Basic questions about a Radio circuit components - Page 1
Forum section: electronics / beginners

author: CircuitQuest
author_rank: Contributor
timestamp: October 05, 2023, 08:33:36 pm
content: I just have some questions about components on a little cassette/AM FM player I'm restoring. If you look at the image I have numbered the components with the questions.Please forgive me if these are basic questions.1. This is the tuning capacitor I believe. It has 4 flat head screws. Are these screws for aligning the radio signal to to the dial? And do they affect AM and FM or does one screw affect AM and another FM?2. Theses are transistors. Is one for AM and the other for FM?3. What do these 2 coils do? One is small and green and the other is large and gold.4. These 2 poles marked GR and Y have tiny threads on them. I believe the threads are attached to the tuning capacitor. What to these poles do?5. This IC is responsible for what? Demodulating the signal?6. Finally these little metal boxes with flat head screw slots. What are they and are they meant to be turned for tuning purposes?The reason for this is that half of the AM dial signal is picking up stations and the first half isn't. FM is fine. The other AM radios I have pick up all stations in the same spot I tested this radio.So I'd just like to know the purpose of these components to see I there is something I can do outside of recapping the board.Thanks for any time you can spare in answering these questions.

author: IanB
author_rank: Super Contributor
timestamp: October 05, 2023, 08:55:46 pm
content: I think the square boxes with screws on top are tunable inductors. They have a coil of wire and a core that can be moved in and out with the screw.But I would not touch any tuning screws. They would have been adjusted at the factory with the help of appropriate test equipment. If you don't have the instructions and the right test equipment to inject and measure signals, then you will have trouble getting the adjustments back to the right settings.I don't really know how to help with diagnosing the fault or repairing it.

author: CircuitQuest
author_rank: Contributor
timestamp: October 05, 2023, 09:08:33 pm
content: Thanks. Diagnosing is half the fun!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 05, 2023, 09:39:09 pm
content: Things labeled "1" are adjustable capacitors for tuning.The cans are, as already said, inductors, either coils or transformers, which are adjustableThe IC labeed "5" may be a lot of things, see if you can get a number from it and look it up. It is probably lots of radio parts in one package., sine there are only two discrete transistors visible. Demod may be part of it.It is really easy to screw up the alignment but what do you have to loose? If you try anything try to pick a station in the middle of the band and tune to that. Make sure the station is in the middle of the band using another radio.You wil discover some adjusts are for AM and Some for FM, Label them with a Magic Marker so you do not get confused.

author: u666sa
author_rank: Frequent Contributor
timestamp: October 05, 2023, 10:13:36 pm
content: Quote from: CircuitQuest on October 05, 2023, 08:33:36 pmThese caps probably went bad 10 times over. I too have an old radio, when my hands get to it, first thing will be checking every cap and recapping as I go along.

author: CircuitQuest
author_rank: Contributor
timestamp: October 05, 2023, 10:34:54 pm
content: How would you remove the glue? I was thinking isopropyl alcohol and chipping away.

author: CircuitQuest
author_rank: Contributor
timestamp: October 05, 2023, 10:36:16 pm
content: Quote from: Wallace Gasiewicz on October 05, 2023, 09:39:09 pmI think I'll try recapping the board first. Those are great tips btw, thanks.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 05, 2023, 11:50:58 pm
content: There are four adjusters on the tuning capacitors assembly. Each band has two tuned circuit - one to peak the wanted RF signal and the other for the local oscillator. Do not adjust any of those four.In the half of the band that does work are the stations in the correct places on the dial?

author: themadhippy
author_rank: Super Contributor
timestamp: October 06, 2023, 12:38:18 am
content: No 6,if you get the urge to twiddle them dont use a metal screwdriver as there often a soft ferrite material that breaks easily and the metal will also intact with the coil

author: CircuitQuest
author_rank: Contributor
timestamp: October 06, 2023, 01:34:09 am
content: Quote from: wasedadoc on October 05, 2023, 11:50:58 pmYes the bands > 800 ish khz are in the right place.

