HI Folks, I have a Sansui AU-20000 that has a 1.4V DC voltage on both speaker outputs. There is not audio output from either channel. The preamplifier section runs perfectly fine (I can connect to an external amplifier and it plays well). I have tested all the Power transistors and they all look good. I cannot find anything wrong with the audio boards too. Do you have any tips / suggestions on what can be done to resolve this issue? I'd like to get this unit working again. Regards Wala.
Take as a fact that there is no working capacitors in a unit with this age all of them need to be replaced no questions asked .
Normally there is no chance that the protection will kick in and lock the output relay while still there is 1.5 volt DC present in the output terminals .
the specific unit might be cannibalized in the past and some clever guy bypassed the relay ...
The specific unit features 4 different power supply's inside one for the protection circuit one for the amplifier first stage one for the preamp and one for the amplifier
Start from there ... check all voltage according o the schematic pop the hatch to see for cannibalization replace all electrolytics and then check soldering psu
expect 85% of the problems of your amplifier to be shelf cured after that .
@ amptech
You surprise me for a person with such a nick name :""amptech"" ( putting resistors before electrolytics for an amplifier of that age )
Kind regards
sakis
PS :
Email me if you need a schematic
Normally there is no chance that the protection will kick in and lock the output relay while still there is 1.5 volt DC present in the output terminals .
the specific unit might be cannibalized in the past and some clever guy bypassed the relay ...
The specific unit features 4 different power supply's inside one for the protection circuit one for the amplifier first stage one for the preamp and one for the amplifier
Start from there ... check all voltage according o the schematic pop the hatch to see for cannibalization replace all electrolytics and then check soldering psu
expect 85% of the problems of your amplifier to be shelf cured after that .
@ amptech
You surprise me for a person with such a nick name :""amptech"" ( putting resistors before electrolytics for an amplifier of that age )
Kind regards
sakis
PS :
Email me if you need a schematic
Sakis, over many years of servicing countless amplifier's and building many...
Iv'e Encountered capacitors going leaky with age I've noticed resistors within vintage equipment going open and or high thus long tail transistor pair going leaky has well...amplifier's can be ikesome with many faults. Even the pcb itself can be part of the fault..
With out the schematic to hand one would start by checking voltages etc ..
Yes if you've got a circuit of this amplifier it would be interesting to see as when there's request for help on trouble shooting we can only give common answers...
Regards.
Iv'e Encountered capacitors going leaky with age I've noticed resistors within vintage equipment going open and or high thus long tail transistor pair going leaky has well...amplifier's can be ikesome with many faults. Even the pcb itself can be part of the fault..
With out the schematic to hand one would start by checking voltages etc ..
Yes if you've got a circuit of this amplifier it would be interesting to see as when there's request for help on trouble shooting we can only give common answers...
Regards.
Thanks for the useful information guys. I have now replaced all the capacitors from both audio boards. Also replaced the two heat sink mounted transistors with MJE15032 and MJE15033 which have better specs. Also replaced both the trimmers for bias and offset with multi-turn units. BUT, the offset issue remains. I did a thorough check of the boards and it all looks good. If I connect 3V on the speaker output (small button battery), the protection immediately kicks in and turns OFF the speaker output. So, I know that the protection is working. I have checked for dry solders and reflowed any that look suspicious. Jasoneyes01 suggested that I turn the bias pot to set DC offset. Is that really the case? Never done that before. Currently, when I replaced the trimmers, I pre-set them to the original trimmer settings. I was wondering if I need to screw in the boards before taking measurements? They have a plate that screws into the body and was wondering if that is where the ground connection comes from? Regards Wala.
Hi,
Here it is the link where you can find the schematic.
Link:Sansui AU-20000 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
Here it is the link where you can find the schematic.
Link:Sansui AU-20000 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
Computerwala, did you check the voltages across the emitter resistors at the o/p these will be the high watt types say 5watt and .22 of a ohm or .47 of an ohm.. A reading of say 20mv or more can be seen.
Biasing sets up the q current for the output devices and if you over set this you notice the heat sinks getting hot!!! When replacing o/ps devices within a amplifier that has a trim a reset is a must or you could damage the replacements..
Biasing sets up the q current for the output devices and if you over set this you notice the heat sinks getting hot!!! When replacing o/ps devices within a amplifier that has a trim a reset is a must or you could damage the replacements..
