Mystery trimmer on Sherwood S-6040CP power board

rlisin

Quad 4 life
Got my hands on an InKel set of MD2200, PD2100 and EQ 200 (power amp, pre and EQ):

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Power and pre were also sold as Sherwood S-6040CP/S-6020CP and as LXI AM 4222/AM4022. I put Sherwood model in the title because I know some ARers have this amp.

The power amp needed adjustments, which I made. At first I missed that 8 ohm load should be connected while adjusting and I couldn't get the right values, but after rechecking the SM I connected an 8 ohm speaker and adjustments went fine. Well, sort of.

I could get the bias to the value range specified in the SM with VR102, but it's not the same on transistor pairs in either channel (off by 2 mV, meaning when I set it to 17 mV for one pair, the other pair in this channel gets 19 mV). Both values are within the range recommended in the SM, so I don't think it's a problem, but if it is, do tell me.

DC offset was more tricky. I could not get it down to near 0 mV using the VR101. According to the SM, values up to 70 mV are fine, but 70 mV seemed scary high to me and I wanted it lower.

But there is a 3rd trimmer on the power board. The SM simply does not mention it and I don't see it on the schematic at all. The reason is, SM is written for the power board part number K40430, and my amp has power board K40430A.

When I moved this trimmer (by mistake), I noticed that it affects DC offset big time, so I set it in the position at which DC offset is 0 mV.

But I hate to not know what exactly I'm doing, so I would like to set it correctly.

So, do you have any idea what the 3rd trimmer does?

It is marked red:

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X marks the spot where it should be on the schematic:

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I have confirmed that the 3rd trimmer is not unique to InKel-branded amp, some Sherwood S-6040CP and LXI AM 4222 also use power board version K40430A and also have this trimmer. It is not on power board K40430.
 
I have the Sherwood version (the S6040) and have the same brown colored pc boards marked K40430A, but I don't have the third pot you have -- I also notice that it doesn't sit above a circle printed on the pcb.
I also notice that yours, like mine, seems to be missing the metal pin on checkpoint 6. Which unfortunately is one of the pins you use in setting offset -- using a fine tip I can contact it, but what I find is a little unnerving -- 4.66V on both sides. When checking point 8, it's an unexceptional 1.7 mV.
I'm not all that experienced in setting bias and offset, though I've done it on non mosfet solid state amps and on a Dyna Stereo 70, and I'm uncertain how to proceed. The amp sounds fine, though it runs hot so I wonder if maybe the manual is wrong (which I just experienced biasing a Pioneer SX 737, where the manual reverses the relevant test points for bias and offset).
Any ideas?
 
As far as I remember the SM is quite badly written and it took me some time and few attempts to figure everything out. The fact that it doesn't exactly correspond to what's inside the amp (i.e. third trimmer) didn't help.

I adjusted 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".
I hope you are able to adjust without the 3rd trimmer.

Recheck after adjusting idle current. In my case, after the adjustments the amp runs nice and only barely warm, even with difficult loads.

Hope that helps. Let me know if you managed to adjust.
 
No help here but, it sounds like the engineer ghosts of the proud days of Sherwood had a hand in it.
 
Both hands. And at least one foot. But mine works without issues and sounds great after the adjustments, so I guess I got it right.
Pity that the matching preamp is rather mediocre. I'm replacing it with a Harman HK-725 pre as soon as I find time.
 
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As far as I remember the SM is quite badly written and it took me some time and few attempts to figure everything out. The fact that it doesn't exactly correspond to what's inside the amp (i.e. third trimmer) didn't help.

I adjusted 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".
I hope you are able to adjust without the 3rd trimmer.

Recheck after adjusting idle current. In my case, after the adjustments the amp runs nice and only barely warm, even with difficult loads.

Hope that helps. Let me know if you managed to adjust.

Hello this my first post. I thank you rlisin and others for all your writing about Inkel MD 2200. I have bought a "mystery" Sherwood s4060/Inkel MD2200 amp. "Mystery" because it hasn't got a name on it.. with the same board K40430A as your amp. I ve got the SM. I have unsoldered the speaker relay and cleaned it and got rid of some noise, when using the speaker switches. The fuses are all on the boards. It hasn't got access to fuses from the outside. . The amp sounds good and is dirty and all original inside with a little rust on the transformators . No hum and distortion. The two channels are about the same volume (listening to the amp) but the left channel has a much higher reading on the led-meter ( it could be a faulty led-meter) Left channel is also warmer when laying my hand on the top of the amp. But its just a little warm even after long hours of playing, and the right channel is still cold. This makes me want to adjust the amp. I have never tried this before.
Questions 1: Does this amp decharge the power capacitors automatic when turned off ? - I can't seem to get a volt reading from the capacitors before i decharge it - and i don't want to get electrocuted
Question 2: rlisin stated that you should use the speaker terminal 6/8 - but which terminal is that ? speaker B / Right ground or ...? I can't se the numbers of the speaker terminals anywhere.
Question 3: Have you recapped the amp, if so how would you describe the change, I wonder if bad caps could be responsible for the differences in temperature between the two channels. And I wonder if I need to change power Capacitors if all looks good ?
 
update i adjusted offset voltage to 39 mv in left channel and found that it was around 1100 mv for right channel. It was so off that I had to use the mystery Trimmer to get the range right for the recommended trimmer and now the Led-meter is the same for both channels :) I love this forum thanks again rlisin!! now for the difficult part: idel current adjustment.
 
update 2 Still only after adjusting the offset voltage. Ive just discovered that when Inkel Md2200 is turned on, but with no signal from preamp. and The Led meter is in "low" position: the left channel is around 4 red stripes and the right channel is around two stripes up. When i turn the signal on from the preamp to both channels . Then both right and left channel have the same max value on the Led-meter.??
 
