16 Ohm Input Not Working on 1960A cab?

[SunTzuBean]

Hi!

I’ve recently run into a problem where my 16 ohm input on my 1960A cab suddenly stopped working. I never flipped the stereo switch while the cab was in use, never plugged another speaker jack into the 4 ohm input while using the 16ohm input, etc.

The cab has the standard Marshall stereo jack. I am not going to hardwire the cab as a 16ohm cab because I have amps that only have 4ohm outputs, and I occasionally use the stereo feature with two separate amps.

I have measured the inputs with a voltmeter and the measurements are as follows:


As you can see, it’s ONLY the 16 ohm input that is having problems. I am very confused as to why this is happening. A replacement jack plate is on its way but I am very curious why this is even happening in the first place. Has anyone experienced this before?

 

[fitz]

Hi!

I’ve recently run into a problem where my 16 ohm input on my 1960A cab suddenly stopped working. I never flipped the stereo switch while the cab was in use, never plugged another speaker jack into the 4 ohm input while using the 16ohm input, etc.

The cab has the standard Marshall stereo jack. I am not going to hardwire the cab as a 16ohm cab because I have amps that only have 4ohm outputs, and I occasionally use the stereo feature with two separate amps.

I have measured the inputs with a voltmeter and the measurements are as follows:

View attachment 113308
As you can see, it’s ONLY the 16 ohm input that is having problems. I am very confused as to why this is happening. A replacement jack plate is on its way but I am very curious why this is even happening in the first place. Has anyone experienced this before?

No personal experience, but the switching board on those 1960 cab jacks are notorious for failures.
It's not a unique problem to your cab.
I'd have suggested an aftermarket replacement, such as Mojotone, but you said, you have one ordered.

 

[Gene Ballzz]

Dirty contacts in the jacks or switch. Or cold/broken solder joint at the most used jack and/or broken trace on the circuit board. Those Marshall jack plates are known for being absolute garbage and responsible for many amp failures, due to intermittent operation! The really sad and stupid thing is that none of the good quality aftermarket retrofit replacements mount with the same footprint! The one linked below is really nice but takes a slightly different size/shape hole. On the plus side, many folks simply mount them from the inside of the back cover, leaving the jacks more recessed! Far superior to the overpriced Marshall junk below it!

https://www.tubesandmore.com/products/jack-plate-plug-and-play-mono-stereo

Jack Plate - Marshall, Switchable Stereo / Mono | Antique Electronic Supply

Switchable stereo/mono jack plate for back of cabinets.

www.tubesandmore.com

Just My ,
Gene

 

[SunTzuBean]

Thanks for the quick replies!

I did a little sleuthing and I finally figured out what the problem was:

The 4 ohm jack sleeve contact does not short out at the blue X when a speaker cable is inserted into the 16 ohm mono input, causing there to be no connection there, which would explain the break in the circuit and why only one mode of cab input does not work.

When I apply pressure to this part of the switch, the ohmmeter reads about 16 ohms as expected.



Any advice for keeping that metal in place? I bet I could replace the whole speaker jack itself, it's just a matter of finding a better quality one. But unsure if I will actually do it, as I know the Marshall jacks are crap.

EDIT: https://www.parts-express.com/Neutrik-NMJ4HC-S-1-4-Mono-Phone-Jack-Switched-090-972 I think these would be a good replacement

 

[fitz]

Thanks for the quick replies!

I did a little sleuthing and I finally figured out what the problem was:
View attachment 113322
The 4 ohm jack sleeve contact does not short out at the blue X when a speaker cable is inserted into the 16 ohm mono input, causing there to be no connection there, which would explain the break in the circuit and why only one mode of cab input does not work.

When I apply pressure to this part of the switch, the ohmmeter reads about 16 ohms as expected.

View attachment 113323

Any advice for keeping that metal in place? I bet I could replace the whole speaker jack itself, it's just a matter of finding a better quality one. But unsure if I will actually do it, as I know the Marshall jacks are crap.

EDIT: https://www.parts-express.com/Neutrik-NMJ4HC-S-1-4-Mono-Phone-Jack-Switched-090-972 I think these would be a good replacement

Try some Deoxit right where you circled that contact, might just be a little dirty.

 

[Dogs of Doom]

That Neutrik jack has the same type of mechanism as the Cliff jack...

That's the same problem that happens on speaker out jacks on JCM2000 amp's. They use the same switch-jack & when it fails, it removes the ground to the other output jacks. Then, when people go to use the other jacks, they blow a trans'...

also, Cliff jacks are considered high quality, in spite of the flaw in the switching mechanism.

 

[SunTzuBean]

That Neutrik jack has the same type of mechanism as the Cliff jack...

That's the same problem that happens on speaker out jacks on JCM2000 amp's. They use the same switch-jack & when it fails, it removes the ground to the other output jacks. Then, when people go to use the other jacks, they blow a trans'...

also, Cliff jacks are considered high quality, in spite of the flaw in the switching mechanism.

I reckon any sort of switch that relies on mechanical motion is bound to fail at some point, that's a shame that happens on the JCM2000's... I hope it doesn't happen to my JVM! I think I'm gonna still try the Neutrik jacks because at least the angle on the switch looks better. So in theory, it should retain its "springiness" longer at least.

 

[SunTzuBean]

Try some Deoxit right where you circled that contact, might just be a little dirty.

Sure, I'll give it a try and report back in this message

EDIT: It did not work sadly

 

[Matthews Guitars]

My solution to stereo jack plates on cabinets is to make a new back board with an old style single speaker jack cup in it. Cut out that stereo switching jack panel nonsense and go old school.

