Testing old style impedance selector

[wallythacker]

I got an older super lead the other day. It has the "pull out-rotate-push in" selector. It seems loose and sloppy. The PO told me the OT had to be replaced 3-4 years ago. I think two are related?

I read this thread figure out if it is hardwired or not:

http://www.marshallforum.com/workbench/56788-ohm-selector-hardwired-not.html

Will an Ohm meter across the pins provide anything useful?

Any other tests/checks?

thnaks

 

[Joshabr1]

It's usually not the connection that's loose it is the actual plug itself. The little metal feet wear the plastic on the inside of the male part itself so it feels loose. The way to check if the connection is loose is inside the amp.

 

[Joshabr1]

As long as it makes contact it should work fine unless its been buggered up in the past. Which may be the case

 

[wallythacker]

The pins are quite dirty. It can't hurt to clean then up. As for making proper contact I guess I'm taking the chassis out to have a good look.

 

[Joshabr1]

Yes. Look at ur selector from the inside. Make sure to discharge caps!!!! See which one u r engaging and check it and the common. They should not wiggle. The rest will though.

 

[ampmadscientist]

I got an older super lead the other day. It has the "pull out-rotate-push in" selector. It seems loose and sloppy. The PO told me the OT had to be replaced 3-4 years ago. I think two are related?

I read this thread figure out if it is hardwired or not:

http://www.marshallforum.com/workbench/56788-ohm-selector-hardwired-not.html

Will an Ohm meter across the pins provide anything useful?

Any other tests/checks?

thnaks

The impedance selector plug is very UN-reliable.
In fact it's probably the most UN-reliable part of a Marshall amp.
And, it has caused many Marshall amps to blow up.

Vibration alone can cause it to fail.
Oxidization also causes it to fail. The contacts corrode.
Changing the setting causes to contacts to wear out and fail.

When the contact fails, the amp is operating without a load.
Or, operating with an intermittent load.
This causes the tubes to blow, the impedance to wander all over, the tube sockets burn and arc, and the output transformer blows.
ALL caused by intermittent connections at this selector plug.

REPLACING the plug: Not a great plan.
1. The NEW selector is probably just as UN-reliable as the old one. It will wear out and fail.
2. The NEW replacement part is a different size. You have to drill the hole bigger, and the screw holes won't line up, either.
3. So, you have to modify your Plexi to install it. A poor way to go.

A better way:
Is to make a metal cover plate for the hole. The screws will line up with the OLD screw holes. The amp chassis remains stock and UN-modified.
Then, mount a GOOD selector switch on the metal plate.
Reliability is increased, without modifying the chassis or the holes.

OR:
Solder the transformer wires directly to the speaker jacks. Bypass the selector entirely.
This makes the amp permanently 16 ohms...etc...no selector.
It's better than blowing an output transformer!

OR:
Use the FOUR speaker jacks, on the back of the plexi.
Two jacks will be 8 ohm (allows for 2X 16 ohm cabinets)
One jack will be 16 ohm (allows for ONE 16 ohm cabinet)
One jack will be 4 ohm.
NO selector connected, only jacks used to select the output impedance.
AND: label the jacks, accordingly!

WHY am I telling you this?
I have worked on guitar amps 40 years. I have learned a few things.
One of the things I have learned: this selector plug SUCKS. And, the replacement ones also SUCK.
You are better off using ONE of the alternate methods (above).

 

[Clammy]

I've played my old Super Basses with those impedance selectors for 20+ years, live and at rehearsal, at high volume, and have NEVER had the plug fall out, or the selector "fail".

These things require MAINTENANCE. The sockets inside the female part, will spread open over time, and they need to be retensioned now and then. It's also a good idea the spread the prongs on the plug a bit. And just like any other electrical connector (input jacks, etc...), they need to be cleaned from time to time. Deoxit works great.

As for testing them. It's easy. All they do is connect the center socket to one of the 3 outer sockets. So, using an ohmmeter set to "continuity", you should get the beep, and zero ohms between the connected sockets. Check each one individually.

Cheers!

 

[Joshabr1]

I've played my old Super Basses with those impedance selectors for 20+ years, live and at rehearsal, at high volume, and have NEVER had the plug fall out, or the selector "fail".

These things require MAINTENANCE. The sockets inside the female part, will spread open over time, and they need to be retensioned now and then. It's also a good idea the spread the prongs on the plug a bit. And just like any other electrical connector (input jacks, etc...), they need to be cleaned from time to time. Deoxit works great.

As for testing them. It's easy. All they do is connect the center socket the one of the 3 outer sockets. So, using an ohmmeter set to "continuity", you should get the beep, and zero ohms between the connected sockets. Check each one individually.

Cheers!

I totally agree with this. I have played 100 watt Marshall's for years at high volume on top of cabs and never even had one work loose. They may feel like they are loose but that is just in the male plug. That style of switch has been 100% reliable for me.

 

[Clammy]

I totally agree with this. I have played 100 watt Marshall's for years at high volume on top of cabs and never even had one work loose. They may feel like they are loose but that is just in the male plug. That style of switch has been 100% reliable for me.

Yeah, I forgot to add that the heads always live on top of 2 cabs (pair of 4x12s or 4x12 on a 2x15), and I play guitar and bass through them, so there's plenty of vibration.

Cheers!

 

[wallythacker]

I guess these things must be OK because I noticed on the JimiHendrix 600 unit run the same type of selector is used.

But, I'm not going to ignore what our resident mad scientist said though. There's a lot of good ideas that he presented.

Thanks for the help from all sides.

 

[Joshabr1]

I guess these things must be OK because I noticed on the JimiHendrix 600 unit run the same type of selector is used.

But, I'm not going to ignore what our resident mad scientist said though. There's a lot of good ideas that he presented.

Thanks for the help from all sides.

Yes. He is very helpful on here.

 

[Joshabr1]

I guess these things must be OK because I noticed on the JimiHendrix 600 unit run the same type of selector is used.

But, I'm not going to ignore what our resident mad scientist said though. There's a lot of good ideas that he presented.

Thanks for the help from all sides.

Yes. He is very helpful on here.