MCS 3233 (JC Penney)

RebelYankee

Member
Hi All,

I’ve been lurking here for a while and from reading through the posts, decided to try my hand at fixing an old stereo. I picked up a MCS 3233 that has no sound from the left channel. I have ordered the service manual from onlinemanuals.com and took a shot at replacing some of the burned out resistors. So before I even started, I must have done something on the right channel of the main amp board because a resistor is burning every time I power it on. I replaced the resistor, and all the other ones that were suspect, and it still burns up. I can’t even check to see if the left channel if working.

Anyway, my question is this: in the service manual, it has boxes with voltage listed and it says “values indicated are dc voltages between the chassis and electrical part”. Do I measure with the power on, power off, plugged in, or unplugged? I know it sound like a ridiculous question, but when I measured with the power on none of the voltages were even close (before I jacked up the right channel).

Appreciate any advice.

Thanks,
Jamie
 
If you have burning resistors, something is likely shorted out. You need to look up how to build a dim bulb tester (DBT). It will limit current in case you have a short in your unit (sounds like you do) and help prevent further damage. You should be testing for what is shorted before applying power again. Once you have found and replaced the faulty component(s), start up on DBT. Then measure voltages before applying full power.
 
If you have burning resistors, something is likely shorted out. You need to look up how to build a dim bulb tester (DBT). It will limit current in case you have a short in your unit (sounds like you do) and help prevent further damage. You should be testing for what is shorted before applying power again. Once you have found and replaced the faulty component(s), start up on DBT. Then measure voltages before applying full power.

This is great. Thank you!
 
Electrolytic capacitors , diodes and transistors associated with those burning resistors are all suspect.
 
I am assuming that the resistors are on the driver board. Simply replacing burned items on circuit boards rarely solves the issue. Something else is causing the high current on the board. First thing is to check your output transistors. If they are well, then pull the driver board to where you can get to all of it and, item by item, check each component for issues. There is a lot of info on this site to tell how to test items. You may have to buy a few test items. I suggest MCM as a place to get decent meters at reasonable prices. At least you will need a meter that will test hfe in case you have to replace transistor and will need to hfe match a batch to find the closest transistors for the job. You will need a meter to test the ECR on the caps unless you just do a full replacement of all caps.

You will need to remove the items from the board to test. I would suggest one at a time. When replacing, it is also recommended that it be done one item at a time. The cascade effect, causing the problems, on a circuit board can be a pain to trace. I too suggest that you create a DBT, dim bulb tester. It will save you from burning up more components. Good luck.
 
Thanks for all the great info. I’m guessing the only way to really fix this is to remove the board.

The burning of the one transistor is a direct result of my stupidity. While the amp was powered on, I hit the pot that adjusts the bias on the right channel and immediately the resistor burned up. I have the amp plugged into a dbt with a 150w bulb and it still burns. The pot has three legs and when I test the legs with a mm, I get readings for all three. On the left channel, only one leg has a reading and the other two do not. Not sure how helpful that is since the left channel didn’t work in the first place.

I have attached a pic that shows the board. You can see the burned resistor towards the 2 o’clock position, and the pot on the right is the one I hit with a screwdriver.

I’ll start researching how to get the board out and look at replacing everything.
 

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Start by removing and testing your large output transistors and work backwards with the transistors from there - use the service manual/schematic for reference.
 
Just a quick update. I changed all the caps on the board and replaced the burned resistor. I also replaced he power capacitors. I can tell someone was in here before me...some of the soldering was crap. I got it all back together and I now have a major short somewhere. 150 watt bulb is full on on the DBT.

Any ideas on where to start looking for the short? Or even some of ideas on how to look for the short.

Thanks,
Jamie
 
Just a quick update. I changed all the caps on the board and replaced the burned resistor. I also replaced he power capacitors. I can tell someone was in here before me...some of the soldering was crap. I got it all back together and I now have a major short somewhere. 150 watt bulb is full on on the DBT.

Any ideas on where to start looking for the short? Or even some of ideas on how to look for the short.

Thanks,
Jamie
I would check your output transistors between all combinations of the base, emitter, collector with the diode checker on the multimeter to see if they are shorted or if one is different than the matching part number from the other channel. They should either be open or about .6 volt on diode-checker depending what leads you check, but should not be close to 0 volt or ohms. It could be a shorted driver transistor as well. What resister is burning? I have a schematic, I could take a look.
 
Just a quick update. I changed all the caps on the board and replaced the burned resistor. I also replaced he power capacitors. I can tell someone was in here before me...some of the soldering was crap. I got it all back together and I now have a major short somewhere. 150 watt bulb is full on on the DBT.

Any ideas on where to start looking for the short? Or even some of ideas on how to look for the short.

Thanks,
Jamie
I'm not sure how new you are at this, but just to be clear, those checks are made with the power off. There are good videos on youtube about how to use a diode checker and test transistors.
 
I'm not sure how new you are at this, but just to be clear, those checks are made with the power off. There are good videos on youtube about how to use a diode checker and test transistors.

Thanks for replying. I am pretty new to this, which is why I started with a JC Penney unit.

The transistors are new, but I checked them anyway. The right channel 2n3055 reads .044v from collector to base and .074v from collector to emitter. The left channel reads .593v from collector to base and .591v from collector to emitter. The other two output transistors are equivalent readings...much lower on the right channel.

