Pioneer SX 780 Power Transformer voltages

[Terry Howard]

Trying to troubleshoot a sx780. Lights up but stays in protection. Trying to swim upstream. No voltage readings on schematics for secondary side of transformer. 30v from red wires but 0v from white and brown outputs. White goes to capacitor assembly, brown to pin 21.

Can anyone tell me what these are supposed to read? If not 0 does this point to bad transformer or does it point to other issue?

Thanks.

 

[kwayde]

Trying to troubleshoot a sx780. Lights up but stays in protection. Trying to swim upstream. No voltage readings on schematics for secondary side of transformer. 30v from red wires but 0v from white and brown outputs. White goes to capacitor assembly, brown to pin 21.

Can anyone tell me what these are supposed to read? If not 0 does this point to bad transformer or does it point to other issue?

Thanks.

On the schematic, look at pin 22 next to pin 21. That is ground. No voltage there. Same for the white wire pins 74 & 75..

One problem these have are the 3 power supply transistors getting very hot and quit making good contact where their legs are soldered to the board. These are Q19, Q20, & Q25.

Sometimes just reflowing the solder on them can get it to work again. If doing this gets it to work again, heat sinks should be added to the transistors to keep them from doing that again.

If that doesn't help, check your voltages at pins 1, 2, 10 & 11. These feed the STK power packs which are known to go bad. Power pack voltages and power supply voltages are on the schematic.

 

[milo63]

Here's your protection circuit, pin #1 on the PA3004 chip has to get 13.5V from Q25, work back from there.

My last SX-780 had a bad 2SK34 FET and shorted D4 and Q21.

 

[Terry Howard]

Here's your protection circuit, pin #1 on the PA3004 chip has to get 13.5V from Q25, work back from there.

My last SX-780 had a bad 2SK34 FET and shorted D4 and Q21.View attachment 3251433

Thanks. Already been down that path. Nothing from Q25. No readings from PA3004 Pin 1 or others. Found a shorted Darlington IC for sure. Replacement on order.

 

[Terry Howard]

On the schematic, look at pin 22 next to pin 21. That is ground. No voltage there. Same for the white wire pins 74 & 75..

One problem these have are the 3 power supply transistors getting very hot and quit making good contact where their legs are soldered to the board. These are Q19, Q20, & Q25.

Sometimes just reflowing the solder on them can get it to work again. If doing this gets it to work again, heat sinks should be added to the transistors to keep them from doing that again.

If that doesn't help, check your voltages at pins 1, 2, 10 & 11. These feed the STK power packs which are known to go bad. Power pack voltages and power supply voltages are on the schematic.


View attachment 3251407

Thanks! That's the first good lead I've had.