Guitar Cord plugs that don't mate well with Ampeg SVT Input jacks?

[nevetslab]

Our Guitar Dept sent over an Ampeg SVT4-Pro that had an issue with a player's cord plugged into the Input jack, ended up using the Tuner Out jack to make it work (6dB lower). I put it up on the bench to verify the problem, but was UNABLE to make the Cliff input jack misbehave. It felt solid, just like the Tuner jack. I was using Switchcraft 280 Plugs on my shop cables, tried one with Neutrik plugs, worked fine there. I didn't have a wide assortment of other brands of cords, but looked at the Switchcraft spec as well as the Neutrik and Rean spec....all were 0.247" OD on the sleeve (Neutrik and Rean stated 6.29mm dia/0.248" OD). The inside dia of the chrome sleeve nut to mount them into the Ampeg panel is 0.253" ID, so it's a decent fit...0.005" difference.

Still, I ended up pulling the preamp PCB out, as it does generate more labor income, and checked the PCB for any issues. As it had been a few years since I had the PCB out, I did find solder joint fractures on one of the pots, many connections on I/O headers, and the circuit trace between the Gain Pot wiper and it's other and, a resistor connecting to the input tube runs on the outside edge of the left front corner, and where the PCB is chopped off at 45 deg to clear the panel handle hardware, the trace looked gamey where the PCB was a bit distorted. I laid in a 28AWG insulated wire from both ends, glued it down with RTV. Repaired all the fractures I found, then put it all back together. All still working fine, so I put it back together, and moved it back into the rack mount case.

Put that up on the check-out bench, and it wouldn't power up fully. Only drew 40W instead of 123W, no pilot light, no relays clicking in, no fan. Swapped power cords, no change. Checked the AC mains fuse (this one didn't have the thermal ckt breaker), that was ok. Removed it from the road case, moved it onto the test bench where I had it running steadily, and now it wasn't powering up there either. ????!!

Pulled the top cover, moved the two power switch wires from the one side to the other side (DPST), just to see if that had anything to do with it.

Powered right up. Oooooo...... I hate it when stuff pulls this kinda stunt. Cycled the power switch over and over, seemingly healthy, so I put it all back together, now checking at every step. Got it back into the road case, got it moved back to the check-out bench (different variac / power analyzer), still working, now supported by the rack panel in the road case.

OK.....I've had enough....pulled the cables, put the covers back on, edited the invoice and set it aside for pickup.

Never did find what it was sent over here for, though I get that sort of thing from the 'smart' guitar techs.

[Enzo]

I have found over the years a few odd plugs that caused trouble. Not usually the Switchcraft or Neutrik. I find the problem is mostly in the tip shape.

A Cliff type jack - but this also can happen in a Switchcraft jack - has the cross blade with a concave cross section. The plug tip is shaped sorta like the onion domes on the Kremlin. Plug into jack, and the blade does not sit exactly the same on all plugs. Especially odd plugs. Common plugs tend to rub the blade in certain spots. The contacts, and for that matter the plugs, tend to be polished at the contact point. Plug in an oddly shaped plug, and it can touch a slightly different spot on the blade - an unpolished spot. And poor contact results.

Some are obvious - like the ones I call telephone plugs, the cover is black Bakelite (or similar) and the plug itself is brass, not nickel plated. The tip tends to be a round ball instead of the pointy thing. But I have also had this happen with the cheap cords with the plastic molded on plugs at the end.

I can imagine the customer had some odd cord used with his pedals, and you would be unable to create the symptom, because you don't have the same cord he used.

Just a thought.

[nevetslab]

I kinda had the same thought. I didn't dig into a box having el cheapo 8-cable snakes with molded phone plugs to see if I had any odd-balls to try. I will have to wander back to our Guitar Dept to see what they have lurking about to cause havoc. I do have some of the Telephone plugs, that of course mate with that style patch bay jacks and DON'T work with these Cliff jacks, nor with open frame Switchcraft jacks, if I'm not mistaken.

Once again, I missed getting into NAMM for not having a pass, which would have allowed me to find some of the odd-ball cord plugs that cost less and cause issues like this if just different enough.

[drewl]

Found a bunch of the old "telephone" plugs that were being thrown away at work.

Since they are stereo I used a few to make Footswitches, and yes they do not play well with all types of jacks, what with the Kremlin domes and what not.

There's also a size of them in between 1/4" and 1/8" which are not used anymore.
I find them in old telephone patch bays.

[SoulFetish]

If you're talking about the internally threaded cliff style with the chrome plated brass nut (see below),


Then I've run into this issue on occasion resulting from slightly over tightening the chrome nut. You can try backing it off just a little, maintaining a good tightness without torque-ing it like you would a switchcraft open jack.

[nevetslab]

If you're talking about the internally threaded cliff style with the chrome plated brass nut (see below),


Then I've run into this issue on occasion resulting from slightly over tightening the chrome nut. You can try backing it off just a little, maintaining a good tightness without torque-ing it like you would a switchcraft open jack.

Yup...that's the style. They weren't over-tightened....just snug, as suggested. I'm heading over to the Guitar Dept this morning to see what more I can learn, having sent it back repaired a couple days ago.

[TimmyP1955]

I think Enzo has it figured out. The thing to do with downbeat staring at you is to put your palms on the sides of the plug and spin it in the jack, like a boy scout trying to light a fire.