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Troubleshooting the Sansui AU-555A

DatGtrPlaR

DatGtrPlaR

Amuse your Muse
I started a thread earlier on restoring the 555A,
https://audiokarma.org/forums/index.php?threads/my-first-restore-attempt-sansui-au-555a.977082/

which was turning into a troubleshooting thing. So, from the advice of others here, I’m doing this particular troubleshooting here. The affected sound on the video happens on outputs of both channels, sets A and B. Here is a more detailed video of what’s currently happening with this amp. I must add that I am a complete novice at audio repair, looking forward to learning much..
 
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I think you'll have to change the 2SA726 transistors. These are known trouble spots, I think they can be replaced with KSA992, but you'll need to do some reading on the AK threads on these to be sure yourself. There is a lot of info on the 555A, see Leestereo's stuff.
 
When in Stereo MODE, are you hearing the same distortion with the balance knob turned fully to the left as when the balance knob is turned fully to the right?
 
DatGtrPlaR said:
Yes… same distortion…here’s a new video
Click to expand...
So, both channels. Hard to tell, but I'm guessing that since both channels are equally affected you have a power supply issue. I'll look at the schematic and report back.

Meantime, have you pulled the pre/main jumpers and determined if the issue lies in the preamp or the amp? Have you done anything besides cleaning yet? If so, did it ever work properly while you've owned it?
 
Ray Gianelli said:
So, both channels. Hard to tell, but I'm guessing that since both channels are equally affected you have a power supply issue. I'll look at the schematic and report back.

Meantime, have you pulled the pre/main jumpers and determined if the issue lies in the preamp or the amp? Have you done anything besides cleaning yet? If so, did it ever work properly while you've owned it?
Click to expand...
I haven’t pulled the jumpers yet.. I’ll try that.. I have not cleaned it yet. I just bought this, was working like this when I got it ..bought for a great price as I would like this to be my first fix it/restoration/mod project.
 
DatGtrPlaR said:
I haven’t pulled the jumpers yet.. I’ll try that.. I have not cleaned it yet. I just bought this, was working like this when I got it ..bought for a great price as I would like this to be my first fix it/restoration/mod project.
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Cool.

Do you have any test equipment? DMM, oscilloscope or signal generator?
 
DatGtrPlaR said:
Got the DMM, signal generator on the way, I’m still trying to find a good analog Oscilloscope that works. Hoping for a good Tek 465B that works!
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Get something newer than a 465 or you're going to be fixing or replacing parts in it.

I got a 2445A for about $100 5 years ago and never regretted it. I was replacing bad caps in 465's 25 years ago.

Like I always say, you don't need vintage test equipment to fix vintage audio. :)
 
DatGtrPlaR said:
I haven’t pulled the jumpers yet.. I’ll try that.. I have not cleaned it yet. I just bought this, was working like this when I got it ..bought for a great price as I would like this to be my first fix it/restoration/mod project.
Click to expand...
Pulled the jumpers.. put the preamp out of the 555a into a known working power amp and got no sound. Ran the preamp of the known working amp into the the power amp section of the 555A and got the same distortion as the video. I’m perplexed as to why I got no sound from the 555 pre to the power amp, as the preamp showed some signs of function in the same unit…. Volume, balance, eq, filters ..etc. seemed to be doing something as in the video
 
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That effectively points the finger at the power amp stages. If you have a voltmeter available, analog or digital, check the DC voltage value at C001. Then check the AC voltage value at the same place.

The 2SC281 in the pre-driver position is a known static generator. It may not be the only problem, but probably isn't helping.

Even though the preamp section seems to be working, there are quite a few 2SC871s in that section, also known to be problematic. Once the repairs are completed, it may be worthwhile to replace those as a longevity/durability measure.
 
Watthour said:
That effectively points the finger at the power amp stages. If you have a voltmeter available, analog or digital, check the DC voltage value at C001. Then check the AC voltage value at the same place.

The 2SC281 in the pre-driver position is a known static generator. It may not be the only problem, but probably isn't helping.

Even though the preamp section seems to be working, there are quite a few 2SC871s in that section, also known to be problematic. Once the repairs are completed, it may be worthwhile to replace those as a longevity/durability measure.
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Thanks for your answer, I am a complete novice, so please forgive me, how do I go about measuring the voltage? I have a Fluke 117, I know where C001 is, but I don’t know anything other than that. The 2SC’s are all transistors correct?
 
DatGtrPlaR said:
Thanks for your answer, I am a complete novice, so please forgive me, how do I go about measuring the voltage? I have a Fluke 117, I know where C001 is, but I don’t know anything other than that. The 2SC’s are all transistors correct?
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You'll have to take the bottom cover off. On the bottom of C001 you should see 2 terminals with wires attached to them. Place a probe on each one at the same time. Measure the DC voltage and note its value. Then, as @Watthour pointed out, change the meter to AC volts and read from the same 2 terminals. The AC reading will tell us if C001 is doing its job.

upload_2021-11-6_6-51-34.png
 
Oh, and with the power off and your meter set to ohms check the 2 fuses between the output transistors. You can see one of them in the above pic in the lower left-hand corner.
 
Ray Gianelli said:
You'll have to take the bottom cover off. On the bottom of C001 you should see 2 terminals with wires attached to them. Place a probe on each one at the same time. Measure the DC voltage and note its value. Then, as @Watthour pointed out, change the meter to AC volts and read from the same 2 terminals. The AC reading will tell us if C001 is doing its job.

View attachment 2387614
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Thanks! And is this when the power is on?
 
When it gets to the point of upgrading, perhaps this will be useful:

http://www.wwdsltd.com/files/SansuiAU-555AUpgradeSchematic.jpg
SansuiAU-555AUpgradeSchematic.jpg
 
Ray Gianelli said:
You'll have to take the bottom cover off. On the bottom of C001 you should see 2 terminals with wires attached to them. Place a probe on each one at the same time. Measure the DC voltage and note its value. Then, as @Watthour pointed out, change the meter to AC volts and read from the same 2 terminals. The AC reading will tell us if C001 is doing its job.

View attachment 2387614
Click to expand...
Thank you…..Well, both fuses were blown between the output transistors. On C001,AC was approx. .025 it was a little jumpy on the meter going between 0 and .040, reading mostly in the .020’s. DC was reading 59.9. What should be my next plan of action?
 
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