During Sansui AU-101 recap, a blown channel

ColinSage

New Member
There was a distortion on the left channel of my AU-101 so I recapped, then went through replacing the transistors... the distortion on the left channel seems to be gone :) but now the right channel makes some quiet, distorted, semi-musical buzzing when I turn the volume up, and not much else.

Reading through other posts, I thought this might be a Bias problem so removed the fuse to check Bias (as advised here: http://audiokarma.org/forums/index.php?threads/sansui-au-101-bias-adjustment.435511/ ). The reading on on the left channel was fine (circa 10.0mA) but was 0.4mA on the right channel. It gives a pretty similar reading not matter how far I turn the variable resistor.

Any suggestions about what this might mean and what I should try next? I've checked the fuse and orientation of transistors but I'm n00b to all this...
 
Agree with mroboto - I've had similar symptoms on my 999 when a biasing fuse blew. double-check the fuse and fuse-holder and (carefully) replace the bias pots.
 
Thanks for the advice both. I'll definitely get some Bourns in. Great idea to check the fuse casing.

I've checked the resistance across the fuse + casing and this is low, so I think OK. Similarly the bias pots seem OK - they have the same resistance on either channel + the resistance changes when I turn them
 
Thanks petehall... I'm not sure which one the emitter resistor would be, but I've gone just ahead and checked all the resistors on the main amp board. No problems unfortunately!
 
In that case TR806, TR808 or D802 are suspect, as long as you are sure all the resistors are ok? like R832, R834 and R836, R838 - these last two are the emitter resistors Pete mentioned.

Measure the voltages you have at the points indicated on the schematic and tell us what they are, that's 3 readings which should be 1.7V, 8V and 21V approx - what have you got?
 
Last edited:
Thanks for the help John. I've had a look at the schematic but can't see how are the points that I need to measure are indicated?

I'm fairly sure the resistors are OK: I've checked them twice in the last couple of days (and re-checked the ones you flagged just now). But my model has wires instead of the emitter resistors R836 and R838 (I've attached a pic!)? And I haven't checked the resistors on the toneboard.

D802 has the same impedences as D801, so I guess that's OK.
 

Attachments

  • 20161114_145551.jpg
    20161114_145551.jpg
    42.6 KB · Views: 36
Thanks John! I've found a different (less pixelated) schematic online and so was able to find them:

Left (working) channel scores: 2.13V, 10.9V, 25.9V
Right (broken) channel scores: 0.36V, 10.23V, 13.5V

(I put the negative probe on the right hand side of R826 for the tests of the right channel and left-hand of R817 for the left channel - assuming this was right as the values seem in the right ball park)
 
Well, I think you have some failed transistors, hard for me to say which exactly - there are only 4 (excluding the OP transistors, are these already tested?) - can you remove and test these 4 - using 'the 6 way test'? - try TR806 & TR808 first - then TR804, and finally TR802.

Do you have replacements for these?
 
Thanks John - I finally got to look at this just now. The four transistors all pass the 6-way diode tests fine. But I've put them back in and the three voltage readings are still the same.

I think that the output transistors are OK. I replaced these last week anyway, but did in-circuit diode testing of these today to check they hadn't blown during my soldering (my technique's dreadful)
 
That trimpot looks like it needs to be put out of its misery. Might be a good idea to replace that too once you have found your immediate issue.
 
I think I would change those transistors anyway, then see how it looks. +1 to changing all trim pots as well.
 
Great! I've replaced the transistors TR804, TR806, TR808 with new ones. I didn't have a new TR802 but swapped this with TR801. Still doesn't work :-(

I've ordered some new trim pots in though - they're pretty filthy...
 
You said you soldering technique is not so great - have you double checked to ensure that you don't have any lifted or broken tracks where you replaced components, or have used too much solder anywhere that has made joins where there shouldn't be any?
 
Thanks slimecity - I think you're right about that being the problem... I checked again last night and the leg from TR806 had lifted slightly and lost its connection to R822. I'll fix this later and hopefully be up and running again
 

Attachments

  • Capture.PNG
    Capture.PNG
    143.5 KB · Views: 18
Happy ending! Reconnecting TR806 and R822 has done it. All up and running again, all the fuzz gone from the left channel, and the right channel working again. So I've closed it all up and am listening again, at least until the new trimpots arrive...

Thanks everyone for all the helpful suggestions and ideas!
 

Attachments

  • 20161130_174654 (1).jpg
    20161130_174654 (1).jpg
    36.5 KB · Views: 13
Great outcome. We've never even seen one of these little amps. People do seem to like them.

This is an example of what we do here, Audiokarma at its finest.
 
Back
Top Bottom