ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Being stupid enough to short the output stage of my beauty BM1700 I am now in the proces of replacing the damaged/ burnt components; it took a while to acquire the various transistors and caps ( just to make sure) and meanwhile I tested the powersupply: with a simple volt meter I found the necessary DC's 15, 35 and 50 Volts, but in AC position I notice also a substantial amount of activity. I have no scope. Also when connected the output pcb a very short while with the power supply there was a heavy hum in the attached speakers along with faint music from the tuner section; and there was heat! Starting with the powersupply: what can be wrong here? I have not yet replaced all the output pcb's transistors: first solve the power problem.
If shorted transistors are still in the output stage, the power supply will see this as a very heavy load.
it will not be able to hold the DC without ripple.You will have hum and heat.
Martin
I tested the powersupply while the output stage was disconnected; I replaced now allready the four 0.39R resistors as well as all transistors and the electrolytic caps on the output pcb. The trimpots give about 1K. Can I now connect the output stage to the power supply? -I have no variac- Speakers (dis)connected? In order to measure again the DC ?
A normal multimeter (DC voltmeter) will be all you need for measuring DC on the output.Measure from the output phase to ground.
Being a little stubborn and ignorant enough I would like to specify my previous question:the output stage, with BD901's and BD902') and all small transistors, caps and power resistors replaced, becomes immediately very hot when connected to the powersupply, however (!) without the heatsink attached.Does it make that much difference when the heatsink is not in place?Or could it also have to do with the powersupply showing values measured as mentioned earlier here? Can I try again again with heatsink properly mounted?(sorry)Aad
Sounds like the idle current(s) running astray.Have you checked them ?
Not yetMulimeter select A?I'll do it after coming weekend
Thanks
Aad
Follow the specs given in the servicemanual.Sometihing like 10-11mV across one emitter resistor.
It is because of a matter of me being wellmannered enough to still react on your suggestion, although I am about to kick the BM1700 to the recycling heap:I have hesitated quite some time before I dared to power up this thing: within a fraction of a second I burnt my fingertip on one of the big R's.( heatsink attached again). How will I be able to measure the no-load current without blowing the output stage again? What did I miss after replacing the power R's, IC's, T's and testing the trimpots (off-pc board) ?
( I will still wait a little while before disposing of this baby)