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update 2 Still only after adjusting the offset voltage. Ive just discovered that when Inkel Md2200 is turned on, but with no signal from preamp. and The Led meter is in "low" position: the left channel is around 4 red stripes and the right channel is around two stripes up. When i turn the signal on from the preamp to both channels . Then both right and left channel have the same max value on the Led-meter.??

I have done some work on my Sherwood S-602CP integrated amp. The S-602CP was also made by Inkel just a couple years before the S-6040CP and they share some parts (although the S-602CP has BJT outputs).

Regarding meters, my experience with my S-602CP is that recapping did improve the performance of the LED meters. Also, there are adjustment trimmers for the meters and a procedure in the SM for calibrating the meters. Does the the S-6040CP have trimmers for the meters, or a calibration procedure in the SM?
 
The mistery trimmer could just be a simple voltage adjustment for the amp or driver stage? I have an integrated amp that uses fixed resistors for bias adjustment. There is one trimmer per channel that adjusts the voltage for the whole amp section. It may be something similar. Kinda redundant sounding if it has bias and DC offset trimmers already though.
 
Update 3.
Wow! im new to this forum. It's amazing how fast people are responding.Roger2 glad to hear about the improvement new capacitors made. I will do that too.I haven't found trimmers for the Led meter. thanks. Lavanne, You might be right regarding the "Mystery trimmer", it was useful. My problem is that i have little understanding of electronics. I can weld, use a multimeter etc, so i'm always looking after someone who has done something similar i'm about to do before me :).
I have adjusted the bias on the right channel . I found out it was easiest to remove the plate cover on the underside of the amp to get easy access to the power mosfet transistors I was able to place alligatorclips on the legs of the "source"legs of the transistors connected to my multimeter. Now I had both hands free to make adjustments of the trimmer v102. I followed rlisin, who started this thread. and set it to 17. But trying the same on the left channel was almost impossible, the values were jumping all over, and i waited 10 min after the amp was turned on, with an 8 ohm 150 watt resistor as speaker-load on the channel. I hope its a bad trim pot. Otherwise i haven't got a clue, but hope it will settle when I change capacitors?


To you who might have the same amp: Sherwood s-6040 /Inkel md2200 / Lxi 4222: what does your led meter show when the amp is turned on in "low" position, but without any signal signal, do you see "red stribes" - if so how many?
 
Update 3.
But trying the same on the left channel was almost impossible, the values were jumping all over, and i waited 10 min after the amp was turned on, with an 8 ohm 150 watt resistor as speaker-load on the channel. I hope its a bad trim pot. Otherwise i haven't got a clue, but hope it will settle when I change capacitors?


To you who might have the same amp: Sherwood s-6040 /Inkel md2200 / Lxi 4222: what does your led meter show when the amp is turned on in "low" position, but without any signal signal, do you see "red stribes" - if so how many?

Hi, glad my thread was helpful! I remember having to use a small drop of a pot cleaner (similar to deoxit) on one or two adjustment trimmers and working them a bit because they had dirty contacts and values were jumping when trying to adjust. So maybe try that, or simply replace the problematic trimmer. It should help.

My amp's meters show just one bottom bar in each channel with no signal. I too had a problem with the meter in one channel when I got the amp, but I don't remember what it was exactly because that particular problem was fixed by my tech. But the problem was something minor, like adjustment issue or bad cap, or maybe resistor in the meter circuit.

I have not recapped my Inkel (apart from maybe something in the meter circuit), it sounds fine and has no problems as is (and it has been in one of my systems and in almost daily use with very demanding speakers for the last 5 years). I remember reading a thread of someone who recapped this amp's predecessor Inkel MD1200, and the sonic results were good, but eventually that amp started misbehaving and blew a fuse - which may or may not be linked to the recap job.
 
Thanks a lot rlisin I was hoping that you were joining :) .Thank you for sharing knowledge and pointing me to this amp. Im very happy i bought this amp for 450 danish Kr = 68 us dollars . It already sounds really good and what's left to do is minor things. Thanks to you and others we can enjoys these amps more than 30 years after they were produced.
I'm still like you, curious about the mystery trimmer, maybe I pose the question in an electronic forum.
Maybe i can get your permission to either link to your pictures of the mystery trimmer on the board and the drawing or copy them ? If somebody can solve the riddle I will let you know in this thread.
 
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