How to repair a fine coarse tuning poti ?

[JHS_Syke]

Hello,

The fine coarse VFO tuning poti of my Systron Donner counter model 6057 does not work good any more. Of coarse a new would be better...

Did somebody try to repair such a device.?

Jörg

[factory]

If both the fine & coarse controls are not working I wouldn't suspect the pot, more likely a fault elsewhere.
Have you checked the pot? The fine is two one turn 1K & the coarse a 16 turn 100K.

David

[JHS_Syke]

They work in principle, but they both do not allow a stable result in frequency.

If I set coarse, and then tune with the fine knob, fine influences the coarse.
I think there is some dirt between the axles. It became better some oil.
Still there is a lot of hysteresis.

I believe it is necessary to look inside the mechanics....

Jörg

[JHS_Syke]

I found that both axles are influencing each other. There seems to be some dirt in between. A little bit oil helped a lot.

But there is still a lot of hysteresis in the adjustment.

Jörg

[WME_bill]

Systron Donner 6057 Counter. VFO tuning potentiometer.
It is a dual 1k for the fine tuning and a single 16 turn 100k for the main tuning, all balanced across the +15v and -15v lines.
Your chances of finding a suitable complete replacement are nil I am afraid. And a specific replacement part from the makers would cost more than you paid for your unit or perhaps it's original cost in 1973.
But a standard 10 turn 100k wirewound pot would be possible, or perhaps better as it will be smoother, an end adjust rectangular cermet pot with panel mounting and some experimenting to fit a knob would work very well.
I have had a look at my unit, and there is not a lot of space, so the cermet pot might be a better alternative.
But I tend to agree with David factory and the fault is more likely elsewhere. Put your meter of the lead to pin 2 on the oscillator board A2 and see if the voltage changes up to +-15 v.
The manual is now scanned and will be on its way to you shortly.
wme_bill

[JHS_Syke]

Sorry for the double post.

In the meantime I found, that the thin axle is not straight and both knobs have a dent.
So I believe that this instrument had a major accident leading to a this problem.

I will tomorrow fumble out the pot assemby and check what I can do.

@Bill thanks in advance

Jörg

[JHS_Syke]

The Pot assembly consists of a planetary gearbox for coarse working on a standard 270 degree 100k pot, and a two standard 270 degree 1k pots direct connected to the thin axle.
The gearbox is made of plastics and very fragile. There was some dirt between the axles.

Electrically the pot assembly is okay. The problem was incorrect mounting in the front plate leading to contact between the knobs.

Now the VFO tuning is much more precise and coarse and fine do not interfere any more.
So far success. But still not precise enough.

The pot assembly is mechanically weak constructed, so it would be better to put a standard 10 turn 100k wirewound pot with a gearbox knob as wme_bill mentioned.

For the moment I quit working on the device and wait for the manual.
Thanks for the support, season greetings, Jörg

[Diabolical Artificer]

I was going through my stash of 10 turn pots this week, I think I have a dual 1k 10 turn if you need it. 10 turn precision pots don't need "oil" or cleaning, they're helical wire wound with a slider. Carefull when soldering the connections, too much heat and you'll loose the connection, result, dead pot.

Some can be dis-mantelled,tricky fiddly job though.

Andy.

[JHS_Syke]

Originally Posted by Diabolical Artificer

I was going through my stash of 10 turn pots this week, I think I have a dual 1k 10 turn if you need it. 10 turn precision pots don't need "oil" or cleaning, they're helical wire wound with a slider. Carefull when soldering the connections, too much heat and you'll loose the connection, result, dead pot.

Some can be dis-mantelled,tricky fiddly job though.

Andy.

Thank Good, and bad enough, this assembly is not a wire wound, but made of plastic 270 degree pots. They are still similar in the consumer market made by a&b tt-electronics. I found them sold by many distributors
The mess was dirt in the coax axles. I believe the assembly was replace and mounted wrong, so knobs could collide.

It is not a good idea to spray something on the resistive structure. This will help only for a short time and kill it for sure in the end.
My strategy was, to open the mechanics and pull off the thick axle and gearbox.
It was really very fiddly. So I could clean the hollow axle and the thin axle separate and lube it. By the way the gearbox was fallen apart. It is made of plastic. some planetary gears in stack with a translation of ca. 1:25 working an very tiny plastic-structures. It is an sensible Instrument and should only by operated with care. I can be overturned very easily.

