MCS 3233 Left Channel Blowing Fuse

[Keneye]

Bought a junker MCS 3233 to try to learn some basics. Left channel keeps blowing 3amp fuse. BUT only if a speaker is hooked up to that channel. Right channel ok. Plays on the right channel and stereo indictator lights up. So what does that mean? No obvious wires touching to create a short. Main transistors? Thanks for any input.

 

[MCS Guy]

Do you have a DC volt meter handy? What is the voltage across the left speaker terminals? It should be less than 100mV. If you see +33 or -33 volts, one of the outputs are blown.

 

[Keneye]

Hi, saw online how to check output transistors. Both left channel ones are shorted. But one right one is too. Yet that channel plays. By shorted I mean if I hook up a continuity light, I get a light on 3 of the 4 output transistors. I will meter tomorrow. But this brings up another issue. Does the issue begin with the output transistor or does something else failing take them out? I have no problem ordering the transistors but don't want to install them and just blow them out again. Thanks for your tip.

 

[MCS Guy]

Its usually just the outputs transistors that blow, BUT you should check the drivers and the bias darlington transistors. If your drivers are also bad, then you will need to check the pri-driver and differential pair. Also look for overheated resistors and bad diodes (there are 2, D501 is a 12V, 500mA zeener and D502 is a regular diode).

The outputs on the 3233/3235 are very common MJ2955/MJ3055's that are still available. The Bias darlington is a MPSA13 and the drivers are a 2N6551/2N6554 by Central semi. The drivers can be substituted with On semi BD237/BD238, but the pin outs are different (ON semi's are E-C-B and the originals are E-B-C). The pri-driver is a MPSA06 and the differential pairs are BC557 (they need to be closely matched if replaced).

80% of the time, its just the outputs.

The 3233 like to blow outputs if driven hard, abused, shorted speakers/wires or bias is not correctly set. Bias needs to be set to 6.6mV with the amp on for at least 30 minutes. VR 501/502 are the potentiometers for bias and it is measured between the emitters of the output transistor pair. Bias is critical on these due to the poor heatsink they have.

Once up and running, they sound pretty good.

 

[Keneye]

Thanks! Bought this for $10 off of Craigslist. The output transistor board is missing its screws, so the seller did attempt to fix this. Luckily it does not look like he did any mods or work. I did notice, when you turn it off, you can hear a cap or caps discharge. So I will check main filter caps too. I have all the toys, Isolation transformer, variac, meters including an in circuit cap tester. Just trying to learn how and when to use them. Luckily forums and the many Youtube channels help. But it is nice to talk to someone once in a while when an issue arrises. Thanks again for the knowledge.

 

[Skywatcher]

Thanks! Bought this for $10 off of Craigslist. The output transistor board is missing its screws, so the seller did attempt to fix this. Luckily it does not look like he did any mods or work. I did notice, when you turn it off, you can hear a cap or caps discharge. So I will check main filter caps too. I have all the toys, Isolation transformer, variac, meters including an in circuit cap tester. Just trying to learn how and when to use them. Luckily forums and the many Youtube channels help. But it is nice to talk to someone once in a while when an issue arrises. Thanks again for the knowledge.

Check the driver transistors and the ceramic caps on them. It sounds like either a driver has gone rogue, or the ceramic cap is bad causing oscillation under load. There is no speaker protection or relay on these so you will hear sound as the caps discharge, and a "thump" on turn-on. Use an 8 Ohm dummy load instead of speakers so if there is DC out, you won't blow a speaker.

 

[Keneye]

Thanks for the info. Waiting for parts. How do you do a dummy load? I do hear the caps discharge when turning off. But no thump when turning on.

 

[Skywatcher]

Thanks for the info. Waiting for parts. How do you do a dummy load? I do hear the caps discharge when turning off. But no thump when turning on.

For testing only, use an 8 Ohm resistor across the speaker terminals (one on each channel) instead of speakers. I have a pair of 100 watt wirewound resistors for "smaller" stuff, and 300 watt for "bigger" stuff. 10 or 20 watt will be fine for most things if you don't plan on turning up the volume any more than a smidge or they get real hot. For idling and setting BIAS or DC offset, small 10 watt resistors are fine. But if you're gonna buy a pair, ya might as well get something bigger anyways.. I don't have the schematic handy, but if you don't hear a turn-on thump, I am guessing it has some sort of "soft start" circuit to prevent that. I am pretty sure this unit was made by Technics (Taiwan), or possibly NEC. Perhaps someone knows for sure and can point to the service manual or schematic for download.

 

[Keneye]

Hi, based on your feedback, I Googled it. D-Lab on youtube has a great video on constructing a dummy load with schematic. Thanks for all your help!

 

[Skywatcher]

Hi, based on your feedback, I Googled it. D-Lab on youtube has a great video on constructing a dummy load with schematic. Thanks for all your help!

Terry is quite the "character" He likes his wine.

 

[Bill Seward]

Do you have a DC volt meter handy? What is the voltage across the left speaker terminals? It should be less than 100mV. If you see +33 or -33 volts, one of the outputs are blown.

MCS Guy, ive got a 3233 and all ibe seen on here is the left channel is the problem, so i have a 3233 and my right side is scratchy and low volume or nothing at all, would that be a output transistor, curious and thanks, Bill

 

[Keneye]

MCS Guy, ive got a 3233 and all ibe seen on here is the left channel is the problem, so i have a 3233 and my right side is scratchy and low volume or nothing at all, would that be a output transistor, curious and thanks, Bill

I had this issue before I knew anything about repairs. So a tech fixed it. Not sure what ge did. But now I routinely replace the caps and transistors in these old units. You can test a Transistor with a meter. Or just swap them and see if the problem moves.