Marshall Super PA 100w Model 1968 no sound input II & IV

[Jacob.SWE]

Hi!

My first post in this forum.

I recently bought a Super PA 100w from 1970, model 1968.

It was in need for service.

I have done the following things to it:
- Tested tubes (all in good shape and bias adjusted to 38mA)
- Cleaned all sockets and inputs
- Cleaned all potentiometers
- Replaced all filter caps, both cans and those on turret board.
- Resoldered loose ground joints.

It went from really bad to 95% good! There are two things that bother me.

- Input II and IV is not working (there is a minimal sound coming through if i strum the guitar really hard. I replaced two jacks with no luck.
- There is some hum left, it gets worse if i turn volume up or adjust treble / mid up. The ground bus on the potentiometers look quiet thin. What should i do?

If i remove preamp tube 1 or PI it is gone.

Cheers!

 

[StingRay85]

If you remove the PI tube, the only sound you can still hear is from the power tubes, obviously.

 

[playloud]

Pics?

Have you checked all ground connections for continuity (including those input jacks)?

 

[Matthews Guitars]

Post pics of everything. Especially the guts inside the chassis. Please make the photos large (1 MB max size per photo) and CLEAR with good lighting.

It may be something simple...or not. Maybe not all the inputs are even connected. Maybe it's a bad preamp tube. The photos will help.

 

[junk notes]

My first post in this forum.

I recently bought a Super PA 100w from 1970, model 1968.

 

[Matopotato]

 

[Norfolk Martin]

"- Input II and IV is not working (there is a minimal sound coming through if i strum the guitar really hard. I replaced two jacks with no luck."

2 and 4 is a bit unusual if they are in the order 1-2-3-4 on the schematic. Each input feeds on half of an ECC 83, and 2+4 are on different tubes. it would be very odd if the same fault had developed on two entirely separate circuits. when you remove "preamp tube 1" are inputs 1+3 still working?

 

[Jacob.SWE]

I am not able to upload or link anything yet. 1mb is impossible to scale down the image to.

Everything is 100% original except for the two new input jacks that i replaced to see if it were the cause of problem. (The old pink wire that is connect high and low input was replaced since i melted it with the soldering iron).

I will try with different tubes.

It has never been modified so all the ground points etc is original.

 

[Jacob.SWE]

 

[Mr.Bungle]

You said you changed 2 jacks. The 4 inputs work in high / low pairs for 8 jacks in total. They are switched so the high gain inputs are 34k ohm to the tube, and 68k for the low inputs. If you only changed one of each pair on channels II and IV, then there could be a switching issue with the other jack. Plug a cable in and measure the resistance from the tip of the cable to the relevant pin on the preamp tube - it will be 2 or 7 (obviously with the amp OFF and drained of all residual B+ voltages - but you knew that). High inputs should read 34k, low should be 68k. Compare these measurements with the working channels I and III. It may just be that the jack switches need cleaning. If all is good there, you need to start tracking along the circuit. Try swapping around the preamp tubes. Try different preamp tubes. Grounding, as someone else has suggested. Then I'd be checking the 100k plate resistors and the .022 coupling caps. The coupling caps look original and could be past their use by date. You would need to check all the voltages on the preamp tubes, although I'm not sure the schematic shows what they should be. The caveat with all of this of course being that I assume you know how to safely work on valve amps (which seems likely given the work you've done!). Congrats on a great amp. I'm jealous!

 

[neikeel]

Unlikely tubes
1&2 are shared
3&4 are shared
Hum and odd mixture of non working channels - check your work on the jacks - are they of correct switching type and wired correctly. May have poor or non existent grounding on those jacks.
Some things you say you have done and how you approached this suggest you are new to this sort of work - take care!