Sansui AU-X901 Protection Mode

No, check the bias current - this will be a measurement taken across those emitter resistor pairs, with your multimeter set to read mV - instructions for this are in the SM - there are 2 settings per channel for this. ('hot side' bias current and 'cold side' bias current)

The reason for checking the bias is obviously to make sure it is set correctly, but also too much bias current could be the very reason why protection is still engaged. Too much bias current will cause more voltage to be dropped across the DBT bulb, thus lowering the available voltage to the protection circuit, thus possibly preventing it from disengaging.

Please do not remove the DBT until we can be sure that both channels respond correctly to DC offset and bias adjustment.

When we have verified this we will check what is preventing protection from disengaging.

I have checked the out put resistors and they check fine.
I'm not able to adjust the bias as when connected across the out put resistors the meter just reads zero.
I have 3 multimeters all of them read zero and the fuse inside the meter is fine.
Please see this picture the transistors marked in yellow become hot if I leave it on DBT.
 
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I have quite a few KSC3503 and KSA1381 , should I use the shotgun approach and replace everything on this board?
 
I have quite a few KSC3503 and KSA1381 , should I use the shotgun approach and replace everything on this board?

No, please don't do that.

Ok for the left channel please give me the voltages (with respect to chassis) 3 readings - as per the indications on the schematic around Q1. (should be -30, -30.5 and -36V)

Please also give me the emitter voltages (with respect to chassis) for Q6 and Q7 for both channels - please be sure to indicate which voltages apply to each channel. (should be +35V & -35V).
 
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No, please don't do that.

Ok for the left channel please give me the voltages (with respect to chassis) 3 readings - as per the indications on the schematic around

Q1. E--33.74, C-0.41, B-33.04

Please also give me the emitter voltages (with respect to chassis) for Q6 and Q7 for both channels - please be sure to indicate which voltages apply to each channel.

LEFT Q7= E -37.80, Q6=E36.34
RIGHT Q7-E -37.91, Q6=E36.68
 
No, please don't do that.

Ok for the left channel please give me the voltages (with respect to chassis) 3 readings - as per the indications on the schematic around Q1. (should be -30, -30.5 and -36V)

Please also give me the emitter voltages (with respect to chassis) for Q6 and Q7 for both channels - please be sure to indicate which voltages apply to each channel. (should be +35V & -35V).
On left around Q1
E -33.85
Near KR10 -39.37
Near KR9 -32.32
 

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From those voltages it looks like (left channel) kDZ1 is open - if I understand your voltage readings correctly it has just over 7V across it, it should have ~6.2V or thereabouts, please replace it.

Need to know all the voltages as indicated on the right channel Before Q6 and Q7, that is kQ2,3,4,5 collector voltages, and those around kFT1 - 9 readings in all.
 
From those voltages it looks like (left channel) kDZ1 is open - if I understand your voltage readings correctly it has just over 7V across it, it should have ~6.2V or thereabouts, please replace it.

Need to know all the voltages as indicated on the right channel Before Q6 and Q7, that is kQ2,3,4,5 collector voltages, and those around kFT1 - 9 readings in all.
Thank you I will update the thread tomorrow with voltages you have asked for. Due to work did not get the time to do it today.
 
From those voltages it looks like (left channel) kDZ1 is open - if I understand your voltage readings correctly it has just over 7V across it, it should have ~6.2V or thereabouts, please replace it.

Need to know all the voltages as indicated on the right channel Before Q6 and Q7, that is kQ2,3,4,5 collector voltages, and those around kFT1 - 9 readings in all.
KDZ1= one end -38.67=== other end -32.27 (LEFT)
KDZ1= one end -38.54=== other end -32.75 (RIGHT)

LEFT= KQ2 +35.52 KQ3 +35.51 KQ4 -36.99 KQ5 --37.15

RIGHT= KQ2 +35.64 KQ3+ 35.51 KQ4 -37.21 KQ5 --37.29

Looks like the diodes are fine, I made an error in measurement probably.

