Need help for a radio repair please. - Page 1
Forum section: electronics / repair

author: RandyDarkshade
author_rank: Contributor
timestamp: June 29, 2017, 02:35:16 pm
content: I have picked up a little Panasonic RF-1630L portable radio. It has MW, LW and FM bands. The issue it has is that MW and LW work fine but FM doesn't. All I get is constant hiss which does not even change when I turn the tuning knob. I have provided a link to a site that has photos of the circuit. Thanks in advance.http://www.radiomuseum.org/r/panasonic_rf_1630lrf1630.html

author: rg58
author_rank: Regular Contributor
timestamp: June 29, 2017, 02:59:20 pm
content: Is it a proper "noise" like other FM radios when tuned to no station? Or does it sound different? Do you own a generator to inject a 10.7MHz signal into the IF path?

author: RandyDarkshade
author_rank: Contributor
timestamp: June 29, 2017, 05:11:21 pm
content: It's the general sort of noise you get when there is no station. No I don't have a signal generator

author: rg58
author_rank: Regular Contributor
timestamp: June 29, 2017, 05:39:23 pm
content: I see. I assume no oscilloscope either? Makes it a bit tough...Do you have a FM transmitter maybe? Often found for the car to connect a MP3 or CD player to it for playback via car radio (in the FM band). Try if you can receive this one. Set it on 87.5MHz first, tune the radio and see if you can find the signal, then set the transmitter to 108MHz and try again if you can received the signal on the radio. To me it sounds like either:1) first oscillator is not working properly / is mis-tuned OR2) input transistor (RF amplifier) is blown OR3) as simple as antenna cable is torn off somewhere near the FM antenna input leading to the PCBHard to tell from here...

author: voltz
author_rank: Frequent Contributor
timestamp: June 29, 2017, 07:50:52 pm
content: The LW and MW bands use the ferrite rod antenna inside the radio and FM uses the pull up telescopic antenna. If that antenna has become disconnected inside the radio, like a broken PCB or loose nut, FM would stop working.Check the telescopic antenna connection inside.

author: rg58
author_rank: Regular Contributor
timestamp: June 29, 2017, 08:51:49 pm
content: That's what I meant to say under 3) that it could be just an interruption from the telescope antenna to the PCB.Here's another thing you can try: Let's check if the oscillator is working!Tune the broken radio to the lowest frequency (87.5MHz) and make sure it is turned on. Now take another FM radio (a small one would be handy) and hold the telescope antenna near the PCB of the broken one (careful, don't touch or shorten anything with the metal rod!). Now tune this test receiver and scan the FM band for a silent carrier (the noise will disappear, like a station without any sound). This signal should be on 87.5MHz+10.7MHz = 98.2MHz on your test receiver. If you found this signal, tune the broken radio a bit higher in frequency - the signal on your test receiver should disappear. You can find this signal again by tuning the test receiver higher by the same frequency like the first one. If the signal in your test receiver does NOT MOVE when you tune the broken radio, that means that your tuning capacitor has an issue and should be replaced or is disconnected from the board somewhere. If you receive NO CARRIER SIGNAL at all, this could mean that the oscillator stopped working OR is tuned to an entirely wrong frequency. If you don't receive the carrier signal around 98.2MHz but way off on another frequency, then again, the oscillator is mis-aligned. NOTE: I think there could be also radios out there that mix the other way around, so if before-mentioned failed, try setting the broken radio to 108 MHz and tune the test receiver to 108-10.7= 97.3MHz where you should receive your silent carrier signal.

author: Nemo1956
author_rank: Contributor
timestamp: June 30, 2017, 05:34:13 am
content: If you are getting hiss from the speaker then the oscillator is probably working or you wont here anything. So have a look at the input connection and the RF front end.

author: rg58
author_rank: Regular Contributor
timestamp: June 30, 2017, 07:13:39 am
content: I also believe the oscillator is working for the very same reason (hence my question about the 'sound' of the hiss). But there could still be a problem with the variable capacitor torn off somewhere on the board and/or the oscillator frequency way off. To confirm this I suggested above procedure. The most likely cause however is a problem with the antenna connection.

author: MosherIV
author_rank: Super Contributor
timestamp: June 30, 2017, 01:45:28 pm
content: Here a silly one :These type radios used something like cord or string to link the tuning knob to a variable capacitor.Is that string/cord there?Does it still move the capacitor correctly?An indication that it is not working is usually the wavelength indicator does not move when turning the tuning knob.(Ignore me if there is no mechanism like this in this radio)

author: rg58
author_rank: Regular Contributor
timestamp: June 30, 2017, 02:19:35 pm
content: He said that the AM bands work correctly, hence the string moves the variable capacitor which is usually combined for AM (capacity up to 350pF) and FM (up to maybe 30pF). So I think we can rule that one out.

author: RandyDarkshade
author_rank: Contributor
timestamp: June 30, 2017, 05:40:17 pm
content: Thanks fort all the replies I can confirm it is now working. It was just a simple dry solder joint I poked at with a soldering iron because it looked odd. Then it burst into life.

author: Nemo1956
author_rank: Contributor
timestamp: July 19, 2017, 05:38:02 am
content: Glad we could help you sort it out.Happy listening