SX-780 Power Packs and Protection Relay

NHDoc

NHDoc

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I have been trying to revive an SX-780 and managed to get the power supply working again but had one bad STK-0050 power pack so replaced it and now still have no protection click and some strange voltages on the power packs. I was hoping someone could give me some insight:

PP1

1: -1.24V
2: -39.3V
3: -5mv
8: 20.8mv
9: 39.2V
0: 1.31V

PP2

1: -1.23V
2:- 39.2V
3: -98mv
8: -98mv
9: 39.2V
0: 0.47V

The PA3004 voltages are

1: 13.55V
3: 43.5mv
4: -25mv
5: 0.8mv
6: -6.98V
7: -35mvdc and 0.68VAC
8: 2.75V

Anyone have any suggestions or can I provide more measurements to help track it down?
 
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Although the voltages are a little off on PP2, don't see anything far enough off to trigger protect circuit.
Haven't looked at one for a while, but AC voltage on pin 7 seems off. Could be the PA3004, or fuse FU3.
Members Merlynski (RIP) and @alexhastings did a lot of research, including DIY replacement for chip at PA3004 chip PCB substitute maybe he could give advice as to what the AC voltage should be.

A workaround with the power packs is to remove the suspect power pack, place a 220 ohm resistor between pins 0 and 8, and another 220 ohm resistor between pins 1 and 3. This will allow you to trouble shoot offset issues (nearly always the 2sa798) nd relay will kick in if all else is okay, but can't run speakers or load.
 
@sregor raises a good point. The schematic is misleading with that 0V designation, it is indeed 0 volts DC but there should be some AC voltage present.
I have never measured the AC voltage on pin 7 of a working unit so I don't know what to look for.
There is a high value voltage divider getting AC voltage from the power transformer that feeds pin 7.

Check the voltage powering the relay R321 could be open or gone high value.

The base voltage of Q26 is too low to turn on the transistor so the IC could be bad.



780prot.jpg
 
Thank you for the responses. I remember I did check the voltage at that spot and was getting good readings close to the value shown on the schematic so am also leaning toward the PA3004 being bad since I replaced all of the caps and the transistor in the protection circuit already. I even removed the relay and checked the coil for resistance and it was good. I did remove the PP2 and replaced it with two 150 ohm resistors and was able to adjust VR 5 slightly to get good readings on pins 1,3 and 8,0 on that side consistent with the readings on the other side with around +- 1.2V on 1 and 0 and close to zero on 3 and 8 but there was no change to the readings on the pins on PA3004 (and obviously it didn't come out of protection). I ordered a replacement PA3004 from a seller on ebay who swears his are NOS from his repair business so that should solve it. In the meantime I may try to bridge the contact pins on the relay so I can hear what the power packs sound like since their voltages should be close enough to produce sound.
 
Hi guys! Pin 7 will indeed have some measurable AC voltage- in both the original chip and the replacement, under normal operation, you should measure about 0.7VAC (from pin 7 to ground). it's clamped to that value by an internal diode, so it should be stable. So the original measurement looks good!
 
Your pin 8 reading is low- it should be around 7.5-8V. have you checked the charging capacitor yet? If you monitor the pin as you turn on, you can watch the voltage rise as the cap charges up
 
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