Hi all.
I recently picked up a Peavey CS-800a with a dead channel. On closer inspection it looks as though a previous tech had been in there and replaced the original SC5278 OpAmp with a 5534N which on paper seems to be an OK substitute except for the operating voltage, am I right? Anyway, I've been right over all other components in the dead channel and ended up replacing a few simple passives (a diode and a transistor) and it runs beautifully if I bounce the SC5278 over from the working channel but just hums and indicates a short on the DBT if I use a 5534N and yes, I've tried a few. I've also tested the Amp using another new 5534N offboard with its own 5-12V supply to no avail, still the same hum & short via DBT
I'm hoping 'indianajo' sees this as I've been reading a heap of threads on here about this Amp and they seem to know a lot about them including ways to improve the sound/performance, especially by swapping the OpAmp for something better. They mentioned something about 'floating' a modern OpAmp because the pinout of the SC5278 was different? Anyone keen to chime in and clarify that because from what I can tell from the PCB tracks and the schematic the 5534N seems to have everything in the right spot for a direct sub. Hmmm.
So close to getting this thing working thanks to being able to simply swap the original SC5278 over from channel to channel proving everything else is schmick.
TIA
I recently picked up a Peavey CS-800a with a dead channel. On closer inspection it looks as though a previous tech had been in there and replaced the original SC5278 OpAmp with a 5534N which on paper seems to be an OK substitute except for the operating voltage, am I right? Anyway, I've been right over all other components in the dead channel and ended up replacing a few simple passives (a diode and a transistor) and it runs beautifully if I bounce the SC5278 over from the working channel but just hums and indicates a short on the DBT if I use a 5534N and yes, I've tried a few. I've also tested the Amp using another new 5534N offboard with its own 5-12V supply to no avail, still the same hum & short via DBT
I'm hoping 'indianajo' sees this as I've been reading a heap of threads on here about this Amp and they seem to know a lot about them including ways to improve the sound/performance, especially by swapping the OpAmp for something better. They mentioned something about 'floating' a modern OpAmp because the pinout of the SC5278 was different? Anyone keen to chime in and clarify that because from what I can tell from the PCB tracks and the schematic the 5534N seems to have everything in the right spot for a direct sub. Hmmm.
So close to getting this thing working thanks to being able to simply swap the original SC5278 over from channel to channel proving everything else is schmick.
TIA
SC5278 is a common as dirt in 1980 fairchild uA741. 5534 differs by having compensation on pin 5, whereas 741 has offset correction. Compensation is a capacitor to prevent oscillation. If you can't get a TI741, I thought I would try OPA604, but it is so fast it might require local power supply decoupling (.1 uf ceramic) and a 33 pf ceramic around the feedback resistor (47k pin 7 to pin 2) to prevent oscillation. Might be easier to try a small ceramic cap (22 to 47 pf) pin 8 to pin 5 on the 5534.
I've never had a cs800a. The shipping is so high on a >70 lb amp that it is more economic to buy switcher supply CS800s, or a QSC cx302. UPS costs much less for switcher supplies, and they sound better. 741 is hissy. OTOH transformer driven CS800 would take a local lightning strike better. I had a tube amp struck by lightning; lost the power switch (arced over) and blew up the anti-pop cap across the power switch.
Enzo usually answers these Peavey questions. He ran a Peavey authorized service center before he retired. I hope he is out fishing and just off the internet.
I've never had a cs800a. The shipping is so high on a >70 lb amp that it is more economic to buy switcher supply CS800s, or a QSC cx302. UPS costs much less for switcher supplies, and they sound better. 741 is hissy. OTOH transformer driven CS800 would take a local lightning strike better. I had a tube amp struck by lightning; lost the power switch (arced over) and blew up the anti-pop cap across the power switch.
Enzo usually answers these Peavey questions. He ran a Peavey authorized service center before he retired. I hope he is out fishing and just off the internet.
Last edited:
Thanks for the reply! Much appreciated.
So with the limited time I had today I tried running a 5534 with a 22pf cap across pins 5 & 8. Still indicating a short via the DBT. I then tried a few other things but no improvement.
All good. I've ordered a heap of ua741s so hopefully I'll just get it working with those. Time will tell!
Thanks again.
So with the limited time I had today I tried running a 5534 with a 22pf cap across pins 5 & 8. Still indicating a short via the DBT. I then tried a few other things but no improvement.
All good. I've ordered a heap of ua741s so hopefully I'll just get it working with those. Time will tell!
Thanks again.
indianajo said:
Best regards!
Your device might feature that offset correction. If given, did you remove the related components before attaching the capacitor?
Best regards!