My first Receiver - MCS 3226

Ross Henning

Active Member
Hi All,
I've been an electronics nut all my life, but only recently I've decided to get more serious about my hobby and really dig in and force myself to learn to be a true technician. I'm not after this as a career, at least not now...maybe when I retire. I just enjoy learning new stuff, and although I've got a good understanding of electronics, I find troubleshooting and repair at the component level to be quite a challenge.

I've acquired all of the gear I need to get started. I have an oscilloscope, function generator, bench multimeter, bench power supply, etc., etc., etc.

My first project is to restore my old MCS 3226 receiver which was my very first receiver when I was around 10 - 12 years old. I knew that I had blown out one of the channels years ago, so I went in knowing that I was going to have to replace transistors on the one side. I also knew that I needed to recap it, so I've replaced all of the electrolytics. After replacing all of the caps, I fired it up and when I increased the voltage on my variac up to line voltage then turned up the volume, I burned out the main power supply fuse. I also found that I had major DC voltage at the outputs.

I went back and started checking things and realized that BOTH channels are now out. I checked the output transistors and sure enough, they were toast. I went ahead and ordered a full set of new transistors so that I would have a replacement for every one as I knew this being a DC coupled unit, there's bound to be further trouble.

Long story short, I've replaced every transistor and still have the same issue. This receiver uses a push-pull configuration with a positive and negative power supply. The DC voltage at the output is negative, and is oscillating. I'm about to pull my hair out trying to figure out where the breakdown is. I'm sure that if I had more experience, or if I had a mentor to help me, the place to look would be much more obvious.

I'm hoping some good soul on this board might be able to give me some pointers as to where I should look, or what I should look for. Unfortunately, I do not have a schematic, and I've not been able to find one anywhere on the internet.

Thanks in advance for advice.
 
First step would be to check the +/- rail voltages, see if they are present and about equal. Next would be to check for a burnt resistor that may feed the driver stages of the amp, ect. If you post your location, someone local may be able to come and take a look and give a hand.
 
Yes, both rail voltages are present and are equal to each other. Visually, I see no obvious signs of a burnt resistor, but obviously that could be the case without a visual clue. I'll try some more poking around for that. Are you thinking that the resistor would be shorted in that case, or open?

Does the fact that the same negative DC voltage is present on both channels, and the fact that it's oscillating provide any sort of clue?

I'm in the NW Houston, TX area in a small town called Pinehurst. It would be great to find someone up here that shares my interest.
 
Does the fact that the same negative DC voltage is present on both channels, and the fact that it's oscillating provide any sort of clue?
Yes.. It does. Have a real close look at your main filter caps its possible one has issues.
 
Yes.. It does. Have a real close look at your main filter caps its possible one has issues.
Thanks, I'll check them, but they are brand new Nichicons, and the voltage measured at each one is pretty equal (one negative and one positive obviously). I've replaced every electrolytic in the power supply, poweramp, and preamp sections with brand new Nichicon caps of equal value to the original parts.
 
I have the schematic (but no service manual) for the 3226 scanned into three jpgs. If you would like them, just click on my username, and start a conversation with me. Include an email address, and I'll gladly send them your way. Maybe it will help.

Rob
 
I have the schematic (but no service manual) for the 3226 scanned into three jpgs. If you would like them, just click on my username, and start a conversation with me. Include an email address, and I'll gladly send them your way. Maybe it will help.

Rob
A Huge Thanks to you Rob. I can't look at it tonight, but I think the schematic you sent is going to help me a lot. It has voltages listed on the legs of all of the transistors, so I'm hoping that will help me find the culprit. BTW...I've put the three pages together using photoshop and now have a single PDF if that would be useful to you. I can email it back to you if so.
 
That would be very nice to have - thanks! It would be easier to trace the circuits out.

Good luck with the repairs. Several other MCS pieces used very similar amp sections - even the same outputs. Models include 3820, 3822, 3823, and 3830 integrated amps.
 
Remove the output transistors and see if it will power up on a dim bulb tested with bulb dim.

If the bulb dims, then check voltages on the output transistor connection points.

If those are whacked, work back through the transistors testing voltages. Remove transistors if needed as you go until you get correct readings.

Then work forward figuring out why the voltages are bad.

Test resistors and diodes.
 
I have not. I’ve re-visited it several times as I’ve gotten more experience solving issues in other receivers. I have nine successes thus far, with only the MCS 3226 that I’ve not been able to find the issue with. I will continue to revisit it as I continue to learn more, but my guess is that it’s one that might stump even the seasoned guys.
 
I got one of those a couple of weeks ago, and so far I’ve really been impressed with it. It’s very modest looking but it’s really a nice unit. You’ll get it and learn from it. I’ve never learned a thing when everything fell into place but when it kept fighting back I got an education.
 
bump....

just refurbed one of these for the store.

these are fairly straight forward machines, I would remove said outputs and check it using just the drivers. everything in the amp section is easily replaceable
 
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