1969 JMP refurbish

[andresound]

9BEFBE34-1CAB-4B48-8347-650DFAA9C22A.jpeg

Have been asked by a friend to get this thing going again. It was sent to a tech to sort a Red plating issue many moons ago with no progress. Buddy collected and brought it to me. The main board looks mostly untouched. However the main transformer is not original and needs a complete electrolytic recap . The front face is also dented from a drop. What do you guys think. Any advice or suggestions before I start? The

[xtian]

One step at a time! Hope yo can tame that rats nest by the e-caps!

[andresound]

Yip.. no rush to finish. Will try get another PT and work slow. Will post progress

[xtian]

What's wrong with the PT? Disconnect its secondaries and test before you give up on it.

[andresound]

Will do. However the primaries are VERY thin gauge! . The amp used to work with this PT, so yes it should be OK. Just looks shit. Maybe I can make a plate for it and remount?

[jelle]

I would get the proper PT, rebuild power supply and add fresh E-caps.
Check the board for DC voltage leakage around the PI output, and add grid stoppers to the power tubes to stop the redplating.
Lift the green wires up a bit at V1 and V2 so they are away from the heater wires to stop hum, and done.
The chassis can be bent back with a wire tensioner but I am not sure I would bother because you could damage the Plexi front.

Happy rocking.

[jelle]

Also, that is still somewhat valuable because a lot is untouched, lots of original solder joints. I would make every effort to keep it that way and only touch what absolutely needs to be touched after verifying it needs it.

[andresound]

Also, that is still somewhat valuable because a lot is untouched, lots of original solder joints. I would make every effort to keep it that way and only touch what absolutely needs to be touched after verifying it needs it.

Thanks Jelle, and all noted.

[andresound]

Also, that is still somewhat valuable because a lot is untouched, lots of original solder joints. I would make every effort to keep it that way and only touch what absolutely needs to be touched after verifying it needs it.

Some progress. The existing PT (not original), was a stand up mount and didn’t line up with existing chassis holes. So I removed one bell housing of the PT, and mounted it horizontally (lying down). Had to epoxy the leads to avoid excess delamination of the core material. I then had to drill and mount a rubber grommet for the 5v and 6.3v supply, which exits from the other side of the PT. Will try complete the PS wiring this week.

The amp had a “hard wired” set of diodes directly on the rectifier tube socket. Have removed this and will restore as close to original as possible although the original circuit was not tube rectified. This came with a GZ34 installed.

[Faze]

Whats the S# on this amp?

I want to compare to my marshall. Mine is an early 50 watt m#1987. 1970 VINTAGE I think. It had the 5V leads for a GZ34 on the PWR but SS Rect. I wired it up to a Tube rect years ago because the chassi was punched for the tube socket, and it had a nice sound. I've since wired it back to Stock. I've had the amp since 1971. Got it used.

[andresound]

Whats the S# on this amp?

I want to compare to my marshall. Mine is an early 50 watt m#1987. 1970 VINTAGE I think. It had the 5V leads for a GZ34 on the PWR but SS Rect. I wired it up to a Tube rect years ago because the chassi was punched for the tube socket, and it had a nice sound. I've since wired it back to Stock. I've had the amp since 1971. Got it used.

This is the serial number and date of manufacture. I will also take amp back to SS with the associated 50+50uf (x3). This is the schematic I will use.

[TUBEDUDE]

Nice job on that rat nest. It cleaned up nicely. Looking forward to clips eventually.

[Faze]

The Schematic you are using is very similar to mine.

2 things to watch out for : you might know this already.

1. the bias circuit turns on with the Standby switch. Not good. This means the output tubes are not biased off when you switch so output tubes red plate for a moment and cause the heater fuse to eventually burn out. There is a mod to fix this so bias is always on. move bias supply to the other side of the Standby switch. Now when you switch standby , the tubes are already biased so no surge.

2. The .22 caps across the standby switch are 600v rated. These will also eventually short out due to the surge and blow the heater fuse. They are not found on later generation marshals. I would eliminate them.

[andresound]

The Schematic you are using is very similar to mine.

2 things to watch out for : you might know this already.

1. the bias circuit turns on with the Standby switch. Not good. This means the output tubes are not biased off when you switch so output tubes red plate for a moment and cause the heater fuse to eventually burn out. There is a mod to fix this so bias is always on. move bias supply to the other side of the Standby switch. Now when you switch standby , the tubes are already biased so no surge.

2. The .22 caps across the standby switch are 600v rated. These will also eventually short out due to the surge and blow the heater fuse. They are not found on later generation marshals. I would eliminate them.

Thank you for the info. I did notice the bias issue. . I will eliminate the .22’s thanks for that.

[andresound]

Progress. Still to slowly fire her up and reform DALY caps. While taking pictures.... I realized I haven’t yet connected the pilot lamp . Will also change out the bias caps to 2x 10uf axial to replace the radial ones. Will looks better!