I am in the process of restoring this Sherwood S-9500c. When I got it, the amp would not power on because of a blown main fuse, the replacement also blew so I traced the problem to a defective rectifier diode and got it to run. I then replaced three power supply electrolytic capacitors, each rated at 500uF since none of them tested near spec. The main filter capacitor, 3000uF remains in place for now. I also replaced the two output caps since they were not original and tested over 50% higher than their stated values.
Listening revealed audible distortion in the LEFT channel so I tested all of the caps and found one bad cap on the main amplifier board. Replacing it did not solve the problem so I proceeded to replace all of the electrolytics in the preamp, tone and amplifier boards. No change, I then started to test the transistors with this handy Peak Atlas Semiconductor Component Analyzer and one transistor in the amp board tested as a diode so I assume this could be the problem.
If I am reading the schematic correctly, the transistor in question is Q205 (SPS41). The transistors on the amp boards are stamped "TP" "109C". I've attached photos and the service manual.
What would be the appropriate replacement for the SPS41 ?
..........BEFORE..........
..........AFTER..........
BAD TRANSISTOR
Listening revealed audible distortion in the LEFT channel so I tested all of the caps and found one bad cap on the main amplifier board. Replacing it did not solve the problem so I proceeded to replace all of the electrolytics in the preamp, tone and amplifier boards. No change, I then started to test the transistors with this handy Peak Atlas Semiconductor Component Analyzer and one transistor in the amp board tested as a diode so I assume this could be the problem.
If I am reading the schematic correctly, the transistor in question is Q205 (SPS41). The transistors on the amp boards are stamped "TP" "109C". I've attached photos and the service manual.
What would be the appropriate replacement for the SPS41 ?
..........BEFORE..........
..........AFTER..........
BAD TRANSISTOR
Some circuits won't give a valid transistor test result, so carefully remove it and test again.
It may be fine. Transistors will test as diodes between the B-E and the B-C junctions.
The C-E should always test as open. Looks like a BC-109C NPN transistor. The 2N3904 NPN is good.
But check both pinouts very carefully before installing, they could differ. Nice work.
How did it test bad, one junction open?
It may be fine. Transistors will test as diodes between the B-E and the B-C junctions.
The C-E should always test as open. Looks like a BC-109C NPN transistor. The 2N3904 NPN is good.
But check both pinouts very carefully before installing, they could differ. Nice work.
How did it test bad, one junction open?
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Thanks for the quick response, rayma. Yes, I did pull the transistors out of circuit before testing, the first one was a little scary but I got the hang of it.
No voltage checks? I'd do that before yanking parts. The Q205 circuit won't give any exact voltage, and does not need to. If the transistor is bad, it will give way-wrong voltages.
And yes, any small NPN will do, BC-109 is popular.
And yes, any small NPN will do, BC-109 is popular.
PRR, thanks for the reply. I am very much a novice, so without explicit instructions I am left to my own devices. Yes, I should have started this thread before replacing parts but I wanted to get into it simply to see if I could get it up and running.
For the presumed good transistor, I get readings of .807 from base to emitter and base to collector.... nothing from emitter to collector
For the presumed bad transistor, I get readings of .789 from base to emitter and base to collector.....emitter to collector shows continuity
For the presumed bad transistor, I get readings of .789 from base to emitter and base to collector.....emitter to collector shows continuity
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It is a greater likelihood which Q205 is 2SA841, please read the Sherwood S9910, they are close to the serial type number。
gage88, thanks for the response. Can you please explain what you mean by "they are close to the serial type number" ?
I have never researched the serial type number at every compamy. But they were two designers. Why the later designers needed to keep the previous designer's code name? but there had been many 2SA841 in Sherwood Co`s story yet, the later designers must used 2SA841, because it were capital belong Sherwood. Although they don't like 2SA841, because they were the previous designer's favorite.
well, last night I purchased these from Mouser
610-BC109C-TL 10 10 0 2.020 20.20
MFG Part No: BC109C TIN/LEAD
Central Semiconductor NPN 30Vcbo 25Vceo 5. / Bipolar Transistors - BJT\u00FE
610-BC109C-TL 10 10 0 2.020 20.20
MFG Part No: BC109C TIN/LEAD
Central Semiconductor NPN 30Vcbo 25Vceo 5. / Bipolar Transistors - BJT\u00FE
I received the replacement transistors today and they have a different style housing, instead of a black cylinder with one vertical edge sliced off, they are whole cylinders in silver. The gain of the new ones measure anywhere from 607 to 637, the good original transistor I pulled from the circuit board measures around 500. Are the new ones acceptable?
You might have gotten original NOS BC109 , which were packed in small metallic cases.
You ordered hamburger:
and were served steak instead, what´s not to like?
to boot , yours has higher gain. WIN WIN
You ordered hamburger:
and were served steak instead, what´s not to like?
to boot , yours has higher gain. WIN WIN
Yes.
In any case, not much difference, some 20%, well within specs.
You might have some problem if gain were way lower, say 100, but in any case experienced designers take care/consider part to part variations.
As is: perfect.
I guess Central Semiconductor does not actually manufacture transistors but resell NOS ones, their catalog is chock full of old and obsolete transistors.
Fine with me, big thumbs up, saves searching around for dubious suppliers and they are there standing behind their products, what´s not to like?
Slightly higher price than others but peace of mind is worth it, and in any case, adds cents (or a couple Dollars) to any repair bill.
In any case, not much difference, some 20%, well within specs.
You might have some problem if gain were way lower, say 100, but in any case experienced designers take care/consider part to part variations.
As is: perfect.
I guess Central Semiconductor does not actually manufacture transistors but resell NOS ones, their catalog is chock full of old and obsolete transistors.
Fine with me, big thumbs up, saves searching around for dubious suppliers and they are there standing behind their products, what´s not to like?
Slightly higher price than others but peace of mind is worth it, and in any case, adds cents (or a couple Dollars) to any repair bill.
I replaced the bad transistor so now the audible distortion is gone, but I want to test it to make sure everything is working properly.
When I recapped the amp and cleaned the pots, I also cleaned the BIAS and BALANCE pots with contact cleaner, working them back and forth, therefore they will now need to be reset. The service manual gives instructions to set the BIAS ADJUSTMENT and OUTPUT BALANCE.
I hooked the amp up to a dummy load and set the BALANCE pot for symmetrical clipping with an oscilloscope.
I am struggling with the BIAS adjustment though. I understand this is done with a volt meter but the service manual for this particular amp gives no instructions as to where to measure from or what the voltage should be. Can someone please help me with this?