HH Sott A436 - need help finding the differential pair

[Bert 1100]

Hello all,

I found a nice HH Scott A-436 integrated amp in my usual swiss goodwill-type store a couple of weeks ago. It is working nicely and cleaned up very well.
I want to recap it fully (Mundorf main filters and the rest Silmic II) and do a few other things like soft recovery diodes and leds for the vu-meters.

Now this amp does not have adjustable DC offset, and I am seeing 50mV and 120mV. Not too bad but not good either. So i want to change the differential transistor pairs to see if I can get that offset lower.

That's where I need your help, can someone please tell me if I'm right thinking they are the ones I drew the green squares around ? all 4 are 2sc1000.

I have some ksc1845 left from my marantz 2220 redo, I think they should be a good replacement, matching the hfe of course.

Also, I am hearing some low-level hiss from all inputs, left and right, that hiss disappears completely if I remove the jumpers to leave only the power-amp side working. There are other 2sc1000 transistors on the preamp and tone boards, am I right in thinking replacing those should get rid of at least some of that hiss?

I know the 2sc458 are famous for being noisy, are the 2sc1000 also suspect ?

Thank you very much in advance - and greetings from the swiss alps.

Bert.

 

[avionic]

Hello all,

I found a nice HH Scott A-436 integrated amp in my usual swiss goodwill-type store a couple of weeks ago. It is working nicely and cleaned up very well.
I want to recap it fully (Mundorf main filters and the rest Silmic II) and do a few other things like soft recovery diodes and leds for the vu-meters.

Now this amp does not have adjustable DC offset, and I am seeing 50mV and 120mV. Not too bad but not good either. So i want to change the differential transistor pairs to see if I can get that offset lower.

That's where I need your help, can someone please tell me if I'm right thinking they are the ones I drew the green squares around ? all 4 are 2sc1000.

I have some ksc1845 left from my marantz 2220 redo, I think they should be a good replacement, matching the hfe of course.

Also, I am hearing some low-level hiss from all inputs, left and right, that hiss disappears completely if I remove the jumpers to leave only the power-amp side working. There are other 2sc1000 transistors on the preamp and tone boards, am I right in thinking replacing those should get rid of at least some of that hiss?

I know the 2sc458 are famous for being noisy, are the 2sc1000 also suspect ?

Thank you very much in advance - and greetings from the swiss alps.

Bert.

Yes those are the diff pairs. KSC1845FTA. The higher the matched Hfe the better.

http://www.mouser.com/ProductDetail...GAEpiMZZMshyDBzk1/Wi8oN7VHZ91Ok7yYQ9zR8%2b88=

 

[Bert 1100]

Nice, thank you Avionic for such a quick reply ! Ill try to post some photos when Im done with the work.

 

[guest118]

bert, there are also a pair of bias resistors you see in the corners of the main amp board. Mine were way off spec. Check Q807E to Q810C, and Q817E to Q820C. Adjust to 8mv +/- 10%. They are the 4 large transistors on the heat sink.

 

[guest118]

btw, did you have luck with the transistors? My spec shows different versions for that application. It shows 2SC372 - 9630 - 35 Volt models not 120.

 

[Bert 1100]

Thank you John,
I did change the transistors, it's working well and the 8mV are easy to adjust.
But as I said in your thread I am now getting 100mV offset on both channels. According to the service manual that is just in spec (max = 100mV) but I don't like it anyway. I thought maybe it comes from the preamp, but it stays the same with the jumpers out.

My diff pair transistors were 2sc1000. My main amp board is a "revision 3".

A lot of things don't match with the manual. and the markings on the boards are terrible, caps shown with wrong polarity, even a diode drawn instead of a cap...

from the manual I was expecting 2sc1000 transistors on the preamp boards, but I found 2sc9633 and 2sc9634 on there, not sure what I can replace them with, and I'd like to replace them to possibly get rid of the slight hiss.

Can you point out exactly which are the bias resistors you are talking about?

Thank you!

 

[Bert 1100]

I'm about to recap an A-437, so I was re-reading this thread...and I said I'd put up some photos of the A-436 when it's done, so...

 

[jheu02]

Resurrecting this thread as I won one on that auction site along with the T-526 tuner. Both in nasty, barn find looking shape, but I wanted the challenge. They haven't arrived yet but I've been looking at the SM of both and coming up with cap lists etc. to check against what's actually installed.

What was the eventual outcome for your offset issue and the rest of the rebuild? It looks great from the outside...how's it sound? Any issues working on the boards?

 

[Bert 1100]

I left it as it was, around 100mV if I remember correctly. I've had a couple other Scotts similar to this one, and all had similar "issues" with the offset. I guess it has to do with the overall design...
It's definitely possible to bring the offset down but more likely requires some thinking ! Just changing the diff pair with matched transistors did not help here.
For the 436 the specs give a "max" figure of 100mV....that says something about the design goals I guess...

Sound wise it was good but not amazing.
The older series like the R75S are quite different, better build quality, MUCH better documented, and to me sound better as well.

 

[GrundigXpert]

I've just had the same issue with a Scott A236S. No offset and no idle current adjustments. Regardless of substitution of differential pair of one channel with perfectly matched transistors for both Vbe and Beta, I had 80 mV on the channel where the differential pair has been changed, and 45 mV on the other channel. I fixed it by modifying the collector load resistor value for the differential, which is also the base resistor for the VAS stage.
Did this by trial and error, till I got zero volts, +/- 10 mV.
Check the idle current afterwards, it may need adjustment.

 

[jheu02]

Can you list all the fuses that are actually in your unit and where? ...mine is missing the main power fuse and the speaker protect fuses are smaller than the "standard" 1/4" x 1 1/4" types, though the fuse holders look like they should be taking the 1/4" diameter types. Also, one is slow blow and one fast blow. So I'm not sure if they are correct. There's also the three fuses F901, 902, 903 on the PS board. SM only lists part numbers for the different fuse types, not location. Just trying to confirm.

Thanks,