Sherwood S-7900A - Printable Version
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+--- Thread: Sherwood S-7900A (/showthread.php?tid=14851)
RE: Sherwood S-7900A - Ron Ramirez - 06-28-2016
Arran - Will do, thanks.
You know, back in the 1980s when I acquired that S-8900A, I didn't give the vinyl covered cabinet a second thought. How did I know that there were better cabinets out there on the better receivers such as Marantz and Pioneer. I was just happy to have a good, working receiver for five bucks and the cost of parts (four rectifier diodes and a 7000 uF electrolytic, maybe $15-20 back then from an electronics wholesaler in Evansville, Indiana, in ye olden days before the Internet and when parts were much harder to find).
But now, after having seen - and owned - other receivers, yes, I see your point about the vinyl-covered cabinet.
RE: Sherwood S-7900A - Arran - 06-29-2016
Ron;
No doubt that the cabinet on the S-8900 was not smashed or damaged, and for $5 plus parts one tends to overlook the esthetic side of things. Still it's rather strange that they used particleboard, the Japanese built stuff often used plywood with a simulated woodgrain, in fact I think that's what my Marantz has along with some private label eight track players I have.
Regards
Arran
RE: Sherwood S-7900A - Ron Ramirez - 12-22-2016
Six months later...
Having finished a Philco 37-640/38-624T project, it's time to turn my attention to this Sherwood once again.
A month or more ago, I ordered enough parts from Mouser to take care of the power supply in this S-7900A, along with two other stereo receivers.
The first thing I did was what I felt would be the hardest part: restuffing the two main electrolytic cans.
I unsoldered everything from them (taking notes beforehand), removed the cans, and cut them open. Here's how they looked:
[Image: http://www.philcoradio.com/images/phorum/S7900A_018.jpg]
I cut away the old electrolytic, drilled two holes in the base, and soldered two wires to the positive and negative leads of new Nichicon electrolytics. The originals were 7000 uF, 50 volts - the new ones are 8200 uF, 63 volts.
[Image: http://www.philcoradio.com/images/phorum/S7900A_019.jpg]
Then I glued the cans back together:
[Image: http://www.philcoradio.com/images/phorum/S7900A_020.jpg]
Tomorrow, once the glue dries, I'll reinstall these into the receiver. I will also be replacing the four rectifier diodes and a few other electrolytics.
This receiver not only has the death caps from each side of the line to ground, but also in the high voltage secondary winding of the power transformer. The originals in the secondary are 0.22 uF film caps (not safety caps). I'm thinking of replacing them with .01 uF safety caps. What say you, people?
RE: Sherwood S-7900A - morzh - 12-22-2016
Ron
You never gave the sch link.
So I am not sure what you call high voltage secondary. But from what I know, the secondary side caps do not need to be the safety type. Only the primary.
RE: Sherwood S-7900A - Ron Ramirez - 12-23-2016
...schematic...who needs a steenkin' schematic?
Well, okay. Here's a partial, showing the power supply:
[Image: http://www.philcoradio.com/images/phorum/S7900A_021.jpg]
C180 and C181 are the caps I was asking about.
RE: Sherwood S-7900A - morzh - 12-23-2016
You call THAT high voltage? Ron, solid state is getting to you.
1. Those are not the death caps.
2. They do not need to be Y-caps.
3. Leave them be.
RE: Sherwood S-7900A - Ron Ramirez - 12-23-2016
C'mon, Mike, I know that isn't high voltage...
I was making a comparison to a tube circuit, in which this secondary would be on the order of a few hundred volts instead of around 40 volts. You know, the rough equivalent to the B supply.
OK. The original film caps shall remain. But I will replace C170 and C171 with safety caps.
RE: Sherwood S-7900A - morzh - 12-23-2016
Yes the C170 and C171 have to be safety Y-caps.
C180 and C181 are so called snubbing caps. They take care of potential unwanted noise from diode switching. they take different forms: sometimes placed across the diodes, sometimes just one cap across the transformer, sometimes like what you have.
RE: Sherwood S-7900A - Ron Ramirez - 12-23-2016
So...first thing this morning, I reinstalled the newly restuffed power supply electrolytics:
[Image: http://www.philcoradio.com/images/phorum/S7900A_022.jpg]
I also installed four new rectifier diodes. And for good measure, I also replaced the snubbers (0.22 uF, 250 VDC), C180 and C181. These are the two burgundy items to the left of the new diodes that look like Chiclets.
I then proceeded to replace as many of the electrolytics under the chassis as I could.
The result:
[Image: http://www.philcoradio.com/images/phorum/S7900A_023.jpg]
No, the 470 uF 50 VDC electrolytic did not get replaced - I forgot to order one.
Perhaps in the near future, I will go back in and take care of that.I also replaced the death caps with new X-Y safety caps.
Finally, I put the bottom and top back on, and reinstalled it in its cabinet. I then lugged the heavy thing upstairs and installed it on my home office desk, replacing the Sansui 2000:
[Image: http://www.philcoradio.com/images/phorum/S7900A_024.jpg]
I will replace the lamps with LEDs in the near future as well.
But for now...that is it for this S-7900A. With this receiver, I can now sort of relive my younger years, remembering how much I enjoyed the S-8900A (FM only) that I owned back then.
RE: Sherwood S-7900A - rocketeer - 12-23-2016
Looks great Ron.
RE: Sherwood S-7900A - KCMike - 12-23-2016
Very nice.
RE: Sherwood S-7900A - Ron Ramirez - 05-22-2018
Five months on...the S-7900A has been pulling nightly duty for us, amplifying the TV sound each night when we watch our one hour of television - DVDs of various old television series (currently Gunsmoke). The Sherwood is feeding a pair of Bozak Urban speakers.
And, might I add, doing a fine job of it.
The switches are just fine. And I just realized - I never did go back in and replace that old 470 uF cap
I really need to do that.
RE: Sherwood S-7900A - morzh - 05-22-2018
If your amp works fine, this means the cap is ok. If it dried up you'd likely notice. It's a good cap, why bother?
RE: Sherwood S-7900A - Ron Ramirez - 05-23-2018
Because sooner or later, it will fail.
KA-BOOOOMM!!!
RE: Sherwood S-7900A - morzh - 05-23-2018
Not necessarily. AMOF electrolytics rarely (statistically) fail in "KA-BOOM" mode. They dry up, they bulge up. When they bulge up, it is either they are bad (like those swollen caps in computers or TVs, which is not the case here) from start, or they are wrong for the circuit they are in (GP caps in Hi-ripple-demand circuits etc, or the WV is super close to the actual voltage at high temps at high ripple). When you see that "2000 hrs life" thing in datasheets, this is when all the max parameters are met at the same time, otherwise the formula is non-linear and the life can extend ... not forever but to tens and tens of years easily.
Plus those caps were mad during the times when they were not Chinese-made, or even Taiwanese. Mostly Japanese or US, or in the worst case - maybe Singapore. No stolen electrolyte formulas etc. My APPLE II power supply still works fine. My Nakamichi deck that I just shipped for service is fine electronically, it needs rollers rejuvenated etc, but not the caps.
And after reforming my Signal generators (from 1950s) work just fine. And the reforming was done due to the long storage time.
So.....piece of mind counts for something, sure. But my philosophy is, if an old (like 70-s) lytic cap is good, it could stay.