Rotel RT850L Tuner - Page 1
Forum section: electronics / repair

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: January 11, 2017, 11:17:24 am
content: I am working on this Rotel tuner which seems to have a developed a noise rumbling/crackling that sounds a bit like a windstorm! It can sound very nice, but the problem has been persistent and intermittent. I have been able to trace the problem to 3 trimmer capacitors in the FM antenna and front end section. A sharp tap on one of these trimmers (usually CT102 or CT103) solves the problem for a while but the windstorm soon starts up a short while later.The problem is identifying the trimmers. They are simply described as 'AT1-53-9 FM ANT Adj' or 'AT1-53-9 FM RF Adj'. The Rotel 'computer number' (CF00000196) is the same for all three, so they are identical parts and certainly do look it, but no capacitance values are given.I have been looking online but I cannot find anything for either 'AT1-53-9' or 'CF00000196'. There is no problem finding replacement trimmers but I am not sure what capacitance value I need to be looking for? Is there a likely value for FM frequencies? All 3 of them have their plates barely meshed so the minimum PF value may be critical and this seems to vary from around 2.5pF top maybe 7pF or more. As far as I can tell, they have just a single set of plates.All three require adjusting for maximum reading on an AC VTVM, which is something that I do not have. Can I use a DMM on AC setting instead?Any suggestions would be appreciated.

author: CJay
author_rank: Super Contributor
timestamp: January 11, 2017, 11:22:16 am
content: Pictures please.The colour, dimensions, style etc. are all good clues when identifying trimmer capacitors.However, a common problem on older trimmers is dirty contacts, the wiper that contacts the moving plate tarnishes (I spent many hours stripping down, cleaning, re-tensioning these contacts as a trainee TV repair tech) so a squirt with a good quality contact cleaner followed by a clean up might just sort the problem for a few years.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: January 11, 2017, 11:36:52 am
content: Here are a couple of photos, but they are not particularly good as the thing is so small. For some reason on the second photo, the metallic parts on the underside look rather dirty, but to the eye they look bright silver.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: January 23, 2017, 10:59:09 pm
content: Since no one has come back to identify the trimmer, I tried CJays suggestion to take the trimmer apart and clean it. I started with the one I had already removed off the PCB. It was surprisingly easy to unclip the bottom plate, clean it and the end of the shaft that it makes contact with using IPA, re-tension the plate and pop it back in. Having soldered the trimmer back in and peaked the signal I did the same with the other two. So far the tuner works fine with no rumbling or crackles. Only time will tell if it lasts, but thanks for the suggestion.

author: CJay
author_rank: Super Contributor
timestamp: January 24, 2017, 08:25:35 am
content: sorry, this kind of escaped me when you posted, I might have suggested some switch cleaner instead of pure IPA but its working now so as you say, time will tell.

author: james_s
author_rank: Super Contributor
timestamp: January 26, 2017, 05:36:17 am
content: Trimmers like that have gotten hard to find, not much uses variable capacitors anymore. If you do end up needing a replacement, it should be possible to measure the value with a proper LCR meter and find something close enough. I suspect it will keep working now that you've cleaned it though, there really isn't much to fail in those.