Anyone want to help me troubleshoot a Polytone 104 "Maxi-Brute" guitar amp?

[Paul Marossy]

First, I'm used to messing with tube amps but I have built, troubleshooted and reverse engineered enough guitar pedals over the years to be comfortable enough to mess with a SS guitar amp.

This is a Polytone Model 104 "Maxi-Brute". It has (4) 10" speakers. It was made in 1979. When I acquired it the reverb, tremolo and octave filter were all non-functional. I got the reverb mostly functional, appeared to be a bad LM301 opamp in the recovery circuit. Also replaced all of the 10uF caps in both PA-378 power supplies, most of them were bad. That helped to improve some of the other weird issues.

So the main issue remaining is the tremolo and octave filter refuse to work. Schematics are are indicating that all of the FETs are NSKE4093. I don't have any on hand but have some on the way. Before I ordered those, I decided to socket all six of the FETs. I tried some 2N5952s in the tremolo section and then for the first time I had FULL volume out of the amp but when I switch the amp to stereo, I get a very loud thump out of the speakers but no guitar. The thump is so loud that I am afraid it will damage the speakers. That at least confirmed that the LFO is indeed working, but how the heck am I getting the loud thump? I am assuming that this is all due to the FETs, and in fact everywhere there is an issue in the circuitry, there is an FET involved. J201s do not work, nor do 2N5457. I can only get stereo mode and full volume out of the amp with the 2N5952s. So based on that, I am convinced the problem(s) lie with the FETs.

So what I think is going on is that, somehow, the wrong FETs are in this amp. It has J113s in it, no NSKE4093s anywhere (aka 2N4093). I am guessing that for whatever reason, all of the original FETs were replaced with the J113s. It makes no sense at all to me why someone would have done that. There really is no equivalent for a 2N4093 as far as I can tell... seems to be in a league of its own. Someone had cut off all of the pins from the second opamp of the RC4739 associated with the tremolo section, which is a weird 14-pin dual opamp package I ain't never seen before. Anyway, it's clear that someone was in here before me doing some bizarre modifications.

Any comments and/or suggestions are welcomed. Thanks...

PS - I drew up a PCB layout so I could follow what was connected to what. I can make it available to anyone that may need it.

[g1]

Please post the schematic you are using.

[Paul Marossy]

Please post the schematic you are using.

Here you go. For the first time ever in my 20+ years of doing this stuff, I paid money for schematics. I was not able to find them anywhere online for free. Any info on these Polytone amps seems to be pretty sparse.  I really want to get this thing working because the tremolo and octave divider were the two intriguing features that attracted me to the amp in the first place. I was not expecting a total basket case. 

Anyway, my Maxi-Brute has the PA-378 power amp in it. Schematics seem to match the PCB pretty much except for a few parts that were apparently added to the feedback loop of U8B (cap & resistor to ground) and a 10K resistor at the +15.6V power supply to part of the LFO circuitry (at D15). U12B is the opamp where someone deliberately cut off pins 8, 9 & 13. The "power amp out" on pin 13 of U12B does not connect to anything. I think it's that way deliberately, I see no evidence of that being tampered with. I assume that this opamp is needed for the LFO to function properly? I have a working opamp there now, and the symptoms I described all were with an intact RC4739 in U12 socket.

[dmeek]

Very interesting and complicated schematic, will take some time to understand it.
Do you have the -7,5V on U12 pin 1? Check that the speed jack tip and ring contacts
are shorting when unplugged. There should be some square wave frequency
at U3 pin 6 and U9A pins 3 and 4.

[Paul Marossy]

Very interesting and complicated schematic, will take some time to understand it.
Do you have the -7,5V on U12 pin 1? Check that the speed jack tip and ring contacts
are shorting when unplugged. There should be some square wave frequency
at U3 pin 6 and U9A pins 3 and 4.

