TR751 pot repair
Hi, here is ZL3JT's take on fixing the pots:
Kenwood TR751 and TR851 Volume/Squelch control repair.
Part number: R23-9402-05
This part is now discontinued but might be repairable. Test each pot resistance to ensure the carbon track is OK. Squelch 50k Volume 20K (some may vary)
1. Dismantle the radio to expose the unit and remove it from the circuit board.
2. Carefully file off the ends of the two rivets flush with the rear plate.
3. Prise the rear plate off, and remove the switch unit.
4. Pull out the rivets, then remove the ‘C’ clip from the central shaft.
5. Remove the central (volume shaft).
6. Dismantle each potentiometer in turn, taking care not to mix the parts.
7. Clean the wipers with contact cleaner on the cotton bud.
8. The centre contact ring of each potentiometer will be dirty and oxidized, clean and polish the contact rings with silver polishing fluid soaked cotton bud. Then clean off the excess, and buff with a clean bud.
9. Temporarily assemble each pot, hold together and test resistance between the centre wiper and the outside terminals as the control is rotated.
10. Reassemble each unit with a touch of lubricant. (WD40 or CRC56)
11. Manufacture new rivets from copper wire, reassemble the units in reverse order of dismantling, and fit new rivets, (see below).
12. Carefully refit the rear plate, and re-rivet by tapping carefully with a small jeweller’s hammer.
13. Test resistances again for full rotation, then assemble the circuit board and re-assemble the radio.
This is a very delicate operation as the units are tiny. Great care must be taken when riveting, and ensure the head of each rivet is directly on an ‘anvil’ otherwise the plastic components will break.
Manufacture of rivets.
The rivet diameter is 0.038 inches, approximately 1.0mm. I used copper wire 1.0mm.
I made a jig in my lathe out of ½ inch brass hex, about 10mm long, drilled a 1.0mm hole in the centre, then cut with a fine jeweller’s saw from one side into the centre hole. This jig enable me to grip the copper wire in the lathe chuck. With about 0.5mm of wire protruding out of the jig, I carefully formed the rivet head by tapping with a jeweller’s hammer as the lathe slowly turned.
Then I reversed the formed rivet in the jig, and at a distance of 25.0mm, and leaving 1.5mm exposed, I used a fine file to reduce the diameter of the rivet to fit the holes in the rear plate. Check the correct length by careful measurement.
Assemble was a little tight as the copper wire is about 0.002 inches too large, but it was not difficult and made the whole unit feel ‘tight’.
Fit the rear plate, and test the potentiometers again, check the operation of the switch etc, before carefully forming the rivet ends over to finish. Make sure the units are held with the rivet heads on the ‘anvil’ as the other ends are formed with a jeweller’s hammer. Take your time, many taps with a 1 ounce hammer, rather than a few with a bigger one. The ‘shoulder’ on the rivet should match the rear plate to take the stress away from the switch and pots while riveting.
If you don't want to tackle this job yourself, find someone who will and use this for a guide!
Q & A via QRZ.com/zl3jt
73 John G8KHS
Family Motto: Si non confectus, non reficiat