Sherwood S-6040CP MOS FET DC POWER AMP

patra710

Active Member
Hi
I have been reading the AK forum for about 2 years now and have learn al ot from it, thanks to all.
I pick up this Sherwood S-6040CP amp but I don't know how to test it. All it has on the front panel is a power, speakers and meter buttons. And on the back is only 2 inputs and speakers connections.
Can I run the preamp from another stereo and will the volume and tone controls work to test this amp.
I have a degree in electronics from ITT tech so I do know a little about things but still so much to learn about stereos.
Thanks for any help that I may get here.
 
I may be answering my own question here. I pulled the jumpers off between the preamp and the main amp on a marantz 2245 and put a signal to the tape input. Hook it up to a scope showing both in put signal and output from the preamp. Played with the volume and tone controls and saw the changes. I used a sine and a square wave.
I think I need to break down a stereo to the basic and understand what happen at each step of the system.
I don't think I could really troubleshoot a problem till I understand what is going on in each stage. Still so much to learn.
 
Hi
I have been reading the AK forum for about 2 years now and have learn al ot from it, thanks to all.
I pick up this Sherwood S-6040CP amp but I don't know how to test it. All it has on the front panel is a power, speakers and meter buttons. And on the back is only 2 inputs and speakers connections.
Can I run the preamp from another stereo and will the volume and tone controls work to test this amp.
I have a degree in electronics from ITT tech so I do know a little about things but still so much to learn about stereos.
Thanks for any help that I may get here.

Congratulation. You got one of the best kept secret in audio. The first thing you need to do is to get a copy of the service manual. You may need to sign up for the HiFi Engine website if you are not yet a member. They are free.

http://www.hifiengine.com/manual_library/sherwood/s-6040cp.shtml

The S-6040CP is one of the better sounding power amplifier in the 100W/ch range. I have 3 of them for a 5.1 setup. The basic circuit is from the Hitachi technical note and similar to the Hafler HD-200, but with much heavier construction and beefier power supply than the Hafler.

If you do a search on AK forum, you will find quite a few references to it such as this one.

http://www.audiokarma.org/forums/showthread.php?t=415613&highlight=sherwood+S-6040CP

Depending on the age and condition, you may want to consider a re-cap of the power supply section. You will definitely need to if you hear hum from your unit. It is normal for old audio equipment. Make sure that there is no DC offset at the speaker output. If you do, correct it with the trim pot VR101L/R. The MOSFET amp usually runs hotter than other bipolar amplifier. If you feel the cabinet is too hot, check the idle current too. All these process can be found in the service manual.

Good luck and enjoy your find.
 
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Thanks Keilau for the information. I took the cover off to look inside and found a blown fuse on one of the power boards and some burn marks on the sub board. The transistors on the bad power board tested bad.
All I've done on stereos before was replace caps. This one will give me a chance to test my skills at some real repairing.
 
This is a very good amp, well worth the effort of repairing it. I went through my share of Marantzes, Denons, Sansuis, NADs and many more, and this amp is now in my main rig. I don't think it will be replaced anytime soon.

Once you have it fixed and get to adjustments, note that they should be done with 8 ohm load connected (I just used an 8 ohm speaker) and amp running with no signal for 10 minutes before the adjustments.

I did idle current first. It was more than double the value in the service manual, and the amp was running quite hot. I used transistor legs as measurement points. Not easy, but I could reach them with multimeter leads ending with pins. You connect the multimeter to the source (right leg) of Q114 (black transistor on the outer right) and source (right leg) of Q116 (green transistor on the outer left), measure and adjust for 17 mv. Then connect multimeter to sources (right legs) of the inner black and green transistors Q113 and Q115 and measure. If it's between 15 and 20 mV, OK, if not, readjust so that both measurements are in that range. They were not the same in my amp.
It helps if you have someone to help, much easier to do this with 3 hands.

I adjusted DC offset using hot (+) speaker terminals for each channel as + measurement points and chassis as ground, with 8 ohm speaker connected. Note that the SM refers to "speaker terminal 6/8" not "checkpoint 6/8" or "P6/P8". The points marked P6, P8 on the power amp boards are for something else, no clue what.

There's a third trimmer on some versions of the power amp board (including mine). See here:

http://www.audiokarma.org/forums/showthread.php?t=579212

I did use it to adjust DC offset (actually had to use it, I could not get it down to near 0 mV using VR101), but I have no idea if it's its intended purpose.

