Identifying 2 diodes?

Are these dead? Do you have a curve tracer?

Michael

Nigel,

Could you tell us what the diodes are in and the location numbers?

Thanks! Tim Schwartz Bristol Electronics

ring

and

Its become one of those back-burner jobs. SMPS on a Yamaha SPX 90. Its now come to the stage of using a variac and 240V/240V isolation transformer to scope the mains side oscillator. These diodes are about the only items left that could be causing the problem, one LV error amp side , problem scenario to check first. Its only 6 to 10W load and putting 2 small transformers in there instread , a few bridges and a 5V regulator would have easily solvered the problem - in hindsight, ditch the SMPS. SMPS starts up at very short mark space and changes frequency and mark space but goes over the required steady state into full drive of 60W or so and heating up the chopper. Large Ds D2,3,9 small ones D4,5,13 I very much doubt they are faulty as they all cold check the same basic diode test.

Got another SMPS, for a Uher amp from 1981, going this week but not this Yamaha one

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

ring

and

I think I've cracked it, one of the most awkward of repair situations. A previous er-um repairman had got there before me. I can only assume that the previous one noticed a component populating a pair of holes , on traces, with no corresponding overlay graphic outline or designation and removed it as suspicious. Numerous such holes, unused and unmarked for this model, on the pcb . I assumed, from the disturbed solder , that he'd tried something temporary at that point but it now looks as though there was originally a small C there that slowed down the HV response to the LV error amp via opto coupler. The optimum error V was about 5 minus 2.2V but it was racing over that up to

5V, as there was no capacitance in the error amp, so cutting off the OC and forcing the HV oscillator to maximum power consumption and all error control lost. Now just a matter of finding the optimium, educated guess/suck it and see, for that C, 100nF works but going by pin spacing must be less than that.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on