FT1 on both side has 5 legs, where do I measure the voltage ?
Thank you in advance
 
That's looking OK, for the other voltages measure on the base legs of kQ2,3,4,5 - Be Careful ! - not to short from the base leg of those transistors to anything else - otherwise bad things could happen. :)
 
That's looking OK, for the other voltages measure on the base legs of kQ2,3,4,5 - Be Careful ! - not to short from the base leg of those transistors to anything else - otherwise bad things could happen. :)
LEFT= KQ2 +8.88 KQ3 +8.83 KQ4 +7.02 KQ5 +7.04
RIGHT= KQ2 +9.18 KQ3 +9.20 KQ4 + 7.34 KQ5 +7.33
 
Well that's looking fine too - good news.

Next - for the right channel only (we will now deal with one channel at a time).
I need you to measure between kQ52 collector and emitter - and provide voltage reading
and between kQ5 collector and emitter - and provide voltage reading

You can measure on any convenient point that directly connects to each location.

For guidance, both of these should be approximately 3.6V.

If you can, please try to adjust kVR52 (for kQ52 reading) - and see if the voltage changes (reduces) - be quick and return it quickly to the highest voltage reading when complete.
And also, please try to adjust kVR51 (for kQ5 reading) - and see if the voltage changes (reduces) - be quick and return it quickly to the highest voltage reading when complete.

Once again - be very careful with your multimeter probes when doing this, I have 'mini-grabber' multimeter leads for this kind of adjustment - much safer.

Sorry this is a slow process, I have to be very precise with my wording - it is so easy to misdirect or confuse you, and cause a calamity.
 
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Well that's looking fine too - good news.

Next - for the right channel only (we will now deal with one channel at a time).
I need you to measure between kQ52 collector and emitter - and provide voltage reading
and between kQ5 collector and emitter - and provide voltage reading

KQ52 measuring direct on the E and C is 2.9v , AGAINST GROUND E= -11.50 C= -8.50 , using KVR 52 I have brought up the voltage to 3.1v
KQ 5 Measuring direct on E AND C 45.32V , using KVR 51 there is no change in voltage at all
 
KQ 5 Measuring direct on E AND C 45.32V , using KVR 51 there is no change in voltage at all

Aha - this is a problem.

Possible causes
kD51 and/or kD53 short circuit (or replaced and fitted back to front? or wrong diode type).
kQ53 and/or kQ55 short circuit C-B (or replaced with sub and orientated incorrectly, or wrong sub).
Or small possibility kQ5 faulty - Open circuit C-E (NB: this is the kQ5 nearest to kVR51)

Please check the above, and replace as appropriate.
 
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Aha - this is a problem.

Possible causes
kD51 and/or kD53 short circuit (or replaced and fitted back to front? or wrong diode type).
kQ53 and/or kQ55 short circuit C-B (or replaced with sub and orientated incorrectly, or wrong sub).
Or small possibility kQ5 faulty - Open circuit C-E (NB: this is the kQ5 nearest to kVR51)

Please check the above, and replace as appropriate.
I replaced Q53 as it was shorted. I replaced it with KSC2383YTA , Pin out was same as old transistor ECB. Q 55 checked fine out of circuit and I replaced it ECB again. The diodes look original. I have compared the voltages across both ends of diodes and they are comparable to left side. However I am happy to replace the diodes.

I am out of my depth here. What number diodes should I use, like 1N4004 etc
 
on supposedly good side, ie Left side the voltage on E and C of Q5 also does not respond to KVR 51 either. Does it mean that side is blown as well ?
 
No, don't replace the diodes - UNLESS they are faulty, 1N4148 are fine to replace them with (1N4004 are huge overkill, and not advised). But it would be nice to KNOW that one or other are indeed faulty before you replace them - I would strongly prefer you don't replace them if they are OK.
 
on supposedly good side, ie Left side the voltage on E and C of Q5 also does not respond to KVR 51 either. Does it mean that side is blown as well ?

Yes it might be a fault affected by the same issue - difficult to know 'what' though....

Can you think of anything you have done to both channels which might match up with the effect you are seeing?
 
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