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 Post subject: Sherwood S-7100A direction needed
PostPosted: Apr Mon 25, 2016 10:39 pm 
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Joined: Dec Thu 29, 2011 8:45 pm
Posts: 1711
Location: York, PA
I seem to have staggered into ownership of a Sherwood S-7100A.
http://akdatabase.org/AKview/albums/use ... Manual.pdf
There is a schematic at the end of the linked file.
I have very limited experience with solid state. I know that the entire receiver should be recapped of all its electrolytics, but Id like to try to get the problems fixed first before going into it that deep. If there is a part that cannot be easily replaced, it might not make sense to invest the time and money into a full recap.

I opened it up and found it is remarkably clean inside and I found no obviously defective components. It actually looks new inside. No bulging caps or burned resistors. No signs of any previous repairs. I cleaned all the controls except the power/volume pot (see below).
I decided to try to power it up and found that while it works, there are a few issues I could use some input on:

1) When switching the receiver off, there is a brief episode of loud static through the speakers immediately after the switch clicks off. I assume this is a cap discharging somewhere (bleeding through?). The power switch and volume control are the same stacked pot. Im told that these are notorious for breaking when cleaned with solvent, so I havent cleaned it. There is no scratchiness in the control.
If this is a bad cap, any ideas which one/ones to focus in on?

2) Both right and left channels work, but the left comes up much sooner on the balance. To get balance, the control is turned about 1/4 of its movement in one direction, with the knob pointer at 9 o'clock. Ive cleaned the control with Deoxit already.
Could this also be a leaky cap/caps? If so, any thoughts on which one/ones to look at first?
Thanks!

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Bill W


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 Post subject: Re: Sherwood S-7100A direction needed
PostPosted: Apr Tue 26, 2016 1:56 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19884
Location: Utica, NY 13502 (USA)
The problem is likely an electrolytic coupling or bypass cap that has lost some of it's capacitance rather than being leaky. This is a very common problem with solid state gear. I usually use an oscilloscope to compare the two channels while signal tracing. An audio signal tracer or sensitive AC voltmeter can also be used to compare the two signals at different points in the schematic.

Dave


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 Post subject: Re: Sherwood S-7100A direction needed
PostPosted: Apr Fri 29, 2016 1:32 am 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 7157
Location: Ohio 45177
I have a couple 7110A receivers. The problem with those is the on off switch breaks and leaves it powered up. And it is not a common part to find, maybe leaving you like I did, having to repair the dang pot and switch itself with great difficulty. As my parts unit has the same issue. Mine has distortion in one channel and recapping it shotgun style did not help any. It was almost a NOS unit in the original box and still has issues. Some day I will rob amp parts from the more worn donor and see if I can make the amp sound clean in both channels. It may be very similar to the 7100 A I think. Just beware that the PC mount loudness tapped volume control if it is the same, is unobtainium. BTW I doubt anything to do with solvent is the issue with the power switch breaking. The NOS looking one broke about the third time I turned the switch off and on and what I found, as I recall, is it is a little phenolic type material lever in the switch on the end of the shaft that cracks and falls apart, just that it did appear flimsy and weak for the job it was intended. My first one in 1976 did not ever have the issue.


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 Post subject: Re: Sherwood S-7100A direction needed
PostPosted: Mar Fri 02, 2018 7:13 pm 
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Joined: Mar Fri 02, 2018 7:02 pm
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I kept the volume control and added a switch as shown in picture. Not too hard.


Attachments:
Sherwood Switch.jpg
Sherwood Switch.jpg [ 73.93 KiB | Viewed 542 times ]
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 Post subject: Re: Sherwood S-7100A direction needed
PostPosted: Mar Sun 04, 2018 3:29 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 499
Location: Fayette County, Pa
The static may be originating from an unequal discharge on each side of the bootstrap when power is removed. Leaky or open caps can cause this, as can dissimilar transistor gains on each side. (Aging or past transistor replacement?) The problem should be addressed and corrected as this condition may cause momentary overloads of the transistors and can lead to later failures. You can verify this condition by measuring the bootstrap voltage while shutting down. The bootstrap voltage should fall at the same rate as B+ bleeds off. If it holds high, or drops too fast you have a clue what is happening. A dual trace scope is helpful to view this instead of just a meter as you can compare both against each other in time.


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