MCS 3233 (JC Penney)

[Racingh11]

Got the 3235, looked under the cover. Looks original. The tuning capacitor seams to have a stripped gear or something. Have to investigate further. Haven't plugged it in yet, I have some other things I have to do first and I'm not sure when I'll get to it. I'm hoping in the next couple of weeks. Amp board looks the same as the pic posted of the 3233.

 

[RebelYankee]

Finally got the new pots in and they are way too small. Gonna have to look for some bigger ones. Also ordered new output transistors.

More to come. Thank you all fo the great advice.

Jamie

 

[Racingh11]

Finally got the new pots in and they are way too small. Gonna have to look for some bigger ones. Also ordered new output transistors.

More to come. Thank you all fo the great advice.

Jamie

I wondered about that, you said they were 1k in one post and said they measured 1.7k across. Combined with the resistors in the circuit, it looks like they are 2k.

 

[RebelYankee]

Yep, they are 2k. Not sure where I got the 1k from. Either way, they are too small and won’t fit. Found some on fleabay from Spain that are the right size. We will see how they do.

 

[Racingh11]

I finally got to that 3235. Only made a few quick checks, but it has power transistors shorted through on one channel. Driver transistor appears shorted as well. Going to order some transistors and turn it on with a variac and see what's going on. I'll have to check the pots right away and see if these are bad first.

 

[RebelYankee]

I finally got to that 3235. Only made a few quick checks, but it has power transistors shorted through on one channel. Driver transistor appears shorted as well. Going to order some transistors and turn it on with a variac and see what's going on. I'll have to check the pots right away and see if these are bad first.

Good luck. I've recapped, replaced all transistors, replaced all resistors, and now I have 2 cut wires. Need to figure out how to replace them since they are the wound ones. Good news is the right channel is working. Bad news is a short in the left channel. It is blowing the fuse.

 

[RebelYankee]

Racing- Has yours been worked on by some else? I ask because if it hasn’t, I have a question.

The input wires to the amp board are shielded. On the amp side, the core goes through the top and is soldered on, and the shielding is soldered to the ground. On the other end, the core is soldered to the pin and the shielding goes nowhere...it’s wrapped up. Does the shielding on yours connect on both ends?

Appreciate the help. I have realized someone did a lot more work than I thought on this...and it doesn’t look very good.

 

[Racingh11]

I just logged on and saw your post. I'll check out the wiring in a little while and get back to you. I just pulled out the shorted power transistors and checked the driver transistors with those removed and found one channel has both driver and power transistors shorted through between all leads. Ordered some replacement driver transistors. I checked the bias potentiometers and both are good.

 

[Racingh11]

Racing- Has yours been worked on by some else? I ask because if it hasn’t, I have a question.

The input wires to the amp board are shielded. On the amp side, the core goes through the top and is soldered on, and the shielding is soldered to the ground. On the other end, the core is soldered to the pin and the shielding goes nowhere...it’s wrapped up. Does the shielding on yours connect on both ends?

Appreciate the help. I have realized someone did a lot more work than I thought on this...and it doesn’t look very good.

Mine has not been worked on. The ground/shield is connected at both ends. I just found the tuner gear split in mine as well.

 

[Racingh11]

The 3235 is done. After replacing driver and power transistors, I turned it on with a variac and it still had high current in one channel. Found and open bias transistor that was sending full current through the bias circuit. It was the original cause of failure. Replaced it and set everything up to spec. Sounds great. It got a full recap too...

 

[Racingh11]

Did some more testing after it was setup and the channel that went bad still had some major thermal runaway going on. I ended up gluing the bias transistor to the heat-sink and it is nice and stable now. I had it against the output resistor like it was stock, but voltage is all over the place that way. Looks good until you run it 1/3 or more power for a bit and the bias will drift from 6.6mV(factory spec) to about 15mV and took off quickly from there. It also varied all over and could not steady. It now is about 10-12mV after running at close to clipping a while and settles down to 6-7mV from idle up to 10 watts or so. The heat-sink runs much cooler, slightly warm even after max power testing.