Thanks amptech. I took a measurement of the AC current while the unit is turned ON and saw approximately 0.3A being used from the mains. No heatsink heating. I'll check the emitter resistors voltages tonight. Also plan on reading the service manual Tauro0221 sent (thanks). I have checked the positive and negative voltage rails on the Power Output transistors and those look good. So whatever the issue is, it has to be coming from the audio boards (it seems). I did measure the resistance of each of the 5W power resistors and they all measured at 0.3 ohms (look good).
These units were NOT built when transistors were cheap, quite the oposite. And they were VERY expensive when new.
IMPORTANT - there are at least 3 versions of the AU-20000, there is no way to tell from the outside and the serial numbers may not be well referenced in this regard.
THe output stage comes in two variants and the driver boards for these are not interchangeable. The difference is the bias servo (Vbe multiplier) location, the early versions had it on the main heatisnks, the rest on the driver heatsink. Regarding this, the driver board section MUST be operated with the lid on it if thermal tracking is to work properly, this also includes adjustment (and makes it quite a bit finicky). This however is not your immediate problem.
If you have 1.4V on both speaker outputs then you have a 'system-wide' problem as it is unlikely that both channels would be damaged in this odd way. I would look carefully at the ground connections, and the place to look is at the pre-out/main-in connectors on the back. Please note I am quoting from memory so I may not be 100% accurate but this is the place where the preamp section ground is joined with the signal ground of the power amp section. I would strongly suspect the wiring and contacts as it is unlikely the protection would not trigger, as the treshold for that is about +-0.6V. Your experiment with the battery also strongly suggests a wiring/contact problem as normally if the actual amp was delivering 1.4V, it does so with an extremely low impedance so you would not be able to budge it at all with an external battery. One more thing you can check is the offset adjustment - see if it works at all. THis may also provide another clue.
IMPORTANT - there are at least 3 versions of the AU-20000, there is no way to tell from the outside and the serial numbers may not be well referenced in this regard.
THe output stage comes in two variants and the driver boards for these are not interchangeable. The difference is the bias servo (Vbe multiplier) location, the early versions had it on the main heatisnks, the rest on the driver heatsink. Regarding this, the driver board section MUST be operated with the lid on it if thermal tracking is to work properly, this also includes adjustment (and makes it quite a bit finicky). This however is not your immediate problem.
If you have 1.4V on both speaker outputs then you have a 'system-wide' problem as it is unlikely that both channels would be damaged in this odd way. I would look carefully at the ground connections, and the place to look is at the pre-out/main-in connectors on the back. Please note I am quoting from memory so I may not be 100% accurate but this is the place where the preamp section ground is joined with the signal ground of the power amp section. I would strongly suspect the wiring and contacts as it is unlikely the protection would not trigger, as the treshold for that is about +-0.6V. Your experiment with the battery also strongly suggests a wiring/contact problem as normally if the actual amp was delivering 1.4V, it does so with an extremely low impedance so you would not be able to budge it at all with an external battery. One more thing you can check is the offset adjustment - see if it works at all. THis may also provide another clue.
I measured some voltages today. Here they are: The bias on all transistors is -1.4V which is what I am also getting at the speaker outputs (-1.4V). The audio boards F-2507 connector pins read (with colors of wires connected to them):
1. Red: 66.5V
2. Blue: -66.5V
3. 3.75V
4. Black -1.4V
5. Brown -1.4V
6. Red -1.4V
7. Orange -1.4V
8. Yellow -1.4V
9. 0V
The Power supply board F-2508 connector reads:
1. Blue -79.3V
2. Red 79.3V
3. Black 0V
4. 0V
5. Orange 35.3V
6. 0V
7. Grey -34.6V
8. 3.8V
9. 0V
10. -2.48V
Wondering if these give a clue as to what's wrong? The F-2507 voltages do not look right. I tried connecting RCA-to-RCA cables across the preamp-out and Main-in. That had no effect. Would appreciate any tips / hints / clues. Regards Wala.
1. Red: 66.5V
2. Blue: -66.5V
3. 3.75V
4. Black -1.4V
5. Brown -1.4V
6. Red -1.4V
7. Orange -1.4V
8. Yellow -1.4V
9. 0V
The Power supply board F-2508 connector reads:
1. Blue -79.3V
2. Red 79.3V
3. Black 0V
4. 0V
5. Orange 35.3V
6. 0V
7. Grey -34.6V
8. 3.8V
9. 0V
10. -2.48V
Wondering if these give a clue as to what's wrong? The F-2507 voltages do not look right. I tried connecting RCA-to-RCA cables across the preamp-out and Main-in. That had no effect. Would appreciate any tips / hints / clues. Regards Wala.