Marshall MA50C input jack

[WesG]

I have one of these that needs a new input jack. The owner has lost the jack inside the amp several times, most recently after I tightened it myself and then bumped his cord with my knee.

The schematic seems to show that it uses a switching jack that shorts tip to earth when not in use, and that it has a second switch pole which earths ring when in use and when not in use.

What I am missing?

I was planning to use a Switchcraft jack (have some on-hand) with a single switch to ground tip when not in use, and point-to-pointing the resistor and cap on the back of that.

Does anybody have a smarter repair for this? The factory jack seems to be a problem with these amps.

Thanks,
Wes

[Leo_Gnardo]

it uses a switching jack that shorts tip to earth when not in use, and that it has a second switch pole which earths ring when in use and when not in use. What I am missing?

I was planning to use a Switchcraft jack (have some on-hand) with a single switch to ground tip when not in use, and point-to-pointing the resistor and cap on the back of that.

On that fancy input jack you're not missing anything. All I can guess is they used the TRS style for 1) more tension to help hold plug in place and B) backup connexion for ground to plug shaft. Brilliant, sort of.

I've put on lots of Switchcraft 12A's to replace plastic. One problem arises if attaching the jack to chassis brings up a ground loop, then you have to put insulating washers between your new jack & chassis, resulting in a less-secure mounting. And if the panel is thick you have to use an L12A with longer bushing threads.

Whether you use 12A, L12A or plastic jack, a dot of glue in the right place can help hold the mounting nut. Usually just a toothpick dipped in thick cyanoacrylate "crazy" glue, I like to use a dab to lock the nut in place. If you're feeling more aggressive, glue nut onto threads but that will make it a super pain to remove the jack in the future. Another handy glue is "Seal-All", same sort of application, you can find it at auto parts stores, hardware, HomeDespot, like that.

[Mick Bailey]

A quick check for input jack potential for ground loops is to plug your guitar in as usual at moderate listening level with your guitar turned up. Then short the plug barrel with a piece of wire or a clip lead to something grounded on the front panel - maybe pull off a knob and short to a pot. If it hums then you'll need to insulate the socket from the panel.

Switchcraft used to do an insulating bush kit, but on a thin panel you can sometimes use a couple of stepped plastic washers of the type that come with insulated sockets. I usually place a washer over the socket, insert it into the panel, then put a single turn of insulation stripped from hookup wire in the gap to prevent future shorts if the nut comes loose, then another plastic washer, metal washer and nut.

[Enzo]

The factory jack seems to be a problem with these amps.

How many of these amps have you seen with broken jacks?

If the nut won't stay on his jack, it sounds like his threads are stripped.

Frankly I'd rather mount a Cliff jack than a Switchcraft metal one.

[WesG]

Wow, busy forum! Looks like I came to the right place.

Leo, thanks for the sanity-check. I have to get this up and running away when I get home and don't want to get trapped by something unexpected. I'm not new to the soldering iron, but I don't normally do guitar stuff. Good tips in there, also, thanks!

Mick, good tip on the ground loop test. The reason I actually started reading the schematic was to try and figure out if there were normally ground currents flowing through that face plate. Impossible for me to tell, however.

Enzo, this is his second input board with the same problem, and it was a common complaint during reviews. The nut doesn't *look* stripped, but obviously the tolerances are too loose. He just wants to use a metal jack if possible and forget about this problem. I've seen Cliff connectors before, thanks for the reference. I may have a good use for their 8-pole CLIFFCONs coming up, providing they are similar in price to SPEAKON. Scanning their website also reminded me that I have stock (somewhere) of Rean/Neutrik jacks that look exactly like the Cliff S2. That might be a better choice for this amp. I wonder if they licensed them from Cliff.

Wes

[Enzo]

Cliff-TYPE jack was what I had in mind, I don't care about the brand.

[J M Fahey]

FWIW I have never used an original Cliff jack, not available here in Argentina , but our own knockoff (actually made by my ex partner when we split business so I got them at cost) and cheap Asian knockoffs ... and *always* applied a drop of glue to threads.
Lately I've become even more aggressive, under the logic that if I have to replace one, it's going to the junk bin anyway, so I crack the nut open with cutting nippers and call it a day.
They are as cheap as dirt anyway.