Unfortunately, I don’t which readings are good. The left channel never worked since I bought it and the right channel blew an output transistor which caused the resistor to burn up (since fixed).

Appreciate all the help,
Jamie
 
Thanks for replying. I am pretty new to this, which is why I started with a JC Penney unit.

The transistors are new, but I checked them anyway. The right channel 2n3055 reads .044v from collector to base and .074v from collector to emitter. The left channel reads .593v from collector to base and .591v from collector to emitter. The other two output transistors are equivalent readings...much lower on the right channel.

Unfortunately, I don’t which readings are good. The left channel never worked since I bought it and the right channel blew an output transistor which caused the resistor to burn up (since fixed).

Appreciate all the help,
Jamie
Seems to me that the right channel is shorted or something in the circuit is. Might have to remove them to test independently. Is the resistor one of the output resistors, what number is it? The bias adjustment could be the death of the transistor. They are touchy on those models and blow transistors if current is set high. The pot needs to be adjusted so the bias transistor is on 3/4 and current bypasses the transistors. Then you slowly turn the pot/bias transistor low and increase the current in the output transistors until at spec(4mV from emitter to emitter on output pair). Which way to turn I don't know. It would have to be figured by checking pot resistance and matching it up with the schematic.
 
Also make sure everything is in the circuit if you power on this thing. If you power it on without the output transistors in the circuit and have a load or speaker hooked up, it will overload the driver transistor circuit and probably blow transistors and resistors. I learned on an MCS 3245 and found out a lot of this stuff the hard way. Went through some transistors before figuring out how touchy and capable that bias potentiometer is. Compared to my Soundcraftsmen amps that don't change the current that greatly from the pot adjustment and probably don't have much chance of destroying things from that point.
 
Thanks for the great information!

The resistors I replaced were all four...2n3055 and MJE2955. One each per channel.

I tested the pots in circuit and think the right channel pot may be messed up. They are 1k variable resistors. These were the readings:

Left channel
Pot turned all the way to the right
Across the bottom legs: 1.72k ohm
Left leg to the middle leg: .7 ohm
Right leg to middle: 1.72k ohm

When I turn the dial all the way on the other side, I get:
Across bottom legs: 1.72k ohm
Left leg to middle leg: 1.72k ohm
Right leg to middle leg: .6 ohm

The right channel pot was a bit wonky. Across the bottom legs I always got 1.25k ohm. When I moved th dial, it reads from 11.9k ohm and sometimes up to 21m ohm. Never consistent.

Could the pot be the problem or could it be something in that circuit?

Thank you,
Jamie
 
Thanks for the great information!

The resistors I replaced were all four...2n3055 and MJE2955. One each per channel.

I tested the pots in circuit and think the right channel pot may be messed up. They are 1k variable resistors. These were the readings:

Left channel
Pot turned all the way to the right
Across the bottom legs: 1.72k ohm
Left leg to the middle leg: .7 ohm
Right leg to middle: 1.72k ohm

When I turn the dial all the way on the other side, I get:
Across bottom legs: 1.72k ohm
Left leg to middle leg: 1.72k ohm
Right leg to middle leg: .6 ohm

The right channel pot was a bit wonky. Across the bottom legs I always got 1.25k ohm. When I moved th dial, it reads from 11.9k ohm and sometimes up to 21m ohm. Never consistent.

Could the pot be the problem or could it be something in that circuit?

Thank you,
Jamie
It could be the problem. If it goes high or open, the bias transistor shuts off and sends full current through the transistors and shorts them out. I would remove the pot and test it again and replace if it seems at all bad. Might be something deoxit or good contact cleaner could fix as well. I have a 3235 showing up this week that kicks out the protection circuit breaker. Probably output transistors shorted through like yours. Will be interesting to see what it is since the 3235 and 3233 are basically the same receiver in a different box. We might be fixing the same problem at the same time.
 
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Oh and the other pot seems to be good. To set them before you turn it on, I would say set it so it's 200-300 ohm to the positive side to center of the pot and the remaining 1.4k-1.5k ohm center to the negative side. That way the NPN bias transistor is on and bypassing current and safely set lower than it needs to be. The positive side of the pot is the side towards the outer edge of the board and negative is the lead towards the center combined grounds. You probably noticed it is a mirror image type of circuit and printed board. If you can do that and it stops blowing transistors, then you can adjust the bias to spec.
And you're welcome, it's fun to help on these things. I get to find out new things and hopefully save you a few dollars along the way.
 
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I just noticed that picture of the burned resistor. I would also check the driver transistors the same way you did the power transistors with the diode checker and resistance. I'm thinking Q510 is shorted through as well since it feeds that resistor that burned.
 
Thanks for all your help!

I will pull the pot and check it off the board. I might as well replace the power transistors while I’m at it.

Let me know how your 3235 is when you get it. Would be interested to know if it’s the same issue.

Jamie
 
Well I pulled both pots off and the right one is definitely bad. It’s a 2k pot and it was reading upwards of 40k at points. The left channel was nice and smooth never hitting too much over 2k. I will replace them both.
 
The right channel 2n3055 reads .044v from collector to base and .074v from collector to emitter. The left channel reads .593v from collector to base and .591v from collector to emitter. The other two output transistors are equivalent readings...much lower on the right channel.
Readings highlighted in red do not sound right. Readings highlighted in blue seem OK.
 
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