1977 JMP 2204 master/preamp volume ???

K

kurtsstuff2003

Active member
Any and all help is truly appreciated!! :)
So what's going on is..with the Master at zero I can turn up the preamp volume and get quite a lot of volume out of the amp and it shouldn't give any volume with the master at zero right?. I'm no amp guru but with some guidance I can usually figure these things out but just need to know what I should be looking for if anybody can point me in the right direction :)
 
Check the grounding of the MV pot. If it doesn’t fully go to ground signal will bleed through. Put a meter on the ground lug then to the chassis. If there’s more than a few ohms resistance it could be the pot. Also measure the resistance of the pot across outer lugs.

Alternatively, clip an alligator clip from MV ground lug to chassis ground and see if the problem goes away.

Preamp control is before the MV. So it’s something downstream. You can also get bleed through at the phase inverter grid. Start with the MV.
 
Thank you very much! Did as suggested and same result. When I plug into the low gain input it acts as it should. I crank the preamp gain with the master at zero I can hear the guitar but barely.
 
Two things I can think of…
1: the pot is messed up. I’d check the resistance of the pot and the taper from off to on.
2: I’d check to make sure it’s hooked up right. Middle lug of the trebble pot should go to the left side of the master pot and the center lug on the master should go to a yellow cap that goes to a green wire on the left side of v3. (Unless that’s one of the weird transition models, where they used some sort of “one wire mod” to cascade things).
 
Chubbs said:
Two things I can think of…
1: the pot is messed up. I’d check the resistance of the pot and the taper from off to on.
2: I’d check to make sure it’s hooked up right. Middle lug of the trebble pot should go to the left side of the master pot and the center lug on the master should go to a yellow cap that goes to a green wire on the left side of v3. (Unless that’s one of the weird transition models, where they used some sort of “one wire mod” to cascade things).
Click to expand...
I was also thinking some kind of botched one wire mod so we are on the same page.
 
kurtsstuff2003 said:
Thank you very much! Did as suggested and same result. When I plug into the low gain input it acts as it should. I crank the preamp gain with the master at zero I can hear the guitar but barely.
Click to expand...
The gain is much, much less in low input. It’s just not pushing the signal as hard through the preamp. I doubt it’s a high vs low input issue.

The master should send all signal to ground that comes into it. If it is sending signal to ground then nothing should be getting into the phase inverter and therefore there should be no noise at the speaker. If it’s working correctly then you need to start looking at areas before the master that could be getting into the PI.

The next place after the master to check is the phase inverter grid. If the master works correctly and shunts all signal to ground then something in the preamp is likely getting transferred into the PI through crosstalk.

Chopstick the green grid wire going into the PI tube from the .022uF PI coupling cap. Move it around and see if the volume changes. This wire is very sensitive to picking up stray signals. The cathode of V2b is a strong signal. Move that wire as well. Just move wires until you hear a change.

Also, post a photo of the guts so we can see what’s going on.
 
Last edited:
I'm gonna start moving wires around and see what happens.. Thanks guys!!
 

Attachments

  • DSC_0001 - Copy (7).JPG
    DSC_0001 - Copy (7).JPG
    2.6 MB · Views: 51
  • DSC_0002 - Copy (6).JPG
    DSC_0002 - Copy (6).JPG
    2.4 MB · Views: 46
  • DSC_0003 - Copy (3).JPG
    DSC_0003 - Copy (3).JPG
    2.6 MB · Views: 48
  • DSC_0004 - Copy (5).JPG
    DSC_0004 - Copy (5).JPG
    2.5 MB · Views: 47
  • DSC_0001 - Copy (5).JPG
    DSC_0001 - Copy (5).JPG
    2.3 MB · Views: 44
1) I’d highly recommend adding shielded coax from the gain pot wiper to the v1a grid. And from high input jack to v1b grid. You have nothing shielded on either triode as it is now.

2) what is up with the twisted orange wire pair running to the ground side of the speaker jack? Coming from OT? That OT wiring coming out the bushing on the chassis looks weird to me.
 
RedPlated said:
1) I’d highly recommend adding shielded coax from the gain pot wiper to the v1a grid. And from high input jack to v1b grid. You have nothing shielded on either triode as it is now.

2) what is up with the twisted orange wire pair running to the ground side of the speaker jack? Coming from OT? That OT wiring coming out the bushing on the chassis looks weird to me.
Click to expand...
Your number 2 question was the first thing I'd asked about and it's 2 orange wires twisted together coming out of the OT . I think at this point it's probably best to get it to a good tech.
 
Please tell me that isn’t electrical tape. I also question the ground for the master volume as it’s not the best soldering.

IMG_3463.jpeg
 
Well!..Turns out the Master Volume pot was bad and Jeremy went through the amp and of course replaced it. He replaced those electrical taped wires as well. The amp sounds really good too. I can't recommend Jeremy enough. The amp landed at his house yesterday and it's on it's way back to me already. He is in communication every step of the way and even sends video's of your amp's progress until finished. He is the guy to go to. :)
 
kurtsstuff2003 said:
Well!..Turns out the Master Volume pot was bad and Jeremy went through the amp and of course replaced it. He replaced those electrical taped wires as well. The amp sounds really good too. I can't recommend Jeremy enough. The amp landed at his house yesterday and it's on it's way back to me already. He is in communication every step of the way and even sends video's of your amp's progress until finished. He is the guy to go to. :)
Click to expand...

Glad it wasn’t that crazy transformer ha.

The amp really does sound good. It’s very much untouched inside circuit-wise. I replaced the MV pot. When I tested it for continuity to ground, it was 5 or 6 ohm off ground with the pot all the way down. Then I lifted the PCB, changed all of the input wiring to shielded coax, replaced bias capacitors, replaced all filter caps, replaced a couple wires for treble pot and presence pot. Biased it up, put it on the scope and it’s all good to go!

Oh…and put the correct black plastic nuts on the input jacks. Someone had put metal ones on there.

IMG_4765.jpeg
 
You must log in or register to reply here.
Back
Top