Hi There,

So in. Your video noise changes with Vibrato Chorus and OFF and entering in OFF. Actually it is most often the other way around on JC120.
You can get a freezer spray and freeze suspected components and noise will disappear when the component causing the noise is frozen.

It is a good idea to change all electrolytics on the main pcb in this amp while you are anyway disconnecting the whole pcb.


The J fets used in this amp are rarely a problem and  they are difficult to replace with other than the part numbers used.
Typically Japanese J Fets are fairly consistent and vary little between units
For instance the 2SK246GR has an UGS around 2V. A number of Western types would qualify but may have a spread of UGS from 0,7-3V that could cause problems. For instance J113 could be used but it is of a different process and has a little higher gm and sound may change.
Other types like 2N3819 or BF245B etc may in some batches be centered around 2V UGS but the spread is typically really wide
Unless noise stops while one of those J Fets are frozen I would let them sit where they are.


Have fun
Bjorn Juhl
BJF Electronics

Quote from: BJF on May 14, 2024, 05:36:48 AMHi There,

So in. Your video noise changes with Vibrato Chorus and OFF and entering in OFF. Actually it is most often the other way around on JC120.
You can get a freezer spray and freeze suspected components and noise will disappear when the component causing the noise is frozen.

It is a good idea to change all electrolytics on the main pcb in this amp while you are anyway disconnecting the whole pcb.


The J fets used in this amp are rarely a problem and  they are difficult to replace with other than the part numbers used.
Typically Japanese J Fets are fairly consistent and vary little between units
For instance the 2SK246GR has an UGS around 2V. A number of Western types would qualify but may have a spread of UGS from 0,7-3V that could cause problems. For instance J113 could be used but it is of a different process and has a little higher gm and sound may change.
Other types like 2N3819 or BF245B etc may in some batches be centered around 2V UGS but the spread is typically really wide
Unless noise stops while one of those J Fets are frozen I would let them sit where they are.


Have fun
Bjorn Juhl
BJF Electronics

Thank you,

Right now with tbe suspect transistor removed there is no noise in any of the three modes.

I was able to find a us seller on ebay with near 100% reviews to source a few 2sk246gr.   Without it being ridiculouslt priced so i will test these to make sure they are in spec when they arrive and go from there. 

Electros should be here tomorrow for the power supply, i wasnt intending in swapping all in the signal path but i should have all those on hand.   

May change the np 1u electros to film if i do change all . 

Thx!

Hi There,

Good job

That's excellent. You can likely find some Toshiba 2SK246GR's. 2SK246 can have Ugs from 0,7V to 6V but  with the GR suffix those would have IDSS at 2,6 -6,5mA and then Ugs 1V5-3V and thus plug right in.

It's a good plan to replace the elkos and yes if you have film 1uF while you anyway have the pcb it will definitely prolong life between service.
The stock elkos would be 40 years old and they were consumer grade at the time to begin with but yes they can cause noise when they go bad.
Elkos have definitely gotten better  with much better manufacturing processes in the last 40 years.

Have fun
Bjorn Juhl
BJF Electronics

Quote from: Locrian99 on May 14, 2024, 11:46:05 AMRight now with tbe suspect transistor removed there is no noise in any of the three modes

It would be a good idea to replace the JFET with something close to make sure it *stays* working.
That would give you a lot of confidence when you get the new JFET.  If it goes bung before that you know you have more debugging to do.

I'm assuming the volume pot on channel 1 is still noisy?   Maybe run some cleaner through it.

Quote from: Rob Strand on May 14, 2024, 07:46:37 PM

Quote from: Locrian99 on May 14, 2024, 11:46:05 AMRight now with tbe suspect transistor removed there is no noise in any of the three modes

It would be a good idea to replace the JFET with something close to make sure it *stays* working.
That would give you a lot of confidence when you get the new JFET.  If it goes bung before that you know you have more debugging to do.

I'm assuming the volume pot on channel 1 is still noisy?   Maybe run some cleaner through it.

Yea measuring what i have on hand hadnt had much luck for close yet.   5952's, j113, a couple thru hole 5457's.   Ill have to do some mmbf 5457's on some adapter boards.   Looking 201's are for sure not going to be anywhere near it.   Yea i had ran deoxit theough all the pota before on channel 2 i was getting more the crackle even after deoxit but seems to clear up now, thought ot could be ac leaking through a cap and part of the "noise".   Ill do some more poking around tonight and see if i can find a close jfet. 

Quote from: Rob Strand on May 14, 2024, 07:46:37 PM

Quote from: Locrian99 on May 14, 2024, 11:46:05 AMRight now with tbe suspect transistor removed there is no noise in any of the three modes

It would be a good idea to replace the JFET with something close to make sure it *stays* working.
That would give you a lot of confidence when you get the new JFET.  If it goes bung before that you know you have more debugging to do.

I'm assuming the volume pot on channel 1 is still noisy?   Maybe run some cleaner through it.

You just had to say that!!!  Lol.   Well i put j113 which was right around 1.9 in and ran a loop on it for a minute.   And theres the noise again.   Seems quieter... but its there. 

