Roland sde 2000

[pucho812]

Got a sick one with a psu issue.
It’s not outputting +/-15vdc. In tracking it down I have unregulated voltage before the voltage regulators. It’s around +/-22-24vdc.
The moment I connect their regulation circuit v+ drops to zero and v-drops to -4vdc. I can disconnect after the voltage regulation but still have the same results.
Even more bizarre is how I am measuring chassis ground at q26 and at pin8 on the tl082 opamp. I have been over this with a magnifying glass and so not see any parts touching.
Pcb fault?
Do you think wiring in some 7815 and 7915 in place of the voltage regulation would be ok?

 

[a soBer Newt]

Have you tried a bench supply to power it up?

 

[Bo Deadly]

My guess would be that some power Si parts are burned up inside and shorting after heating up. So transistors in the regulator are suspect. But I think it's actually more likely to be the rectifier. Even though you measure a voltage, adding a little load and time (heat) could cause it to go short. Try adding 220-470 ohm resistors (whatever you have to handle that wattage) loads to the rectifier, wait 5 minutes for them to heat up and measure again along the way. If the 22V was really being shorted, the fuses would have blown or at least stuff would get really hot. So my money is on the rectifier or maybe some other Si part in that path if there is one.

 

[pucho812]

My guess would be that some power Si parts are burned up inside and shorting after heating up. So transistors in the regulator are suspect. But I think it's actually more likely to be the rectifier. Even though you measure a voltage, adding a little load and time (heat) could cause it to go short. Try adding 220-470 ohm resistors (whatever you have to handle that wattage) loads to the rectifier, wait 5 minutes for them to heat up and measure again along the way. If the 22V was really being shorted, the fuses would have blown or at least stuff would get really hot. So my money is on the rectifier or maybe some other Si part in that path if there is one.

Fuse did blow. Rectifier was replaced as was Filter caps after the rectifier. The board has a cable connecting those parts to the voltage regulation circuit located on the other pcb.
when not connected the si portion is working as it should within about half a volt of what the manual says. I also can disconnect the output of the regulation section and with both cables(in and out) of that portion of the circuit, I still get pin 8 of the tl082 measuring continuity to chassis.
Schematic and service notes. so far everything checks out except the +/-15VDC

https://www.synthxl.com/wp-content/uploads/2018/04/Roland-sde-2000-service-notes.pdf

 

[Bo Deadly]

Fuse did blow.

Well you definitely have a short then. Not clear how you were able to see how something dropped to 0 and -4 if the fuse is blowing though.

What's the voltage on the upstream side of the pass transistors?

So you say that you can disconnect the regulator circuit from the load and the regulated voltage is still 0 / -4? Then the short is local to the regulator board?



Seems to me the pass transistors can't be the problem. Not clear that it could be the op amp either.

Maybe diode D22 or cap C33. If one of those has failed short, it could drag down the transformer and account for the -4.

 

[pucho812]

Well you definitely have a short then. Not clear how you were able to see how something dropped to 0 and -4 if the fuse is blowing though.

What's the voltage on the upstream side of the pass transistors?

So you say that you can disconnect the regulator circuit from the load and the regulated voltage is still 0 / -4? Then the short is local to the regulator board?

View attachment 93386

Seems to me the pass transistors can't be the problem. Not clear that it could be the op amp either.

Maybe diode D22 or cap C33. If one of those has failed short, it could drag down the transformer and account for the -4.

1.when the unit hit the bench f3 and f4 fuses were popped. I replaced them with fuses of equal value along with the bridge rectifiers and filter caps out of precaution. Without the regulator circuit, I measure around +/-24vdc. I also get the +5v, -5v and 12v the unit uses.
2.With the regulator circuit disconnected.
I can measure pin 8 of the opamp in the regulator circuit to chassis.
3.when I connect the regulator circuit the fuses don’t pop anymore and I measure 0v and -4v instead of +/-15vdc. Power Transformer feels warm but not warm enough you can’t touch it.
4. Regulator circuit Caps and diodes tested ok out of circuit. No fried resistors.

 

[Bo Deadly]

1.when the unit hit the bench f3 and f4 fuses were popped. I replaced them with fuses of equal value along with the bridge rectifiers and filter caps out of precaution. Without the regulator circuit, I measure around +/-24vdc. I also get the +5v, -5v and 12v the unit uses.
2.With the regulator circuit disconnected.
I can measure pin 8 of the opamp in the regulator circuit to chassis.
3.when I connect the regulator circuit the fuses don’t pop anymore and I measure 0v and -4v instead of +/-15vdc. Power Transformer feels warm but not warm enough you can’t touch it.
4. Regulator circuit Caps and diodes tested ok out of circuit. No fried resistors.

What's the voltage on the upstream side of the pass transistors?

So you say that you can disconnect the regulator circuit from the load and the regulated voltage is still 0 / -4? Then the short is local to the regulator board?

 

[pucho812]

What's the voltage on the upstream side of the pass transistors?

So you say that you can disconnect the regulator circuit from the load and the regulated voltage is still 0 / -4? Then the short is local to the regulator board?

Yes correct

 

[pucho812]

what is perplexing is pin8 of the ic measures continuity to chassis. Anything after pin8,r72,d22 etc all have continuity to chassis.R69 and q26 do not but That’s because there is jumper from the factory that I removed in testing preventing q26 and r69 to have continuity to chassis
This happens when testing with the power off.