Help needed with Sherwood S-8800 bias and voltage settings

[redpackman]

I've got a Sherwood S-8800 (not the 8800a). I realize that there's a service manual for the 8800a on HiFiEngine etc. and it's been helpful, because there are many similarities between the 8800 and the 8800a.

However, the board(s) on which the balance and bias setting variable resistors are mounted are not the same. On the 8800a, as clearly displayed in the above mentioned service manual, they are mounted high and low on the board. In the 8800 they are mounted at the top of the board, but I am not certain which is the bias and which is the balance pot.

Does anyone have a service manual for the 8800 (NOT the 8800a)? If so could you copy and post or send me a copy of the appropriate portions showing whether the bias pot is closer to the outputs or on the other end of the board, AND as importantly could you send the instructions for adjusting the bias and balance pots for the 8800. I Assume that the procedure (once the pots are properly identified) is basically the same, but you know what they say about people who "assume."

A schematic for the 8800 would be helpful too, but perhaps that's asking way too much.

I've also got a 120hz mild hum in BOTH channels that stays pretty steady (you can hear it as soon as you turn it on when listening with headphones) and it only increases when you turn the volume WAY up (with no other signal). With any music, it's hard to hear, but it's there. Ideas there are appreciated as well.

 

[gadget73]

See if the S-9000 schematic describes your power amp section. I have a sneaking feeling it may be the same.

http://akdatabase.com/AKview/albums/userpics/10007/Sherwood S-9000 Service Manual - Rev2.pdf

120 hz is usually a main supply filtering issue, though if the output transistors are way off balance it might cause some hum.

Worst case, mark where they are currently set to and give one a tweak while monitoring the voltage at the output cap. If it doesn't move much when you tweak the pot, you are moving the bias adjustment.

 

[redpackman]

Thank you for your input. I'm afraid the layout looks quite different. The S-8800 is quite similar to the S-8800a except for the placement of the bias and balance pots on the driver boards and the presence of one more multi capacitor near the main transformer. I'm sure there are a myriad of other differences, but those are two obvious ones to me.

In my case I did adjust the pot on the left (farther away from the mains) with the volt meter attached to the + terminal of the output capacitors. That seemed to adjust the voltage on those capacitors most readily. The other pot did not affect it much when gently adjusted. Using the specs from the 8800a I put the voltage at those points at half of the B+ voltage (70v DC) and adjusted the voltage at the output caps to around 35v. So far so good. No hot outputs and no magic smoke...yet.

 

[Goldie99]

The (a) S-8800 schematic at least is included in the service manual at hifiengine: https://www.hifiengine.com/manual_library/sherwood/s-8800a.shtml

Not sure exactly how you'd set up /check bias, so I'd leave it as is for now.

 

[stoN_Cold]

Ideas there are appreciated as well.

First replace all capacitors in PSU & Pow. Amlifiers, with new!

 

[Binkman]

stoN_Cold... Yes... it all starts in the PSU. And I agree don't screw with the bias pots till you get a manual. (may have to pay for a service manual and or try to find a 'Sherwood' experienced tech shop. Do you have an O'scope?

Miss Sophia!

 

[redpackman]

The (a) S-8800 schematic at least is included in the service manual at hifiengine: https://www.hifiengine.com/manual_library/sherwood/s-8800a.shtml

Not sure exactly how you'd set up /check bias, so I'd leave it as is for now.

The schematic for the S-8800a has some significant differences with the S-8800. See my original post above.

 

[Goldie99]

The schematic for the S-8800a has some significant differences with the S-8800. See my original post above.

While I haven't checked for the differences - if you check pages 11 & 12 of the pdf, you'll find the schematics are marked (by Sherwood) as "S-8800", the S-8800a schematics are on pages 9 & 10

 

[redpackman]

While I haven't checked for the differences - if you check pages 11 & 12 of the pdf, you'll find the schematics are marked (by Sherwood) as "S-8800", the S-8800a schematics are on pages 9 & 10

View attachment 1264571

I am not sure here, but I think Sherwood had three different models of the 8800, the S-8800, the S-8800a and the S-8800FET. I am open to correction from Sherwood experts. But if my impression is correct, I'm still in the dark. Thank you for writing though.

 

[Binkman]

I have the 8600a also a FET model.

Now lets back up.. I doubt any bias adjustments will resolve the 'hum' issue. Audible 'hum' increase with vol. 'gain' is audio path and usually ground loop associated or a filter leak.. cap(s) in supply side or smaller service caps.

So clarify is hum present on all audio selects am, fm, and tape phone connected? Equipment wise how did you identify 120hz?
hang in there.. and don't do anything stupid like shot gunning.
bink

 

[Moving Ahead]

As above.......
A cap coupled unit? can generally take an educated guess which trimpot is which by inspecting timmer track values. Besides using 4 dmm when aligning will definitely tell you whats going on.

 

[redpackman]

I have the 8600a also a FET model.

