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hugeshows hugeshows is offline
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Default Restoring a Sherwood S-5000

Hello all,


Well I've recently finished up the electrical portion of my S-5000 restoration. I've learned quite a few things along the way, and even though I've done several of these before, I made a couple mistakes this time that taught me a few things about how this amp operates.

First off, let me cover the mistakes I made in posts previous to this.

Mistake #1

Changing the chassis capacitor in the way I did it was a bad idea. I had a fundamental misunderstanding of the way that capacitor was designed to work in this amp, and it wasn't until I noticed increased hum that I realized my blunder. Simply using a single cap as is so common in radios and other types of amp doesn't pan out well with the S-5000. The reason is that the original cap is actually two caps connected in series and tied in the center to the chassis. One end of these caps are on hot, the other neutral. The result is a cancellation effect. A single cap simply dirties the chassis currents by connecting it to only one leg. One option would be to simply remove the cap for safety, but after thinking long and hard about it I began to ponder whether the cap was there to deal with inductive spikes on the power transformer when switched on or off. Without any way to answer that question, I simply punted and restored the original capacitor back into the circuit. The added hum was gone instantly. I'll write up that whole AC line redo in a little while...

Mistake #2 (JRC, I think this might be your answer to phono hum)

Believing my eyes and the schematic. The bias supply turns out to be a finicky thing on this amp. If you read the schematic, either Sherwood OR Sams, you see that there are three capacitor sections in the supply. In the original amp, these are all in a twistlock can and share a common positive, rather than a common negative like just about every other twistlock can. If you look at the pictures I posted earlier, you see that there are several things tied to the "ground" on this can, but that is incredibly misleading. You see, while this cap has a common positive, it is NOT on the can body!! There is a terminal that has the common positive, and it is NOT connected to the can. Whoever drew the schematics made the same assumption I did, that the common positive is connected to ground. It is not. It is connected only to the center-tap of bias winding, the DC filament return, and the ground bus that for the entire preamp section!! If you follow the schematic or your eyes when you rebuild the bias section, and you connect the common positive to ground, you will be instantly greeted by a NASTY hum as soon as you connect a magnetic phono cartridge! JRC, since your bias supply was rebuilt, and kinda sloppily, you may wish to redo that section and be sure that the common positive is not tied to the chassis!

I got everything dead quiet in this amp until I tried the phono section which was working beautifully before my rebuild. It now had a horrible hum, and so I knew it must have been something I had done. I poured over the amp with a fine-tooth comb, uncovering 3 mistakes from the factory in the process including two joints that were never soldered, one was the center-tap of the AC primary at its chassis junction!! The other was a tone control resistor that was connected but never soldered to the pot. Obviously, this amp had some pretty green hands making it. After hours of frustration, I started measuring things. I compared the bias circuit carefully to my restored version. After finally getting my meter out and measuring the original cap (still in place but disconnected) I discovered the truth! The schematics are wrong and that circuit connects to the center tap of the bias secondary ONLY, and is floated from the chassis. One wire snipped out and voila! A dead quiet phono pre!


I've got a lot more pictures to take and upload, and I'll probably get to that tomorrow. But I wanted to go ahead and correct my mistakes for the record now, lest anyone else go down my erroneous path with the bias and AC supplies.


Cheers,

-forkinthesocket