So the red represents noise.   It seems to me it is originating at the 47k going into the drain of q10.   Whether that makes sense im not sure but clearly its not the fet.   So ill put that back in and replace r70 and see what happens.

Edit: its not at gate of q9 that kind of looks like it is.

2nd edit At about 30 minutes of a loop running through it with the 2sk246 back in and the noise hasnt came back.    Not ready to deem it fixed.   When the rest of the house ks awake ill try it at a respectable volume and see 

Quote from: Locrian99 on May 14, 2024, 11:21:14 PMSo the red represents noise.   It seems to me it is originating at the 47k going into the drain of q10.   Whether that makes sense im not sure but clearly its not the fet.   So ill put that back in and replace r70 and see what happens.

I'd check the DC across the JFET Q10, with chorus in and out of circuit.  You might see a small DC voltage possibly fluctuating.

The cause may well be on D6.  Perhaps even the PCB or soldering.   The gate of Q10/anode of D6 isn't a good place to probe when the JFET is on.  You could check the DC voltages from the cathode of D6 back to Q12/R76.

Quote from: Rob Strand on May 15, 2024, 01:08:21 AM

Quote from: Locrian99 on May 14, 2024, 11:21:14 PMSo the red represents noise.   It seems to me it is originating at the 47k going into the drain of q10.   Whether that makes sense im not sure but clearly its not the fet.   So ill put that back in and replace r70 and see what happens.

I'd check the DC across the JFET Q10, with chorus in and out of circuit.  You might see a small DC voltage possibly fluctuating.

The cause may well be on D6.  Perhaps even the PCB or soldering.   The gate of Q10/anode of D6 isn't a good place to probe when the JFET is on.  You could check the DC voltages from the cathode of D6 back to Q12/R76.

Currently its not noisey (i know at this point not to trust that.
Q10
Chorus off S-13.95 D-13.97
Chorus on s-13.95 D-13.29 (i an also hearing a slight volume drop in chorus mode i think, its been a bit since ive played a functional 120, dont recall if thats normal, its very slight).

D6 cathode- 26.2
R73- 26.2/28.9
R76-29.1/28.9
Q12-C-28.9 B-0, E-im not positive my probe is making contact, getting 0 which looks right...base im also not 100% im getting contact got the board screwed back down and its late dont feel like pulling it back out again til im doing the electros.   

Amp has been on for awhile no noise.

Well i ran it at about 5 on the volume for 1/2 an hour with a loop noise never came back.

Electro's arrived i am going to start changing those out.

Feeling pretty good this is solved.   Apparently r70 was the issue. 

Quote from: Locrian99 on May 15, 2024, 08:51:52 PMWell i ran it at about 5 on the volume for 1/2 an hour with a loop noise never came back.

Electro's arrived i am going to start changing those out.

Feeling pretty good this is solved.   Apparently r70 was the issue. 

It's one of those tricky intermittent problems.   

You would think if R70 was faulty the problem would still be there with Q10 shorted. The act of soldering in the area may have pushed R70 into a good region, for a while at least.

Nonetheless the problem does seem to be in the area and R70 is one of the few things left, other than soldering and PCB tracks.

If you have at the old R70 maybe you can push/pull the leads and see if you can get it to measure off-value.

Fisdling around with it, consistently measures 47.1 pretty good for a 40
Year old carbon comp.   Perhaps it was a bad solder joint?   I didnt have noise when it was shorted or the jfet wasnt there.   It was when i put the j113 in that it came back, whether that was due to the intermittency or not im not sure.   Lucky for me this amp wont leave my music room so i dont have to worry about it coming up in a gig setting, worse case it'll annoy me
Again thanks for all your help don't think i couldve tracked this down without it.   

I changed all the main filter caps, deoxited all the pots.   Still getting that weird pot noise a little but only if i turn the pot quickly....  Seems pretty healthy now.  Actually substantially quieter than the jc 50 i had awhile back from the same era.   

Quote from: Locrian99 on May 16, 2024, 01:47:24 AMFisdling around with it, consistently measures 47.1 pretty good for a 40
Year old carbon comp.   Perhaps it was a bad solder joint?   I didnt have noise when it was shorted or the jfet wasnt there.   It was when i put the j113 in that it came back, whether that was due to the intermittency or not im not sure.   Lucky for me this amp wont leave my music room so i dont have to worry about it coming up in a gig setting, worse case it'll annoy me
Again thanks for all your help don't think i couldve tracked this down without it.   

I changed all the main filter caps, deoxited all the pots.   Still getting that weird pot noise a little but only if i turn the pot quickly....  Seems pretty healthy now.  Actually substantially quieter than the jc 50 i had awhile back from the same era.

It could have been the soldering on R70.  I have gone over PCBs with an eye loupe many times.  It's very hard to tell you have fixed the problem on these intermittent faults.   You can only test it or bang it.

There's no DC on either side of the pot in that circuit so the most likely fault is a bad connection.

You could try resoldering the pot.  Sometimes the resistive track on the pot can get a crack.  You can also get cases where the pot leads connecting to the resistive track get bad connections.  A fudge fix which sometimes works is to put a large resistor across the wiper and one of the pot pins (say 10 times the pot value).