Tricky way to extend 100MHZ FM range to 108MHZ
 

Hi
lastly I found in the Haifa flee a real treasure : Nordmenda Transita 1961 2/608

The radio is in amazing beautifull and working condition.
The UKW sound is crystal clear , unbelievable for 60 years old radio ,and one of the ancient FM German radio.
But in the bad side (always something must crush the first excitement)
the FM range is limited for 100MHZ only
As I know nothing about Radio technician,and I know too that it's almost impossible to extend the original tuner I found a tricky solution for 5 $ investment.
Look please in the pictures.
If someone may found a better solutiun without taking a risk to destroy my jewel I'll be glad to hear about


Your's Alon

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

Nice radio. Most early FM radios sold in Europe only tune to 100MHz like this.

You may be able to adjust the FM oscillator so that the radio will tune a different range. You won't be able to extend the range but you can shift it up, say to 91.5MHz-104MHz. A circuit diagram will help you work out what you need to do if it isn't obvious.

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

Hello Alon,

here's an elegant electronic solution using a 1 IC frequency shifter.
To receive frequencies above 100MHz, you toggle a switch and everything is shifted down by 10MHz, i.e. a station on 103MHz appears as 93MHz on your dial.

No tinkering is necessary on the receiver's FM converter which is really nice.

Best Regards

jhalphen
Paris/France

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

You can buy 'band expanders' for when Japanese cars are imported into the UK, as the Japanese FM radio band is 76-90MHz so the original car radio isn't much use (unless you only listen to Radio 2).

They cost around £10-15, and just got inline with the aerial lead (plus take an 8-15V dc supply) and shift the 90-108MHz range down into the 76-90MHz range. You end up with the two ranges overlapping.

I would guess they use a circuit very similar to the one posted above. They work very well, and are convenient small metal boxes about 4cm x 2cm x 2cm that you could tuck away inside the chassis of most radios nicely.

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

You should be able to extend the HF end of the range by simply opening the associated oscillator trimmer. This will be mounted either on the tuning gang itself or in the tuner can.
Setting the trimmer to minimum capactiance will allow the radio to tune higher, possibly even beyond 108 MHz.
It will move the bottom end (88 Mhz) as well, and this can be corrected by adjusting the oscillator coil/slug.
This is quite a simple procedure if you know the ins-and-outs of radios, but it may be rather daunting if you don't.
I have a 1970's Philips transistor portable that stops at 104 MHz, but a quick tweak on the trimmer and coil got it covering from 88 to 108 with plenty to spare.
Of course once you've done this, the dial markings are wrong!

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

Not all FM sets will have sufficient pull in range to do this, notably early valve sets, you might be lucky to get 90-106 mhz.
Remember most valve sets use permeability tuning.

Andy, What you are saying is very hit and miss, when opening the osc trimmer, adjusting the slug will of course offset what you have done with the trimmer, so the range is unlikely to be the full 88-108mhz. Then of course you need to do the same with the aerial side, and then tracking could well be out.
I would leave the set alone but if necessary build the frequency shifter as in the earlier postings.

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

Trevor - yes, adjusting the coil will move things 'down' again, but you end up with increased tuning range because you've taken capacitance out of the system by reducing the trimmer. A source of 88 MHz and 108 MHz would be handy, but BBC Radio 2 is a good LF marker and most of us have a station around the top end to use for the HF end as well.
Of course you are right about the tracking, but once again it's a simple matter to reset this.
It does help if you have spent 30 years 'at the bench', but I'm sure even a relative novice could do these jobs if he takes it slowly and (above all) one step at a time.

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

any chance you could describe the process for http://to.ly/aeJk this lovely unit. its just 0.4 off picking up resonancefm 104.4 in london that i listen to through the day. a tweek would be a joy.
ta
wai pei

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

Very difficult to explain, wai. I'm not familiar with the radio and as you have limited experience you will be poking around in the dark.
However, I'll try!
First, using another radio find a strong station at about 108 MHz. Use this as a 'marker' for the 108 MHz end so you know what you are listening for.
Do the same for the 88 MHz end.
Now you need to find the trimmer capacitor for the FM local oscillator. This may be inside the FM 'can' or mounted on the tuning capacitor. Tune the radio to a station at the 100 MHz end and gently adjust the trimmer so the station moves downwards (ie towards 90 MHz).
Move it only a couple of Mhz for now.
Now check to see where the 88MHz station has gone. It may have moved below the bottom end of the dial. Now find the coil for the local oscillator. This will probably be close to the trimmer. Adjust it to bring 88MHz back to where it should be.
Repeat this 'trimmer then coil' until the radio is receiving 88 MHz at the low end and 108 MHz at the high end.
The adjustments are highly interdependent so be prepared for lots of tuning back and forth!
Job almost done.
Now the local osc is covering the right range, the RF circuits need to be adjusted to match:
The RF trimmer(s) and coil(s) will probably look very similar to the ones you have already adjusted.
Tune to a weak station at the 88 MHz end and carefully adjust the RF coil(s) for best reception.
Now tune to the 108 Mhz for a weak station and adjust the RF trimmer(s) for best.
Again the adjustments tend to offset each other, so you'll have to repeat this three of four times to get good tracking.

*Watch out for fragile ferrite cores. These may break if adjusted without the correct tool.

**Note for purists. Yes I know, but I've done this scores of times over the years and it works well. Of course the dial markings are out once you've done it. Like trousers, shares can go down as well as up. Do not immerse in water, do not eat or leave near a naked flame. Contains nuts and small parts. May upset the servants.

Re: Tricky way to extend 100MHZ FM range to 108MHZ
 

Thanks so much for the swift reply (and apologies for my late one), how exciting and yet still kind of baffling.

Simon.