Help with Yamaha 5014c pwr mixer.

[Shadrock2]

I have a Yamaha 5014c powered mixer and one of the power amp channels has quit. I suspect that this is caused by a failure in the limiter circuit on the main board but I want to verify that power amp is working first. I think this because on last use the clip led flashed on for a substantially longer time on the B channel than the A channel. In fact with no signal the B channel led did not flash but as soon as any signal at all was present this led started flashing.

I have fixed a few amps before but I have never worked with switch mode power supplies. Also, this unit is an order of magnitude more complicated than anything I have tried to fix. I have a good dvm and an old Heathkit signal generator and scope.

I downloaded the service manual for a 5016cf from a link I found in a previous post here. That mixer is not exactly the same but they use the same power amp and the differences in the power supply board are readily apparent. I also contacted Yamaha and they want me to buy the service manual cd. I might have done this but they also insist that I register on their site which I attempted to do but they sent back an error of invalid email address. An address that they replied to not an hour before. I have not attempted to sort that out yet. The 5016 instructions for disassembly were spot on though.
The whole service manual pdf is quite large but I can post relevant parts because I have CorelDraw6 which allows me to extract individual pages and re-save as a .pdf. I have attached the PS and PA schematics + the diagram of the PA test circuit.

So... there are procedures for testing the power amp and power supply boards. I really don't think the ps board has any problems otherwise the "A" power amp or the main mixer section would show a fault condition. Those seem to all work fine. My problem is with setting up the connections for the ribbon cable to the power amp. To do it exactly like it shows in the diagram I'd have to come up with that 14 pin header which I don't know how to find. The +/- BH and +/- BL are already there although those 5.6k resistor would need to be added. The signal ground and sig_A, sig_B, clp_A, clpB and mute connections are all available on different smaller headers on the PS board which passes them straight through. I have a couple 4 pin computer supply connectors that I can file down and make work for those and I can make up the signal buffer for my sig gen. The fly in the ointment is the "DC" connection which goes back into the PS board via a 10k resistor. I want to just make the listed connections according to the test diagram and see if each of the amps makes sound. Do you think I can get away with that and do any of you think that will prove my initial theory that the amp section is ok?

Sorry this post has turned out really long. And likely will be a big ask altogether.

[Enzo]

Not sure I understood all the wires and connectors thing. I have always serviced the circuits while in their units.

You have the advantage of a working channel. Myself, I;d start by checking for odd DC voltages. Any offsets not present in the good channel?

And I'd agree, the SMPS is probably fine.

This is not a basic amplifier circuit, this is a complex sophisticated rail-switching circuit.

[Shadrock2]

Thank you so much Enzo for the reply

The connector issue is to duplicate the setup Yamaha uses to test the amp unit.
There is a 14 pin ribbon cable that makes the non HV supply connections to the power amp section.
I had removed the top which holds the main board with all the controls. It is connected to the Power supply board with 4 multi-pin connectors.
The power amp board has all the components down so I'll have to flip it over and support it somehow.

So I tried to see if I could just measure any DC offset on the speaker outs.
And when I tried to power up it shuts off in less than a second.
I tried this again with the 4 cable connectors to the main board reattached and same thing.
I tried to see if there is some kind of switch that detects if the top is loose but I didn't see anything.
So maybe a ground fault? Any idea's on how to proceed?

I new this was going to be difficult.

The mixer has a switch that determines the power output of the amp. I think that's the "rail switching" that you mentioned. The output sections look like pretty standard bipolar outputs. This surprised me a little because I was expecting a class D amp since the unit is so small.

[Shadrock2]

It knows if the top is screwed on cause I just put it back together and it stayed on.

10mv DC on the B channel speaker outs and 15 mv DC on the A channel.

[Jazz P Bass]

Thank you so much Enzo for the reply

The connector issue is to duplicate the setup Yamaha uses to test the amp unit.
There is a 14 pin ribbon cable that makes the non HV supply connections to the power amp section.
I had removed the top which holds the main board with all the controls. It is connected to the Power supply board with 4 multi-pin connectors.
The power amp board has all the components down so I'll have to flip it over and support it somehow.

So I tried to see if I could just measure any DC offset on the speaker outs.
And when I tried to power up it shuts off in less than a second.
I tried this again with the 4 cable connectors to the main board reattached and same thing.
I tried to see if there is some kind of switch that detects if the top is loose but I didn't see anything.
So maybe a ground fault? Any idea's on how to proceed?

I new this was going to be difficult.

The mixer has a switch that determines the power output of the amp. I think that's the "rail switching" that you mentioned. The output sections look like pretty standard bipolar outputs. This surprised me a little because I was expecting a class D amp since the unit is so small.

These Yamaha amps have ground connections all over the place.

That is what you must duplicate if the unit is apart & on the bench.

The 'rail switching' is not a feature of the amp.
It is a function of the output circuit.
Class G, I think it's called.

Look on the schematic, it is the mosfets after the output section.

Aside: that 'clip detect' circuit gave me a headache trying to follow it.

[Shadrock2]

Thanx for info. Didn't even know about class G amps. I'll have to study up.

Been looking at the clip detect circuit and I certainly don't understand it... at least not yet.

WIll see about grounding and get back.

[Enzo]

In a nutshell, most music is lower amplitude stuff, with occasional higher level peaks. Think big kick drum hits twice a second with all the rest much lower. So for most of the music lower voltage is enough, say 35v. Then we can also have 90v rails for just those peaks. And a switching circuit to control it. So while lower sounds play, anytime there is need for a peak, the higher voltage is switched on allowing the peak to go full height. That is more efficient than just having the 90v do everything.

[Shadrock2]

Thanx for that tidbit of information. I see, at least crudely, how that section works works now that I've read about class g amps and someone has explained it to me. It at least partially explains why they can claim 500 watts out of 6 TO-220 like devices when my BGW amp uses 8 TO-3's on a heat sink 4 X larger.