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Sansui AU-6900 Woes .......

camgilbert said:
When i hook up the Orange and Brown leads from the Main Caps I still have a DEAD SHORT.
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Dont be offended - I did this myself the other day - main cap polarity is correct?

Do you have a capacitance meter to test each main cap individually?

Probably not the issue but - Are you satisfied none of the outputs or pre-drivers are contacting the heatsink?
 
Hi guys,

I wish I had done that earlier.

After taking out the 8 output transistors the amp turned on properly.

I suspect if I need to solder them in and out one more time I will have no trace left on the circuit board.

So does this mean the issue is with the output transistors only?

If so, how do I test? cause as you know from above, these ones that were currently installed were brand new On Semis.

Thanks.
 
Why don't you retest each OP transistor (and the drivers for that matter) using the '6 way test' with a multimeter on 'diode test' range. And then re-install one NPN/PNP pair at a time, testing for shorts with your multimeter on 'resistance' range after installing each transistor. Only when you get a clean install with no shorts detected with the multimeter should you apply power - on DBT of course. Your TO3 mica washers should be free of cracks and splits, and no oversize holes for the transistor legs and screws, thoroughly clean and inspect each one carefully for solder/metal swarf embedded in them, replace any damaged ones.

Note: I spent about 20 minutes last night checking you had the OP & driver transistors in the correct positions - I was checking your picture with the mixture of 2SB545 // 2SD188 and NTE88's, (it seems I wasted my time as you had the MJ's installed!) I also checked some of the wiring against my pictures of an AU-7900 restore that I did.
 
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Hi guys, finally some good news.

After methodically going through tests with DBT and slowly adding transistor pairs, I was able to isolate a bad PNP output transistor and replace.

I now have the unit plugged in without the DBT and it powers on but relay does not click in.

I have tested most of the resistors and diodes which seem good.

I assume now I need to work back from the OT's through that circuit?

Any hints on this?

Status 9162017.jpg
 
camgilbert said:
I now have the unit plugged in without the DBT and it powers on but relay does not click in.
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What wattage bulb do you have in the DBT ? should be 60W, as with a lower wattage it could very well be normal for the relay not to activate.

I suspect you simply have high DC offset causing the relay not to activate. So, you need to find a measurement point for DC offset that is before the relay contacts, because if the relay won't close there is no connection to the speaker terminals, so you can't use them as measurement points for checking the DC offset. I suspect high DC offset and the protection circuit is preventing relay closure (as it should!). Find those measurement points, (at the junction of a pair of emitter resistors) and check the DC offset there and adjust. Just get the relay engaging, don't spend time adjusting the DC offset finely with it on DBT, then check setting the bias - just to see if it will adjust - but turn it way down before removing the DBT this is important.

Any questions just ask.
 
good advice! Some amps dont like clicking on the dbt....
Edit
Ooops, no dbt , and no click.... Just caught that.
 
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Thanks guys, yep, working with no DBT (prior to connecting to AC directly, on DBT the 60W bulb literally was off when amp clicked in, I figured that I was pretty safe)

Ill have a look at comments on how to test DC offset.
 
So I checked the DC offset for the mains and the right channel was 30mv and the left channel was about 13mv.

I also measured after the bias pots and after slight adjustments the left and the right were both about 18mv.

Most of the Tr's have been replaced except the A798's, C1475's (which ironically are not even in the service manual) C1951's, D438 and none of the diodes.

Protection circuit still not clicking in.

GBC-ASUSDESKTOP 9-17-2017 11.28.48 AM.jpg
 
Ok sow what you are saying is that you are measuring the DC Offset PRE-relay (not on the speaker terminals), and you are getting those mV readings?

If that's the case, then yep, I would start looking at the protect circuit itself. Does the schematic tell you what voltages you should be getting around TR601-603?
 
As a major NOOB here, I wondered if I could get some guidance on where to take my measurements.

Also should I be measuring with or without DBT?

I did test the DC Offset on the speaker terminals but do understand that the issue is probably very close to the relay.

With the two images below (the Schematic and the physical board) could anyone offer the test points for my measurements to try and get to the bottom of this.

Thanks in advance.

GBC-ASUSDESKTOP 9-17-2017 8.51.31 PM.jpg20170917_205732.jpg
 
sansuilove said:
Chapter 6 on Service Manual - Adjustments.
page 12.
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Yes, that is correct, however that procedure is only for bias adjustment. Thank you for posting that information - I went to check and it made me realise there is no DC offset adjustment with this amplifier, having reconditioned one AU-7900 I was sure there was an adjustment, :( but apparently not.

The fact that the DC offset is sufficient that the amplifier goes into protection mode suggests a fault in one or both of the power amplifier stages as yet unresolved.

For DC offset (when in protection mode), You need to measure (with no DBT) at the junction of R67 & R69 (0.47 Ohm cement resistors), to chassis ground. And the same for R68 & R70 - these are the emitter resistors for the OP transistors and are mounted on the F-2596 main amplifier board, the board with the fuses and the relay. (be careful ! - no 'probe slips' allowed here ;) )
 
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And checking voltages on the DBT is fine - its just that your readings will be lower than what they would be, off the DBT.

So if you want to be careful - measure first on the DBT, if the readings are generally OK and not too far off what they should be - if all else is safe, take the amp off the DBT and check the readings again - if they are close, they should come up a few mV or V.
 
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