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HP 8640B stiff main tuning

 

I recently fired up my 8640B after a few years of non use and found the main tuning dial almost impossible to turn. After warming up a while it loosened up a bit but still remains very stiff.

I realize there is a problem of the old grease hardening up on the various shafts, as I already had to clean and relubricate the bandswitch gearbox shaft and bearings. The fine tune mechanism also had this problem of hardened grease.

It appears the main tuning shaft of the cavity oscillator is suffering from the same problem. If I look at the tuning shaft where it enters the cavity I can see evidence of hardened grease. But the problem is how can I clean the hardened grease from the shaft and bearing without complete disassembly of the cavity itself?

Does anyone have experience in taking the cavity apart to relubricate the shaft and tuning plunger threads?
 

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You didn't store it on its back, did you??? If so, that could be the problem.? You might try storing it face-down for a few days/weeks before taking the cavity apart.?

?

There's some discussion of a similar phenomenon here ( https://www.ve7ca.net/TstH86.htm ), in which the migrated grease causes intermittent electrical contact.? It's easy to imagine that the loss of lubrication in certain areas could cause the shaft to bind up.

?

-- john, KE5FX

?

?

From: [email protected] [mailto:[email protected]] On Behalf Of mike.kusiak@...
Sent: Friday, May 20, 2022 3:36 PM
To: [email protected]
Subject: [HP-Agilent-Keysight-equipment] HP 8640B stiff main tuning

?

I recently fired up my 8640B after a few years of non use and found the main tuning dial almost impossible to turn. After warming up a while it loosened up a bit but still remains very stiff.

I realize there is a problem of the old grease hardening up on the various shafts, as I already had to clean and relubricate the bandswitch gearbox shaft and bearings. The fine tune mechanism also had this problem of hardened grease.

It appears the main tuning shaft of the cavity oscillator is suffering from the same problem. If I look at the tuning shaft where it enters the cavity I can see evidence of hardened grease. But the problem is how can I clean the hardened grease from the shaft and bearing without complete disassembly of the cavity itself?

Does anyone have experience in taking the cavity apart to relubricate the shaft and tuning plunger threads?
 

No, its been sitting on the bench in operating position. Electrically, the oscillator works fine with no problems, so I don't think its a problem with the internal grease migration. Its just that the tuning dial is very hard to turn. Other shafts and bearings in the instrument show the same symptoms of stuck bearings. The original grease seems to become like glue with age.

Doing multiple searches I have not found any information on the cavity disassembly. Its not immediately obvious how to take it apart and given it otherwise works fine I am hesitant to proceed. I was hoping to find someone who has actual experience in disassembly.

The best info I could find on the actual physical construction comes from a drawing in the HP Journal of Feb, 1973. It shows the shaft and tuning plunger, but lacks details.

Mike, WA2YGA

8640B cavity.pdf
8640B cavity.pdf

 

Avoid dissassembly. Get service manual.
Try some gentle solvent then lube on the shaft , perhaps Iso Alcohol?

Jon
 

I have just put
8640B Service Manual Volumes 1 and 2 inc CCT Pn 08640-90215 May 1991 .pdf

Onto this site in the files also other info including 8640 note for sticking tuning
Paul

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Froggie the Gremlin
Sent: 21 May 2022 16:46
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 8640B stiff main tuning

Avoid dissassembly. Get service manual.
Try some gentle solvent then lube on the shaft , perhaps Iso Alcohol?

Jon
 

On Saturday 21 May 2022 07:14:21 am mondial wrote:
Other shafts and bearings in the instrument show the same symptoms of stuck bearings. The original grease seems to become like glue with age.
The issue here is the same thing that causes fans to not work well when they age. The original lubricant is made up of a number of different fractions, and over long periods of time the lighter ones evaporate, leaving the heavier ones behind.

The solution is to remove what's left of the original lube and put some new lube in there.

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
 

Roy is right.
In many greases, one of the major heavy components is soap. Look at what
happens when soap residue hardens. But, be careful; not all soaps dissolve
in water. Meths can sometimes help.
73 de Brian, VK2GCE

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Roy J.
Tellason, Sr.
Sent: Monday, 23 May 2022 1:06 AM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 8640B stiff main tuning

On Saturday 21 May 2022 07:14:21 am mondial wrote:
Other shafts and bearings in the instrument show the same symptoms of
stuck bearings. The original grease seems to become like glue with age.
The issue here is the same thing that causes fans to not work well when they
age. The original lubricant is made up of a number of different fractions,
and over long periods of time the lighter ones evaporate, leaving the
heavier ones behind.

The solution is to remove what's left of the original lube and put some new
lube in there.

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies.
--James
M Dakin
 

Thanks for all the suggestions. I have been able to clean off the grease residue on the other bearings which are accessible for disassembly using isopropyl alcohol.

The problem with cleaning the tuning shaft bearing is that it is internal to the tuning cavity itself and not accessible unless the entire cavity assembly is taken apart. That's the real problem. So far, I have not found any info online regarding experience with disassembly, so it appears that no one has actually attempted (or posted info) on doing so. HP provides no info as they considered it a factory service only assembly.

What I will try first is to soak the visible end of the shaft in solvent and then follow with a light lubricating oil like ATF. Hopefully some will enter the bearing and this will do some good in loosening it up.
 

Hello we have three 8640, with varying amounts of shaft resistance, but all are OK to use.

I suggest that position the gen panel up, then apply alcohol or degreaser on the shaft to flush out the thick grease,

It may take some time to soak in the degreaser and dissolve the old congealed greases, gently warming the cavity and shaft and allowing it to sit a day may loosen it.

Finally re lubricate with a selected oil.

The cavity is a precision assembly and may require special tools and fixtures to disassemble and then realign. I am sure the frequency calibrations will be affected.

I would avoid disassembly if at all possible.

Bon Chance

Jon
 

Some have reported great success using Tri-flow as a lubricant on hardened grease that TenTec used on their vfos.? Reports have it that TenTec used whatever tractor grease the farm store had on the shelf.
--
Thanks,
Randy
randy.ab9go@...

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