UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Help! Problem with my Technics Amp cutting out .

PJL · 8th Jul 2020, 10:25 pm

Remember the fault signal we have disconnected also reports a DC offset on the speaker outputs. A DC offset will kill a speaker.

The way the fan stall works is the -15V that powers the fan is fed through R371 and charges the capacitor C371. If it continued to charge it would pull the emitter of Q354 negative and Q354 would conduct and send the fault signal to the ST module. The ST module then cuts the power. However, there is a resistor R358 in series with the fan and as the fan rotates it produces pulses across R358 which pass through C383 and momentarily turn on Q371 that discharges C371 before it can trigger the fault.

Either there is a faulty component in this circuitry, or the current pulses produced by the fan are insufficient to trigger Q371. You would really need an oscilloscope to see what's going on.

One option is to lift one end of R371 (68K) and refit R391 which would disable the fan stall check but leave the DC offset detection working.

8th Jul 2020, 10:25 pm	#51
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Help! Problem with my Technics Amp cutting out . Remember the fault signal we have disconnected also reports a DC offset on the speaker outputs. A DC offset will kill a speaker. The way the fan stall works is the -15V that powers the fan is fed through R371 and charges the capacitor C371. If it continued to charge it would pull the emitter of Q354 negative and Q354 would conduct and send the fault signal to the ST module. The ST module then cuts the power. However, there is a resistor R358 in series with the fan and as the fan rotates it produces pulses across R358 which pass through C383 and momentarily turn on Q371 that discharges C371 before it can trigger the fault. Either there is a faulty component in this circuitry, or the current pulses produced by the fan are insufficient to trigger Q371. You would really need an oscilloscope to see what's going on. One option is to lift one end of R371 (68K) and refit R391 which would disable the fan stall check but leave the DC offset detection working.