Howdy!
I'm relatively new to the world of fixing amps but I have already restored Pioneer SA-508 and Marantz 1040. However, third time is definitely not a charm. <:
I recently bought Sansui A 930P, not the best shape but externally I'd give it 6/10 can be polished and detailed though to make it 8/10. Coming to the point, though.
The aforementioned amp did not work when I bought it. It turned on, but no speaker noise, cracking, nothing, nothing on neither channel or selector. Well the selector itself didn't really work, pressing the buttons caused the buzzer to beep but no LED was being lit.
I started investigating and now I have it in pieces, the main board is the one that's giving me problems. There is some weird voltage on transistors but I can't really get what's going on. I checked all the trans and they seem to be ok, no short circuits. I've also checked other main components (resistors, caps, chips) and they seem to be ok as well.
The problem is that sometimes on the transistors I have 50v on all three legs BEC, the thing is worse because it sometimes is left, sometimes right channel. I tried to bend the pcb, tap the components... no place which would change anything. The only reaction was while spraying IPA on some of the components. However, not always and not repeatable, I cannot replicate the change of the channels. I don't have a clue what might be causing it. I even had 50v on all 3 transistor connections with NO transistors being present in that channel.
If anyone wants I can post photos or service manual for this model.
TIA for any suggestions,
mistahke
I'm relatively new to the world of fixing amps but I have already restored Pioneer SA-508 and Marantz 1040. However, third time is definitely not a charm. <:
I recently bought Sansui A 930P, not the best shape but externally I'd give it 6/10 can be polished and detailed though to make it 8/10. Coming to the point, though.
The aforementioned amp did not work when I bought it. It turned on, but no speaker noise, cracking, nothing, nothing on neither channel or selector. Well the selector itself didn't really work, pressing the buttons caused the buzzer to beep but no LED was being lit.
I started investigating and now I have it in pieces, the main board is the one that's giving me problems. There is some weird voltage on transistors but I can't really get what's going on. I checked all the trans and they seem to be ok, no short circuits. I've also checked other main components (resistors, caps, chips) and they seem to be ok as well.
The problem is that sometimes on the transistors I have 50v on all three legs BEC, the thing is worse because it sometimes is left, sometimes right channel. I tried to bend the pcb, tap the components... no place which would change anything. The only reaction was while spraying IPA on some of the components. However, not always and not repeatable, I cannot replicate the change of the channels. I don't have a clue what might be causing it. I even had 50v on all 3 transistor connections with NO transistors being present in that channel.
If anyone wants I can post photos or service manual for this model.
TIA for any suggestions,
mistahke
Inspect all the solder spots for cold connection. If parts heat and cool, they can loosen the wire from the solder. I would reflow most spots and see if that changes anything.
The voltage readings (with or without transistors) have no meaning unless you interpret them
with a schematic.
with a schematic.
There may be a problem finding an A-900P schematic or service manual but what was your reference point for the other (black?) meter lead in each case? Was it chassis ground, power supply ground or speaker output common ground?
Thank you for all the answers!
I looked at the amp board with my x6 Mantis and didn't see any cracks or lacks of continuity. I am planning on reflowing it. Unfortunately my good soldiering gun died, the other one is not suitable for this kind of work and I am currently waiting for my rework station to come. So at one point in the future that seems to be an option.
I do have the schematics, on an exemplary transistor the voltages should be -0.6 -1.2 and -59 I have -59 on ALL three legs - no it's not dead trans, the transistor has been checked and is ok, not shorted, the same high voltage is on the passthrough points even if all 4 transistors of the line are out. What's more sometimes the high voltage situation changes form LEFT to RIGHT channel.
As stated, I do have the schematics that I will attach here. I measured it to the chassis/radiator ground making sure first in various points that the PCB ground is well connected to the chassis.