accidentally acquired a Sherwood 7100a

Rsclj

Active Member
accidentally acquired a Sherwood 7100a (schematics needed)

My Father had this when I was going up...I heard my first Zeppelin album on his AR thru his Sherwood...so I see one on the auction site and put in a low bid figuring I wouldn't win...but I did...its absolutely spotless...not a ding or scratch to be found...BUT it has a weak, almost non existent channel. ..I intend to recap the entire thing...I have downloaded the SM...as this missing channel also happens in the headphones I assume it is in the final output amp...I could shotgun the entire amp and replace each component but as I am trying to learn to troubleshoot properly...I would like to approach this differently. ..the problem is the schematics from both places I have downloaded the SM from are incomplete. ..does anyone have a full schematic of this so I can check voltages and attempt to solve this problem logically rather than blindly replacing every part till it works. Thanks!
 
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First thing I'd do is give the function switch, tape monitor switch, stereo/mono switch, etc. a good deOxit bath...then work those switches a few times & see what happens. Faderlube on the pots as well. Might be all that it takes to get her singin'.
 
Be very careful. These Sherwoods had plastic based controls that do not like most cleaners. Make sure the cleaners states it is safe for plastics. I used a Rat Shack tuner cleaner that so stated that with success.
 
Yeah the famed on/volume switch feels solid...and doxit D100 is all I'll be using on this one
 
It's always best to fix the problem before preventive maintenance. Yes clean the set and Deox the controls in hopes that all it needs. But fix, then restore!

Does the set have Pre-in's and Main's-out?
 
No pre amps in mains out in this little amp...
All 4 power ( d315e) transistors test as no short...
 
Good news!
Can you swap the Pre-in's to the amps and see if its a Pre or Main issue?
 
Without a complete schematic im no certain how I would do that...even with a schematic im not 100 % certain how I would do that...but I might have enough of one to check voltages
 
I noticed a slightly puffy cap on the FM board and though I migh try a phono input to see if the right chanel drop was still present...it was :(
 
This may be of interest. I had one of those in pristine condition and it sounded fabulous with my KLH 23's albeit underpowered for them. If you get this working well, I'll bet it would sing beautifully with some KLH 17's. Definitely worth the effort.
 
Dang it, the SHERWOODS had those lousy on-off/volume knobs too? Man, I hate those. I'm currently window-shopping for an amp or receiver in the 50wpc-80wpc range, something for a new pair of speakers, and the US-made Sherwoods were on my radar.
 
Ok...I swaped the inputs to the main amp...the issue switched sides...I assume I move on to the pre amp??
 
Because my schematic is incomplete. ..im uncertain if there is much if anything between them
 
Correction...the problem switched sides on the balance control...but the out put stayed stronger on the left speaker...that is a constant...so that is leading me to think its the amp....however all of the voltages are even on both sides of the amp at the transistors
 
Swapped the position of all 4 output transistors. ..no change still driving the left chanel only
 
I'm a little confused on what works and what doesn't, could you bypass the preamp section entirely and see what works/doesn't work completely? (bypass and play music through the amp section, both channels)
 
sherwood6019mainamp-1.jpg


OK Steps taken to date...

1. fired her up an found a weak almost non-existent right channel in both speakers and phones.
2. downloaded SM and opened her up to see any obvious problems...none to be seen
3. cleaned all pots and switches with a plastic safe cleaner
4. pulled and tested all output transistors (all consistent with each other and no short detected)
5. tested voltage at final output resistors (R619/20/a/b) all found to be consistent (around 29 vlt if I recall)
6. tested all transistors in the amp and pre-amp (IN CIRCUIT) for variations in readings from one channel to the other...none found...no obvious shorts either.
7. fired her up and tested all voltages at each transistor (Amp and Pre-Amp) compared against schematics AND opposite channel...all were within 1-2 vlt of spec and each sister transistor in the opposite channel.
8. Noticed some waxy crud between C603b and R602b (as seen in upper left of pic) pulled Cap and tested...within spec, replaced
9. swapped the "IN" leads (as seen upper center and right) under the board and noticed a shift...all volume now came when balance knob was turned all the way right instead of left...opposite direction produced a weak almost non-existent signal from the opposite channel BUT the volume all still came from the LEFT speaker (just now it happened with knob turned to opposite channel)
10. swapped all output transistors location from TR608/9a to TR608/9b and vise verse (even though they had tested fine...this had no effect and was more a last ditch hope)

Conclusions...
1. I'm confused!
2. the swap of the inputs having the same audible output tells me that it's the main amp...but all voltages seem fine and consistent from channel to channel.
3. a recap in in order but I was hoping to order the "correction" parts at the same time...could a bad cap do this and still allow for the test results I have seen?
 
I am assuming that you have a DMM but no signal generator or oscilloscope. There are web sites (search) that will generate a constant tone that you could inject at the tape or aux. inputs. Then with your DMM set for frequency, you could probe through the circuits looking for that frequency and then measure the AC voltage of it, comparing the left and right channels. You should be able to find where they differentiate.
 
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