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Home Forums General Discussion & Questions General Discussion & Questions Strategy for Changing Capacitors and Trim Pots on FM Tuner

Viewing 17 posts - 1 through 17 (of 17 total)
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  • #39350
    Glitch
    BRONZE Member

    I would like to hear peopleโ€™s thoughts on changing out capacitors and trim pots on something like a working FM tuner board. Specifically, what to do when one doesnโ€™t have the specific equipment to follow the adjustment procedures in the service manual.

    1)ย ย ย ย ย  Do nothing. The FM is working OK, donโ€™t fix what is not broken

    2)ย ย ย ย ย  Change just the capacitors. The capacitors are old and will eventually fail. Donโ€™t tempt fate by changing the trim pots.

    3)ย ย ย ย ย  Change the capacitors and trim pots. They are both old and have either failed or will fail.

    3a.ย ย ย ย ย ย  Change only the trim pots for functions that one has the equipment to test/adjust the pot settings

    3b.ย ย ย ย ย  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.ย ย ย ย ย ย  Change all of the trim pots. Set the โ€œpointerโ€ on the new pot to match the old one.

    3d.ย ย ย ย ย  Some combination of a, b, c

    What do you normally do and what is your rational for doing it?

    Glitch

    ( I know I forgot choiceย  4) If you have to ask, you probably shouldn’t be doing this ๐Ÿ˜‰ )

     

    #39351
    Dillen
    Moderator

    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

    #39352
    Glitch
    BRONZE Member

    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

    #39353
    chartz
    GOLD Member

    Radio alignment is difficult and requires a lot of experience, even with the proper tools.

    I for one wonโ€™t touch it with a barge pole โ˜บ๏ธ

    The only thing I do is the adjustment of 19 kHz frequency.

    Location: Burgundy
    #39354
    Glitch
    BRONZE Member

    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.

    #39355
    chartz
    GOLD Member

    They canโ€™t go bad but resistors and caps may change values and re-alignment required anyway, even when the device has never been touched. Sometimesย especiallyย if the tuner was never revised!

    Some tuners never get misaligned though, and thatโ€™s impressive. I have a Beomaster 5000 (1967) that still has its original components and settings and it works perfectly.

    I bought two FM3โ€™s of the same period, one had its IF completely out and the other was still perfectly aligned. Thatโ€™s pretty unpredictable.

    Now some designs rely more on trimpots than others.

    Location: Burgundy
    #39356
    Glitch
    BRONZE Member

    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

     

     

    #39357
    chartz
    GOLD Member

    Oh yes, thisย wasย solved and documented!

    Youโ€™ll need more than an oscilloscope, like a HF generator, a distortion meterโ€ฆ

    Location: Burgundy
    #39358
    Glitch
    BRONZE Member

    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

     

    #39359
    chartz
    GOLD Member

    Oh good! I read too quickly. All-in-one then. This thing is incredibly cheap. Have fun and share!

    Location: Burgundy
    #39360
    Glitch
    BRONZE Member
    #39361
    chartz
    GOLD Member
    Location: Burgundy
    #39362
    Glitch
    BRONZE Member

    Good idea to look at the chip spec sheet for clues.

    I did some more reading and EMF likely refers to electromotive force. I think that last time I heard that term used was a very, very long time ago in college. It isn’t clear to me why it is being stated in that way. My best guess is that they are trying to indicate making a measurement on a signal generator with a low impedance built-in load with a high impedance measuring device (i.e. without a 50 ohm load at the scope)?

    The spec sheets may have provided a clue to another question that I had. Which aerial input to connect the signal generator, the 300 ohm or 75 ohm? Seems like the 75 ohm is the proper one(?).

    I’ve been playing with the TinySA to get a feel for how it performs. So far, the settings on the TinySA match what I measure on my scope. I still need to work out ways of measuring what I need with the equipment that I have. For example, I don’t have anything that can measure a 10 uV signal in the MHz frequency range. I think the many of the measurements that I need to make will have to be done indirectly.

    Glitch

     

    #39363
    Glitch
    BRONZE Member
    #39364
    hcraig244
    BRONZE Member

    Fascinating stuff……I have always thought it best to shy away from this subject due to the test equipment required and also the expertise required to perform it, but would love to learn more and explore the use of the TinySA piece of kit, I do have a BM4400 that has a less than perfect reception.

    Craig

    #39365
    Glitch
    BRONZE Member
    #39366
    Glitch
    BRONZE Member
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