 

[jimmyo]

Those Marshall stereo/mono jacks are junk. Learned after some experiences with them failing to swap these out to single 16ohm input like the older cabs. I don’t like intermittent or failing connection between the cab and a cranked tube amp.

Maybe there is a more robust replacement switching jack you could order and use, if you are trying to keep that functionality?

 

[Dogs of Doom]

I'll have to take a pic, but, I bought a used JCM900 A cab & it has the stereo plate.

The guy, whom I bought it from, toured on the Van's tour some time back. He told me, that he replaced the original nut, on the 16 ohm input w/ a metal beefy one. All the guys on the tour did that also, & it supposedly made the jack more secure. Mine still works fine...

Not sure how that solves the switch from failure, but, he claimed that everyone on that tour thought that it did...

 

[Leonard Neemoil]

That's the same problem that happens on speaker out jacks on JCM2000 amp's. They use the same switch-jack & when it fails, it removes the ground to the other output jacks

That's the first thing I thought about. Is it safe to just solder a jumper wire from one side of the ground switch to the other like the ground mod for the JCM 2000?

 

[Dogs of Doom]

That's the first thing I thought about. Is it safe to just solder a jumper wire from one side of the ground switch to the other like the ground mod for the JCM 2000?

no...

because the PC board has different wirings for when that switch is connected, vs not. &... The difference is series vs parallel. Imagine hooking your speakers up in series & parallel at the same time. You'd be shorting out the circuit...

The way this is supposed to work, is that you can switch between 2- distinct ½'s 8 ohm mono x's 2 (stereo), or 16 ohm or 4 ohm. 4 ohm would have to have all speakers in parallel. 16 ohm would have to have series parallel. That break, in the ground, enables the 16 ohm & stereo, otherwise you might as well rip the switchplate out...

I'm wondering if something isn't wired right, or, if the PC board fried, shorting something...

I would think, that, the switch should be "off" when there is nothing plugged into it. It's supposed to engage, when you plug into it. would make sense, because parallel would be 4 ohm, which all grounds would be parallel, as would the leads, whereas series parallel, the grounds should not be all connected, but, the connection should form a loop...

 

[SunTzuBean]

I'm wondering if something isn't wired right, or, if the PC board fried, shorting something...

Visually inspecting the board, nothing is fried, shorted, or otherwise out of place. But there is a tiny, tiny space between the switches that I have circled in the picture. And remember, all other parts of the plate work fine!

 

[Leonard Neemoil]

no...

because the PC board has different wirings for when that switch is connected, vs not. &... The difference is series vs parallel. Imagine hooking your speakers up in series & parallel at the same time. You'd be shorting out the circuit...

The way this is supposed to work, is that you can switch between 2- distinct ½'s 8 ohm mono x's 2 (stereo), or 16 ohm or 4 ohm. 4 ohm would have to have all speakers in parallel. 16 ohm would have to have series parallel. That break, in the ground, enables the 16 ohm & stereo, otherwise you might as well rip the switchplate out...

I'm wondering if something isn't wired right, or, if the PC board fried, shorting something...

I would think, that, the switch should be "off" when there is nothing plugged into it. It's supposed to engage, when you plug into it. would make sense, because parallel would be 4 ohm, which all grounds would be parallel, as would the leads, whereas series parallel, the grounds should not be all connected, but, the connection should form a loop...

That's what I thought...well, not all the technical stuff you mentioned, but I didn't think it would work anyway.

Thanks for the clarification.

Cheers!

 

[Gene Ballzz]

My guess is that if you really dissect it, you'll find that its not actually the jack that has failed, but instead one or more of its solder conections to the board or a faulty trace on the board. Repacing the switch may cure all the problems, just by resoldering it all, even if the switch itself isn't the culprit. Then again, it may no, if the board is toast! They are junk in many ways!
Just Sayin'
Gene

 

[SunTzuBean]

My guess is that if you really dissect it, you'll find that its not actually the jack that has failed, but instead one or more of its solder conections to the board or a faulty trace on the board. Repacing the switch may cure all the problems, just by resoldering it all, even if the switch itself isn't the culprit. Then again, it may no, if the board is toast! They are junk in many ways!
Just Sayin'
Gene

The way I dissected it was testing each PCB trace from point to point to see if there was a connection, to ensure connections from solder joint to circuit board trace to solder joint worked. Frustratingly, at first every single stretch of solder joint, circuit trace, to solder joint worked as on the circuit diagram. This includes the switch, which still works absolutely fine.

Then I assumed that despite not working, there HAD to be some solder trace broken, and that I had to be doing something wrong. I followed the 16 ohm signal path from start to finish, bridging the solder joints with a separate alligator clip to bypass the PCB traces. I then would check to see if the resistance on the multimeter changed. I did this until I got to the negative speaker jack end on the 4 ohm side. There should be a direct connection from the sleeve of the shorting jack to the negative speaker lead. As soon as I bridged that gap, the resistance read a little less than 16 ohms. Jackpot. (no pun intended)

After this I removed my alligator clip bridge and simply pushed with my finger the top of the shorting jack down and again got the readings I expected on the multimeter. But sufficient to say I ruled out the switch, as I thoroughly tested each connection!

My first course of action before all this was to touch up the solder joints!

 

[Chris-in-LA]

My solution to stereo jack plates on cabinets is to make a new back board with an old style single speaker jack cup in it. Cut out that stereo switching jack panel nonsense and go old school.

Except that OP also runs amps with 4ohm output and needs the switching function.