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.
Please post the schematic you are using.
Quote from: g1 on December 17, 2024, 08:18:33 PMPlease 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. :o 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.
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.
Quote from: dmeek on December 18, 2024, 11:47:25 AMVery 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. :cheesy:
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?
Quote from: J M Fahey on December 19, 2024, 08:55:24 AMUsually 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! :lmao:
Quote from: J M Fahey on December 19, 2024, 08:55:24 AMThen 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.
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.
Polytone Maxi-Brute Harmonic Divider.gif
Quote from: dmeek on December 19, 2024, 10:36:53 AMHere 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.
Polytone Maxi-Brute Harmonic Divider.gif
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.
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 :tu:
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.
Quote from: J M Fahey on December 19, 2024, 12:44:16 PMDon´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.
Quote from: J M Fahey on December 19, 2024, 12:44:16 PMBut 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.
Quote from: J M Fahey on December 19, 2024, 12:44:16 PMThis 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 :tu:
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!
Quote from: J M Fahey on December 19, 2024, 12:44:16 PMI 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.
Quote from: J M Fahey on December 19, 2024, 12:44:16 PMAgain: 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.
Quote from: J M Fahey on December 19, 2024, 12:44:16 PMI 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.
What are the voltages when Stereo/Mono is switched with the fets removed?
The datasheet says -1V to -5V is the pinch-off voltage.
Polytone Maxi-Brute.gif
Quote from: Paul Marossy on December 19, 2024, 01:25:26 PMYeah 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
(https://i.postimg.cc/bJJHXmpX/Fetswitch01.gif)
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?
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).
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"
Back in post #2 you noted the cut leads on U12B. This is important. Pins 8 and 13 will
interfere with the FET switching. make sure they are cut or bent out of the socket.
Also I would remove or clip R50 and R51
One thing I noticed on the layout, the fets have the middle leg bent forward, whereas the small bipolars have the middle leg bent to the back.
Since the fets had all been changed out by someone else previously, make sure they are not installed 'backwards'.
Quote from: dmeek on December 19, 2024, 02:47:13 PMWhat are the voltages when Stereo/Mono is switched with the fets removed?
The datasheet says -1V to -5V is the pinch-off voltage.
Polytone Maxi-Brute.gif
I will check that, but let me ask a question first: Is that stereo/mono switch on my PCB drawing wired correctly? It looks like it came from the factory that way but it seems odd to me that in stereo mode it's grounding some of the pins on U11 but in mono mode it's connecting only to +15.6V. I don't quite understand how all that works to be honest. I guess the LFOs are basically being put out of circuit by applying the +15.6V?
I did replace the 6800uF caps in both power supplies tonight, so at least now there is sound in stereo mode but still no tremolo. In mono mode I can hear the LFO bleeding thru, makes a bit of a whirring buzz sound that changes with the speed control. I also got the octave filter to work when I tried different FETs for Q7 & Q8 (2N5952s) but the effect is always on and the footswitch does nothing. Also was not reliable, sometimes on certain notes it would get waaaay louder. Need to revisit that now that the PS filter caps have been replaced.
After the big filter caps in PS were replaced the sound was generally a lot better, more responsive bass. I still have a 120Hz hum/buzz, not horrible but it's there. Wondering if isolated jacks would help with that. Seems like there may also be some ground loop issues going on.
Sure is a lot of strange problems on this amp lol, but it sounds pretty good with the reverb on, although that is kinda noisy. I'm not sure what the purpose of the FET is on the reverb recovery circuit... is that supposed to be like a noise gate or something? I currently have a 2N952 there, I think it sounds better. Will have to play around with that some more.
Quote from: dmeek on December 20, 2024, 09:36:23 AMBack in post #2 you noted the cut leads on U12B. This is important. Pins 8 and 13 will
interfere with the FET switching. make sure they are cut or bent out of the socket.
Also I would remove or clip R50 and R51
Noted! I will undo my fix for U12B and see what results that yields. That U12B area had me confused for a while. Because output of U12B says "to power amp", I thought maybe it was supposed to be connected to something, and someone had been in there before me messing with something. In any case, I had come to the conclusion that it had been left unused. I wonder why on earth did they do it like that? Perhaps they didn't know any better? :-\
EDIT 12/28/2024: I lifted R50 & R51 and made U12B as it was when I acquired the amp. None of that helped. :-(
Quote from: J M Fahey on December 19, 2024, 06:44:11 PMQuote from: Paul Marossy on December 19, 2024, 01:25:26 PMYeah 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
(https://i.postimg.cc/bJJHXmpX/Fetswitch01.gif)
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?