Now lets back up.. I doubt any bias adjustments will resolve the 'hum' issue. Audible 'hum' increase with vol. 'gain' is audio path and usually ground loop associated or a filter leak.. cap(s) in supply side or smaller service caps.

So clarify is hum present on all audio selects am, fm, and tape phone connected? Equipment wise how did you identify 120hz?
hang in there.. and don't do anything stupid like shot gunning.
bink

I am not messing with the bias settings. I don't have the equipment. I was able, using a digital meter, to set what I believe is the balance pot (the one physically farther from the output transistors. It's the one on the left in the photo.) so the voltage is around 35v, half of the B+ voltage which is 70 v DC.

I have not checked whether the hum is present in the phono or tape mode, but I will. The hum (it is mild) is present as soon as the unit is turned on and stays the same until the sound is turned probably 2/3 of the way up and then it increases some. It is mild enough to be drowned out when music is playing, but it's there. It is present in both FM (there is no AM on this model), and in AUX setting. I too am suspicious of the filter caps.

 

[awillia6]

I have not checked whether the hum is present in the phono or tape mode, but I will. The hum (it is mild) is present as soon as the unit is turned on and stays the same until the sound is turned probably 2/3 of the way up and then it increases some. It is mild enough to be drowned out when music is playing, but it's there. It is present in both FM (there is no AM on this model), and in AUX setting.

Sorry, I haven't read through the whole thread. Have you checked for ground loop as the source of the hum. Easy way is to float your receiver by pulling everything that could be grounded, then using an ungrounded source such as an iPod or smartphone. If no hum, then you've got a grounding issue (and good luck with that!). If it's still there, time to signal trace through the guts of your receiver (good luck with that as well!). I don't envy you either way.

 

[redpackman]

I did run the iPod through it. The hum was present.

One other symptom I just discovered. I've been testing this unit with headphones (expendable, but I could tell if it was running rather than hooking it up to my EPI 100's which I DON'T want to fry). Well anyway, I checked the DC current at the speaker hookups and all was well as both channels came in around 10 mV. Hooked up the precious EPI's (don't worry, they're still fine). Turned on the unit. At low volume there's a little hum. As the volume goes up the hum remains but the music overpowers it and it sounds pretty good.

Now the new symptom it didn't display with the headphones (probably because of the LOW voltage required to drive them). As I get to about 40% on the dial the sound starts to pulse. The best way I could describe it as "motor boating."

Usual culprits?

I think I'm going to sell this on eBay as a kinda working "as is" unit.

 

[gadget73]

Motorboating is usually poor power supply decoupling, so we're back to power supply capacitors again.

 

[redpackman]

Thank you. That's the kind of input a guy like me needs (and I'm not being sarcastic).

For some of you guys that fact is as plain as 2 + 2 = 4. But for the rest of us, we need that kind of specific clue. It may NOT guarantee a correct diagnosis, but it's better than anything else we've got.

That's why this forum exists and why guys like me who don't simply want to dump it on some guy to fix for $125 an hour. I know that guys like me can blow this machine up, too, when we mess with it...but once again, that's why we're here. To learn and to venture out with a little more information into what, yesterday, was the unknown.

 

[Binkman]

Well.. advice was offered in post #5. power supply inspection is norm but usually with an O'scope. So quite a few amateurs don't have one so they simply go for ps caps. also called 'mains' caps. just assuring a doubt taken care of. Also inspection and testing rectifier diodes and other diodes. Also general inspection of wiring and solder quality or suspect solder age....it's all relative.

Make notes and don't burn the midnight oil. Take breaks.. you'll make better decisions.

 

[ivandezande]

She's making sound, that's good. I worked on a 8800a it was gross to say the least but you'll get through it. Change out some caps to deal with that hum as others have said.

 

[podbaydoors]

Thank you. That's the kind of input a guy like me needs (and I'm not being sarcastic).

For some of you guys that fact is as plain as 2 + 2 = 4. But for the rest of us, we need that kind of specific clue. It may NOT guarantee a correct diagnosis, but it's better than anything else we've got.

That's why this forum exists and why guys like me who don't simply want to dump it on some guy to fix for $125 an hour. I know that guys like me can blow this machine up, too, when we mess with it...but once again, that's why we're here. To learn and to venture out with a little more information into what, yesterday, was the unknown.

Hi,
Did’ja ever get your Sherwood working up to snuff? I am about to receive one and start having fun with it. The current owner sez it’s working but that remains to be seen. I am one of the folks that presume that 55 year old electrolytic caps are suspect at best and consider that replacement of such is a preordained part of service for vintage gear like this. I also have a nice supply of dial cord ‘cuz that seems to have a finite service life too. I don’t think the 8800 contained any IC’s, but the A variant probably did and back then it was a status thing and advertised as such.

best,

Lou

 

[gadget73]

curious what they were using for an IC. Probably just an op-amp if I had to guess, maybe one of those TO-5 cased 741 units. Those have been around forever.