Loose/faulty Amphenol plug on Leslie 147 amp

[juanbastonoeste]

Hello all,

 

The male amphenol plug on my leslie 147 amp is loose so that if I move the leslie cable at all, the sound tends to cut in and out until I find a good position again - then everything's fine as long as the cable isn't moved again.

 

I've encountered this problem in various leslies in the past, so I'm guessing it's very common. My question is what's the most effective way of fixing it?

 

[B3Nut]

There should be a wavy retaining spring around the part of the connector that's under the chassis. It's probably hanging there off the wires by now. You don't want to run it like that if so. Push the connector back in and work the spring back under the bosses on the connector. It'll go back into its little groove. You'll want to clean and possibly resolder the pins, and clean the female contacts on the cable as well as tighten them. You'll have to take the cover off the female end, then you'll see the contact springs. They lose tension over time.

[juanbastonoeste]

Gotcha. For the female contacts part, are you referring to the the actual leslie cable I'm using, or is there a cable inside the amp chassis? And how do you tighten the contact springs?

[MoodyBluesKeys]

The actual Leslie Cable. Contact spring tightening - I always did it with a small screwdriver, carefully bending the contact (each one is shaped a bit like a letter "U" and the U bulges a bit when it gets loose) on both sides of the U. Not enough room to get long-nose pliers in there.

Bend a little, look at socket. Do this for all 6 connections, then put the socket on the plug (without putting the cap on the socket) and see if it feels firm without being way to hard to connect.

 

The connectors are the same type as early tube sockets, and were not really designed for many plug and unplug cycles. Only the 5 and 6 pin sockets were designed in this way, the 8, 9 and 11 pin use a different design (although can also be problematic).

 

Added comment: Make SURE that none of this stuff is still connected to any power. The 147 Leslie has the full line voltage between pins 1 and 6 (IIRC).

[mate stubb]

The 147 Leslie has the full line voltage between pins 1 and 6 (IIRC).

 

Not quite. Pins 1 and 6 are signal, but everywhere else will be line voltages.

[MoodyBluesKeys]

Its been too long since I wired one up. Yeah, 3&4 are AC power. I do still have a new 25' 11-pin Leslie cable on hand, and several 5 and 6 pin plugs & sockets.

 

I remember running into a fair number of Leslies that the 6 pin connector had the retaining ring come loose. Seemed rather common, probably because the cables were heavy and not super flexible.

[WesG]

Probably because the design is ridiculous by today's standards. If I were running a backline company, I would change to a different connector entirely.

[Musicale]

Yeah like one of those great barrel jacks that you plug wall warts into!!

 

Musicale

[CEB]

FWIW - These things never really were intended to be portable.

 

A smart idea I think Wes posted once was to keep a spare female connnector on hand to plug the males into to protect the pins when not hooked up. My biggest problem over the years seems to be have been pins getting bent once in a while.

[OB Dave]

The connector in the amp being loose, in and of itself shouldn't cause the sound to cut in and out, so you probably have a broken wire in the connector, either at the male (amp) end or the female (cable end). On the male plug, you can often see that the solder has broken loose and reflowing fresh solder will often do the trick.

 

You definitely want to get that plug nailed back down though because if it moves the wrong way you can short 120v to the chassis, which will be no fun for anyone.

[CEB]

That is no joke. You can get shocked enough with a organ in good order when using a vocal mic with no wind screen and if you make metal contact with the organ and the polarity is off.

[juanbastonoeste]

OK. Thanks for the advice. I'm a bit concerned going under the hood of my amp to fix this issue, as I don't have electronics experience and have heard that there are high voltages potentially being held at various places in the amp circuitry..I'm also not confident as to how I would properly drain the capacitors of their voltage to make it safe for such a repair. Any advice on how to fix this without hurting myself?

