5e3 hum?

[chuckb]

Hey guys, I have a 5e3 clone here to fix a hum problem for a friend. He has it in a head version and it's ptp, but with terminal strips instead of a board.With no guitar connected and both vols off no hum. When I turn up the vol the hum is there. Now, when the guitar is plugged in the amp is fine,no hum. The hum is only there when the guitar is not plugged in. He has only two inputs which are wired correctly, but when I go near the 1st input on the inside of the amp with a stick the hum gets louder. Any suggestions on what I vcan try to eliminate this hum? I checked every conection and grounds and they all seem fine,good solder joints and no loose wires. I tried moving some of the wires around with a stick while the amp was on but nothing affected the hum.

Chuck.

[Jazz P Bass]

There should be "grounding" jacks installed.
Meaning, there is a normally closed contact which connects the + input lug to ground when nothing is inserted.

[chuckb]

There should be "grounding" jacks installed.
Meaning, there is a normally closed contact which connects the + input lug to ground when nothing is inserted.

Hey Jazz P Bass, there is two switchcraft grounding jacks installed.

Chuck.

[Tom Phillips]

Sounds to me like one or more of the input jack switching contacts is not working correctly.

[chuckb]

Sounds to me like one or more of the input jack switching contacts is not working correctly.

Hey tom, These are new switchcraft input jacks and the contacts are working as they should. I replaced the old ones with new jacks ,suspecting the same problem you mentioned but no luck.

Chuck.

[Tom Phillips]

If you connect the center contact of the switchcraft jack inside the amp to chassis ground does the hum stop?
That's my brute force method to bypass all wire tracing and contact/wire checking.
HTH,
Tom

[Jazz P Bass]

What I would suggest is to measure the resistance of the first preamp tubes grids to ground.
Pin 2 & 7.
No input in the jacks.
Amp off.
They should both read the same.

[chuckb]

If you connect the center contact of the switchcraft jack inside the amp to chassis ground does the hum stop?
That's my brute force method to bypass all wire tracing and contact/wire checking.
HTH,
Tom

I did your brute force method and it increased the hum considerably'

Chuck.

[Tom Phillips]

...increased the hum considerably

Interesting. Gotta think about that. (?)

[chuckb]

What I would suggest is to measure the resistance of the first preamp tubes grids to ground.
Pin 2 & 7.
No input in the jacks.
Amp off.
They should both read the same.

I checked out the resistance on pis 2&7 and both are the same 1.078 ohms.

chuck.

[chuckb]

Interesting. Gotta think about that. (?)

Hey Tom, Sorry bud, but i did the test again from ground to the center tap on the switchcraft jack and the hum dissappeared. So what does that mean?

Chuck.

[Jazz P Bass]

Something is not wired correctly.
Shouldn't the grids be 64K from ground?
1 meg in parallel with a 68K = 64K.

[chuckb]

Hey guys, i found the problem ,both 1M resistors on the inputs were installed wrong,"my bad". i have them correct now and hum is gone. Thanks for your help in this matter,much appreciated.

Chuck.

[Tom Phillips]

Glad to hear you found the problem Chuck. Maybe a night sleeping on it then taking a fresh look did the trick. Those jacks, with the resistors mounted right on the jack terminals, can be confusing to a new builder. They really play tricks on my eyes sometimes.
Tom

[chuckb]

Glad to hear you found the problem Chuck. Maybe a night sleeping on it then taking a fresh look did the trick. Those jacks, with the resistors mounted right on the jack terminals, can be confusing to a new builder. They really play tricks on my eyes sometimes.
Tom

I hear ya Tom, I've wired of these before , but I have mis-wired a few also. i just need to pay closer attention next time. Thanks for your help.

Chuck.