Behringer Amps and Isolated Jacks

[kfaria]

@Enzo:
>"Then you will have to make larger holes in the panel."
No worries there.
>"So you have a couple broken jacks, but is it really necessary to use metal?"
It is obvious that the plastic jack bushings do not stand up to strains, and that is the reason for replacing with metal.
>"But why not TRY it as RG suggested."
My concern was if the designers had a reason (other than price) for using plastic.
>"The schematic says nothing about where the circuit common is grounded to chassis."
Can you provide a link to the correct schematics?

@Mick and Gtr_tech:
"Insert your lead and connect up your guitar."
Good points....

[Enzo]

The reason they use the plastic is because they have planned out the grounding scheme. If you do something different, it is not going to HARM the thing. All it potentially will do is hum more. Plastic jacks are not cheap substitutes. Switchcraft makes both kinds in most of their jack types. If ther is any price differential at all, it is incredibly tiny. Price is not the reason. I think I said before, it is not rare at all to find both types on an amp.

I'm not trying to sell you on them, I just want it be known that it is a misconception to think of plastic jacks as cheap.

You want to know the biggest reason for stripped threads on those jacks? Allowing the nut to get loose. ANy time you have a loose nut, it allows the jack to move and that gives it a mechanical advantage against the threads.


I run a pro shop, and I cannot say plastic jacks are unreliable. Are they easier to crack or strip? yes. Does it happen a lot? Most amps live their lives with their original jacks of either kind.

[g1]

I'll agree with Enzo, the jacks should not be condemned just because they are plastic. Some seem worse for getting stripped than others. In my experience the ones with fine threads and metal nuts are the worst for this. The Rean types with plastic nuts did not strip ever. The newer switchcraft or neutrik with more coarse thread and large nut (commonly used on Fender now) do not seem as prone to stripping.
One thing I will say in defense of plastic jacks. I've seen plenty of vintage Fender amps with bent chassis at the input jacks. When the amp (with guitar cord plugged in) falls on it's face with metal jacks it bends the chassis. With plastic jacks the jack breaks. The work involved in straightening a chassis front can be major, sometimes not even possible to get right.

[Enzo]

And stepping on the cord, also good for bending panels.

[Mick Bailey]

The work involved in straightening a chassis front can be major, sometimes not even possible to get right.

Last year I had to completely strip a Silverface Twin Reverb to straighten the chassis. It was severely dished in the middle where the panel had been caved in with something very heavy. It took my hydraulic press and lots of shaped Iroko and aluminium blocks to get it right, plus a lot of time. Bent metal work-hardens and stretches on the crease, so when you come to strighten it, it wants to bend somewhere else.

[Gtr_tech]

You can test for hum before you make any change;

Insert your lead and connect up your guitar. Turn the guitar tone/volume up and have the amp set to a low-ish volume. Now, touch a piece of wire between the metal jack plug body and something on the amp that is connected to the chassis - a screw, pot nut or whatever. If it hums that's what you'll get with non-isolated sockets. If there's no change then it won't matter if the sockets aren't isolated.

Turning up the guitar vol will likely mask any gnd hum increase with p-up hum. Sure you can plug in a guitar, just leave the vol at zero. or plug in a cable and short the tip to the sleeve. I have a bag full of lab grade 1/4" to banana jack dealies that I use for this all the time. I just bridge the jacks....

[Gtr_tech]

I will use metal jacks and fly leads to the board on things that are used at least semi professionally. I do it for reliability since the plastic ones have a tendency to fall apart, get sloppy connections, break loose from the board, etc. I can say I have not had a single amp come back with jack problems.

[Mick Bailey]

Turning up the guitar vol will likely mask any gnd hum increase with p-up hum. Sure you can plug in a guitar, just leave the vol at zero. or plug in a cable and short the tip to the sleeve. I have a bag full of lab grade 1/4" to banana jack dealies that I use for this all the time. I just bridge the jacks....

You're checking the difference in hum levels - any ground loop hum is in addition to Pickup hum. Turning the guitar volume down acts the same way as not having it plugged in at all - it shorts out the 1st preamp stage (unless it's configured with a blend pot) - same as a shorting input jack. As the amp relies on a signal between its hot input and the signal ground (as opposed to the chassis ground), shorting it out often reduces the chances of evaluating what hum level may be present when the guitar is being used. Some amps don't hum at all with the guitar volume turned down, but hum violently with it turned up - way more than pickup hum. I have an Ohm 75 right now that does exactly that.

Why would you plug in a cable and short the tip to the sleeve - Isn't this the same as not plugging it in at all?

[Enzo]

Maybe so, but you are assuming the internal shunt is making 100%. BY plugging into the jack, we eliminate that potential detail. It takes 5 seconds, so why not try it?

Never think up reasons not to check something.

[Mick Bailey]

I should have mentioned the shorted plug being in/out is from the amp's perspective. Of course, you'd need a plug in there to get a ground connection on a plastic socket unless the chassis is out.

I still maintain that you'd need to check for hum with the guitar volume turned up - a shorted input in many cases won't produce anything, which is the point I'm making about checking - why wouldn't you do this, for the sake of rotating a control?

[Gtr_tech]

In my experience, if gnd loop hum is an issue, it doesn't care of there's signal present or not. Its not a signal path issue we're looking for, its gnd noise. And that will make itself apparent with no signal so long as the pre and master controls are up enough to hear it.

Its been a while since I had to sleuth out a gnd loop issue, but seems to me the last time was for a local amp builder. He had built something and had what sounded like pwr supply hum, but no ripple to speak of. The hum came and went when the gain ctrrl was turned, and got noisier at the lowest setting. It ended up be'n where he had the isolated input jack grounded. I simply found a quiet gnd for it and the noise was gone. I never had to plug anything into it to notice the issue.

[Mick Bailey]

In my experience, if gnd loop hum is an issue, it doesn't care of there's signal present or not. Its not a signal path issue we're looking for, its gnd noise. And that will make itself apparent with no signal so long as the pre and master controls are up enough to hear it.

Its been a while since I had to sleuth out a gnd loop issue, but seems to me the last time was for a local amp builder. He had built something and had what sounded like pwr supply hum, but no ripple to speak of. The hum came and went when the gain ctrrl was turned, and got noisier at the lowest setting. It ended up be'n where he had the isolated input jack grounded. I simply found a quiet gnd for it and the noise was gone. I never had to plug anything into it to notice the issue.

My experience differs slightly, in that I've had a small number of amps where the hum is controlled by the guitar's pot when the input is grounded to chassis. Usually cheap transistor amps - starter packages and the like - the ones most likely to have a 'less than optimal' socket to begin with. This suggest two scenarios; one where grounding the input jack creates a 'classic' ground loop of difference in potential between grounding points (independent of input signal presence, or whether the jack is shorted or not), and another where noise is being routed via the guitar circuit to the signal input. It has to come down to the grounding scheme and input circuitry, and whether the amp is double-insulated (like some Roland amps).

[kfaria]

Follow up to my OP......
Found a schematic and indeed the isolation seems to be necessary.
Kept the original isolated jack and super glued it to the chassis.
During that process one of the pins connecting the input jack module to the front board, PCB307070, broke off.
Then the joy began.
The seven mounting screws for the front board were gooped with red Loctite.
Stripped three of the philips head screw slots before thinking.
Used a Weller soldering iron to heat them and viola, they came out.
Thanks for all your suggestions.
.....Kurt

Edit: The GMX212 was purchased at a pawn shop.
The jack was already stripped.