Plexi Crankling & Squealing Noise

[vahid_vs]

Hello all,

I bought a '74 JMP Bass, it worked perfectly but after changing one of the power tube sockets (it was broken) and 50 + 50 caps with F&T ones, the amp makes loud crackling and squealing sound (sometimes it produces normal sound but after 5 seconds, it will make very loud noises).

I really appreciate it if you help me.

Thanks

[Joshabr1]

If u will post some pics maybe a sound clip it would help these guys help u out

[vahid_vs]

sure thing. I'll post some pics and a sound clip.
And one more thing, When I use input I, it doesn't sound that loud even with volume on 10, and when I use input II, it doesn't make any sound.

[vahid_vs]

When I pull the power tubes, the amp is really quiet.

I measured the voltages:

V1 = Pin1: 147 / Pin2: 0 / Pin3: 0.993 / Pin4: 3.03 / Pin5: 3.03 / Pin6: 150.6 / Pin7: 0 / Pin8: 0.993 / Pin9: 3.01

V2 = Pin1: 130.5 / Pin2: 0 / Pin3: 0.731 / Pin4: 3.03 / Pin5: 3.03 / Pin6: 222 / Pin7: 131.2 / Pin8: 132.2 / Pin9: 3.01

V3 = Pin1: 176 / Pin2: 17.1 / Pin3: 27.2 / Pin4: 3.03 / Pin5: 3.03 / Pin6: 166.6 / Pin7: 17.89 / Pin8: 27.2 / Pin9: 3.01

V4 = Pin2: 3.01 / Pin3: 365 / Pin4: 364 / Pin5: -33.4 / Pin6: 366 / Pin7: 3.01

V5 = Pin2: 3.02 / Pin3: 365 / Pin4: 364 / Pin5: -33.4 / Pin6: 366 / Pin7: 3.02

[vahid_vs]

Here are some pics:

[neikeel]

The problem looks to be related to the new socket wiring and the implementation (I can see odd wiring and sleeving there. Is that a bias adjust resistor on V4, but not on V5? There are scraps of wire and insulation too.)

Maybe needs a tidy with the OT primaries going along the chassis floor beneath the secondaries and the NFB wire. These little details can make a difference.

[vahid_vs]

Yes, that's a 1 ohm 1 watt resistor, I installed one for V5 too, it's under the fuse holder.

[vahid_vs]

Here is a sound clip of the noise, I flipped the standby switch and after 5 seconds it made the crackling sound again

[vahid_vs]

Please HELP ME

[neikeel]

First unsolder the white wire from pin 3 and re-route the wire below all the other wires ( ie the ground wire and the heater red wire and resolder.
Move the red and black heater wires to the back of the chassis.
It looks like your 1 ohm resistor on that socket will not be doing anything as I see pins 1 and 8 joined, then a ground wire to the lug and then the resistor with dodgy insulation going somewhere under the fuse holder, correct me if I am wrong.
Your heater wires on that socket look to have a dry joint on pin 7?? Might be worth gently reflowing all the solder joint that you made during the cap and socket change too

Next thing to check is that your preamp filter cap wires under the board have not been pulled off when the board was lifted.

Make sure your OT primary wires and secondary wires are not sitting on top of each other and the purple presence wire is away from the primaries too.

Make sure your purple presence wire is clear of other wires at the front of the chassis and that you hav enot pinched any wires under the board stand offs.

[maxwedge572]

The soldering on V5 looks really poor and the same with the fuse holder there. Is V5 the socket that you replaced? I'd start all over again and move the wires as Neil says. Get yourself some solder wick because a solder sucker sprays solder all over your chassis.

[vahid_vs]

Thanks, I'll do it today.

One more thing can I extend OT primary wire (just the white one) with another wire, it's a little short?

[neikeel]

Thanks, I'll do it today.

One more thing can I extend OT primary wire (just the white one) with another wire, it's a little short?

Try not to. It will reach if you route it correctly.

[vahid_vs]

Thanks everybody, after changing the power socket with a new one, and shielding OT primary wire, the noises went away.

Now I'm trying to fix the problems the amp had before, when I close the Input I volume all the way to 0, I can still here the sound of guitar, and when I hit the the standby switch to off it makes a popping sound.

[bobtec]

My old 63 Bandmaster was dirty..The ground wires had oxidized under the solder and were broken...I spent a $1,000.00usd sending my amp to a Texas Fender Guru trying to solve the problem,got ripped off bad. and all I needed to do was clean it real good.. I bought some magnifiers glasses and really looked at every part..Some Deox-it and a rag a little fine grit file will do wonders...Did the same with my SLP smoke and sweat and beer gets in....