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I am an experienced solderer with very rudimentary electrical understanding needing help with a Sherwood s-8600 (solid state, 1967-ish) that had been working fine for several years since i was given it by original owner.
Recently decided it might be wise to change out the power supply capacitors given its age. With schematic in hand I ordered them, but the day before the parts arrive, while listening to it, the 3 Amp fuse blows. I replaced it with properly rated fuse and it blows instantly.
Inside, aside from a slightly bulged out capacitor, it appears that 2 of the 4 diodes attached to the two big power caps were damaged (DMM in diode setting showed 0.00 in both directions; I believe these four diodes are arranged in what I think is called a rectifier bridge?) Anyway, i thought I'd replace those four diodes as well, but cannot figure out whats required. The schematic/repair manual just lists them as 'silicon rectifiers' with no specs in the parts list.
The tiny numbers printed on diodes themselves turn up absolutely nothing after exhaustive google searches. I happen to have four working UF4007s pulled from a Bottlehead kit i modified, but thought I'd better get a better idea of what specs i should be considering first. Any help is appreciated!
Follow Ups:
While their 1000 PIV rating is certainly enough, the 1 A. capability of the UF4007s is not big enough for use in "sand" power circuits.
How much power is the Sherwood receiver rated to deliver? That number will allow diodes with a large enough current rating to be selected. To put things into perspective, an old Hafler DH200 rated at 100 WPC used a 25 A. rated bridge.
Eli D.
Thanks Eli, (i think i recognize your name from the old bottleheaders asylum?) Anyway, heres the info pasted from the Sherwood service manual--
"POWER OUTPUT: music power: 80 Watts at 4 ohms, 50 watts at 8 ohms, continuous power each channel: 30 watts at 4 ohms, 20 watts at 8 ohms for 1.0% dist."
Is that all thats needed to calculate the required diode specs? thx again
3 A. rated UF5408s might be OK, as you seem to indicate your unit has 4X discrete parts. Can you post a photo that shows both the diodes and a coin.
Please notice that the claimed power yield is badly inflated. Those spec's predate the strict FTC rules. RMS power with inaudible THD is the info. needed. I'm thinking 15 WPC into 8Ω is about right. You don't want more than 0.3% THD.
Eli D.
Heres a photo of diodes, all disconnected from capacitor end:
Those old diodes are not especially big. 3 A. rated parts should be fine. At 1000 PIV, the UF5408 is overkill for this job, but because of economies of scale they tend to be the lowest priced 3 A. rated UFnnnn parts.
UFnnnn parts are so much quieter than the similar rated 1Nnnnn parts that putting them in, even if the old diodes are intact, represents a nice upgrade. The background gets "blacker".
If you have doc. for the receiver that includes a schematic, please scan the schematic and upload it.
Eli D.
Partial scan of schematic:
Happily, a bipolar PSU is present. Please observe that 26 V. peak is 18.4 VRMS. As power = V 2 /R, the max. possible power yield into 8 ohms is 42.5 W. In the real world you get less .
Put new 'lytics and new diodes in and pray your O/P transistors are OK.
Eli D.
nt
Did you test those diodes OUT of circuit?
You have to unsolder one leg to test those.
charles
nt
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