• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Scott A436 transistor replacement

P

Pasagile

New Member
Hello,
I've been putting my hands inside a Scott A-436 lately. After replacing all capacitors, I checked for bias according to the service manual instructions. (https://retronik.silicium.org/DOCUMENTS/Audiovideo/Scott/Scott-A-416-A-426-A-436-Service-Manual.pdf).
When I checked for the 0.2 mA DC current as written on the manual, transistor Q815 blew up. I'm not sure what happened, I knew where to put the probes but maybe shook a bit too much...
This transistor was a 2sc372. Can you help me find a replacement for this pair, or help me understand what is their role so that I know which parameters to match ?
Also could use guidelines on how not to keep blowing up my amp :(
Thank you
 
Register to hide this ad
My work blocks the link but as far as the transisor itself, it's nothing special, just a small signal general purpose amplifier. Nearly any NPN small signal gen purpose transistor will work from a 2n3904, a BC546, to a KSC1815YTA. Mind your pinout though.
 
ohm470k said:
My work blocks the link but as far as the transisor itself, it's nothing special, just a small signal general purpose amplifier. Nearly any NPN small signal gen purpose transistor will work from a 2n3904, a BC546, to a KSC1815YTA. Mind your pinout though
Click to expand...

ohm470k said:
My work blocks the link but as far as the transisor itself, it's nothing special, just a small signal general purpose amplifier. Nearly any NPN small signal gen purpose transistor will work from a 2n3904, a BC546, to a KSC1815YTA. Mind your pinout though.
Click to expand...
Thanks a lot. Should I replace the equivalent on the other channel too, or no need ?
 
Pasagile said:
Thanks a lot. Should I replace the equivalent on the other channel too, or no need ?
Click to expand...
I wouldn't necessarily worry about them on the other channel. Check the known problem transistor list on this site and see if they are on the list. If they are, then shotgun them. Replace any complementary ones though too. For example, if you have a 2sc372, it's complement is the 2SA495, I believe. if you see any of them around the 2sc372 replace them with the complement of whatever NPN you use to replace the 2sc372. 2n3904 and 2n3906 , for example, are complementary.
 
Pasagile said:
I don't know, but I could count with the manual. How does it matter ?
Click to expand...
well, you asked how you could not blow up your amp.

One way is to be meticulous about your method of replacing capacitors. This would include:

1) keeping a precise account of the original capacitors as a source of reference
2) testing the amp after replacing a limited amount of capacitors to help isolate potential problems
 
Pasagile said:
Thanks a lot. Should I replace the equivalent on the other channel too, or no need ?
Click to expand...
Need, no. Personally i like both channels to be the same though so if this were mine, I'd probably change the bad one, verify everything is happy vs the working channel, then change the other for symmetry.

just my 2 cents and personal preference. From the guy listening to an amp that I replaced a blown set of outputs with used pulls of a different type in one channel only :)
 
malden said:
well, you asked how you could not blow up your amp.

One way is to be meticulous about your method of replacing capacitors. This would include:

1) keeping a precise account of the original capacitors as a source of reference
2) testing the amp after replacing a limited amount of capacitors to help isolate potential problems
Click to expand...
What could have gone wrong replacing the capacitors ? Seemed pretty straightforward.
I understand this approach can only be better, but I was not in good conditions to do that, and didn't really have that much time.
 
ohm470k said:
I wouldn't necessarily worry about them on the other channel. Check the known problem transistor list on this site and see if they are on the list. If they are, then shotgun them. Replace any complementary ones though too. For example, if you have a 2sc372, it's complement is the 2SA495, I believe. if you see any of them around the 2sc372 replace them with the complement of whatever NPN you use to replace the 2sc372. 2n3904 and 2n3906 , for example, are complementary.
Click to expand...
gadget73 said:
Need, no. Personally i like both channels to be the same though so if this were mine, I'd probably change the bad one, verify everything is happy vs the working channel, then change the other for symmetry.

just my 2 cents and personal preference. From the guy listening to an amp that I replaced a blown set of outputs with used pulls of a different type in one channel only :)
Click to expand...
Thank you, I'll take that into account, must admit that I like symmetry too.
 
Pasagile said:
no need to reply further
Click to expand...
these threads may be useful to someone years from now, not just for the current problem

The point I was trying to make is that keeping track of what is replaced, even if it's just a capacitor, could spare someone a big headache.

Also, testing each stage of the amp after replacing components is a good practice.
 
Last edited:
Hello again, I had time to work on the amp and turns out the damage affected more components than I thought. On the right channel which we were discussing, Q813, Q814, Q816, Q819 went down, with some diodes as well. On the left channel on which I had also shorted something (the lesson is learned) all the transistors went down (including outputs) except for the differential ones Q801 and Q802. Also several diodes.

I you are willing to help me some more, I'd like to ask you a couple questions:

should I check resistors and condos that appear fine ? If so, should I test all of them (please no...) or test a few based on something ?

some transistors passed the diode test, but showed out of limits resistance between certain pins (I think often B-E and B-C), can I assume they survived ?

are there transisors I should match between channels ? Or some I should match for hfe as sometimes advised for the diff pair ?

-could other circuit board have been affected ?

Thanks a lot in advance if you take the time to answer
 
You must log in or register to reply here.
Back
Top Bottom