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AU-666 +20V DC offset on each channel?

B

buler4

New Member
Hi AK

I have an old AU-666 that has been recapped and gotten new transistors all around since it was in a really bad shape.
However when i turn it on now it has a DC offset of around -28V on both channels measured with no load, and I have run my head into the wall as to why.. All new transistors are picked from countless hours of research with previous rebuilds found around the forum. And as such the current input pair transistors are ZTX694B but they are not HFE matched as i have no acces to a meter that can measure it.
They are however from the same reel which would make one think that they are relatively close.

Adjusting the trimpot does nothing, nor does it help to bypass the preamp

Worth noting is that bias does not seem to affect the offset, and no components seem to produce any notable heat above lukewarm
The crowbar protection has been bypassed as long as im working since it kept triggering

What am i missing? 28V offset is off the charts from anything i have experienced before.
 
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Have you checked the pin-outs of every single transistor you have replaced? - especially the ZTX694's
 
check whether you have positive supply voltage at B5 (+38V), I think at pin 1 and -32V at B7
or at pin 6 of the driver board.
 
To have the same voltage on the output of both channels suggests an error has been made, very likely the exact same error for both channels. I strongly advise you to recheck your work, assume nothing, and check all that you have done.

If you are seeing -28 Volts at the output this means that TR813 & TR817 are conducting, whereas TR811 & TR815 are not. What have you used as a replacement for TR809, originally = 2SC281 ? - very easy to get the pinouts wrong for this transistor compared to its replacement. ;)
 
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Hyperion said:
To have the same voltage on the output of both channels suggests an error has been made, very likely the exact same error for both channels. I strongly advise you to recheck your work, assume nothing, and check all that you have done.

If you are seeing -28 Volts at the output this means that TR813 & TR817 are conducting, whereas TR811 & TR815 are not. What have you used as a replacement for TR809, originally = 2SC281 ? - very easy to get the pinouts wrong for this transistor compared to its replacement. ;)
Click to expand...
Okay im done checking now. Only faults i found was on tone control (F-1279) but even with that unpluged nothing changes. I have today tried to isolate where the fault lies and have found that even with no other boards than a single driverblock, and no power to the output transistors I still get upwards of -30V on the output.
So one must conclude that my fault lies on the driverboards. I did check power supplied to the boards and that too is correct +-40V

Now comes the question as to why this happens? wondering if i choose a bad replacement transistor

I have 3 original transistors left on the driverboards.
TR-813 --> i have a replacement called BD14016
TR-811 --> forgot to order replacement
TR-809 --> I have a replacement called KSC1815

I havent changed the bias stuff since I forgot one transistor and read somewhere that I should really only touch it if setting bias was troublesome, however it dials in easily

Output transistors had to be changed too and are now
2sA627 --> MJ21193G
2sD188 --> MJ15001G

other replacements on the board
CDC8002 --> KSC2690
XA495 --> ZTX694

Caps:
Nichigon BP 47uf
Nichigon 100uf
 
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Tried soldering in the old CDC8002 again, which lowered the offset to 10V, as far as i know this might just be the lower gain of the old CDC over the new KSC
 
petehall347 said:
isnt a ZTX945 NPN ?
Click to expand...

My guess is it's ZTX795A PNP.

@buler4 , did you also swap transistors on the ripple filter boards on the underside of the chassis? If so recheck and note here what you used.

You could try removing all output transistors and driver cards and check your voltages again per schematic at the card sockets.
 
AnalogueBen said:
My guess is it's ZTX795A PNP.

@buler4 , did you also swap transistors on the ripple filter boards on the underside of the chassis? If so recheck and note here what you used.

You could try removing all output transistors and driver cards and check your voltages again per schematic at the card sockets.
Click to expand...
Yeah they too used the dreaded CDC ones, i swapped them with

CDC8002 --> KSC2690
CDC9002 --> MJE172

output form the ripple blocks all check fine

Voltages in sockets are
Row 1: 41,7V
Row 6: -41,8V
 
TR813 and TR811 are your driver transistors, I would not change them unless they've failed. And if you change them do so with a complimentary pair.
BD139/BD140 to use as an example.
Same goes for the KSA1220A/KSC2690A, use as pairs, like for CDC8002/9002 pairs.
 
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