Crest 4801 Amplifier in Protection Mode

[kazarkazam]

I have a Crest 4801 amp that has a blown 17,000mf power supply cap. and both channels in protection mode. So I replaced all 4 power supply caps, yet the amp is still in protection mode. Also, there was a small glass fuse blown on the display board that controls the rectifier on that board and runs the fan and LED's. Replaced it and the fan and LED's work fine, but the amp just stays in the protection mode with the red Temp/DC lights on both channels.

Per some suggestions on this forum, I checked all the small electrolytic caps on the power supply board and the display board. All checked out ok. Does anyone know if the protection circuit is on the power supply board or the display board (the one in the front with the LED's).

I also measured DC at the large coils next to the relays on the PS board. 84 volts DC at each coil. So I pulled the amp modules out and checked all the transistors for shorts (there were none) and resistors for opens (none found) diodes look good and caps are okay. Still, both channels in protection.

So I'm back to looking at the PS board and the display board. Any advise would be helpful.

One more thing, the dialetric from the blown capacitor was cleaned off the PS board and did not appear to do any damage to the board. It was dry and just flaked off the board.

Any help would be appreciated.!!!

[kazarkazam]

I have a second Crest 4801 that is operating fine. Checked the DC offset of this good amp at the coils by the relays and it is negligable.

So I started swapping boards. First the display boards with the LED's. Display board from bad amp to the good amp. Display board works fine.

Next, power supply boards. Took the power supply from the good amp to the bad and and bingo!! The amp started working. It came out of protection, relays engaged and no DC at the coils.

So I took the so called bad power supply board and put it in the good amp. Same results as the bad amp -- stays in protection and DC at the coils.

So, the output sections are okay... in both amps. Looks like it is something with the power supply board--it is seeing DC at the coils. This is the board that had one of the large filter caps blow. I replaced all of them. Checked and the rest of the caps on the board and they are fine. Resistors and diodes check out fine. There is an IC chip on it. Have no idea what it's for or how to test.

Any ideas?