Sherwood S-8000 woes!

G

gkargreen

Well-Known Member
I just did the following post on AA, and am posting here as I need as much help as I can get on this one!

Thanks for all that helped in the previous message below, however, I am still having numerous buzzing/hum issues with this receiver and am at a loss to figure out what is going on. To recap, this unit has been recapped in the power supply and the coupling caps, and all voltages are right on or close to the money. The problems right now are a high buzz level in the amp in the aux mode as well as in the phono mode, this shows up because the volume pot has to be at least 12-1 to get any real volume out of it, which seems really odd. Additionally, the phono stage volume pot can not be cranked to max without a nasty hum, almost like a motor boat sound showing up, which definitely shown on the scope as well, I can watch it go from absolutely flat to this weird, almost sawtooth wave on the scope. I had the problem of 56 vac on the chassis no matter the orientation of the original two-wire plug, following the recommendations below I replaced it with a 3-wire and the 56 vac is gone, but not the overall hum! I worked on an S-8000 model II (7868) and did not have any real issues with that one, but this one really has me scratching my head! One problem I uncovered was that the light bulb socket holder that holds the 3 lights on the side had been left to float as some jackass had to replace the #51 bulbs and used the wrong bulbs with a longer body, like a #47 style, which the #51 is not. Thought that fixing that back onto the chassis would do the trick, but no luck. Any suggestions on what to look for appreciated, thanks!
 
I should also note that when there is no active device, like the CD player connected to the AUX input and the input shorted, there is NO hum or buzz in the rest of the receiver, including the receiver section and the phono stage, it is almost without any noise. I still have the motorboat sound when the phono gain is turned up past 2-3 o'clock, and the volume control is still way down in gain. Thanks!
 
sounds like a ground problem. I had issues on my S-7000 with the input jacks losing ground connection. I don't know how they are constructed on the S-8000, but on the 7000 the jacks are pressed through a panel. I don't remember if the panel has a ground wire, or if it relies on the mounting screws but a look back there should tell you. If no wire, it has to be the mounting hardware. Make sure that is good in either case.

The phono stage may be acting up for the same reason, but I did have another ground issue on mine. The 7000 has the phono circuit built on a sort of sub-chassis that is attached to the main chassis. The ground for the circuit was through the mounting hardware, and it had become less than perfect. Cleaning the area and putting some solder from the main chassis to sub chassis fixed it. Mine wasn't motorboating but I was getting random crackles and pops.
 
OK, a quick check of the input jacks show that the jacks are pressed into the mounting flange which is ISOLATED from the chassis, shorting the flange to the chassis cured all of the hum. But now, I have to find where the original grounding scheme worked to see if there is a problem with that, thanks for the help gadget!
 
Check for a black wire bundled into the rest of the wires running back there. I seem to recall mine all run through a plastic tube or something up to the selector switch. Its probably grounded up there. I think mine is also isolated from the chassis, but its been a while since I was in there. I rebuilt it and basically haven't done anything but use it in several years.
 
Thanks gadget, I think I have the problem. You are corrected about the bundle, I traced that to the input selector switch which then lead to a long ground wire onto which all the grounds were connected (except for the power supply grounds) and grounded at one point on a terminal strip. While the ground wire and the terminal were around 0 ohms, the terminal connection via a rivet to the chassis was around 10k! As it was, this is in the area of a leaking power supply cap I replaced and had to clean up the corrosion around, apparently some of hte corrosion got under the rivet even though it looks perfectly clean. I let you know more once I remove the rivet and clean up and re-ground the rivet again.
 
Aluminum oxidizes, may just be a bad connection. I seem to recall having to use an auto center punch on the back side of the RCA jacks on mine to get them to behave properly. There is a rim on the back of the jacks, I just put a dimple or two in each one on that rim. The idea was to punch through the oxidation and get good contact. Unfortunately you can't just solder aluminum or I'd have done that.
 
OK, that cured the hum once I cleaned and put the terminal back in place, making sure there was a good connection. Thing is, when I removed the rivet and the terminal strip, there was NO corrosion on either the bottom of the terminal strip that connects the ground or on the copper-plated chassis, not sure what the problem was but its fixed now. Just a few more issues with the balance and its button up time!
 
Cool. Doesn't take a lot to make resistance when you aren't carrying any current to speak of.
 
buickguy62 said:
Don't you just love AK, like having a super-tech standing over your shoulder!!!!!!
Click to expand...

True that, at AA is like having a bunch of idiots yapping on about something they know nothing about! Gadget, got one more problem (its a general vintage audio problem I am experiencing more frequently...) regarding the volume pot on this Sherwood, it has gotten significantly out of tracking with one section up and the other down in value, and its an odd value, 300K! In the schematic that most closely resembles my make (S-8000 type 1) the pot is a 500K, this one is clearly marked 300K, but good luck finding one of those! I am considering using a 500K like the schematic, what do you think? Obviously the loudness tap will not be there, guess I will have to live without that. Thanks!
 
Maybe see if Mark Oppat (oldradioparts.net) can rebuild yours with stock value parts. I don't know off the top of my head what changing that would affect, but it may interfere with the phono stage eq. Sometimes those are a bit load dependent. Broadly speaking going up is probably less problematic than going down though.
 
Thanks gadget, I checked the schematic again today and found that indeed it is marked 300K, just like the pot in the Sherwood. Going through a box of old pots I found a dual 250K that is tracking nicely and has a loudness tap on it, so I am thinking of using that for the replacement, what do you think? Thanks!
 
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