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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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15th Nov 2018, 10:24 pm | #1 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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RCA AR88 tuning gearbox
Hi guys
Looked for information on how to service the AR88’s tuning gearbox but didn’t find what I wanted so had to learn the hard way, then wrote this which I hope other AR88 owners will find helpful. When cleaning the gearbox in my AR88D I found that there is a snag in the gearbox which causes a slight backlash at abt log scale 18 / 85. Therefore I got hold of a 2nd hand gearbox fm UK with the intention to learn how it works and how to fix it before doing any surgery in my radio. Started by cleaning the ”new” gearbox with white spirit to get out all grease and dirt, then dismantled it and noted all parts which can be summarized as follows: 2 plates, front and rear which are held together by 5 stanchions riveted to the front and held with a a nut and spring washer on the back 10 sprockets, several are fitted in pairs and one is actually 3 sprockets on the same shaft. The 4 large sprockets and 1 smaller are actually made of 2 sprockets each held together with a slight offset and spring loaded to minimize the backlash. 6 shafts The capacitor shaft has one large sprocket with 2 end stops which are NOT 180 degrees apart - thus mark the sprocket so you put it back right. - 3 ballbearings with a race in the shape of cup and 14 small balls each. The cones are fixed to the shafts. 7 bearings with 1 bigger ball at the end of the shafts, 2 have balls at both ends and the rest at one end. All are set into adjustable races (end of a screw with a lock nut). The main dial shaft has no ball bearing and is kept in place with 2 washers and a locking ring. This ring is hard to get off but easy to put back. Dismantling was easy but one has to keep track of all the bearing balls. After much headache came up with this method to assemble the unit: - Grease all bearings with a thick ballbearing grease and insert the balls in respective race. The grease will hold them in place. Remove all adjustable races. Insert the main tuning shaft, the vernier scale shaft, the main dial shaft in the front plate. Insert the capacitor drive shaft in the backplate Put the the sides together and fix with the nuts. Attach the adjustable races at the end of respective shaft (3 at the front and4 at the back) so they are fixed to the plates but barely enter the inside. Place the large balls at respective race as you go along. Capacitor shaft - tighten the adjustable race (on the front plate) so that the shaft is held in position. Tuning knob shaft - tighten the adjustable race (on the back plate) Vernier scale shaft - tighten the adjustable race (on the back plate) Next insert the twin large sprockets and place it between the 2 lowest adjustable races (one on each side) and tighten the screws. Insert the single large sprocket between the upper 2 adjustable races and tighten the screws Check that the gears work OK and adjust the races so that all gears are smooth running Check that the Capacitor shaft is in the right position. The adjustable races enable you to: Adjust the position of the large sprockets to avoid damaged cogs. Correct the position of the capacitor shaft if you it wrong. Based on my own experience I recommend working on a tray with raised edges in order to keep all bearing balls. Have fun Jan |
16th Nov 2018, 8:33 am | #2 |
Heptode
Join Date: Feb 2009
Location: Rotherham, South Yorkshire, UK.
Posts: 559
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Re: RCA AR88 tuning gearbox
Morning Jan.
Thanks for taking the time to pass this information on. I have 3 AR88 LFs 2 AR88D and 1 CR91. Kevin. |
19th Nov 2018, 10:48 pm | #3 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: RCA AR88 tuning gearbox
Hi Kevin
I hope that the info will be useful for somebody, anyway I had some fun playing with the gearbox. Quite a collection you have. I have only one AR88D which I got 50 years after i sold my first AR88D Jan |
20th Nov 2018, 12:53 pm | #4 |
Heptode
Join Date: Jan 2003
Location: Tewkesbury, Gloucestershire, UK.
Posts: 693
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Re: RCA AR88 tuning gearbox
Hi Jan,
Thanks for your very useful post. I have filed the procedure in my AR88 folder! One day I shall give my AR88 LF a full overhaul and it will be very useful.
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Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm |