Polytone Studio 212 Power amp repair - Sourcing NSDU57 NSDU07

[DrGonz78]

NASA? We have lift off!!! Turned out to be just the damn CABLE that runs the test signal from computer to the amp, thank GOD! Well the amp sounds great and everything is working good. Got me really worried that I might have blown out an opamp and started researching those RC4739DB chips. Those chips are really hard to source reliably too.

[Enzo]

Think about it. What would it take to "overload" an op amp and fry it? And how many times have you ever seen it? I never have. I don't count things like plugging the speaker cord from another amp into the input.

A signal is just a signal. The op amp is good out to its power rails. So what does your preamp run on? I am guessing 15v. That means your signal has to hit 15v peaks to even clip. That is over 10vRMS. You COULD get over that on the input stage. If you put in over 15v, then it would push the IC beyond its rails. That IS bad for the IC, as far as the IC is concerned it is like applying reverse voltages. Look at just about any Peavey solid state amp, and the inputs will have clamping diodes across each input to prevent exactly that. That is a pair of diodes wired from the signal input to each power rail and wired in reverse bias. The few Polytone schematics I have looked at lacked such clamps, but if yours has them, it is further protection.

And that is only the input. All the following stages are already limited to rails. The op amp only has the 15v rails to work with, so they have no way to push signal past that.

SO if you indeed crammed 15volts and more of signal into the poor thing, it could have been damaged, but mostly, all over large signals do is clip. Your working amp is evidence. That is why it is amusing when someone writes in he thinks he may have turned his fuzztone up too high and blown his preamp.

[52 Bill]

Got me really worried that I might have blown out an opamp and started researching those RC4739DB chips. Those chips are really hard to source reliably too.

But at least they are just dual opamps and with a little rewiring a standard 8 pin can be used as a replacement. In fact i think that PAIA used to make an adapter board/socket to do this.

Anyway, glad that you got it working again.

[DrGonz78]

One update about this amp... I have spent some time fixing the reverb circuit as it there was a bad connection on the preamp board, got it working. The amp has a modulator which I would assume is a chorus function or something. Oh well without a schematic that part of the amp is really hard to trace out and what is a Polytone mod or just a hack job. The amp obviously suffered from someone tinkering with it over the years. There is even a burn mark on a square shaped film cap, so somebody was a bit clumsy. Anyhow my main observation to report is that the repaired power amp module hisses a bit, almost like a hum. Here are my notes that I took while working on the amp a couple of days ago.

Notes: Basically, if I unplug the RCA plug(input signal) to the repaired amp the noise goes away. So right there I can say the noise is not coming from filter caps or something on that amp module. I then think that perhaps the signal from the preamp is causing the noise and I try plugging the RCA into the other amp, no noise. So it's not the preamp causing the noise and I begin to think it might be electrolytic caps that surround the LM391 chip that drives signal into the power amp. So I plug in either RCA preamp input plug into the repaired amp and no hissing noises. All that done with original working power amp RCA input unplugged. At this point the only time I get the hissing noise is when I plug in both RCA inputs to both power amps. The hissing always comes from the one speaker that is hooked up to the repaired amp module.

So I have checked all my grounds and just can't figure out why I am getting this noise. I have heard that these amps were noisy and perhaps having the two speaker hooked up causes this to happen. Still could be something to do with the modulator not working and this is the end result. I am at a loss. Honestly it's not too bad of a hiss, but could be better.

[Enzo]

Modulator?



Scope the things, what is on the output? You could have instability causing RF oscillation that only occurs when both grounds are connected.

[52 Bill]

Modulator?

I think he's talking about the Harmonic Divider circuit, a sort of Leslie simulator. If I remember correctly it split the signal into highs and lows and them sent the split signals to alternating power amps. Kind of like a Magnatone stereo vibrato.

Later on they came out with the Modulator circuit that used a bucket brigade chip. They used to sell a lot of amps to accordion players, so a Leslie simulator was a good feature for them.

If the amp has the Harmonic Divider, it will have some digital chips like a CD4520 and a CD4023. If you swap the two preamp outputs going to the power amps, does the noise still stay with the repaired power amp? Are both speakers original?

[DrGonz78]

I have been scoping the amp and the signal still looks good with the scope set on 1 volt. Problem does not follow when swapping the input signal between power amps and actually I have new behavior to report. When I first took my notes about the noise primarily coming from the speaker hooked up to repaired amp, it was before I did work on the preamp circuit.

While I had the amp apart I set the bias even colder on the repaired amp. In fact, I turned the bias pot all the way the direction that puts the amp at coldest idle setting. Basically, hooking up both amps to speakers and now the repaired amp speaker is now making the humming noise and it is now on the originally good power amp/speaker. I don't think this has anything at all to do with the preamp as scoping the preamp just shows a nice clean sine wave. I do get a nice clean sine wave on both outputs of the power amps. Can different bias settings make one side more dominant and louder? Before I set my bias lower on the repaired amp it was very dominant and now it is opposite. Kind of interesting. Perhaps it is the old filter caps but there is no hum with signal inputs removed.

So it is too late to try anything else, but tomorrow I am thinking that I should set the bias the same on both power amps. At that point they will be at their coldest points. Perhaps then the blend of the noise will go between each speaker evenly and sound better. Still Enzo's point about RF needs to be investigated too.

Here are some pics of the effects knobs and preamp etc... Not many pics of the guts of this amp online so here are a few. Thanks.

Note: I can see there is noise on the scope of which ever channel now has the hum. I saw that it changed to the other power amp after the bias setting was lowered in my example above. So the noise is shared and these amps are just noisy perhaps. My hope is that balancing the noise will yield a workable solution. If my thinking is correct that bias in each amp needs to be perfect balanced between each module so one speaker is not more dominant.

[ReadyTeddy]

Bias level should not effect "dominance" or perceived loudness. You can check it easily with your scope; put a signal into both amps that will drive the outputs to a few volts peak-to-peak. Then notice if the output level changes with bias. The excess gain of the power amp, that is, the ratio between the open-loop gain and the closed-loop gain, will try to get the amp to jump across the crossover region quickly. That is why crossover distortion usually looks like a quick jump as opposed to a longer-in-time dead zone. Once out of the crossover region, the excess gain should also keep the output of the two amps equal, despite any difference in bias. The power amps should not be biased all the way cold--that will only create crossover distortion, which can sound pretty ugly. Starting from coldest bias, you will see crossover distortion. As you turn up the idle current, you will see the distortion disappear, and then, if you have a distortion analyzer, you will usually see the total harmonic distortion continue to go down as you turn up idle current until it reaches a minimum, at which point the distortion will increase with increasing idle current. There is probably a fairly broad region of idle current at which the amps will work well. At some point, it would be helpful if you could connect resistors in place of the speakers and turn the power up until the amp gets fairly warm, then check the idle current. If it hasn't crept up too high, your bias setting, as long as crossover distortion remains invisible at all normal temperatures, should be just fine.

[DrGonz78]

Thanks for that last post ReadyTeddy. It is definitely not the bias change that made the noise switch speakers. I guess I got confused that the bias had been the key element that changed the behavior. Anyhow before(fixing reverb circuit on preamp) when I would switch the RCA input cables between the amps the noise did not follow. Now the noise is following when switching RCA inputs between power amps, and I swear it did not behave this way before the preamp fix. The noise is now coming through the input that goes to the non-repaired power amp. So I am gonna have to spend some time I guess tracing that line through the preamp and see if there is something that is causing the noise. I have checked my grounds but will double check everything over again.

[52 Bill]

I would be checking grounds for a possible loop causing the hum. The power amps both use the same power transformer and therefore share a single ground. For that matter the preamp supply also shares the same ground so running grounds on all of the audio cables can cause a ground loop.

Did you fix any grounds on the cables? Are the Molex plugs securely connected? Etc.

[DrGonz78]

Yeah all the grounds are looking good, but I am still checking. I am gonna just keep checking ground connections until I am 110% sure. The other aspect of the amp has to do with leslie type modulator. I neglected to see what was connected to the four tweeters. I just assumed that they were connected from somewhere and did not realize till just now that the blue(ground) is connected to first tweeter. The red wire is missing completely and that would probably explain very well why that modulator circuit is not working. The four tweeters are connected in series and it also has an output jack too. I have no idea where that red wire went and it must have been hacked out by someone long ago. I am gonna have to try to call Polytone again, but that has not proven to be very effective. Here is a pick of the four tweeters to get a visual perspective.

Oh and the molex connections seem to be just fine as far as I can tell so far. I also have cleaned the heck out of anything that appeared even remotely oxidized and then checked it with ohms readings.

[DrGonz78]

Just wanted to update on this guy! So with the blue (ground) connection running to one side of the tweeters and nothing connected to positive, I was pretty sure that would be what was causing my ground noise. I got busy with a bunch of repairs so I had to put that thought on the back burner. In the end it turned out to be the culprit. So it must have been introducing some ground noise hum on the one repaired amp side before I fixed the reverb on the preamp circuit. Then it switched outputs to hum only on the other amp and at that point I noticed the tweeter had the negative ground connected. Removed the ground from the negative terminal on the tweeter and problem solved. I also cleaned the heck out of the input RCA terminal plugs and inputs for good measure. Now the amp is as good as I think I am gonna get it for today. I am curious still how the modulator effect works and if is effected on the tweeters. I think someone long ago removed the functionality or the modulator and disconnected the tweeters from the circuit. Oh well the amp still sounds great without the effect anyway.

[52 Bill]

I am curious still how the modulator effect works and if is effected on the tweeters. I think someone long ago removed the functionality or the modulator and disconnected the tweeters from the circuit.

The modulator was supposed to sound like a Leslie/Magnatone vibrato. A big market for Polytone used to be accordion players, who seemed to want to sound like Hammond organs. Later on they created the RS Modulator (Rotary Speaker) that was around a bucket brigade chip. A much better sounding unit.

In all of the years that we sold those amps, we probably sold one with the stereo modulator and harmonic divider.

[prulup]

Hi there, I have a strange problem with this amp (Polytone mini brute, it has 378 power amp schematics). It burned down: R17 100ohm resistor, drivers, LM391, one 0.15 5W resistor, power transistors. I replaced the resistors, the LM391 and the drivers. I checked the voltages with bulb limiter inserted and without the power transistors. I got:

MJE 182 (NPN)
E 0
B 0,6
C 29

MJE 172 (PNP)
E 0
B -0,6
C -29

I know the MJEs have a different pinout than PU57s.
The bias potentiometer works, I can set the -0,6V on the MJE 172.
On the pin 7 of the LM391 is cca 1,2V.

and now the strange thing:
When I put the power transistors the bulb starts to light and I have about 90mA on the power resistors (0.15 ohm). The bias potentiometer has no influence on it. The power supply voltage gets down from +-29 to about +-8V.

When I take the LM391 out and connect the pwr transistors on, I get these voltages:

MJE 182 (NPN)
E 28,9
B 19,8
C 29,4

MJE 172 (PNP)
E 28,8
B 28,2
C -29,2


Any suggestions please...? Thank you

[pontiacpete]

Where did you get your LM391?
I had always suspected the ones I got were counterfeits, so I gave up on the repair