JR RX Parts?

If you can identify the part specification, I'm sure you could pick one up from Maplin (assuming you are in the UK). Or try MacGregor (again, assuming you are in the UK).

Tim

Try MacGregor, their service department is really helpful 01753 549111

HTH John.

These things are almost unobtainable. Toko and Murata make them, but very few places actually stock them, and the importers like to deal in thousands.

Depending on how expensive a receiver it is, and how well you want to preserve its excellence, your best bet ois to raid scrap electronics kit.

A trashed ghetto blaster with an FM section normally nets you a 10.7Mhz one suitable for first stage DC receiever filtering, and many, but not all, AM radios of the smaller sort use the 455khz ones in the AM IF strips. In general they are all eithet 2 terminal (pass through) or 3 terminal (4 or 6 element) types and more or less interchanegable BUT getting a different one and putting it in will be sub optimal for the actual pass band and skirt response.

Your local tip probably has half a dozen suitable bits of junk on any given day.

Occasionally receivers for 'spare or repair' come up on Ebay. Also e.g. a cheapo Hitech feather reciever has one halfway decent 455 flter in it (4 element). So that's another source of almost sensible spare parts.

Or send it back to JR...and get the right part fitted?

That's the impression I got, but I was hoping that maybe someone knew how to get the part from JR somehow. You can get specific parts for much more expensive and specialised equipment - why should R/C gear be different?

Someone suggested checking out dead cordless telephones - however as I don't know what the bandwidth spec is for the JR part it's a bit of a gamble. Obviously less than 10KHz...

That did occur to me - there must be a heap of dead R700 RX's floating about - although if they've died in crashes it's fairly likely the ceramic filter is shattered in them too :(

It's a matter of economics. Here in Australia, an R700 costs about AUD$130. I can't imagine there's a tech who would even look at a device for less than a $70 minimum charge, and they're obviously free to charge whatever they like for the actual repair.

So fixing this RX is one of those things where it's barely worth my mucking around trying to find some solution, but certainly not worth getting repaired professionally.

Thanks for the suggestions.

Russ.

Pretty much all 455khz filters are designde for NBFM or AM wich really menas they are designed for 10khz channel spacing (AM is 9khz).

What varies is teh acciracy with which they do it. 6 pole filters are flat to 6khz or so, then drop like a stone to give huge rejection at

9khz. 4 poles are rather more rounded, and don't have as good rejection of next door channels.

The rest of the specs are more to do with which exact compromise shape you want.

So all will work, but some will work better than others.

I'd take the risk and tear apart some dead radios.

Not necessarily.

Well don't throw it away. Some time you will need it for spare parts :D

I forgot to say that the data on trying to obtain filters was fairly european specific. I didn't realise you wre in 'Kangaroo Kountry' You may find far east suppliers that have the things in stock.

A google search will reveal the Toko and Murata local suppliers, may be worth a phone call or two.

I'll give it a go - though I suspect they'll be talking about reels of 1000. That said I did see some listed in a Farnell catalogue, so that might be an option.

Russ.

The dead part is marked M6J N V which might correspond to 6 poles?

I guess the only way of confirming its selectivity is by having a transmitter 10KHz away from the RX, which is tricky as TX crystals are only available at 20KHz spacings, or am I missing something (RF is not my forte).

Sure will - in worst case it might end up in a park flyer - even without the filter it actually behaves perfectly.

Russ.

Dunno. Are you sure it IS a ceramic filter, and not a capacitor? Matches neither murata nor Toko part numbers..how do you know its dead?

Mmm. Is it a 2 legger or a 3 legger.

And what has broken?

Well, it's big, 11 x 6mm & 3mm thick. It has 4 contacts - two on one side are both grounded, and one of the other pins had around 455kHz, which I assume was the output of the mixer. I know it's dead 'cause the casing has been impacted, in fact after it was removed the end fell off and inside one could see two stacks of small square plates side by side (each plate is about 3-4mm square).

In fact, after a little Googling, it looks pretty much exactly like this:

this may be a Chinese copy of another manufacturer's product.

When the device was in circuit, the RX worked but behaved as if it had been pulled off frequency - only a few channels would work, they'd contrl the wrong servo and for some of them, only for half the control throw. Once the device was removed the RX worked perfectly, giving the same walk-test range as before (approx 50m). I tried it with another TX next to the RX while I did my walk test, and it was completely stable, however the other TX was

160kHz away, so it wasn't that relevant a test really.

I am a little confused that I didn't need to bridge the input & output pads on the RX with the filter removed.

Also, as you might guess, I know bugger-all about RF, but I'd have thought a filter in an FM radio would need at least 16kHz of bandwidth to pass hi-fi quality audio?

Russ.

OK. Fairy nuff. Count the plates. My guess is 6 of them. 6 little Xtals all fragile...the 6 elemnet filters are much mire precise than teh 4 elements.

Weird. There is still a bit of tuning left in the discriminator. Hardly narrow band tho. How many coils in your set, and is it DC?

Yes.

Yes, so what? AM radio is not hi fi quality audio. You are very lucky to get much over 4khz bandwidth out of AM radio..

FM runs at 10.7Mhz IF, not 455khz, and is typically 250-400Khz bandpass. 400 is best for quality, unless you have stations 100khz apart, which you sort of do. Although any given transmitter spaces them about 300Khz apart I think. The idea being that you are not likely to pick one up 50 miles away that is only 100khz away,...RC is narrow band FM, and only needs 4khz or so. It behaves far more like AM in terms of signal quality etc.

I am extremely puzzled that it did work when removed. The facts you note with it broken are reasonable with it being WAY off tune with some elements broken. Perhaps there is enough coupling from other components.

If you can find a filter that phsically fits, you only need about 6khz bandwidth, and a 50dB stop band of about 10khz is more than enough.

The PDF you posted is a suitable part. E or F preferred, but almost any would work better than what you have now.

If you do find a source of filters, please post it here. There is a group of us looking to try and buld a halfway decent receiver, and that is the hardest bit to source.

That occured to me after you mentioned the 6 pole thing. There probably are

6 ceramic plates, plus 2 for the external leads. I can't see where the ground connections go to, but I assume there's some kind of plane in there.

In my searches I found a Murata filter with identical packaging, however it is 450kHz, I had another check of the receiver and the signal on the input seems to vary between 448 and 455 kHz, but not above 455 - I can't tell exactly as the only tool I have is 150MHz digital scope. I'm thinking, therefore, that the IF might be 450.

It's a SC unit (I don't think JR do DC) it has 6 coils.

Fair enough, I assumed that FM radio used the same filter - as I said, I know bugger all about RF :)

Cheers for that - always good to learn something :)

Mysteries of RF I think.

Sure - I've got a few ideas, I'll see how it goes.

Russ.