I did check for -7.5V supply, that's working. I also checked and cleaned ALL of those switched jacks. I'll check for sq waves on those pins when I get a chance. Have this pesky thing called "work" that I have to do ATM. 

[J M Fahey]

WEIRD amp.

Not sure what they were trying to achieve?

Just (wild) guessing: maybe they wee accused of being way too conservative? Which is not bad at all in the Jazz Guitar community they are so popular in.

So they hired an Engineer who decided to add some special effects to flavour the sauce?

Only he went wild and chose 2 barely usable effects nobody else does, either as a pedal not even less "fixed" inside an amp.

Octave divider is a "use once for 1 song and forget it" effect type, can´t wrap my head about the "harmonic divider" or something.
If they wanted to be "unique", they certainly achieved that.

Is there a YT demo showing them?

Maybe somebody disliked them so much that he lobotomized them by cutting IC pins OFF?

Usually against blind shotgunning, much prefer to test circuit functionality but here I have no clue as to what they are even supposed to do, so guess best course of action is just to **carefully** remove suspect or visibly mutilated parts in that area, PAMPER THE IRREPLACEABLE PCB , replace them with fresh ones.
Then cross your fingers, pray, and hope for the best.

Lighting some black candles or stroking a rabbit´s foot might help

Worst case, you still have a loud and otherwise functional Polytone amp.

Used to the ultra compact cubes, can´t imagine a 4 x 10" one, can you post some pictures?

[Paul Marossy]

Usually against blind shotgunning, much prefer to test circuit functionality but here I have no clue as to what they are even supposed to do, so guess best course of action is just to **carefully** remove suspect or visibly mutilated parts in that area, PAMPER THE IRREPLACEABLE PCB , replace them with fresh ones.

Yeah this is not the approach I would normally take but after studying the schematic for quite a long time, I came to the conclusion that I either had a bad FET and/or a bad IC chip somewhere. I got replacements for all of the active components in the amp and thru the process of elimination I arrived at the only possible thing it could be is the FETs, and indeed they are not the same as what the schematic calls for - and there is apparently no equivalent to a 2N4093 that I can find. I wish this amp could talk to me and tell me WTF happened to it! 

Then cross your fingers, pray, and hope for the best.
Used to the ultra compact cubes, can´t imagine a 4 x 10" one, can you post some pictures?

Will do when I get a chance. After today I have 12 days off so I won't have work getting in the way.

[dmeek]

Here are a few notes. I breadboarded part of the circuit.
2N4093 is available at Mouser and Digi-Key though not cheap.
-if that's the problem.

You cannot view this attachment.

[Paul Marossy]

Here are a few notes. I breadboarded part of the circuit.
2N4093 is available at Mouser and Digi-Key though not cheap.
-if that's the problem.

You cannot view this attachment.

Thank you! That will help me to know if the waveforms I should be seeing are present, and if they look like that. I am pretty sure the LFO is working, but will verify all that. Problem is getting signal to the power amps. Currently it's just a very loud thumping and nothing else. Makes me think the FETs are not operating correctly.

That is basically the signal path to the speakers, although there are no speaker jacks, the speakers are connected directly to the power amp with slip on wire terminals (don't know what the technical name is for those). I know when both power amps are getting a signal because the amp is hella loud. Otherwise, at full volume I can easily talk over it. Not very loud.

I have some TO-18 2N4093s on the way. They have to make it from east coast to here in NV... hoping to have them by Saturday. Pretty sure the problem is that the wrong FETs are in this amp. Everywhere something is not working at all, or not working optimally, there's an FET involved.

[J M Fahey]

Don´t worry.

2N4093 is a very common plain vanilla NCH FET used only as a switch, easy peasy, even the humblest FET can do that.

They used it because it was common way back then, but many modern cheap ones can do its job.

