Alesis Active M1mk2 dead PSU

[xtian]

I have a broken PSU. It draws a slug of current a startup, probably filling the reservoir caps, then settles quickly at 1.5 watts. There is no DC at connector J5 (far right side of schematic). I measured 154vDC (with 4.5vAC ripple) on the hot rail as shown.

I could use a plan of attack for finding the fault.

[Enzo]

OK, first: these are EXTREMELY dangerous to work on. The mains are directly rectified and filtered. Connect yourself and the entire current of the wall outlet is available to cook you. Use an isolation transformer.

Measurements to chassis ground are not meaningful. the circuit references V- of the rectified mains. Look on the drawing, pin 5 of U1 connects to this common line. That is from where you take readings on the primary side.

On the secondary side, you do take readings with respect to ground.

I start on the secondary, D10-11-12-13, any of them shorted?

Hold a scope probe NEAR to transformer T1. Does it pick up any activity? In other words is it switching? U1 controls it all, look on pin 7, remember to reference pin 5, do you have good voltage? I expect something like 5v, 12v or even 15v, look up the data sheet for that IC.

[xtian]

I see now that CHGND is isolated from the -ve rail by C5. I measured 334vDC between pin 5 of U1 and the junction of D1 and D2.

I measure only 14 mV DC from pin 7 to pin 5 of U1.

That led me to test R3/R4--these appear to be very high resistance, probably burned.

Using diode testing mode on my multimeter, D7 tests ok, but similar (BAV21) diodes D5 and D6 do not pass.

So that's four components I know about.

[Enzo]

D6 has a transformer winding across it, so won;t test right in circuit.

[xtian]

Oh thx. Just replace resistors and try circuit?

[Enzo]

You said D5 was bad, what was wrong with it? Lift it and check. If the two resistors burnt, then what was the failure current path? A shorted C14 or C8 could do it, plus it would be across D5 making it look bad, so check them. IS R5 OK? U1 itself MIGHT be shorted.

[xtian]

SUCCESS!

I lifted D5, R3 and R4 and found all of them are still good (just misled by testing in-circuit). Then I looked at C14, which measured close to shorted (20 ohms). I destroyed the cap while trying to remove it. But when I replaced it with a good cap, powered up slowly, and everything worked!

All good after playing AC/DC for five minutes--keeping fingers crossed, but this looks good.

Thanks for your help, Enzo!