One of the reasons why I thought it could be the FETs is because the Vgs off number on the 2N4093s is pretty high compared to others I have on hand. Curiously, I get worse results with the 2N4093s that I bought the other day. There definitely seems to be a connection with what FETs it likes but I don't understand exactly what it is. Seems like some of that was connected with the two power supplies. Some things have improved since I replaced the 6800uF caps in both power supplies (which needed to be done anyway), like at least now I don't have no sound at all when switching to stereo mode - there is sound just no tremolo. I think I know why that may be per one of dmeek's posts, so I'll check into that.
Distortion circuit has always worked with no issues. Reverb driver is finicky about what opamp is in it. Original LM301 in U7 position seems to be dead, worked with an LM741 but not other opamps. Don't understand what is going on with that. Sometimes there is loud pop when turning off amp with reverb level up high, other times not. Not sure what the purpose of the FET on the reverb recovery circuit is... what is that supposed to do exactly?
Quote from: g1 on December 21, 2024, 08:49:10 PMOne thing I noticed on the layout, the fets have the middle leg bent forward, whereas the small bipolars have the middle leg bent to the back.
Since the fets had all been changed out by someone else previously, make sure they are not installed 'backwards'.
I'm not 100% sure on whether or not someone changed them all. The soldering looked original on all of those FETs but there was what appeared to be a date code on some of the J113s that looked like 1982 to me. If that was the case, with the amp being made in 1979 and the person I bought it from saying that he bought the amp in 1980, that would suggest that they were changed out by someone some time after he bought the amp. I could be mistaken, maybe that's not a date code and the "82" is coincidental. Were J113s even being manufactured back in 1979?
As far as those legs go, I'm 95% sure this PCB was designed by hand, so those pads probably ended up being wherever they could get them on the board. It almost looks like it was designed around a TO-18 package to me.
Maybe.
Way back then FETs were a newfangled premium component and deserved expensive metallic cases.
Today?
Apparently not worth even a humble plastic TO92 case because these are being discontinued, everything is shifting towards SMT cases, SOT23 and similar.
In any case, what matters is pinout; if you need to do some lead bending, so be it.
Different pinch-off voltage?
Sure, but in any case switching line applies 14-15V to gates, way above any pinch-off voltage, so they are forced to work.
I suggested testing the Distortion FET switcher, basically to *measure* voltages around it and take that as a reference.
All others must work about the same.
Best FET in the World will not switch if proper voltages are not applied, that why I insist you measure them.
J111/12/13 are modern stuff, straight designed fr switching, almost nobody makes FET preamps any more.
Doubt they were around in the late 70s
Quote from: J M Fahey on December 27, 2024, 11:50:07 AMMaybe.
Way back then FETs were a newfangled premium component and deserved expensive metallic cases.
Today?
Apparently not worth even a humble plastic TO92 case because these are being discontinued, everything is shifting towards SMT cases, SOT23 and similar.
In any case, what matters is pinout; if you need to do some lead bending, so be it.
Different pinch-off voltage?
Sure, but in any case switching line applies 14-15V to gates, way above any pinch-off voltage, so they are forced to work.
I suggested testing the Distortion FET switcher, basically to *measure* voltages around it and take that as a reference.
All others must work about the same.
Best FET in the World will not switch if proper voltages are not applied, that why I insist you measure them.
J111/12/13 are modern stuff, straight designed fr switching, almost nobody makes FET preamps any more.
