Troubleshooting a JCM800 2203 100W

[sonolink]

Hello everybody

A friend asked me to fix his JCM800 100w head a few days ago. Symptoms were basically noise and scratchy pots. I inspected the inside and my first thought was to replace tubes and power caps so much they looked old and dusty.

So I've replaced these and the pots now seem much quieter (although not DEAD quiet, maybe they need replaciong too) but the amp seems to hum quite a bit. When moving VR1  (preamp volume) the noise changes in frequency. To the left it's quite bassy, towards noon it goes treble then bassy again after 3 o'clock.

I've chopsticked the preamp tube sockets and V1 and V2 joints seemed to pop quite a bit everytime I touched the pins or played with the wires until the Mains fuse blowed. So I reflowed the joints and replaced the fuse and now they seem ok and quiet when chopsticking but the hum is still there...

I've tried different preamp tubes (the old ones, new JJ ECC83S and a used but good EH) and no go.
If I touch VR1 (preamp volume) with a metallic object the hum goes REALLY loud (like if a guitar jack was plugged in without the guitar). Maybe this is normal...


The noise in question with Master Volume very low and when moving VR1 (preamp volume):
https://soundcloud.com/thesoundkitchen/jcm800hum

Any ideas?
Thanks a lot
Sono

 

[sonolink]

Fotgot to add that I've got a Bias Master device. I read the values for each power tube and they are:

V4 - 21.5
V5 - 21.0
V6 - 28.8
V7 - 21.6

(V4 and V7 are the ones on the outside)

 

[sonolink]

Tried pulling out outer tubes....same thing

No hum when V1 is pulled out...

 

[sonolink]

Nobody?

Pretty please...........

 

[Deepdark]

Tried pulling out outer tubes....same thing

No hum when V1 is pulled out...

Try out some more tube at v1. Some one exhibit more hum than other (leak)

 

[sonolink]

Try out some more tube at v1. Some one exhibit more hum than other (leak)

So you don't think it's something wrong with other components or solder joints, etc?
I'll try other tubes then.

Thanks for your help

 

[pucho812]

what are the filter caps  in the power supply looking like? good, bad?

 

[sonolink]

I replaced power caps AND tubes. They're new

 

[Bo Deadly]

Check continuity between the chassis and earth ground and other bits that are supposed to be shielded. If it was exposed to moisture, some parts may have corroded and conductivity may have been affected.

Also check the heater wires. If they are not draped properly, they are a good source of hum.

 

[sonolink]

I think I did check ground scheme but I'll have another go
Tomorrow I'll post some pics.

Any other ideas?

Thanks again for all your help

 

[CJ]

spray the pots with De-Oxit or similar spray

V6 is running hotter than the other three tubes, why?

did you buy a matched quad set of JJ's?

measure the screen voltage on all 4 power tubes, Be Careful!  keep one hand in your pocket, no pocket billiards!

is there a hum balance pot or heater pot?

measure the AC voltage across the first filter cap, Be Careful! one hand in pocket, no pocket pool either.

make sure volt meter not set to ohms

pull the first preamp tube , is the noise gone? pull the next tube, is the noise gone?

turn down presence control if it is on 10. any improvement?

more lead dress around with chop stick near input jack/V1, and change?

report back immediately or else....

 

[sonolink]

CJ

Thanks a lot for your help

V6 is running hotter than the other three tubes, why?
did you buy a matched quad set of JJ's?

I have no idea why there is that mismatch. I did buy a matched quad set indeed

measure the screen voltage on all 4 power tubes, Be Careful!  keep one hand in your pocket, no pocket billiards!

I measured 45V on pin 4 and 520V on pin 5 of each power tube

is there a hum balance pot or heater pot?

nope

measure the AC voltage across the first filter cap, Be Careful! one hand in pocket, no pocket pool either.

I can't seem to be able to read AC where I think I should place the probe....
I've placed it on each of the three big caps positive terminal
I've attached a pic. could you please tell me where I should place it please

pull the first preamp tube , is the noise gone? pull the next tube, is the noise gone?

When I pull V1 it's dead quiet. When I replace it and pull v2 same thing

turn down presence control if it is on 10. any improvement?

No change in hum

more lead dress around with chop stick near input jack/V1, and change?

No. Hitting the shit out of those joints won't do any suspect sound since I reflowed everything.

BTW hum is on all the time from the moment that the Stand Bye switch is flipped, WITHOUT any guitar jack plugged in.
Any chance that the Bias Caps should be replaced?