Metallic case original ones are still available by the thousands, that´s how common they were, but they charge "museum" prices, Mouser asks $6.61 for the ones they have in stock, no need to pay that much.
https://www.mouser.com/c/?q=2n4093&sort=pricing

I used them a lot, but the plastic cased ones, called PN4093 ("part number"4093 go figure) and still have a few stashed somewhere.

Mouser has them for all of 50 cents each.
Currently non stocked but that means they are still very much in production, so "others" will have them (Digikey, Farnell, etc.)
https://www.mouser.com/c/semiconductors/discrete-semiconductors/transistors/?series=PN4093&srsltid=AfmBOorb2Ku6gas7wqsBRPXCaV81SfgDvib-XI8UhUFmXkH9SZziRHj7

Just as an example: Digikey *Spain* has them for 0.3 Euro, about U$ 40 cents.
https://www.digikey.es/es/products/detail/rochester-electronics-llc/PN4093/11528016?srsltid=AfmBOooSZvOLeFKtGcpW6k2pBfgfvbsziL3x0ZvG1MZVAJaBudBRKtMj
Guess Digikey US also has them.

But not even that, most *any* N channel plastic FET can do that audio switching job (they are usually advertised as "choppers"); I would use any of these, only paying attention to pinout.
Write D G S labels on PCB with fine point Sharpie but in any case, I would leave them as last ones, it is very unlikely they failed.

This is the modern widely available FET I suggest:
https://www.mouser.com/ProductDetail/onsemi-Fairchild/J112?qs=ljbEvF4DwONoDl3CPnuhug%3D%3D

Stock:
    39,608 Can Ship Immediately 

ONSEMI © Semiconductor Components Industries, LLC, 1997
March, 2023 − Rev. 5
1 Publication Order Number:
MMBFJ113/D
N-Channel Switch
J111, J112, J113,
MMBFJ111, MMBFJ112,
MMBFJ113
Features
• This Device is Designed for Low Level Analog Switching, Sample
and Hold Circuits and Chopper Stabilized Amplifiers
• Sourced from Process 51
• Source & Drain are Interchangeable
• These are Pb−Free Devices
As usual, doublecheck pinout matching.

I would start with ICs first, check connections, replace what´s *missing*, cheek cracked solder pads and tracks, etc. , before replacing perfectly good parts.

Again: pamper the PCB, use good solder wick to fully clean pads before removing parts, etc.

PS: Using N Channel FETs as switches was much the rage in the 70s 80s (then people started using analog switching ICs such as 4016 4093 etc.) , this substitution I suggest also works on old Acoustic amps and many more.

EDIT:s ad to know you bought the expensive OEM ones.

I do NOT suspect them, zero stress applied to them, but instead they may not be getting the proper control voltages, or there are cracked/broken tracks or pads in the Audio path.
Any connectors too.

[Paul Marossy]

Don´t worry.

2N4093 is a very common plain vanilla NCH FET used only as a switch, easy peasy, even the humblest FET can do that.

Yeah that was my thinking but it does not work at all with 2N5457, J201, or the J113s that are in it now! It works sort of with 2N5952s but it's still not right.

But not even that, most *any* N channel plastic FET can do that audio switching job (they are usually advertised as "choppers"); I would use any of these, only paying attention to pinout.
Write D G S labels on PCB with fine point Sharpie but in any case, I would leave them as last ones, it is very unlikely they failed.

That was my thinking, any FET ought to work but that has not been my experience in this case.

This is the modern widely available FET I suggest:
https://www.mouser.com/ProductDetail/onsemi-Fairchild/J112?qs=ljbEvF4DwONoDl3CPnuhug%3D%3D

Stock:
    39,608 Can Ship Immediately 

ONSEMI © Semiconductor Components Industries, LLC, 1997
March, 2023 − Rev. 5
1 Publication Order Number:
MMBFJ113/D
N-Channel Switch
J111, J112, J113,
MMBFJ111, MMBFJ112,
MMBFJ113
Features
• This Device is Designed for Low Level Analog Switching, Sample
and Hold Circuits and Chopper Stabilized Amplifiers
• Sourced from Process 51
• Source & Drain are Interchangeable
• These are Pb−Free Devices
As usual, doublecheck pinout matching.