Doubt they were around in the late 70s
I measured Q5 & Q6 in circuit this morning. Here is what I measured with 2N5952s (with J113s no sound will pass in "stereo" mode):
In "mono" mode--
Q5 G= 0.01V, S= 0.01V, D= 0V
Q6 G= 0.01V, S= 0.01V, D= 0v
In "stereo" mode--
Q5 G= -5.5V, S= -5.5V, D= -4.85V
Q6 G= -5.84V, S= -5.84V, D= -4.84V
On that stereo/mono switch, it is directly connecting +15.79V (red wire, point C on my PCB layout) and -6.07V (brown wire, point D on my PCB layout). The switch connects these two together when in "mono" mode. In "stereo" mode, these two are disconnected. I assume this is what forces the FETs to become switches? It does not seem to work very well as when you flip that switch it is VERY noticeable - not a huge pop but the speakers are moving a lot of air. Don't know how to describe the sound exactly. Like SCHARMF? Anyway, I imagine that should not be happening when everything is functioning properly. Is that switch really supposed to be connecting those two voltages together like that?! Something seems very odd about that to me... but I also don't work on things with bipolar power supplies much.
I've been looking for leaky caps too. I checked all of the 1uF caps on the main PCB, they all seem OK to me. Do you think it's possible that I have some leaky diodes? I checked them all with multi-meter, and none of them are shorted or open. I read somewhere that someone had a leaky diode in a Polytone amp, but I don't recall all of the details. I have never heard of that before. These are all 1N4003s according to the schematics. They certainly look like they are but I don't see any markings on them. just a pink-ish band on the cathode end. I did not test any of these diodes while the power was on however.
Here is an updated PCB layout. Corrected a few minor errors and/or omissions. I also drew up the power supply PCB layout since none exists online and I wanted to have one.
Looking at the LM391 data sheet, it says on page 7 that "the signal ground must return to the power supply alone, as must the output load ground. All other grounds -- bypass, output R-C, protection, etc., can tie together and then return to supply. This ground is called high frequency ground." I'm not sure what that means exactly or if that power supply is doing the grounds the way the the data sheet shows on page 12. I thought the speaker grounds and the rest of the signal grounds were supposed to be isolated from each other... is that just with tube amps?
Just trying to identify what sounds to me like a ground loop. Wondering if the input jacks, which are not isolated from the chassis, are picking up noise and amplifying it. Could also be the power supplies are a little mis-biased. Need to check on the bias after I've rounded up the appropriate resistors to check the crossover distortion.
OK so I have made some progress: I put R50 & R51 back in circuit. The I tried a NOS IC chip for U9 & U11 one more time, but this time put the original J113s back in and now the harmonic divider works! It's a very interesting effect... it's not like anything I've ever heard. Closest thing would be the Bigfoot FX "Magnavibe" DIY vibrato I built like 10 years ago now, but in stereo. That DIY guitar effect actually IS a vibrato, not a tremolo, and I verified that via oscilloscope. It's here if you want to check it out: https://revolutiondeux.blogspot.com/2012/01/bigfoot-fx-magnavibe.html
Anyway, I really like this harmonic divider thing. Very different but once you learn how to use it, it's pretty cool.
BTW, it did NOT work with the 2N5952s and the NOS IC chips - it would not pass any sound and only got the very loud thumping. So despite the notion that any old FET should work, this circuit is very picky about what FET it will accept.
So now my remaining major issue is getting that octave divider thing to work. That is still not functional. Another thing that seems to be FET related as it doesn't work with the J113s in it now but sort of works with 2N5952s, but they seem to always be biased on and the footswitch doesn't do anything. I guess I'll eventually figure that out too. It's gotta be the FETs associated with that circuit (or maybe the diodes?) because U4 is verified to be working.
This morning I verified that the input jacks do NOT want to be isolated from the chassis. Did some lead dress stuff - got the long length of twisted AC wire for the four pilot lights separated from the parallel shielded reverb cabling, and that reduced the noise level somewhat and also eliminated a strange oscillation problem I had when the volume was turned up past a certain level. I had a hunch on that one, based on some past experiences.
Rev/Dist & Harm Div footswitches are confirmed to be working. I've been trying to find a picture of the footswitches/pedals that would have been available for this thing but that seems to be 10x as rare as this amp is. I think it would have had LED indicators on it but I am not 100% sure. I may make some sort of footswitch arrangement for it down the road.
Anyway, happy to have finally made some progress on this thing. It had multiple issues to figure out! I was about to give up on it today. :grr
Quote from: Paul Marossy on December 28, 2024, 07:18:26 PMSo despite the notion that any old FET should work, this circuit is very picky about what FET it will accept.
Not sure where that was said. 'Any old' would still require it be n-channel jfet with same pinout.
2N5952 has a different pinout than the PN4093 or J113, so it will not work if installed with the same orientation.