I would like to hear people’s thoughts on changing out capacitors and trim pots on something like a working FM tuner board. Specifically, what to do when one doesn’t have the specific equipment to follow the adjustment procedures in the service manual.
1) Do nothing. The FM is working OK, don’t fix what is not broken
2) Change just the capacitors. The capacitors are old and will eventually fail. Don’t tempt fate by changing the trim pots.
3) Change the capacitors and trim pots. They are both old and have either failed or will fail.
3a. Change only the trim pots for functions that one has the equipment to test/adjust the pot settings
3b. Change all of the trim pots. Measure the resistance between legs of each of the old ones and set the new ones to match.
3c. Change all of the trim pots. Set the “pointer” on the new pot to match the old one.
3d. Some combination of a, b, c
What do you normally do and what is your rational for doing it?
Glitch
( I know I forgot choice 4) If you have to ask, you probably shouldn't be doing this ;-) )
Strategy for Changing Capacitors and Trim Pots on FM Tuner
17 replies
1. Possible.
2. Possible.
3. Not possible without the proper instruments for setting the trimmers.
3a. Possible.
3b. One of the worst things you could do.
3c. Worse than 3b.
4. Asking never hurts.
Don't forget:
5. Get the proper instruments and learn how to do it right yourself.
6. Leave the alignment to someone who can do it.
In your case, respectfully keeping 4 in mind, I'd say 6 would be the cheapest and fastest way, and you will end up with a good result.
Martin
Martin,
Thank you for your sage advice.
Your reply is pretty much what I suspected. I had to ask in case the settings of the trim pots were far less critical than what I expected. With old equipment it is sometimes hard to judge which is the lesser of two evils, old worn out parts versus new out of spec parts.
My game plan is to stick with "1" if there are not issues, then maybe try "2". I'll research what is needed for "5". My bench equipment most likely falls into two categories, too new (modern digital with all of the digital issues) or too old (like vacuum tube old, my wife's grandfather's test equipment). My gut feel is this is a job for quality analog equipment(?)
I'm not too sure if "6" is really an option. One of the reasons that mothballed the stereo equipment years ago is that all of the really good electronics technicians that I knew either retired or otherwise closed down shop. Besides, option "5" sounds like more fun.
The strange thing is that I really have no interest to listen to FM (same goes for cassette tapes). I just like the idea of having nice, vintage stereo equipment that is fully functional.
Glitch
Radio alignment is difficult and requires a lot of experience, even with the proper tools.
I for one won’t touch it with a barge pole ☺️
The only thing I do is the adjustment of 19 kHz frequency.
Jacques
Isn't most of the "alignment" done with the variable inductors?
My assumption is that these don't "go bad" and are (very much) to be left alone. Is that true?
Glitch
Note: It is probably obvious that I don't have ANY experience with messing with the tuner sections. I do have general, theoretical, engineering knowledge and can follow along with most topics.
They can’t go bad but resistors and caps may change values and re-alignment required anyway, even when the device has never been touched. Sometimes especially if the tuner was never revised!
Some tuners never get misaligned though, and that’s impressive. I have a Beomaster 5000 (1967) that still has its original components and settings and it works perfectly.
I bought two FM3’s of the same period, one had its IF completely out and the other was still perfectly aligned. That’s pretty unpredictable.
Now some designs rely more on trimpots than others.
Jacques
I have looked around for the equipment needed for working on the FM tuners. I originally started looking at used, professional grade equipment. I found myself gravitating towards the equipment that we had in our electronics lab at work in the 1980-1990's time-frame. Though this equipment has dropped significantly in price since new, most of it is still out the price range that I would want to spend for this project. Additionally, I have the usual concerns about the operating state of such old equipment.
While researching FM tuner repair and alignment, I came across a device that might fit my needs. It is called a "TinySA" (https://www.tinysa.org). This appears to be able to generate the signals that I would need. Does anyone have experience with this piece of equipment? If so, would a TinySA and a 200 MHz scope be sufficient to do the basic adjustments?
chartz: I was reading some old posts in the Beoworld archives. Did you ever get your BM8000 to tune to the higher frequency stations?
Glitch
Oh yes, this was solved and documented!
You’ll need more than an oscilloscope, like a HF generator, a distortion meter…
Jacques
The TinySA can generate signals in the FM frequency band with various modulations, as well as sweeps. I can compute FFT's with my scope which should give an idea of the distortion?
Glitch
Oh good! I read too quickly. All-in-one then. This thing is incredibly cheap. Have fun and share!
Jacques
Please Log In to Reply
You must be logged in as a BeoWorld member to post replies to forum threads.
or
Join BeoWorld
Register to access the forums, reference material, and exclusive member benefits — including our monthly prize draw.
Become a Member