J113s are what is in it now and it does not work!

I would start with ICs first, check connections, replace what´s *missing*, cheek cracked solder pads and tracks, etc. , before replacing perfectly good parts.

All of the ICs were in sockets and I've already checked them all.

Again: pamper the PCB, use good solder wick to fully clean pads before removing parts, etc.

Preachin' to the choir bro. ;-) Not my first rodeo. Yeah, been using de-soldering braid and being very careful. I've worked on far more delicate things. The main PCB is fairly robust but the power supply PCB not so much.

I do NOT suspect them, zero stress applied to them, but instead they may not be getting the proper control voltages, or there are cracked/broken tracks or pads in the Audio path.
Any connectors too.

When I started on this project I had no reason to suspect the FETs but thru the process of elimination I have come to suspect them. I'm getting all the right voltages. Already have looked for broken tracks, etc. but maybe I'll do that for a fourth time just for the heck of it. :-D

Attached is the data sheet I have for PN4093. 2N5952 seems closest but still not quite right. I have not found anything that is a direct replacement for a 2N4093.

[dmeek]

What are the voltages when Stereo/Mono is switched with the fets removed?
The datasheet says -1V to -5V is the pinch-off voltage.

You cannot view this attachment.

[J M Fahey]

Yeah that was my thinking but it does not work at all with 2N5457, J201, or the J113s that are in it now! It works sort of with 2N5952s but it's still not right.

That was my thinking, any FET ought to work but that has not been my experience in this case.


J113s are what is in it now and it does not work!

When I started on this project I had no reason to suspect the FETs but thru the process of elimination I have come to suspect them. I'm getting all the right voltages. Already have looked for broken tracks, etc. but maybe I'll do that for a fourth time just for the heck of it. :-D

Attached is the data sheet I have for PN4093. 2N5952 seems closest but still not quite right. I have not found anything that is a direct replacement for a 2N4093.

Just thinking aloud: then maybe you do not have a FET problem but something else.

These FETs do not amplify or create any effect or anything, they just let Audio through or not.

Here is one which is easy to test functionally, Q2.
Switch Distortion  ON/OFF.
Does that work?
IF so, Q2 is working.

Even more, I would inject 100mV 1kHz at input and check for signal level/presence/absence on nodes C21/R19 and on U5 pin 6




If not: does voltage change on nodes R20/D10 or R20/R21?
I would expect 0V (Dist ON) or -15V (Dist OFF)

What dmeek already suggested.

Notice FET is switched ON by merely applying 0V to gate and OFF by applying -15V to it, which is WAY higher than needed to turn ANY FET OFF

I reluctantly suggested shotgunning at first but clearly we are getting back into actual troubleshooting.

Maybe because it works? 

[g1]

Agree with JM that there may be other issues rather than the fets which are just being used as switches.

A jfet is low resistance (or 'on') in a non-powered state (or when no voltage applied at gate).  These are N-channel so they require a negative voltage at their gate pin to turn them 'off'.
Source to drain resistance should measure less than 100 ohms with amp un-powered.
Removing the fet from the circuit will simulate the fet in it's 'off' state (which is high resistance from source to drain).

[dmeek]

I'm going the other way, thinking it is the FETs. The thump when switching he mentioned happens when the gate voltage is leaking into the drain-source. Though I don't see why any
garden variety N-CH FET wouldn't work in this circuit.

The FET on the left bypasses the effect, the one on the right monos the two amps.
Still don't know where the output of U12B is going.

I'm imagining they were trying to make a kind of stereo chorus effect but didn't want to
use an analog delay chip. Then when it didn't sound like a chorus they made up the story about the "harmonic divider"