the same way, pin 1 down?

They are in sockets, right? REmove all three and power up. IS the power still dragged down or does it stay up at 15v? If the voltage returned, then obviously one or more of the op amps was bad. If the voltage stays down, something else is shorted.

SO then first check the reverb driver - a simple two transistor push pull. If both transistor short, BAM your 15vs are shorted together.

I see at least three pairs of little diodes on that small board, check them all for shorted.

The ICs have little pairs of bypass caps on their powr supply pins, but BOTH of a pair would have to be shorted to drag down both 15v rails, so that is not likely the problem.

[Techknowman]

Its funny that 4558's came up.
I just got a ROSS PC-8400 Power mixer that blew the 1A, +15VDC fuse to the mixxer and had a bad 4558 in the mixer channel.
Quick fix!

[Danlampton]

Thanks for the help guys. I was doing my tests without the input channel board. I did check it without the opamps and the 15s were still low.

Unfortunately the customer wanted to return the unit to the seller so I closed it up and sent it back to him. Wish I could have got it working but on to the next project. I do appreciate the assitance.