Roland KC500

[Vincec811]

Hi! I am trying to repair a KC500 keyboard amp. It took a power hit. I have the power board working fine now by replacing output transistors. When I connect the pre-amp board to the power board the +/-15Volt supplies go to 0.8V and 1.6 volt respectively. They measure full output before connecting to the preamp board. There are 11 op amps wired to these supplies. Any idea how I can troubleshoot the one(s) that are probably bad? Does anyone have a schematic of a KC500? Roland wants $41.00 and at least 4 weeks to deliver.

Thanks!

[booj]

If you can tell what the IC's are, just change them. It'll turn into routine after the third or fourth chip. You can also check the p.s. voltage after every couple of chips or so. Make sure you've got plenty of solder flux.

[Vincec811]

Hi Booj!

Yes that is exactly what I am doing now and the amp is slowly coming back to life. I have replaced 5 of the op amp ICs and the short is gone from the + and - supplies. I have a solid 15V on each. But the preamp output signal is still pretty low. So my belief now is that the power hit killed all twelve M5218AL chips! One more question -- The collectors of what I believe are the bias supply (self biasing amp- no pot adjust) have 2 watt power resistors in them from the +/- 52V supply. These are the only transistors getting warm and the resistors get about 1 watt each at idle. The resistors measure 400 ohms but they appear to be a little burned so I can't read them. I read about 20 volts across the resistor so its dissipating about 1 watt. The base to emitter junctions read 0.6 volts on each so they seem ok but are close to hot. Is this normal? Output transistors are cold and have already replaced all of these.