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Sherwood s8900 - transistor sourcing help

S

special_20

Active Member
So, bought a non-functional Sherwood S8900. Got more than I bargained for - bad driver board instead of a bad filter cap. The boards are not matched so it appears someone may have been in here before me. I'm pretty sure root cause of problem is a bad capacitor (leaky 4000 uf filter cap on the same channel). I've been able to figure out the value of burned resistors and smaller transistors. I have not had much luck yet figuring out replacements for the larger transistors. Wondering if anyone can help me identify suitable replacements


Gallery

Transistor 1
Bad Board: RCA SK3024 8414
Good Board: Motorola 2n4239-7224 (this is a current mouser recognized part; likely a replacement?)

Transistor 2
Bad Board: RCA 4039 H9 CL
Good Board: RCA S522 CKJ 3B (really poor silk screening not sure on 's' vs '5' and 'B' vs '8')

Wondering if anyone might have:
  • a manual or schematic (this is the 8900 non-'a') version
  • A working unit they can share a picture of to confirm correct part numbers
  • advice on suitable replacements for the T0-1 style transistors cited above
Assistance is appreciated!
 
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Schematic for S8900a available on hifiengine, don't know if the "a" model would be different but worth a lock.
 
Wont work they are different beasts. the a is full compliment with darlington outputs. the non A is possibly cap coupled (need schematic to tell).
Yes you will definitely need a schematic.
for the driver transistors think about the TO-39 NTE128(npn) /80v and complimentary NTE129 (pnp) You should pick them up for a few dollars a pop and the 4dbmax NF is a good feature. Don't just replace the failed transistors. You should match across both channels for full benefit.
With such damage experienced I would check outputs first for failure. Also see if the on switch still works as they can be the Achilles heal. You definitely need a DBT for this repair.
 
AK member Drummerboy75 has the S8900 (not the "A" version) and may be a resource. There is a schematic available at http://elektrotanya.com/sherwood_s-8800a_s-8900_sm.pdf/download.html, and it shows DISTINCT differences between the 8900A and 8900.

The outputs are in fact capacitor coupled.

A suitable replacement might be the Fairchild 2SC5242 or OnSemi NJW21194G, although they are TO-3P packages which will fit TO-3 sockets but must be securely heat-sunk.

As Moving Ahead pointed out, there could very well be collateral damage, such as emitter resistors and driver transistors. All the circuit should be checked for damage. Effective replacement driver transistors might be a Fairchild KSC2690A.
 
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power switch works.

Guy I bought it from "showed me" that it turned on and off. Intact channel works find. Hum out of bad channel. I'll pull the output transistors and test later this week.

Based on inspection, I'm pretty sure the 4000uf cap on that channel is bad (brown goo). Do you think that'd take down both the pre-driver board and output transistors when it went "open"?

I'm beginning to suspect that this was a serial failure (i.e. transistors smoked. Someone tried a repair by just replacing transistors. The transistors smoked again and they binned it). I'm thinking of proceeding on the assumption the good board is "stock". I've found a few replacement transistor suggestions for the "good board" transistors; the bad board transistors are a mystery and may have just been what the shop had. Alas they are on a piece of paper at home right now. Will update with some ideas for transistor replacement and results of testing output transistors.

Thanks!
 
Disclaimer: I'm learning a bit here. I've done a few recaps and adjustments but never troubleshot damage this extensive (or old). I appreciate the input, but some bits of advice might have gone over my head :)

So, some progress this evening, but the rabbit hole goes deeper. Switch is good. Pulled leads from big output transistors associated with bad board. One output transistor is dead (shorted). Other is not shorted and appears fine. Output transistors may have been serviced in past as there are 2 RCA and 2 Motorola units bolted to the back (sigh).