EDIT: I did a quick search and if your outputs are blown, you might have a problem. 2SK343 and 2SJ99 are not easy to find, and quite expensive. The only source I'm finding for Hitachi originals is a seller on the auction site, with prices around $20 for 2SJ99 and $30 for 2SK343. If you need to replace all 4, that would be $100, quite a lot to repair this amp. There are parts with the same model number on alibaba (cheap), but I'm not sure if it's the same part, possibly something else with that number. 2SK344 and 2SJ100 are possible replacements, but those could be even harder to get. You might want to look for another S-6040CP (or its Inkel/LXI counterpart) with a blown channel and use it for parts.
 
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Thanks Keilau for the information. I took the cover off to look inside and found a blown fuse on one of the power boards and some burn marks on the sub board. The transistors on the bad power board tested bad.
All I've done on stereos before was replace caps. This one will give me a chance to test my skills at some real repairing.

You can get the geniune Hitachi MOSFET from some chinese supplier on Ebay at reasonable price. Spend a little extra to get the matched pair and from established China seller. There are too many imitations and you have to be real careful. Whenever it is too cheap to be true, it usually is.

You may have problem finding the exact MOSFET used in the original Sherwood. Hitachi from the same family with higher current and voltage rating should work ok. The last time I checked, the Digikey website had the information I needed.

This Sherwood amp sounds a lot better than the Hitachi HCA-7500 which commands a premium at Ebay this days.

Share your trial and tribulation of the repair with us. Good luck.
 
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Looks sell the Hitachi I guess. Some people can't resist wipers :)

The Hitachi HMA-7500 is a very good sounding amplifier too. But the high price is due to its collector item status.

http://www.ebay.com/itm/HITACHI-HMA-7500-Stereo-Power-Amplifier-Mos-Fet-Hi-End-Top-Line-Free-Delivery-/141669514218?pt=LH_DefaultDomain_0&hash=item20fc293bea

I regret selling my Pioneer SA-9100 integrated amplifier dirt cheap years ago. Now, it commends hundreds of dollars on Ebay. So are many of the Japanese receivers from that era. They are definitely sold for looks.
 
Can you link to further info on this point?

I have two of of these myself, future refurb projects. :)

I presume that you already downloaded the service manual from HiFi Engine.

I have the Hitachi technical note in paper form only and cannot find it right the way. You can find some information from Paul Kemble at Angelfire.com.

http://www.angelfire.com/sd/paulkemble/sound7g.html

You may be interested in Bob Cordell's JAES paper too.

http://www.cordellaudio.com/papers/MOSFET_Power_Amp.pdf
 
Thanks to all for the information. This forum is great. Over the last couple of years I have been hitting the electronics dumpsters. I have found,
Pioneer SX-450 SX-SX-780 SX-SX-939 SX-1050 SX-3700 SX-3800 SA-800 QX-848
TX-606
Kenwood KA-305 KR-9600
Sanaui 441 6060 R-50
Scott 370R
Yamaha CR-1000
Fisher RS-2007
Marantz 2245
Quadraflex 777 240R
Sherwood S-6040CP
Most of them will power up and play but all need work. This is my first week of retirement so now I got all the time I need to start going through these one at a time, stage by stage. I always had a passion for electronics and vintage stereos and with the help that I can find in this forum, I think I will be successful.
 
I have one of those. Has intermittent offset in one channel and the protection is not working right.

You found all those Pioneers in the trash ? And that Marantz ? I think I want to move to your neighborhood.

Anyway, the one I have is deemed not to be repaired because of corrosion. I'm told it was in a basement for a while.
 
I had one of these, shouldn't have sold it. The electronics dumpsters around here are guarded, deposit only.
 
Of the two S-6040CP amps I have, one I have never hooked up because the DC offset did not settle the couple of times I plugged it in. On one channel rose steadily to above 100mv and kept climbing. I never investigated or even attempted to adjust offset. Point is, I don't know how it sounds.

The other, to be honest, was a bit sloppy sounding. But that is in original state and no doubt the caps need refreshed.

But I have faith that these will sound very good once they have been gone through. I also have a couple Sherwood integrateds from the early 80's. One of which is a S-602CP that shares some parts with the 6040. Both of my integrated Sherwoods sound very good.
 
I picked up this same little amplifier. I am also having trouble bringing the right channel DC offset down. The left I set right down to 4mv, but the right i cant get lower than the 230s. I replaced the trimmer I'm using to adjust as the original was shot and all over the place. My board does not have the mystery trimmer someone took a picture of. Does anyone have any advice on where to look next?
 
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