Extending VHF on a FenManII

[Mike Phelan]

I'm thinking aloud here, so bear with me.
Just in the process of restoring one of these - mainly cabinet work and cosmetics on the chassis, as the PO has done a good job on the electronics.
In theory, like many of this era, VHF only goes up to 100MHz.
Tuning is carried out with three slugs (aerial/mixer grid/LO) mounted on a hinged plate that is moved vertically with a boss that rides on a cam on the AM tuning gang spindle.
The cam moves the slugs in a linear manner.

I want the scale to go up to. say. 102MHz
It is a simple solution just to screw the LO slug up a bit, and align the other two on a good signal (my Advance B4 sig gen only goes up to about 82MHz )

Just as simple to make a larger diameter boss to fit on the existing one, which is about 3/8" diameter.

Of course, either of these would lose me the lower end by the same amount - not a problem.

The gap from 98-100 is almost twice that from 96-98 - that is purely the form of the cam.

I could make a new part-cam to screw on the existing one, so it only takes effect from 98 upwards but is then steeper. Easy to test the idea by just raising the slug carrier so I can see how far it needs to go for about 102, and I can make the part accordingly.

Comments, anyone?

[Radio_Dave]

Originally Posted by Mike Phelan

Just as simple to make a larger diameter boss to fit on the existing one, which is about 3/8" diameter.

Of course, either of these would lose me the lower end by the same amount - not a problem.

This is exactly what I did on one of mine, I simply slipped a piece of tubing over the peg. I lost Radio 2 but it works well up to about 103Mhz

Originally Posted by Mike Phelan

I could make a new part-cam to screw on the existing one, so it only takes effect from 98 upwards but is then steeper. Easy to test the idea by just raising the slug carrier so I can see how far it needs to go for about 102, and I can make the part accordingly.

I did this on my second one by using a piece of 2mm copper wire, soldering it to the outside of the cam and bending it to the required shape. You'll keep Radio 2 but the stations get a bit crammed between 100 to 103Mhz.

My third one is on the bench and I trying to think of another solution... hopefully this thread will help

David

[Paul Stenning]

The only comment I can make is that it is usually possible to shift the range of the FM band fairly easily. For example on my Philips Reverbeo the scale goes up to 104MHz and the pointer goes beyond that, to probably about 105MHz. The bottom marking is 88MHz and it goes down to about 87MHz. I had no problems shifting the band by 1MHz so that it tunes about 88 to 106MHz. This allows me to get Saga 105.7FM at the top without losing Radio 2 off the bottom. This was done by tuning in Radio 2 then setting the tuning down 1MHz and then tweaking until I found Radio 2 again. Saga then appeared just off the top, where expected.

Stretching the band would be more difficult and would require some sort of design change rather than just an adjustment tweak.

If you only want to get 2MHz further up than the scale goes, you may be able to just shift it the same as I did. See how much beyond 100MHz the pointer actually goes. There is probably enough movement in the slugs to get to 102MHz.

[Mike Phelan]

I take your point here, Paul, but on this set it would have been relatively easy to shift the entire band up with a sleeve as David did; this would have the same effect as screwing the LO slug up and realigning the other two.

However, despite the fact I only need to go down to 89.3 (R2), I think I would like to keep most of the scale calibration correct as it is, and just make the top bit go up another 1.5MHz or so by adding a bit to the top of cam. This should not affect the tracking at all.

To put it all into context, the scale is 88-100, but the actual cursor extremes are 85.5 - 100.5. The spacing between 94-96-98 is 1.1", but 98-100 is 2.25"; the cam looks as if it seems to flatten out at this point to spread the top end, so it should be possible to add a small plate to the cam from 98 upwards, to make the end of the scale linear so 100 becomes 102 on the scale, and tunes to 102.5.

I need to finish off the chassis first, get it all working and calibrate the scale, and then do some measurements with a vernier caliper on the cam and slug plate travel.
Will end up on Success Stories, hopefully.

[Radio_Dave]

Originally Posted by Mike Phelan

it should be possible to add a small plate to the cam from 98 upwards, to make the end of the scale linear so 100 becomes 102 on the scale, and tunes to 102.5.

Hi Mike, This is effectively what I did (I think) but, as I mentioned earlier, it will bunch 4 or 5 stations into a very small space on the tuning scale and make tuning difficult.

I think it might be better to replicate the cam profile but lift it up from a lower, more empty, area on the scale, probably just after R2

[arjoll]

One of these might be useful - they're used here to convert the 88-108 FM band down to the Japanese band for used imported cars - one of the 10MHz ones ($30/GBP10) would probably do the job nicely for you, dropping 108 down to 98 etc. Better still no modifications, just put it in line with the aerial. The one thing to watch is if you have local stations 10/12/14MHz apart (depending on model) you can have problems.

[paulsherwin]

Originally Posted by arjoll

One of these might be useful - they're used here to convert the 88-108 FM band down to the Japanese band for used imported cars - one of the 10MHz ones ($30/GBP10) would probably do the job nicely for you, dropping 108 down to 98 etc. Better still no modifications, just put it in line with the aerial. The one thing to watch is if you have local stations 10/12/14MHz apart (depending on model) you can have problems.

That's an interesting solution. I've not come across these units before. Presumably you get lots of secondhand Japanese cars imported into NZ. Quite a few make their way here too, but the radios are usually changed by the importer - not only is the FM coverage wrong but the AM channels have 10kHz separation not the 9kHz European standard.

Paul

[arjoll]

There have been a lot imported since the late 80's. I can't see why they don't replace the stereos, but my parents bought a Demio a few years ago with one and seeing they only listen to Concert FM on 90.0 and National Radio on 720 it hasn't been a problem.

The MW-AM band in the imports is 9kHz spacing and covers the normal range, its just the FM from 76-90 which isn't a lot of use.

Band expanders are falling out of favour a bit with more use of 100-108, and CD players are so cheap now I don't know why people bother!

[Mike Phelan]

I've not heard of one either - does it just move the band down linearly? I need the top end to move up. Moving it all linearly is easy by putting a sleeve on the cam follower, but I am tempted to modify the cam or make another, to either steepen the 98-100 end or make the lower-then-88 end go up to 102 in the opposite direction as per Radio Dave.

The decision will rest on me graphing all the stations I need and and ensuring that the modified cam spreads them out enough, or just leave it as is with a sleeve on and the scale incorrect. We'll see. I will post pics in SS when I have done it.

[arjoll]

They are designed to move FM broadcasts down by 10, 12 or 14 MHz (depending on the one you get), so that for example a station on 98 MHz will show at 88 MHz with a 10 MHz band expander and therefore be tunable on a Japanese radio. For your application I was thinking of it being able to move a station on (say) 106 MHz down to 96 MHz so you can tune it in.

Of course you'd then need to switch it out if you wanted to listen to anything below 98 MHz (which would be 88 MHz).

Might be good for anyone who doesn't want to modify their radio.

[arjoll]

Just noticed the Dick Smith link isn't working properly now, I've attached an image of the product section I linked to earlier. Apologies for the size of the image, any higher JPEG compression and it was beginning to look pretty scratchy!