Here is a rundown of part compliments. If I'm gonna do this it sounds like I might just have to bite the bullet and replace everything (or is that excessive given I have a working channel?).

updated gallery

Item 1
Bad channel: RCA 3024: found a thread that suggests a 2n3053; cross references suggests an NTE123A; or NTE128 Any obvious winners or someone have a better reference?
Good Channel: Motorola 2N4239: part is available but is 6 bucks! Any other suggestions?​

Item 2

Item 3
Bad Channel: Motorola 2N3055: Current part; lots of options...
Good Channel: RCA S356: not finding anything...​

So, what are the key things to know about replacing vintage transistors? I've found datasheets and suggestions for replacements. What values are most important to match when replacing transistors?
 
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Ugh. This is NOT the unit you want to learn on.

I got the print now I think, I'll look at it later and see if I can figure anything out.
 
OH, I forgot to say - brown goo is not necessarily cap discharge, it is usually glue used by the manufacturer. When it is white or grey it is good, but over time and heat it turns brown, AND CONDUCTIVE. Then it is bad.
 
JURB said:
OH, I forgot to say - brown goo is not necessarily cap discharge, it is usually glue used by the manufacturer. When it is white or grey it is good, but over time and heat it turns brown, AND CONDUCTIVE. Then it is bad.
Click to expand...

Well, I can add this to the list of things to test. Bought a combo transistor/resistor/capacitor tester I've been looking for an excuse to use. If I can find the time, I'll start testing the big can capacitors to confirm which are good and bad :)

If you've got a schematic, please feel free to share and/or PM me!

While yes, this is a basket case, it's got some room to work in and seems like a relatively simple layout (just going to be a headache diagnosing the bad without a good schematic). I figure I've got 15 bucks into it. If I can keep the replacement parts within reason, even if it doesn't come out, I get an education :)
 
On second look I might not. Thing is ambiguous. Says something about an S-8900FET ???

I'll probably never find the site again so later I'll just Dropbox it. Got company coming right now and I am trying to catch up on everything, like email and all the threads I am in here and there. Plus I got work tomorrow. But I work afternoons so I do that kind of stuff at night.

Anyway, luckily you got the good board as a guide. I do not like the looks of the bad one, PC boards is one place where black is not beautiful. You might as well get started on that. you have to remove all the components from the burnt area and start scraping the top with a jeweler's screwdriver or something. If it breaks through then it does, because if it is that carbonized it is conductive. Keep scraping until all the black is gone. A little slight discoloration is alright. Stick an ohmmeter in it with the probes real close on the highest range you have. It should read open circuit. Later, if there is a hole you have to hand wire it and then probably the best thing to hold it all in place is hot glue. If you epoxy it and ever have to change one of those component you have a hell of a time with epoxy.

And don't bother putting the transistor sockets back in if it has them. I had a similar receiver burn out a guy's woofers because of a transistor socket. Whether you want to eliminate them in the good channel is your call.

With any luck, it looks like the hole is going to be smaller than an inch diameter. that is good because things were not so jam packed like in the new stuff. I KNOW that is repairable, but a one inch hole in a board in something built today pretty much condemns it.
 
The Nte123a is a TO18 case. The PD would probably be not up to it for this driver location. Its more suited for earlier stages. The NTE 128 is TO-39 which will be a suitable replacement. NTE stuff comes in individual static free bags so there isn't any fear of fakes.

If this was my machine and it had all the troubles you've detailed, I wouldn't be content until all semiconductors were changed. Sure thats a lot of work (about 30-40 items), but there would be no other way to get the channels balanced. The machine is 45 years old with a rough history. Theres also plenty of large caps in there so reckon you'll spend easily $100+ on parts trust me you'll end up getting more than 1 parts shipment.

Before I ordered parts I'd do a quick continuity check on the pots. see if you get a smooth reading through the dial. Sherwoods have a bad rep for faulty pots. DON'T DEOXIT.

As for the main amp board, can you do an ultrasonic clean?
I personally like the transistor sockets. I understand their frailty, but thats what makes a; Made in USA Sherwood a 'classic Sherwood'. There are many features in there that you'll shake your head at but that will reinforce the love -hate relationship most Sherwood users develop. If you do remove them take note of the pin out.