Jörg

[Diabolical Artificer]

Have you fixed it then? I looked in my pot stash the dual gang 1k I have is only one turn.

Andy.

[JHS_Syke]

Yes it works now for the moment.

The counter had a sever crash, and may be the pot assembly was changed.
Under close inspection I found deformations in the chassis.

Some caps are rotten too. It is not possible for the moment to say, whether the device meets specs ore not.

[Diabolical Artificer]

Under close inspection I found deformations in the chassis.

Sounds like it's suffering from courier syndrome : )

Andy.

[JHS_Syke]

Originally Posted by Diabolical Artificer

Sounds like it's suffering from courier syndrome : )

Andy.

Hi Andi,

Yes and no !

The damage was not done recently, when I bought it from junk.
Maybe I faced this instrument centuries ago, when a college of mine repaired it.
It was really rare. I had an severe accident, destroying the original pot-assembly.
The knobs could be still used. But I have after all the centuries only a very rough idea about this event. ;-)

Now the problem is caused by mechanical wear.
DAUs react with time delay, trying to turn the pots over the end. ;-)
This was leading to mechanical hysteresis in the coupling element between axles and the Pots itself.
I think it can be fixed with some paper glue in the gaps.

Jörg

[JHS_Syke]

Finally success.

Filling the gaps in the coupling elements with a water based paper glue reduced the mechanical hysteresis dramatically.

Thanks for the nice discussion.

Joerg

[Diabolical Artificer]

Well done Joerg.

Andy.

[JHS_Syke]

So far it workes, but very unstable as an result of the mechanical design.

Finally I got an offer for a brand new pot-assembly from an other manufacturer.
When I buy it, my wife will kill me. Only 500$ without import fees...

So I try a 10 gang pot and an additional stereo pot beside. Max. 25€ and a hole to drill. This sounds better for the moment.

Joerg

[joebog1]

500$ for a pot!!! Just buy the standard one, the solid gold and diamond model is only for audiophoolery amplifiers. .

Joe

[JHS_Syke]

Yeah.!! And pure golden sparks for my car.!

The reason for this price is:

It would be full custom manufactured special for me.!
Six elements two gears concentric shaft and tree pot-elements.
- Half is freight, tax and fees
- Plus Administration and handling
- And the manufacturer is the one and only in the world.

About 20€ would be okay for regular merchandise in a mass production.
In the end this price is really okay.!!!

( An I-Phone is about 2$ hardware cost when ready to ship... )

But I believe that a new original pot-assy does not solve the problem.
I will stay fumbly.

So I will try the DIY upgrade with a standard 10 gang pot 100k and an extra concentric shaft double stereo pot 1k and 100R for fine and extra fine tuning.
The 1k pot has a resolution of 50kHz/degree.
The 100R pot will have a resolution of 5kHz/degree.

@Joe
I know some guys, who believe in pure silver power cords !!!

Joerg

[factory]

Originally Posted by joebog1

500$ for a pot!!! Just buy the standard one, the solid gold and diamond model is only for audiophoolery amplifiers. .

Joe

Trouble is it's a stack of three pots, one of which is multi-turn 100k, the rear are two single turns 1k joined as one, very difficult to get a good picture due to everything else around it. Guessing it's an AB modpot of some type, but didn't check the size (they are made in ½ & ⅝ square).


These style of pots are used in lots of other kit, but will vary depending on the values in the stack.

Could it not be possible to ask in the wanted section for some similar and mix & match the sections in the stack to what you need?

This would be much better than adding extra holes in the front panel, that you won't be able to hide if the correct part is found.

David

[JHS_Syke]

This is exactly the same as mine !!
It is an Allen-Bradley Series 70 Mod Pot.

The problem is not electrical, but it is not any more possible to adjust the VFO exactly.
My quick repair worked so far for the moment. But fine tuning is only chance.
The two 1k pots and the thin shaft do not any more fit exactly together. So the frequency is jumping to and fro.

I have worked out a new plan to repair the old one, maybe there is no need for a new one.

Joerg