Bell MRC12 Powered Mixer

[m1989jmp]

Hi,

I've got this mixer on the bench that a customer tried to clean and ended up shorting out some parts on the power amplifier board.

All scorched parts were replaced and I powered it up with a bulb limiter. Limiter glows bright until I disconnect the ribbon cable that supplies uPC1298's with +-B5 voltage. All voltages are fine with that cable disconnected.

I suppose uPC1298 is toast since cap and resistor combo on pin 12 were burned?

Any help is greatly appreciated.





Here's a full schematic link:

https://www.dropbox.com/sh/3p8yz89kv...IFoc7AQAa?dl=0

[g1]

Were the uPC1298's replaced, and if so, where did they come from?

[m1989jmp]

Were the uPC1298's replaced, and if so, where did they come from?

That's the thing; I didn't find a reputable enough source...

Most on eBay are from China, labeled NEC Japan but I guess they can be fakes.

Maybe this one:

https://www.ebay.co.uk/itm/UPC1298V-UPC1298-ZIP-IC-circuito-integrado-Calidad-envio-rapido-desde-Espana/253800870874?hash=item3b17b5f3da:gaAAAOSwcmBbaG5A

Does anyone have a decent source?

[The Dude]

https://www.audiolabga.com/mal_cart/..._button=Search

Oops! Might not be practical to ship to Croatia?

[m1989jmp]

https://www.audiolabga.com/mal_cart/..._button=Search

Oops! Might not be practical to ship to Croatia?

Actually, shipping cost is almost the same as from anywhere in Europe and there are no additional costs (customs, taxes, etc.) so it's not impractical. Thanks!

The only factor is shipping time, that would be the only downside.

[m1989jmp]

I powered it up with U602's voltage supply disconnected and lightbulb limiter stays dim. That would indicate that U601 is fine.

From your experience, when these kind of integrated power amp drivers short (uPC1298V) do they take out any of the associated circuitry?

[m1989jmp]

Well, after replacing the shot uPC1298V, I'm now getting sound on both channels.

However, the channel that was repaired now has some ugly distortion to the point that sound cuts if there's too much bass dialed in. I guess something else around that uPC1298V took a beating.

Any suggestions before I start probing around with oscilloscope?

[Jazz P Bass]

I am curious where you purchased the uPC1298V.

They have been out of production for a number of years.

That said, I would make sure pin 6 (phase comp),7 & 8 (bias) are soldered well to the IC.
It goes without saying that the power, signal & ground pins must also be secure.

[m1989jmp]

I am curious where you purchased the uPC1298V.

They have been out of production for a number of years.

That said, I would make sure pin 6 (phase comp),7 & 8 (bias) are soldered well to the IC.
It goes without saying that the power, signal & ground pins must also be secure.

I got them off eBay from someone in France:

https://www.ebay.co.uk/itm/1pcs-PC12...IAAOSw0A9aLp5s

[m1989jmp]

The B+ rail is down to a couple of volts, I'll disconnect the power supply today and go from there.

The way PCBs are stacked on the heatsink makes it hard for me to measure things while the amp is on.

I suppose I can remove transistors from the heatsink while testing without load or signal passing, they aren't dissipating any heat under these conditions?

[m1989jmp]

Well, silly me, I was measuring the voltage with (-) clip attached to the heatsink. With it clipped to the ground point between 2 main filter caps everything is fine.

Since I have one working channel I've started comparing measurements. The most logical place to start is bias network. I've attached a picture below that shows high negative voltage on all Q611 legs. The other bias network (working one) has around -1VDC there.

I suppose this bias voltage is tapped off VCC coming into uPC1298? It seems that bias voltage coming off pin 8 is high to begin with.

[m1989jmp]

I am curious where you purchased the uPC1298V.

They have been out of production for a number of years.

That said, I would make sure pin 6 (phase comp),7 & 8 (bias) are soldered well to the IC.
It goes without saying that the power, signal & ground pins must also be secure.

Do you suspect some of these uPC1298V's available for sale are busted/B-stock? I've checked the soldering, everything looks fine (B-,B+, ground etc.).