Glad to see Jurb is on board with your restoration. He'll certainly set you on the right coarse- He's got more patience than I'll ever have.
 
Moving Ahead said:
http://elektrotanya.com/sherwood_s-8800a_s-8900_sm.pdf/download.html

pages 30 & 31
Check driver board serial - AO31R29-OA
Click to expand...

Yes! that part number matches the board. We have a schematic and some voltages. I totally missed that page for the S8900. Looks like the schematic is well labeled, I'm still not seeing the board outline to match. Anyone else have better eyes?

Next question: The ID labels on the circuit diagram match the printing on many of the original parts. Do these (assumed RCA) part numbers Sherwood used have any means of cross referencing or confirming values? I'm not coming up with much info when I google "RCA Transistor XYZ". Any tips or tricks to this?
 
and I think I've answered my own question

Based on the NTE website, I have the following transistor part numbers identified (Schematic #: Part #: NTE Equiv: JIS Equiv)

Q205: SPS817: NTE123AP: 2n3904
Q206: SPS41V: ?
Q207: SPS4199: NTE123AP
Q208: SPS41: NTE123AP
Q209: S2104: NTE192; 2n3435
Q210: S522: NTE192
Q211/212: S409F: NTE128
Q213/214: NTE130: 2n3055

So that's about 45 bucks worth of transistors in just the pre-drivers. The big caps are going to be another 45. The rest is small value/small dollar. This is starting to feel spendy for an unknown commodity that isn't named Marantz, Sansui or pioneer. Are these sherwoods worth dumping the money into? They don't seem to have the cachet or demand as some of their successors (marantz, sansui, pioneer). I've also got a few other amps waiting in the wings...

This weekend, time permitting, I'l put the ESR meter on the big cans and start demounting the predriver board to clean up and assess scope of burn/damage.
 
I have a s-8600a late 60's. yes the sound is worth the cost... don't know if you should do all that transistor work, though. if it's just cleaning up and some testing. but yes if matched pairs of transistors. The design is very very good so it will be interesting if you put the time in with hearing new parts in a good design. As mentioned don't screw the pots with harsh cleaners.. some of these units had 'pot grease' and only need a shot of plastic safe cleaner like "d5"

What esr meter are you using against the uf caps value? if the caps have not been activated in a long time then the meter readings won't tell much.. but not saying they are bad. If mains caps they should hold a charge but a lot of controversy on method of measurement with newer cap. meters on large caps.

LOL :D I've always said it's not a 'cheap hobby'. but hey it's a lot cheaper than many.
 
FYI: Nte products can be overpriced. I recommended the NTE128/129 TO-39 package as this case style is getting harder to source and the NTE 128/9 are suitable for this application.

If youre going to proceed i can help make a few suggestions for transistors.
 
First that PC board has to be scraped. Should we call that debrided ? (that is when they remove dead tissue/bone frags from a joint in your body I think, correct me if I am wrong)

The top you scrape until you see the board color all the way around the hole. On the bottom, do this so you have no dark between the copper foils. Right under them is OK as long as the top of the board is cleared.

Yeah, the word debrided, doesn't that mean removing debris from like a knee joint after injury ? This is kinda the same thing, though I might not have the right medical term.
 
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no progress this weekend. To busy crying in to my beer sunday after the vikings loss. Gotta hand it to the vikings, they find a new way to disappoint their fans every single year... Hoping to get a chance to break out the soldering iron soon and do the "debriding". Probably close to the right term given the level of scorch. I did get a chance to do a closer visual inspection and found a bit of char peeking through to the back. So yes, that is probably going to be the crux of a move forward decision. Thanks for the input guys!
 
The board may be salvaged using clear urethane epoxy to reinforce and insulate the charred area. Been there, and with a lot higher voltage supplies on a burned board.
 
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