have this amp for repair. switches off after a 15min operation.
Output offset is about 2.2V, weakly adjustable with the offset pot. so any speaker connected draws DC current, heating up the sink.
The fan near the transformer was mounted reversed, so it drew air out of the cabinet, instead of blowing in, as the heatsink blower blows the air out.
the input pair has only a beta of 35, and that is far to small for the bias resistors and the 4mA collector current. Will try the NTE 2696 suggested as a replacement for the 2SC3200.
These transistors measured 502 and 565 beta. keep ya posted...
My fear is that in overload the input BE diode is zenering clipping resulting in beta degradation.
Output offset is about 2.2V, weakly adjustable with the offset pot. so any speaker connected draws DC current, heating up the sink.
The fan near the transformer was mounted reversed, so it drew air out of the cabinet, instead of blowing in, as the heatsink blower blows the air out.
the input pair has only a beta of 35, and that is far to small for the bias resistors and the 4mA collector current. Will try the NTE 2696 suggested as a replacement for the 2SC3200.
These transistors measured 502 and 565 beta. keep ya posted...
My fear is that in overload the input BE diode is zenering clipping resulting in beta degradation.
Soldered the NTE2696 as diff input pair, placed the pcb back to to heatsink, resurfaced with thermal compound, and measured the offset. could be easily adjusted to 5mV. Bias was quite high , 100mA per transistor, so reduced to 50mA , rotating the trimmer clockwise. Mounted new fans in the proper air direction, center fan in, heatsink out.