Friday, December 24, 2021

Christmas Eve

 And I know my audience will remember all those who are busy doing other things, and can't be home for the Holidays.

 The Kids will be over for dinner tonight, and the whole clan will be here on Christmas Day for dinner and a gift exchange.

ANNNNNNND.......I (finally) found the problem with the Heathkit IM-5248 IM Distortion Analyzer.

See this little guy marked "320K"?

It's a 1% tolerance, precision resistor, used in a section of the metering circuitry. They used a 1% resistor as this part, and another 1% resistor in a 20:1 voltage divider, to set a precise voltage for the bias level in this stage.

Only it's not 320k!

It measures 400k, a full 25% out-of-tolerance. This threw off the bias point by a similar amount, effectively killing this stage of the meter amplifier, and making it impossible to calibrate.

I'm very familiar with carbon composition resistors drifting off value with age, but this is the first time I can recall a 1% resistor going this far off the reservation.

So with this instrument calibrated and back on the shelf, I can get back to the one that "buffed out a bit" and most likely go down another rabbit hole getting that one back in service.


  1. Replies
    1. Most definitely! They'll probably laugh at me for getting the new snowblower, but oh, well....

  2. That's very weird for a metal film one-percenter to go that far out. I presume you had a replacement for it.

    Merry Christmas - enjoy your time with the family and the accompanying feast!

    1. I went through my box of 5% resistors and picked a 330k that measured 321k. I think that's "close enough" as it's within both 5% for a 330k, and 1% for the MF I removed. I think I've read about MF resistors doing strange things with high-voltage across them, but this one only had 15V on it, so go figure.

      Hope you guys have a very Merry and Blessed Christmas this year!

  3. Merry Christmas, drjim & all the assorted other members of the fam! If you need any of the 87" of snow we've had since late September, I'll look for a way to ship you some. :)

    1. HAH!

      We've had two light dustings, and some rain. Gonna be bad for the crops this year!

      A a very Merry and Blessed Christmas to you and yours!

  4. Merry Christmas to you Jim, SLW, TLG and everyone.

    Like you, I've seen carbon comps go high in value as they age but never seen a metal film that far off.

    1. Thanks, SiG! Wishing you and yours a very Merry Christmas.

      Yeah, finding that one took a week. I finally resorted to pulling one lead of every resistor in the circuit so I could measure them. First thing I noticed on this one after I wicked off the solder was that one lead was barely through the hole. I thought maybe that could be part of the problem, and then I measured it..... OUCH!

  5. Merry Christmas to y'all! And Yay for finding the errant resistor!

    1. Thanks, NFO!

      Yeah, it was driving me bonkers. And because a 1% resistor is considered a "premium" part, I didn't suspect it until I found it!

  6. Merry Christmas! And I have to borrow the top infographic. Nice.


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