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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 16th Oct 2017, 10:10 am   #1
Linnovice
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Join Date: Jun 2014
Location: Chelmsford, Essex, UK
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Default Trimmer resistor changes?

Morning all. I need little bit of advice regarding trimmers. I'm in the process of recapping and installing new trimmers on a Revox B77 HS. On the Monitor Amp board there are two 470k two legged trimmers mounted between lines of components. I've been trying to get two legged trimmers but can't find a supplier. I have been told that its possible to convert a three legged into a two legged variety but no one will tell me over the phone how to do it. Can you help please?
It is an odd one. On the schematics I have (downloaded from Studer site) it shows two variation of board. One has no trimmers onboard, just fixed resistors. The second has two trimmers installed but they are three legged and running along the long length of the board with easy access. The board on my machine, as I said, has two two legged trimmers mounted between lines of resistors and I can't find any drawings for this variation.
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Old 16th Oct 2017, 10:19 am   #2
Boater Sam
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Default Re: Trimmer changes?

If you use 3 legged ones, connect one end to the centre wiper then cut off the connecting pin at that end or if the spacing is wrong then, cut off the centre pin.
Bit rare to have 2 pin preset pots. Usually they use 3 pin and do the connection on the pcb.

It is a bad idea to not connect the unused end of the track to anything as if the wiper ever lifts off the track, the pot is open circuit and something may not like that.

If the other model just uses fixed resistors, could you not do the same and not have presets? Sounds like they will be difficult to adjust anyway in the middle of a pcb.
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Old 16th Oct 2017, 11:52 am   #3
Linnovice
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Default Re: Trimmer resistor changes?

Hi Sam, thanks for that. Correct me if I’m wrong but if I bend up the back leg of the three and solder it to one of the other legs would that do it? The leg spacing would then fit the board. Adjusting is not difficult as the originals were fitted with serrated thumb wheels and I’ve managed to get a modern version that fits the new trimmers.
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Old 16th Oct 2017, 11:54 am   #4
Linnovice
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Default Re: Trimmer resistor changes?

Oh, and the reason I can’t use fixed resistors is because the board has undergone quite a few revisions by ReVox. It would not be practical to try and revert this board back.
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Old 16th Oct 2017, 2:37 pm   #5
Boater Sam
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Default Re: Trimmer resistor changes?

Yes, as long as you have centre and one end leg shorted it is fine.
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