Polytone Studio 212 Power amp repair - Sourcing NSDU57 NSDU07

[DrGonz78]

So I am repairing this Polytone and it is a stereo amp. There are two individual power amp modules in the amp, but one of them needs repair. Something caused the MJ802 to short it's base to the collector. I ordered the replacement output transistors MJ802G and MJ4502G where the G indicates PB free. The drivers are shorted out too. After searching high and low I cannot find a good replacement part for the NSDU57 and NSDU07 drivers, they are National brand plus obsolete. So I looked into the MPSU57 AND MPSU07 type replacements and they too are really expensive. Only found some on Ebay too.

Any have ideas what other transistors might work in this application to replace the NSDU57/NSDU07 drivers?

The two schematics attached represent the power module I am working on but a bit different. The polytone 378 shows the LM391IN chip and mine has that one too. My power module is using MJ802/MJ4502 and not the 2N5880/2N5882 output transistors.

[Enzo]

The little "case 152" tab tops from Moto. They went away long ago. And the odd base lead in the center format. I think the MJE243/253 pair ought to work, and any number of TO220 like TIP31C/32C. You'd have to mount them sideways to get the legs in the right holes.


And why not call Polytone and ask if they have a recommended replacement part?

[pontiacpete]

I used the suggested mj182 and 172 on the left of the page, they worked fine.
Although another polytone that came in once with the LM391 presented problems

[52 Bill]

Polytone used both the 2N5880/2N5882 and MJ802/MJ4502 at different times, but all will work with the MJ output transistors. As the others have stated, almost any medium power transistor will work as driver replacements. We used to use the TIP series transistors, but the MJE transistors fit the board better. Be sure to mount the drivers in contact with the two bias diodes. We used to use heat sink grease and a cable tie to pull them together. Also be sure to check all of the emitter resistors for the drivers and the outputs.

If your version of the power amp uses the LM391 chip, I'd expect the chip to be damaged as well. It is probably the weakest link in that circuit design. It was fairly common to have to replace outputs, drivers and the driver chip when they went into meltdown mode. I was so glad to see them get rid of that power amp design.

Be sure to power up with a light bulb limiter when you power up the amp. In fact it was these amps that got us started using light bulb limiters.

[DrGonz78]

Great information! Yeah I was looking at that LM391 chip and reading the schematic, pretty sure it got hit hard too. The amp came to me with all the screws floating around inside the amp and the power modules were not screwed together to the cab, they were just floating around. Reverb spring was just floating around inside completely detached from the tank. Thank god the preamp is working great and the amp sounded great with just one power amp running the one speaker. I will no doubt ask for more advice on this amp as I get it all together. Bias settings is one question that will come up later. As Enzo said I should try calling Polytone and I will do just that tomorrow. Try to get as much info from them as possible. Thanks again!

[52 Bill]

If the problem occured because a loose screw shorted something out, you might get lucky and not need the driver chip. I've seen the heat sinks come loose from the power amp chassis, the screws that hold the preamp chassis come loose and fall into the amp, and of course the speaker holding T-nuts will always strip out of the front panel and fall into the amp.

We never had much luck in talking to anyone there unless we were ordering amps. But that was 25 years ago when the amp you are working on was new. Maybe it's better now. We sold a lot of those amps for a while there, but we also had a lot of them come back for repairs. I always assumed that a big part of the problem was the fact that the power amp sat inside a closed speaker cabinet with no way for the heat to dissapate. I remember going to a cousin's wedding which was held in a nice sunny garden. It was probably 85 degrees and sunny. The band was set up under a small tent and I couldn't help but notice that the bassist was using a MiniBrute. The amp was sitting in the sun baking away. I half expected the amp to blow up, but it never did. Well at least not that day.

If you do need a driver chip, PM me and I'll look through my parts, I probably still have one.

[J M Fahey]

Agree, I'd use BD139/140 because I have them one yard away, but pinout does not match , so a little leg bending is necessary.
I'm also worried about the LM391.
I would never ever buy a Hong Kong one on EBay.

[DrGonz78]

It appears to me that someone had tried to fix or at least was in the process of working on this amp when it came to me. The screws inside the amp were put there in a paint cap so they would not get lost and the speaker was disconnected. So I am pretty sure the screws were not the culprit to initial power amp failure.

When the power amp blew out it took with it the emitter resistors(I think that is what they are called 0.15ohm 5 watt cement wire wounds.) Measuring them I read open on my meter, so they are definitely fried. I have TIP31C/32C in my shop so I will probably go with those as my driver. I also am thinking the LM391 chip is toast too, since there is a fried resistor R17 that leads right into pin 13. So just getting to the point of putting all the parts in but slow to go until I have all dead parts replaced. Thanks again all for the suggestions on part replacements and this thread is far from over.

[DrGonz78]

Okay I have finally started to finish repair on this power amp module. I have new output transistors installed and all the emitter resistors replaced. I went with TIP31c/32c and bent the legs around to get them to the correct pin orientation. Thanks so much to Bill for helping out with getting a LM391N chip, very cool.

I am attaching a pic of a connector that is screwed down on the collector side of the mj4502 transistor. What is this clip used for on this power amp? When disconnecting it there was some thermal grease on it, and I am not sure if I need to add a dab of heatsink compound. My guess is that the grease was just there from some over flow from the transistor. Anyhow just have not had an amp that used this type of clip so it is interesting.

So I am gonna double check everything tomorrow and start some checks on the amp. When I get to biasing the amp what is the correct procedure for this amp? I know about about testing for voltage across the emitters and that sort. However, could anyone here give me some pointers on this matter? Figuring you guys have experience with this exact type of power amp and I have none.

Thanks again to all contributers to this thread and especially Bill...!!

[Jazz P Bass]

That device is tracking the temperature.
If that is where it was when you disassembled the amp, then put it back with a small smear of thermal compound.

It could be a diode.
Or a thermistor.

There is a note on the 378B schematic about D3 & D4 must contact Q ,Q (Q3 & Q4?)
But there are two & you show one, so it may be something else that is not shown on the schematic.
Or do you have two sensors?

[DrGonz78]

Yeah it's just one sensor Jazz. My board is the PA378A one and yeah the schematics I have found don't really have to many extra notes. I have two identical amp modules in this stereo amp and the other one is working. So I can always go to the other one as a good point of reference.

[DrGonz78]

So I got testing some voltages on the amp and checking the bias. Right away it was drawing a bit of current and I had 94mv and 92mv across each emitter resistor. So that seemed quite high so I adjusted the bias trimpot a bit and it basically just ends up on 0v across each emitter resistor. That is identical to the other amp module. The main thing that confuses me is I started at 40, 60, and 100watt bulb in the limiter. 40 watt shined brightest and 100 watt was very dim, so that seemed fine. Although when I brought down the bias none of the light bulbs light up at all. Why would the light not shine at all? Only when the repaired amp's module bias was set high did the light bulb even light, is that normal? This behavior was the same for any wattage of bulb that I used.

MJ802
E= -33.23v
B=-33.29v
C= -0.3mv

MJ4502
E= 33.2v
B= 33.2v
C= 20.4mv

Anyway I get about 8mv on the output and that seems fine. I can say that the other power amp module reads 0v across emitters and I get 2.3mv on the output. I am pretty sure it is time to pull it off the light bulb limiter and start signal testing. I know that to bias the amp it will take hooking up the scope if I want to get it just right. Too late tonight so will have to continue testing this tomorrow.

Edit: One more question. When I test signal through the amp can I just hook a scope up without a load? Also, when I do hook up a load for testing do I need a load on each power amp output? I figure the amp worked fine with the other speaker disconnected so it must be fine. When ever I get stereo amps it gets me a bit confused...

[g1]

The bulb does not light because the current through it is so small with the bias turned to 0. If you checked current through the mains fuse it would be very small.
You don't need a load for signal checking, just for power output tests and biasing. You only need to load one side at a time.

[DrGonz78]

So I was happy with the sound of the working side of the amp the way it was biased. I elected to basically match my readings of the working amp to the repaired one. So far so good, sine wave is basically the same and the readings across each emitter resistor are in good balance. Slight notch of cross over distortion on the scope indicates the good amp is a bit underbiased. I agree with Juan about underbiasing the amp a bit, especially with these 100-120watt amps. I am doing a little repair work on the speakers and then tomorrow will do some louder speaker testing. After that time to put the beast all back together. Thanks for all the help along the way with this one!

[DrGonz78]

Well the good news is the power amps are both working. The bad news is now I have a preamp problem, go figure. So, it is very low signal coming out of the preamp now with a lot of white noise. 100% positive that it is from the preamp so divide and conquer begins now. My guess right off the bat is that I might have overloaded the signal for testing and fried an opamp, but thats just a guess. Oh well back to the drawing board to research all these chips. I will try to hunt down a schematic and will probably have to call Polytone to find one. Might have to start a new thread later for help on the preamp.