Help with 1960 Lead cabinet speaker switch

[RLW59]

12.8 DC resistance is a good reading for 16 ohms AC impedance. Solidly in the typically range.

Series/parallel should always match up to the individual speakers. If one or more speakers randomly vary a bit higher or lower, series/parallel might be a bit off, but if all four speakers read 12.8 then series/parallel should read 12.8.

13.3 is also in the typical range, so none of us thought that reading was concerning. But the jackplate couldn't have changed the individual speakers -- they must have been all 12.8 all the time. So with the speakers measured, it's clear that something in the switching circuit was adding resistance all along.

 

[Gene Ballzz]

I don't want to be a doubting Thomas, but before I re-heat something that may not be fault, can you folks look at some clearer pics. Thank you.

Cable just tested fine, and these are the heavy duty Hosa cables I have.

So do I try reheating those six contacts or do they look ok and it's probably inside the switch that needs to be cleaned?

Karl, @Back to Marshall ,

There are at least several possible/probable failure points involved here:

A> Poor, oxidized, intermittent contacts in the rather cheap style switch, that are often prone to failure, in the real world.
B> Poor, oxidized, intermittent contacts in the jacks, that we are not even sure if they are genuine Cliff UK or the cheaper "knock offs" that Marshall has been putting in their amps for a while now! I would suspect that genuine REAN/NEUTRIK versions would be as good as the Cliff UK.
C> Faulty trace on the printed circuit board, possibly caused by flexing from weight on the jacks, from heavy speaker cables being horsed in and out and pushed side to side over the years, rough handling of the switch, etc, etc.
D> Faulty solder connection(s), due to lead free, RoHS solder. This stuff is well known to develop mostly invisible, hairline cracks, as it is more hard and brittle than lead solder. But hey, at least you don't have to worry about your kids getting lead poisoning when they "lick" your circuit boards!

My biggest suspicion is:

E> Any single and/or combination of any of all of the above!

With all that said, I'd love to get one of these jack plates in my hands, to disassemble, dissect and determine if it could be repaired/upgraded with a more sturdy, short bat toggle switch, real Cliff UK jacks and all solder connections re-done with real solder! While such a repair/upgrade would likely cost at least as much as a new, OEM replacement, it would most probably be rock solid and generally bullet proof instead of just a new piece of the same old junk!

The concern was the 6.6 reading for the 4 ohm jack from his first set of measurements.

Even if he doesn't use the 4 ohm jack, it suggests there may be more going on than just the intermittent problem on the 16 ohm jack.

On a hardwired jack, I'd be inclined to write it off as a spurious reading. But on a switching jackplate that already has an intermittent open circuit, it strikes me as a red flag.
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Many people assume that the Marshall jackplate problems have to be simple, straightforward, fixable issues.

But there have been threads describing really weird failures. Like all the options make sound, but all 4 options are way off on impedance. Usually people with problems end up hard-wiring or replacing the jackplate instead of fully diagnosing the cause of failure so it's hard to know exactly what goes on with the jackplates.

(One possibility I haven't seen explored is that sometimes PCB's can become conductive.)
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Intermittent opens while playing are the absolute hardest things you can subject an amp to. So far, the 16 ohm setting has either worked or not worked, and when it's worked it hasn't cut off and on while playing.

But with a jackplate that's started acting flakey there's a distinct chance disaster is lurking around the corner.

Absolutely "correctomundo" on all points there!

Just Jackin'
Gene

 

[Gene Ballzz]

I decided NOT to try re-soldering the circuit board that looks ok. I'm going to assume the inside of the switch is going bad.
I'm in the process (almost done) of re-wiring the cab, which is easy with the connectors already on the wires, to Series-Parallel 16 Ohms. I already had a jack and a plate I could use.
Before I disconnected anything I labelled all wires as to original location, so If I get the opportunity to replace the original style of speaker/Impedance selection I can.
Individually, all speakers measured 12.8 Ohms.
Re-wired, and tested prior to the jack I also get a new cabinet reading of 12.8 Ohms. I know with a multimeter speaker resistance always reads lower than actual. Since the readings are all consistent, I'm going to assume I'm right close to 16 Ohms. Does that agree with you folks?
I figured it was better to be safe and just wire the cab for a consistent known value and not take a chance of it changing and ruining a brand new amp!!

I see this as having been the wisest option, for now! And yeah, your readings now appear to solid and consistent, just what you want!

On another note, if you live in the US, I'd love to have you ship/mail me that wonky plate for inspection and possible repair/upgrade design. If successful, in return for your willingness to be the "guinea pig" I'd only charge the price of the parts for the upgrade and return postage. If it turned out to be just junk, I could either throw it in my own bin, or send it back to you on my dime, in alternate return for your assistance, and let you throw it in your own bin!

This has been an ongoing issue with these jack plates for many years, and it seems a wonderful idea to come up with an elegant and solid solution that doesn't require cabinet modification or deletion of the rather handy features provided poorly by these inferior jack plates.

Just Thinkin'
Gene

 

[RLW59]

Segue/tangent now that the issue has been fixed...

I've never heard of problems with the Plug & Play brand switching jackplates. But I don't know how many have been sold, and I'm sure lots of them have gone into other brands of cabinets. It's possible they fail at the same rate as Marshall's jackplates but we just don't read about the fallout.

Very few brands do switching jackplates. I'm quite confident that Marshall has made and sold more switching jackplates than everyone else combined. And pretty sure the vast majority of them work fine forever.

If they were all ticking time bombs doomed to eventual failure, 1960's would have such bad reputations that Marshall would discontinue the feature, save a few bucks in production costs, and charge more for "new and improved", "vintage hardwired" cabs.

It's like Gibson headstock breaks. It's a real problem that perhaps happens more often than it should, but it really only happens to a a fairly small minority.

 

[chocol8]

A static resistance of 12.8 is right where you would expect for an impedance of 16 ohms.

If you only use the cab at 16 ohms, a straight jack is much safer than a switching setup.