author: rteodor
author_rank: Frequent Contributor
timestamp: October 06, 2023, 04:21:11 am
content: Haha. I used to play with these radio boards all the time. Brings back memories when I could crash all the RF alignments and and do it back all over again. The trick was to use another properly tuned receiver (they were all quite similar) to jump the signal from the one being tuned to the good one as I worked on each possible tune circuit on the signal path. Nowdays I would just use a wonderful TinySA.You said that half the AM band does not work and that the stations kept their place. The first thing I would look at is #1 in your picture. That is a mount of variable capacitors both for AM and FM. It is the nastiest component of all. If isolator material gets damaged it is (was for me) pretty much doomed. I could not fix those or if I fix them they would break again quite fast. In this case I can not give another advice other than go look for replacement.Quote from: CircuitQuest#2 it is hard to say, one might be the FM IN and the other one the FM LO. Look at the size of the coils they are connected to. If they are connected to the big air coils (like #3) then its FM. If they are connected to ferite antenna or other coils with core on them then they should be AM.Quote from: CircuitQuest#3 are FM coils. Do not bend them or you will have a lot more fun than you might wantQuote from: CircuitQuest#4 probably test pointsQuote from: CircuitQuest#5 hard to say from over here. Is audio getting out of it ? Back in the days I would just probe every pin with an audio amplifier/headphones, of course with a capacitor in between.My guess is that chip would be the entire AM (RF+IF) and also FM IF. But I might be wrong.Quote from: CircuitQuest#6 are (probably all of them) AM&FM IF tuning coils. For the FM they are probably tuned to 10.7MHz and for AM at 455KHz. If you say that that FM works and AM works (even partially) then it a good idea to leave them as they are.

author: fourfathom
author_rank: Super Contributor
timestamp: October 06, 2023, 05:25:51 am
content: If a cap has gone bad it's probably an electrolytic, and these will be in the power and audio sections. Those are unlikely to affect only a part of the AM band, or for that matter any of the RF function (if you have audio on part of the AM band). The RF caps are ceramic or film, and typically last longer than you or I will live.Losing part of the AM band could be due to a bent plate on the tuning capacitor, but that part on this radio looks like it will have insulating sheets between tuning cap plates so a short is unlikely. I suppose an insulator sheet could be torn, and that could cause it to short out, probably at the lower end of the tuning range (as rteodor says above).So those are things that probably *aren't* bad, which I admit doesn't help much...

author: warner112aa
author_rank: Newbie
timestamp: October 06, 2023, 02:08:54 pm
content: The reason for this is that half of the AM dial signal is picking up stations and the first half isn't. FM is fine. The other AM radios I have pick up all stations in the same spot I tested this radio.So I'd just like to know the purpose of these components to see I there is something I can do outside of recapping the board.pikashow download

author: wasedadoc
author_rank: Super Contributor
timestamp: October 06, 2023, 02:50:46 pm
content: Quote from: warner112aa on October 06, 2023, 02:08:54 pmRecapping the board is unlikely to fix that problem. Nor will twiddling the adjustments.In the opening post you stated that the "first half" of the band is not working. Please clarify what that means by stating the range of frequencies that is working and the range that is not. Also can you discern if there is a sudden point on the dial where stations up to that point work perfectly and after that nothing is received? Or does reception get poorer all across the working half as you approach the other half?In the gaps between the stations in the working half how does the static compare to anything you do hear in the non-working half.Are you using fresh batteries? Low batteries could give the symptom you are experiencing.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 06, 2023, 03:46:26 pm
content: As already mentioned it is probably the tuning capacitor (1) that's gone bad. There's a lot of tiny metal plates in there that move in and out another set of metal plates and since they should not touch there is isolating material (plastic?) between them. After turning the dial for years the isolation wears out and the plates touch. You should hear a cracking sound from the speaker when turning the dial from the working part to the non-working part, that's the sign. Replacing (1) should solve your problem, but the 4 tiny screws would probably need adjustment after that, 2 of them are to adjust the dial so that the stations appear in the correct spot (1 for AM, one for FM). The other 2 are for tuning the RF stage, after setting the first 2 adjust these for maximum signal at about half the dial.

author: CircuitQuest
author_rank: Contributor
timestamp: October 06, 2023, 06:14:36 pm
content: Quote from: wasedadoc on October 06, 2023, 02:50:46 pmfrom low khz AM range to the 800 khz range there are 3 stations that all my other radios pick up. They are the 3 big stations in that frequency. Nothing but static in that range. To be honest > than 800 khz isn't exactly clear but I still pick up the stations. I would not say that there is a point where all of a sudden the stations pop up and work perfectly.The gaps sound the same. Static. No popping or crackling.Not using batteries. Using a 9v 200ma adapter.