I just recently found a nice near mint FM4 tuner. Hooked it up to my 33-303 set and do like the sound a lot.
I do have a small issue however; Radio stations are received perfectly in the lower part of the FM frequency bandwidth. (up until 99 MHz.) but when I joggle further in the regions of 100 - 101.50 MHz stations do not show any strenght anymore.
102 - 103 MHz also, no strong stations. Region 104-104.40 MHz gives total distortion/noise.
It seems as if 105MHz and up is OK again.
Anybody tips & tricks?
I do have a small issue however; Radio stations are received perfectly in the lower part of the FM frequency bandwidth. (up until 99 MHz.) but when I joggle further in the regions of 100 - 101.50 MHz stations do not show any strenght anymore.
102 - 103 MHz also, no strong stations. Region 104-104.40 MHz gives total distortion/noise.
It seems as if 105MHz and up is OK again.
Anybody tips & tricks?
Only off the wall thought is that there could be some weird aerial mismatch and that that part of the spectrum is being cancelled out. A bit like putting a deliberate stub onto an aerial lead to filter an interference issue or using aerial cable of the wrong impedance.
First thing has to be to try it on another known good outdoor aerial.
If it proves to be a faulty tuner then repair and realignment is highly specialised and not something that could normally be successfully attempted as a diy repair.
First thing has to be to try it on another known good outdoor aerial.
If it proves to be a faulty tuner then repair and realignment is highly specialised and not something that could normally be successfully attempted as a diy repair.
Mooly, thnx for the quick response. I checked my input source already by trial and error with several sources.
It seems this should not be my problem.
It seems this should not be my problem.
Parasitic oscillation generating wideband noise around 100MHz?
Were the 'weak' stations actually low in signal strength or strong but noisy?
Were the 'weak' stations actually low in signal strength or strong but noisy?
If you are 100% sure that the tuner itself is the problem then I'm afraid its not really a diy proposition unless you got lucky and found the problem was something more mundane such as a dried out or leaky electrolytic cap. Its not totally beyond the bounds of possibility it could be something like that.
Anything requiring replacement or alignment of any of the front end RF circuitry (electrolytics excepted) though is definitely not diy. You need all the correct gear to get the alignment correct.
Anything requiring replacement or alignment of any of the front end RF circuitry (electrolytics excepted) though is definitely not diy. You need all the correct gear to get the alignment correct.
@DF96, the weak stations were low in signal strenght. Only around 104 a strong noise. No sound at all. I am thinking about replacing caps. Anybody suggestions on which ones to start with?
RF DIY is possible. It just needs RF knowledge, RF experience and RF test gear. I designed and built an FM tuner. However, the average audio DIYer should keep well away from RF front-ends.
Ideally you would use a wideband scope as a first method of checking the caps and see if any had any unexplained HF noise across them. Electrolytics are 'safe' to replace because they will not affect the alignment. Which to replace... replacing in hope isn't the correct way to go about it but if that is all you can do (no test gear) then start with the small ones dotted around the RF stages. A recap involves replacing all the electrolytic.
Goes without saying. Unless you are experienced in RF work then its not diy friendly. Even moving a part physically can knock the alignment off as the alignment takes into account the effect of stray capacitance and so on from neighbouring parts.
the service manual can be found at electrotanya. there are various modifications listed. one for reason of an unreliable part. this tuner has ton's of adjustment points.
so really need an RF generator and some means to probe the signals.
the frontend has 4 LC filters with varicaps that have to track the dial freq. could be that one is is off, suspect the first section, damaged by lightning.
so really need an RF generator and some means to probe the signals.
the frontend has 4 LC filters with varicaps that have to track the dial freq. could be that one is is off, suspect the first section, damaged by lightning.
or the dc voltage on the varicaps is missing as these need a high voltage for low capacitance -> high freq..
In order to fix a problem it is usually helpful to first diagnose the problem. Blindly changing caps is more likely to introduce new faults than fix an existing fault.
You need to ask questions like:
when did the fault start?
did it start gradually getting worse, or suddenly changed from good to bad?
does anything temporarily fix it?
Even so, a fault which affects only part of the tuning range is most likely to be a fault in the RF front end, so unlikely to be fixed by an audio DIYer unless he is extremely lucky.
You need to ask questions like:
when did the fault start?
did it start gradually getting worse, or suddenly changed from good to bad?
does anything temporarily fix it?
Even so, a fault which affects only part of the tuning range is most likely to be a fault in the RF front end, so unlikely to be fixed by an audio DIYer unless he is extremely lucky.
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