Pics:







Thanks a lot for all your input and help
Cheers
Sono

 

[CJ]

check your power tube voltages again,

pin 3 should be plate

pin 4 s/b screen

pin 5 s/b grid and have a negative voltage, probably the 45 you measured without the minus side,

measure the 12ax  pins also>
V1  P1= (pin 1)  K = (pin 3)  P2= (pin 6)  K2 = (pin8)  do the same for V2

measure the ohms from output center tap to plate wires, turn off amp and discharge caps for this test

me thinks white is B+ and red and brn plates could be wrong

 

[andyfromdenver]

my favorite way to play a 2204/2203 is -> Lo input, master at full or just off, preamp gain down low like a single volume control amp.  That setting has a killer sparkly clean that is not Fender and is awesome.  the problem is that it exposes any noise in the amp.
I was assisting a touch over beginner friend fix his 2204 cause it had a horrible hum especially with the aforementioned setting. He said it did it since he bought it and he had tried all new tubes and no change.  My thinking was, "ok let's recap it, which I helped him order parts and execute.  guess what?  no change!  (the hum really sounded like a bad filter cap).  On a whim, I started pulling tubes starting in the pre until the hum disappeared.  It was just a bad tube!!!  At least he got to spend money on new caps  :
I know you tried new tubes already, but if you have some known good, it is worth a shot, did you pull from the pre up to the PI until the hum stopped?  only other thing I can think is did your heater center tap get disconnected or???

best wishes!

edit: sorry I just saw you pulled the pre tube and hum disappeared.  another quick test.  with guitar plugged in and volume on guitar reduced to zero does the hum disappear? (thinking maybe a failed switching jack connection) might be nice (albeit a pain) to check the underside of the board, maybe a stray wire or something is bridging?  dunno, looks like the pre is untouched, but not sure.  did you post voltages for v1? dc on 2-8, 1-6, may as well check heater voltages too ac.

you could hail mary redo the whole pre, but that is like a shotgun approach.

 

[sonolink]

Hello

Sorry for the delay, I've been away gigging. So I've been thinking about the mismatch on the power tube voltage these days while I was working and I've come to one conclusion: this amp was designed so that there's only one pot to set bias, so the mismatch had to come from the tube OR a faulty component on that path. So I swapped tubes positions: the mismatch followed the tube. I guess it's a hot one this one

The other great factor for the noise was grounding as has been said in this thread so I've reflowed all solder joints between input and V1 and inspected them carefully and I noticed that one of the input jack connectors was not original. So I checked continuity to ground and noticed that the switch of the input didn't do contact properly, so it wasn't grounded properly when no jack was inserted. So I adjusted the terminals with some pliers and now the noise is mostly gone. There is a residual "purring" but signal to noise relation seems now ok.

I guess the purring will disappear when I finish reflowing all the rest! I'll try to find some original or similar connectors to replace both input jacks though.

Any other ideas or suggestions are as always very welcome. Thanks a lot to everybody for your time help and ideas. I can never have enough learning with you people and I seriously appreciate it!

Thanks again
Cheers
Sono

 

[Whoops]

So I checked continuity to ground and noticed that the switch of the input didn't do contact properly, so it wasn't grounded properly when no jack was inserted. So I adjusted the terminals with some pliers and now the noise is mostly gone.

That was it

 

[andyfromdenver]

Hi Sono.
good job on the bad input switching contact.
Not sure what you mean about purring.
cleaned pots, wiggle the tubes a bit and seat them well, replace the jacks and you're good! (i hope ). I wouldn't heat anymore traces personally, doubtful that work is bad.
an aside, some/ (many?)guitar tube amp OTs aren't perfectly balanced on the primaries, due to winding and extra taps etc.  if you have a matched quad in, just set it for the high tube and let it be what it is . all imo. if you have a bunch  of a particular tube, you can forge a match unique to your OT.

if that's all obvious, my apologies! 

 

[sonolink]

Andy
Thanks for your input. Sorry I didn't reply before, I only saw your post today
By purring I mean a residual noise that sounds a bit like when a cat purrs. I think it comes from my house electrical installation anyway. This week I'll take the amp to my studio where I know the installation is proper and quiet and I'll check it.
I replaced the input jack connector with a new one and cleaned the pots and it's sounding good

if that's all obvious, my apologies! 

Not at all. I appreciate your input very much
Thanks again for your time and help
Cheers
Sono

 

[CJ]

worked on one of these the other day, horrible sound,

found that the first two pramp tube sockets had gone conductive, replaced those, waveform got better,

got rid of the hot/cold input mess and just stuck a standard 1 Meg/68K x 2 input scheme,

ditched the cathode follower so we have 3 stages into the inverter and that really helped the sound,

the clincher was replacing the little yellow square caps with some good stuff, results:

customer: "this thing sounds better than it ever has! , sounds like a Marshall now!"

go figure, we have a Fender preamp feeding a Marshall output circuit,

still has the master volume, might have moved the tone stack up a notch,

here is the old board with the offending suck caps>

 

[Whoops]

Nice CJ.
Whats the problem with those caps? Are they prone to failure?

Thanks