RF circuit adjust - Page 1
Forum section: electronics / rf-microwave

author: Bhutt
author_rank: Contributor
timestamp: April 20, 2020, 01:46:24 pm
content: I posted in the beginner section maybe that was the wrong place. Is it possible to adjust part of a RF circuit back to relatively close specification without original service adjustment procedures? Have a 30 year old Icom that someone has tampered with a few of the IF transformers, turned them all the way in. As a beginner hobbyist I do not know how this would be done or even if it is possible. I have put details in the beginner section under Beginner needs help with receiver.Cheers,

author: TheMG
author_rank: Frequent Contributor
timestamp: April 21, 2020, 12:50:26 am
content: The service manuals for most older Icom radios can be found online. What's the model? What kind of radio is it (HF, 2m FM, etc)?Do you have an RF signal generator that you can use to feed a modulated signal to the radio? If so, you may be able to tune it by ear, if you can get the radio to produce some kind of audio from the signal generator. Crank up the signal generator until you can just begin to hear the signal, then start adjusting the first transformer until you hear a peak in signal strength. If the signal gets too strong to hear a difference as you near the peak, reduce the generator level until you get a noisy signal again. Move on to the next transformer down the line, and repeat. Once you have them all peaked, go through all the adjustments once more.That's assuming you can get anything out of the speaker (if it's really badly out of alignment it's possible you might not be able to get your signal generator to produce audible output from the radio). Also assuming there isn't a fault in the radio, which is entirely possible (why did the previous owner mess with the adjustments so much?).

author: Bhutt
author_rank: Contributor
timestamp: April 21, 2020, 11:58:06 am
content: Good morning,I do have the service manual, I was able to download from internet when I got the radios. That is what is puzzling, the pots that have been tampered withare not listedin theadjustment procedure. They are L83, L84 and L85 on the RF board and have been turned all the way in. There is one more on main board as well that has been tampered with. I will attached two pictures of the three that have been turned all the way in.L86 that is close to them is part of the adjustment procedure.Without instructions I am at a loss as how to proceed with them as I do not understand how they interact with the circuit.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 22, 2020, 01:04:09 am
content: Now that I've been able to go through the manual and figure out what is going on in that circuit...L83,84,85 are part of the 70.4515MHz bandpass filter for the 1st IF. If I'm not mistaking, they serve as impedance matching circuits for the two crystal filters (the non-descript white boxes in the schematic, referred to as FL1).In the same step where you are adjusting L86, also adjust L83,84,85 for peak signal.

author: Bhutt
author_rank: Contributor
timestamp: April 22, 2020, 10:21:23 am
content: Good morning,Thank you for taking the time to look into this, it is very much appreciated.Once the millivolt meter shows up from the Island I will run it through the adjustment procedure again and let you know how it went.Stay safe,Bob