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good advice

jdg123

jdg123

Active Member
I have taken on one of my biggest projects ever, a blown Sansui AU-919. I have cleaned it up, replaced all the electrolytics and black flag caps and the output transistors with 2sa2223A and 2sc6145A. There was only one obvious thing wrong with the amp upon tear-down: one of the emmitter resistors (R33/5W) was completely cooked as were its output transistors. From the service manual I gather I should check all the transistors on the driver board F-2839. Does this mean I should remove every one of those tiny dudes and check them individually or can I run a simple test in situ?

Lastly, and this is a long shot: does anyone have a better image of page three of the service manual? My xerox image of the amp's top view is useless for finding test point for bias current.

You guys are great and I'd really like to tap your ocean of good advice and bring this amp back from the dead.

Cheers!
 
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If you think you need to test the trannies independantly, yes you will need to test them out of circuit You may not need to do this however if you reckon you have tackled the problem already. I usually test the outputs and work back to the pre-drivers, then further back if problems persist.
 
slimecity said:
If you think you need to test the trannies independantly, yes you will need to test them out of circuit You may not need to do this however if you reckon you have tackled the problem already. I usually test the outputs and work back to the pre-drivers, then further back if problems persist.
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I don't know why that resister cooked. It is the only thing I saw was bad when doing the recap. Many things can let out their magic smoke without leaving behind evidence I guess. I'll have to keep going backwards as you suggest until I find something else wrong and assume the up stream then caused the downstream all based on an assumption: the resistor was OK until something else failed :-)
 
jdg123 said:
Lastly, and this is a long shot: does anyone have a better image of page three of the service manual? My xerox image of the amp's top view is useless for finding test point for bias current.
Click to expand...

I think even a good copy would not show it well, use the description at the top of the page for where to put your probes - but much much better here if you have mini-grabbers, and clip them on without power applied, otherwise you risk shorting something out.

This is a tricky adjustment - take extreme care.
 
I'm back...having replaced all the transistors on the power board 2839 (as well as all the electrolytics everywhere and all black flags)... but I'm hesitating to try the start up and adjustments! Which boards are done first?

An other scary thing: I have yet to find a bad transistor save the outputs; should I go deeper into the transistor list and to other boards replacing transistors?

It seems likely from all my reading that one of the black flags (sorry, I tossed them all having tested most and found them to have the expected Low pF values) caused the failure...

I can find no records of someone explaining the deaths of their AU-919s. Maybe we need an autopsy board...
 
OK. I have and use one but only (so far) after I have found the problem and as a precaution. So it seems it is the only way to check ones' work...to see if the problem was solved...

I know the protection circuit/relay takes about 11 seconds to click in; if the bulb does not then immediately dim I'll shut it down and look further into the unit :-)

Cheers!
 
Just to report: dim bulb tested it for 15 seconds...the bulb never went dim, protect relay never disengaged (no click). Sadness...
 
What wattage bulb are you using?

Have you done a search for AU-919 reconditioning threads and read them?
 
I am using a 60W bulb in my dim bulb tester.

I have read as many threads on the AU-919 as I can find, even ones just tangential. The one best bit of advice, to test boards as you replenish them, is hard to enact unless they mean that one should tear down the amp, fix one board and reconstruct the whole and test. That does not make sense! How would you know if you had done right or wrong until the unit just worked? And if the flaw that blew the amp was a board you had not gotten to then POW! You blow it again. Did I miss the part where the whole of that process was explained.

I know it is a lot to ask that one should teach for free. I know, I have done it as a free math tutor. I suspect this might be tougher. I'll ask though: has anyone got a line on the written process???
 
jdg123 said:
I don't know why that resister cooked.
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If a speaker was connected to the output when the output transistors "checked out" It would have sent rail voltage through that emitter resistor until
A. The speaker wiring opened up.
B. the emitter resistor opened up
C. the protect relay "reset".
 
The transistors that you need to take a close look at are, in order Tr29//Tr31, Tr25//Tr27, Tr23, Tr19//Tr21.

Here is a rough description of what you should look for:-
TR19 & TR21 set up voltages across the various resistors and diodes (D05 & D07) in a chain, in the middle of this chain you have TR23 which picks off a voltage from the chain and uses it to turn itself on hard , (when VR03 resistance is at max). This causes the bases of TR25 and TR25 and in turn the bases TR29 and TR31, to become much closer in voltage terms because they are effectively shorted together by TR23 turning on, this starves their bases of turn on voltage, and thus current, and thus gives you minimum bias. Dim bulb tester brightness will definitely be affected if this is not working properly.

The above is the condition you are looking for, you need to find out why this isn't happening, it could be a back to front electrolytic capacitor, or diode, a shorted zener, an open resistor, or open trimmer resistor - anything like that.

And BTW it has nothing to do with teaching being free - it is that novices/learners can make the most horrendous screw ups and they don't even realise what they have done. Which makes helping them not exactly a thankless task, but bloody difficult to say the least.
 
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I have been tied up a couple of days but will take this data to heart. I will report back. I am a slow student but diligent :-0
 
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