GrisPato
Super Member
That is correct.
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Barn find Sansui AU-7900
- Thread starter Kylek1443
- Start date
Well guys. Sadly the short still exists. I replaced the 4 transistors today. Put the amp back together and the DBT lit up. At least I know my DBT build actually works. No blown fuses on the Sansui. The power light doesn’t come on when it’s plugged in to the dbt.
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GrisPato
Super Member
Nice DBT! We just have to get it go dim.
Now it's time to measure your rectifiers, filtercapacitors and the mica insulators of your output transistors ( measure resistance between collector, which is the whole case to the heatsink. When we can rule them out we continue to go downstream of the powersupply.
Now it's time to measure your rectifiers, filtercapacitors and the mica insulators of your output transistors ( measure resistance between collector, which is the whole case to the heatsink. When we can rule them out we continue to go downstream of the powersupply.
I have brand new filter caps for it. Think I should just go ahead and swap them out?
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yes center pins were cut off. I used silicone thermal paste that is non electrically conductive. I was just thinking if you agreed I might as well replace the filter caps and see what it does.
GrisPato
Super Member
It's your amplifier so you're free to do what you want to do.
Did you follow Hyperion's advice to disconnect the power amp board from the powersupply and test the powersupply? Did you measure the rectifiers? Just takes a few minutes to do and wil give you a lot of information to decide how to proceed.
I can easily take the power amp board out but I would have no clue how to test the power supply. Let’s say hypothetically it IS the power supply.. then what? They don’t make replacements for these do they?
You don't need to remove the board to do the tests.
I gave you instructions (maybe not clear enough?), on how to test for a shorted rectifier, or other problem with the PSU.
This is Vintage Electronics, you are not in a World where you can easily find a replacement board, you have to work with what you've got, which means learning how to 'troubleshoot to component level'
And when you have found the faulty part, you then have to deal with the physical differences between the old parts compared to available new replacements. You don't need to remove the board to do the tests.
I gave you instructions (maybe not clear enough?), on how to test for a shorted rectifier, or other problem with the PSU.
This is Vintage Electronics, you are not in a World where you can easily find a replacement board, you have to work with what you've got, which means learning how to 'troubleshoot to component level'
I guess I didn’t understand your instructions. It’s a learning curve. I don’t even think I’m at rookie status yet . I can’t even figure out how to use my crappy multimeter correctly.
So I’ve been messing around with my DBT and the amp connected. If I take out the 2 8A 250v fuses from the amp board, and turn on the DBT the power light comes on and there is an audible hum from the power supply. Putting the fuses back in and turning on the DBT the power light doesn’t come on, the hum is gone and the bulb on the DBT is bright. Does this possibly mean there is a short in the power supply? I have no problem taking the power supply out and tearing it apart to clean it. I just wondered if this meant anything to you guys.
GrisPato
Super Member
Unregulated power supply is: transformer-rectifier-filtercapacitors
Regulated powersupply is:
Transformer-rectifier-filtercapacitors-regulator(transistors, zenerdiode, resistors, capacitors)
As Hyperion mentioned this is discreet circuit so build from singular components in a network/circuit.
So by taking out the fuses you have just the transformer connected.
Best you can do is measure the rectifier diodes one by one for a short. This normally shows up easy. Testing the filter capacitors can be more difficult because it's possible they don't leak at the voltage they get from the DMM. Best way would be to connect your DMM probes to each tab of the capacitor and switch it on for 1-2 seconds and rad out the voltage over the capacitor if this is smaller dan the rail voltage (48-50vdc) by a discerning margin they leak.
Regulated powersupply is:
Transformer-rectifier-filtercapacitors-regulator(transistors, zenerdiode, resistors, capacitors)
As Hyperion mentioned this is discreet circuit so build from singular components in a network/circuit.
So by taking out the fuses you have just the transformer connected.
Best you can do is measure the rectifier diodes one by one for a short. This normally shows up easy. Testing the filter capacitors can be more difficult because it's possible they don't leak at the voltage they get from the DMM. Best way would be to connect your DMM probes to each tab of the capacitor and switch it on for 1-2 seconds and rad out the voltage over the capacitor if this is smaller dan the rail voltage (48-50vdc) by a discerning margin they leak.
Forgive me if I ask dumb questions. You guys have been a major help but sometimes I feel a little embarrassed to ask things. I checked the rectifier bridge and the numbers seem to be bouncing all over the place. If I were to replace those, can you give me a link to the diodes I would have to buy? Also, you said if the DBT is bright, turn it off immediately. Don’t I need power going through the amp to test certain things with the MM? I feel like the problem has been narrowed down to the 8 output transistors, the rectifier bridge (D601,D602) or the filter caps.We need to get you measuring stuff by following our instructions. We need you to tell us when you don't understand the instructions, and for you to describe what you don't understand.
No, you can use your multimeter (with the unit powered off) to do most of the tests. Some tests need to be performed with the power on, but with anything that might be causing the short disconnected, as I described, I think clearly enough? in that long post of mine a while back. Perhaps Gris @GrisPato can sanity check it for me? to see if I have left out any key instructions for your level of understanding.
I am sorry if I have confused you,
I try to include as much detail as possible because that's my nature, I also feel that the detail needs to be there so I don't lead you to make a mistake. Of course the key thing is not to overload you with so much detail that you get spooked by it all, I am not very good at gauging how much that is. No no you’ve been an amazing help in explaining things. Some things go a little over my head but we aren’t in any rush to fix this. Do you agree that most likely the problem could be/is either the PSU, output transistors, filter caps, or bridge rectifier diodes? Is it ANY possibility that an electrolytic cap could cause the short? Some caps look to be deformed from age and heat. For example:No, you can use your multimeter (with the unit powered off) to do most of the tests. Some tests need to be performed with the power on, but with anything that might be causing the short disconnected, as I described, I think clearly enough? in that long post of mine a while back. Perhaps Gris @GrisPato can sanity check it for me? to see if I have left out any key instructions for your level of understanding.
I am sorry if I have confused you,
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Unlikely **Unless** someone has either replaced the capacitor(s) and put them in the wrong way round. OR disconnected the wiring and put that on back to front. (or another combination that does the same thing). All pretty unlikely.
If the soldering and wires on the main PSU capacitors look original then your thought about it being OP transistors or bridge rectifier causing the short will almost certainly be correct.
(Whatever you do, do not try reversing the capacitor wiring to see if that fixes it).
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Unlikely **Unless** someone has either replaced the capacitor(s) and put them in the wrong way round. OR disconnected the wiring and put that on back to front. (or another combination that does the same thing). If the soldering and wires on the main PSU capacitors look original then your thought about it being OP transistors or bridge rectifier causing the short will almost certainly be correct.
(Whatever you do, do not try reversing the capacitor wiring to see if that fixes it).
from the looks of it I am the only person to do anything on this amp. All caps are original. So… it looks like our next step should be replace the OP transistors and, hell, might as well replace the rectifier diodes as well?
GrisPato
Super Member
Did the bulb go dim at this occasion? Just to be sure.
About replacing rectifier diodes, filter capacitors. Sure you can do this but you wouldn't be the first one that is left with the same problem.
I would advice you to test the rectifier diodes for shorts. This means you localise where one starts and where it ends. Put the DMM in diodetest and measure both ways. You will have to do that four times. Test also the mica insulators from the outputs. One lead on the case one on the heatsink. Make sure you make good contact and measure resistance, should be OL.
Eight times ofcourse.
Otherwise you replaced a lot of components to find out it was something really simple.