Peavey 400sc problem

[ea3898]

Hi all,

I have been working on a 400 sc amp that has a problem that I have not been able to solve. The + 15 volt rail is reading 12 volts. The -15 volt rail reads -15 volts and both the +60 and -60 volt rails read +60 and -60 volts respectively. I have replaced the + 15 volt zener to no avail. When U101 or U201 is removed from the circuit the voltage does come up to 14.5 volts but not to 15. I have attached the schematic for the model that I am working on. Any ideas on this would be greatly appreciated.

[Enzo]

The zeners don't work alone. How about the 2.7k dropping resistor, has it gone high in value?

R9 and R10, 15 ohms, they turn 16v into 16V2 for the U101/201. I am assuming the schematic has a small error in that U101 runs off that 15 ohm isolated rail, but U201 appears to be wired right to the 16v. Are both 101 and 201 wired on the isolated side? meanwhile, where are you measuring these low voltages, at the U101 socket or at the zener? What voltage drop is there across R10? In other words what kind of current is being drawn through it?

[ea3898]

Thanks for the reply Enzo,

Both of the 2.7k resistors are reading 2.7k. The schematic is just like the circuit, u101 and u100 use power across the dropping resistors and all of the other op amps use the voltage straight from the zeners. R9 and R10 are only dropping about 0.01 volts each. The voltage measurements that I gave were at the zener diodes referenced to chassis ground. All of the voltages are the same at the op amps with just
.01 lower at u100 and u101.

Just a little background information about this amp. When I received the amp pa2 had very low and distorted output. I replaced a shorted q206, q207,and q213,and an open q200. Resistors r204, and r230 were open. Cr202, cr205, and cr218 were shorted. At some time since I have been working on the amp cr118 has shorted as well. The shorted diodes were what pointed me to the low voltage problems that I am having now. The amp is actually functioning right now with the exception of the voltage problem, so I am afraid to leave it powered for any length of time.

The only place that I have found the 16v used besides the op amps is across a dropping resistor attached to the collector of q103 and q203. When I had unplugged u201 and saw the voltage come up I thought that I had found the problem. I then replaced u201 and saw that I had the same problem. I then discovered that if either u101 or u201 were removed the voltage rises. This is what made me concentrate on this part of the circuit.