 

[RebelYankee]

I replaced the bias transistors and get a little bit of sound from the right channel and nothing on the left, power supply was buzzing like crazy. Unfortunately, the person who worked on this before me did some weird stuff so it’s really hard to track down the issues. I think I will shelve this project until I learn more. Need to find me a virgin amp now...

 

[RebelYankee]

Update...and advice needed.

So decided to have another go at this thing. I pulled the power board and replaced all the caps. I also pulled the amp board and removed the output transistors and checked them all. The output transistors tested fine except for one. I replace it and a few suspect resistors. Good news is that I no longer have a short. I’m pretty excited about that. Bad news is I get a hum from both channels and no output. I tested the voltage and get pretty much either 33v or -33v everywhere, including both the speaker outputs...both 33v. So am I looking at bad resistors?

Thanks,
Jamie

 

[Racingh11]

Update...and advice needed.

So decided to have another go at this thing. I pulled the power board and replaced all the caps. I also pulled the amp board and removed the output transistors and checked them all. The output transistors tested fine except for one. I replace it and a few suspect resistors. Good news is that I no longer have a short. I’m pretty excited about that. Bad news is I get a hum from both channels and no output. I tested the voltage and get pretty much either 33v or -33v everywhere, including both the speaker outputs...both 33v. So am I looking at bad resistors?

Thanks,
Jamie

Well, I'd say it looks like the positive rail output transistors(NPN) would have to be either shorted through to voltage or powered from the stage before it. Since you said you checked the transistors for shorts, it must have its base powered from the stage before it. One way I like to narrow it down is to short a driver transistor from base to emitter and see if the voltage at the speaker output goes low, near or at 0v. If it goes low then it is being powered from the stage before it and similar methods could be use to check that. If it stays high, then the driver transistor is likely shorted through. Either way, a load should not be hooked up at full voltage. That is transformer voltage you're hearing and you will likely short another transistor doing it.

 

[RebelYankee]

Thanks. I’ll give it a try.

 

[Racingh11]

Just looked at the schematic and it might be possible for resistors R521 or R522 (depending on what channel it is), being open and could cause the positive side to turn on. Transistors for current source Q505 or Q506 open(dead), neg feedback Q503 or Q504 open, input Q501 or Q502 shorted. That's what I'm seeing that could cause that condition, but there are other less likely combinations that could do it as well.

 

[RebelYankee]

Thanks. I know I replaced almost all those, but that was before I had a short. I’ll go ahead and replace all the resistors and transistors again and see what happens.

So how has your 3235 been running?

 

[Racingh11]

It was good while I had it and tested, then it sold right after. The buyer really likes it, but had trouble getting stations in the pole shed he lived in. I gave him some antenna advice and haven't heard from him since. I have a pair of 3233s I'm doing soon and a couple of 3230s, one done already, a 3245 yet, and a 3260 ready to go. I don't know if there is much wrong with the remaining units, might be just a recap and tune. The first 3230 needed quite a bit of work on top of the recap and tune. Turned out great though and is for sale as well. I got some MCS 8310 speakers with one. Was surprised how good they sounded for a 2-way speaker and very good lows for an 8 inch woofer. Sounded a lot like a 10. Very good in their 50 watt range but the highs get to be a little much at loud volumes. Very efficient setup too. Those are being sold to someone with a 3237 I fixed up next week, should be a good pair I think.

 

[MCS Guy]

Racingh11,

Where to you measure to get the bias voltage? The Sams photofact does not give me anything on it.

 

[Racingh11]

Racingh11,

Where to you measure to get the bias voltage? The Sams photofact does not give me anything on it.

It's measured from emitter to emitter on the output transistors which is also the outside leads of the large 3 watt .33 ohm output resistors. Should be 6.6mV which works out to be 10mA.