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Old 4th Jun 2015, 2:04 pm   #1
shortwaverx
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Default AR88 Gearbox Servicing.

Hi All,

The gearbox, on my AR88D, has some backlash on the frequency dial. There is no actual backlash to the tuning capacitor, but I do find it a bit annoying. The tuning control is not as smooth as I feel it could be too.

Can anybody please give me some advice on the removal, servicing and lubrication of the gearbox please? I have read many posts on here, and I am looking for a bit more information regarding the gearbox removal, cleaning and lubrication. I have been advised to ontain some graphite grease.

Electronically, the set poses no problems to me and I am slowly working on its restoration.

Any input will be gratefully appreciated.
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Old 4th Jun 2015, 2:26 pm   #2
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Default Re: AR88 Gearbox Servicing.

The frequency scale is driven round by what is most often an incomplete-circle gear on the capacitor shaft. The scale has a smaller gear on it so 180 degree rotation of the capacitor gangs gives about 340 degree rotation of the tuning scale.

This partial gear is really a pair which are sprung against each other for anti-backlash. This gear also provides the limit stop for the whole tuning mechanism and is the known weakspot in the AR88 gearbox. Many have been broken. The only available replacements come with the rest of the AR88 attached!

Very early AR88 had whole circle gears in this place, and were much stronger.

Spinning the flywheel tuning and letting it hit the stop is the cause of damage.

If you're really really lucky, someone made a botch of reassembling things and didn't pre-tension the antibacklash spring. If you're unlucky, some teeth have dropped off, allowing the anti backlash gear to slip and release the spring.

David
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Old 4th Jun 2015, 4:49 pm   #3
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Arrow Re: AR88 Gearbox Servicing.

From memory, the large anti-backlash gears need about 3 to 4 teeth of overlap. But the really important one - and the most difficult to set right - is the smallest gear. Prior to - and during dismantling - it is a really good idea to take photographs - and make a few pencil and paper drawings, too. I also use 'Tippex' on mechanical parts like this as a method of recording the relative positions of the parts: that greatly assists re-assembly.

Setting up the gear box requires two pairs of hands! I solved that problem by making a fixture out of a block of wood with some small metal clamps to hold the gears in their loaded positions whilst the top plate was re-attached. I recall writing all about it on this forum - but it was a long time ago. I no longer own that fixture - it was one of the many things that I parted company with when we relocated, but should the need ever arise again, I simply make another: done it once, can do it again.

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Old 4th Jun 2015, 10:50 pm   #4
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Default Re: AR88 Gearbox Servicing.

Thanks for your suggestions gents. I'm hoping to have a couple of days off work next week, so I'll have a bit more time to look into the cause of the backlash. I also think that a strip down, clean and lubrication will help to keep this radio working for another 70 years. I have some experience with split gears, working on my EA12, which ended up in my garage as I couldn't get the IF stages to tune up correctly in SSB mode.

I may well pick up another, rough but working set for future spares.
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Old 7th Jun 2015, 6:54 pm   #5
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Default Re: AR88 Gearbox Servicing.

If you have a Texaco oil distributor there, the Texclad 33 (IIRC) is a 33% graphite grease designed for open gears.

It sticks to almost anything, especially your hands and clothes. Hard to get out of clothes, but wonderful stuff for any sort of gears.
Here I got mine in a caulking gun style tube. I put it into a sealed can and have used it for decades on any sort of gearbox on electronics, rotors, VCR's etc.
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Old 7th Jun 2015, 7:07 pm   #6
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Default Re: AR88 Gearbox Servicing.

Similar - if not the same, in essence - is molybdenum disulphide grease. Racal specify that for the worm drive in the RA-17 receiver. I have a large tin of the stuff, which has a B.P. logo on the front. I used it during from my motorcycling days and now use it for all types gear lubrication.
As Frank has said, it sticks to almost anything, especially your hands and clothes - from which it is difficult to remove - but wonderful stuff for any sort of gearing.

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Old 7th Jun 2015, 7:26 pm   #7
David Dunlop
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Default Re: AR88 Gearbox Servicing.

Hello Ian.

Have you seen this website?

www.radioblvd.com

There is an interesting section on the AR88 gearboxes some ways into the site and I think more than one type of gearbox may have been used during the overall production run of these receivers.


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Old 8th Jun 2015, 2:57 pm   #8
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Default Re: AR88 Gearbox Servicing.

Thanks for all of your help with this. The backlash appears to be caused by the small split gear and I hope it was just badly installed at some time. I can't see any missing teeth, so I will remove the gearbox when I have time to do so.
I have some molybdenum disulphide grease and some graphite grease, so I will see which one of them is still in good condition. The molybdenum disulphide grease, I have, seems to separate out in the bottle. It also has a tendency to try to escape from the bottle too. It may be too thin as it is left over from my days of tinkering with clocks and watches.
I removed the cabinet yesterday, to find a pool of oil which had escaped from one of the bath tub capacitors. This was duly removed and replaced with three 0.1µF capacitors on a piece of tag strip. I will bring the original to work and clean it out later on this week. We have a hazardous waste bin here, so I can dispose of the remains in the correct manner. Some of the bath tubs have Wima capacitors in them and some are originals.
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