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In Reply to: RE: Dynaco FM3 tuner posted by Cougar on July 25, 2018 at 19:27:13
As I was emailing Cougar the files he requested, I came across this bit of information above, which I found years ago in an article about general electronics repair (VCRs, TVs, etc). I took note regarding indiscriminately changing out carbon composition resistors in an RF stage.In the James Lin "Improving Dyna's FM-3" article, he mentions a previous Audio Amateur FM-3 article by Ben Pochland, where Pochland replaced the carbon comps with carbon films, supposedly without issue.
Lin goes on to say that he (Lin) then replaced most of the FM-3 resistors with metal films and metal oxides when updating his tuner, including those in the IF and RF stages.
I admit to being rather cautious with regard to this, as well as with replacing a tuner's ceramic or mica caps with film, for example. Since they may act differently at RF frequencies, and I don't begin to claim to understand RF, I tend to leave the type of cap and resistor as the designer(s) intended.
In the output stage, or power supply, that's a different story.
Just an FYI for anyone who's thinking about doing a wholesale replacement of every cap and resistor in their tuner.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
Edits: 07/27/18Follow Ups:
It's worth noting that the author of that article used 100 MHz as an example. I don't think that was a shot in the dark. There should be little if any difference replacing carbon comps with film up to frequencies that include the FM IF (10.7 Mhz). But, in the front end where tuned circuits reach 108 MHz, I wouldn't do it. That's not to say the resistors shouldn't be replaced. The value of carbon comps can change significantly with age. Just replace them with known good resistors of the same type as already discussed.
--------------------------
Buy Chinese. Bury freedom.
I appreciate you saying that, simply because it tells me I'm not being overly cautious. Too many modders dive in with their "Ready. Fire. Aim!" approach, in my opinion. An amp may be more forgiving to that method, but understanding RF isn't for the average RF-uneducated solder slinger to guess about.
You make a good point regarding checking and replacing old, drifted resistors. Just do so judiciously, would be my suggestion.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
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