[OB Dave]

Unplug the Leslie amp and let it sit over night. Most of the time the internal resistance of the cap will be enough to bleed of any voltage stored. You don't want to bank on this though. Pull the amp and pull the two 6550 power tubes so you can flip it on its back. Identify all the capacitors. Some will be point to point wired ones just floating inside the chassis, but you especially need to worry about the big can capacitors, which are mounted to the chassis sort of like tubes. I forget how many there are in a 147 but one of them is two or three capacitors in one package, with four (?) terminals on the bottom. Sorry, this is all from memory. On each capacitor, you want to short each terminal to all the other terminals on that capacitor. Most of the time that's only one other terminal. but watch for the multi pin guy. Best way to do this shorting is to clip an insulated alligator test lead to one terminal, and touch the other end of the test lead to the other terminal(s) on that cap. I've seen guys do it with needle nosed pliers, but I personally wouldn't trust pliers to complete the circuit for something like this. Anyway, on the four terminal cap, go ahead and do all 12 permutations just to be safe. You may get a loud pop if there's a charge still stored, so be prepared to be startled. Isn's this fun?

 

Most important thing of all - only put one hand inside the amp until you know all the caps are discharged. Electricity always finds the easiest path to ground, and you definitely do not want that path to be up one arm, across your chest, and down the other arm.

[WesG]

Loose connector CAN cause this issue in a 147, i think - the audio signal could short to the chassis.

 

Discharging the caps is best done through a resistor to avoid stressing old parts, but the caps should be self-discharged after a day anyhow, no real need to worry unless you are impatient.

 

IIRC the 147 has one can cap with four sections. You don't short these to each other - you short each section to ground. One terminal will probably be obviously the ground, otherwise, consult the markings on the side of the cap. Bt I'm pretty sure the chassis is grounded in a 147. There is also at least one decent sized cap under the deck.

 

Don't fret about this of you're not experienced, Leslie amps are really simple. Just don't break anything.

 

If you have an experienced friend helping, ask him if your amp has death caps, if it does, clip them out.

 

Wes

 

 

[WesG]

CEB - I should mention, an unmodified organ in good working order should not be able to shock you. If you're getting shocks, and the organ has not been modified, I would immediately suspect the power tranformer in the preamp, and also check the the one in the Leslie.

 

Hammond and Leslie relied on single-insulated power transformers to provide galvanic isolation from the mains. These can leak current from the primary to the secondary winding. Testing this is the very first thing in the service manual.

 

I was reading recently that some top-level acts will meter instruments to the mic ball as part of setup. That seems like a smart idea.

 

Wes

[Threadslayer]

FWIW - These things never really were intended to be portable.

 

Good point. I never thought of it before, but in a church or theater it would have been:

1) Install once

2) Run forever

[CEB]

CEB - I should mention, an unmodified organ in good working order should not be able to shock you. If you're getting shocks, and the organ has not been modified, I would immediately suspect the power tranformer in the preamp, and also check the the one in the Leslie.

 

...

 

It is the same type of shock singing guitarist will get once in while when your lips hit a mic and the polarity of the PA circuit and guitar amp are different. Not sure what causes it. I just use a foam windscreen and that stops it.

[WesG]

With guitar amps, it can be a grounding issue, which doesn't happen in (stock) Hammonds. It can also be leaky death caps, which doesn't happen in (stock) Hammonds and later Leslies.

 

It can also be transformer leakage, that is a defect.

 

The quick lesson is that it should happen on NO unpolarized equipment EVER and if it does, you have equipment that needs repairing, it is not a "polarity problem" - throwing the polarity switch merely hides it.

 

The wise techs that meter the strings-to-mic-ball voltage are looking for a hazard, but when they find it, they will also fix the underlying defect at the earliest opportunity.

 

If you are getting shocked by equipment and need a foam windscreen to fix the problem, you've basically thrown a plastic bag on a sucking chest wound. It's a solution that should be reserved only for the trip to the hospital.

 

Note: some ancient guitar amp designs do not use mains-isolation transformers, such as the DanElectro amp-in-a-guitar-case. These are defective (and dangerous) by design.

 

BTW - incorrectly-wired stage power is why I consider double-insulated mains transformers to be safer for musical instruments than grounded chassis. Luckily that is not an issue that we need to worry about with stock Hammonds. No grounded chassis..although only single-insulated transformers.

 

Wes

 

PS: CEB knows this, but for future surfers: I am not an electrical professional