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Old 19th Apr 2009, 11:18 am   #1
'LIVEWIRE?'
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Default 'Beat Note' interference on a Roberts R200

Not sure whether it's an Image frequency or what, but there is a 2 or 3kHz beat note on Radio Gloucester 1413kHz, which at this location is a strong signal from a transmitter at Bourton on the Water about 8 miles away. I thought maybe the beat note was due to an oscillator harmonic from 945kHz, but there is no station on this freq. nearer than London, thus no signal strong enough to cause this type of interference. No other Radio I have tried exhibits this phenomenon, and CAREFUL adjustment of either the IF or Osc. circuits doesn't cure it. What might be the cause? The radio is the later model with AF117s, which I have replaced with AF139s

(PS I was going to use amateur radio terms such as 'QTH', 'QRM', & 'QRG' for location, interference, and frequency respectively. Would these be acceptable in future?)

Last edited by 'LIVEWIRE?'; 19th Apr 2009 at 11:22 am. Reason: Correcting typographical errors, etc.
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Old 19th Apr 2009, 12:21 pm   #2
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Default Re: 'Beat Note' interference on a Roberts R200

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Originally Posted by 'LIVEWIRE?' View Post
(PS I was going to use amateur radio terms such as 'QTH', 'QRM', & 'QRG' for location, interference, and frequency respectively. Would these be acceptable in future?)
Following debate in the Moderators office the answer is no. Sorry.

Although these terms mean something to those in the Amateur Radio fraternity they are by no means common to the majority of our members. This might limit the response to your posts to those from members "in the know" regarding QTH etc etc. It might also be seen as contravening forum rules B8 and B9 which cover the use of plain English when posting, in the event of complaints from members unfamiliar with those terms.

The use of amateur radio terms would, in addition, be very incongruous in threads such as this one concerning a Roberts R200.
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Old 19th Apr 2009, 2:39 pm   #3
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Post Re: 'Beat Note' interference on a Roberts R200

The AF139 is not a suitable transistor for this postion, replacing these with AF127 will probably cure the problem.

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Old 19th Apr 2009, 2:54 pm   #4
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Default Re: 'Beat Note' interference on a Roberts R200

A beat noise is also an indication of a flat battery ........ I once found a AF239 (same spec as an AF139) in a rough sounding Bush TR130 where once was an AF117 and a replacement AF126 worked better, although I gather an AF239 may just work OK in some sets. I can sell you three AF126s if you need them.

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Old 19th Apr 2009, 3:34 pm   #5
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Post Re: 'Beat Note' interference on a Roberts R200

Remember the AF126 etc have transposed base and emitter leadout to the AF139 which is a different beast altogether. Not best suited for MW LW sets.

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Old 20th Apr 2009, 7:18 am   #6
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Default Re: 'Beat Note' interference on a Roberts R200

I double checked to ensure that I had the transistors connected the correct way around, Geof, especially as I fitted them on the print side of the PCB to avoid the fiddly job of inserting them between the IF cans. I used AF139s because AF127s seem hard to come by and pricey these days-in fact I think Cricklewood Electronics, for one, sells AF139s as substitutes for 127s(don't quote me on that!!)though an old Mullard data book does specify the 139 as 'a mixer/osc. for use up to 860Mhz'


Howard, thanks for the offer-this particular radio is now back with it's owner, but I may PM you re the AF126s-they'll come in handy for other things.
Point taken about the use of 'Q' code terms, folks
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Old 20th Apr 2009, 8:11 am   #7
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Default Re: 'Beat Note' interference on a Roberts R200

Livewire, did you get to the bottom of the beat note problem, though?
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Old 20th Apr 2009, 10:15 am   #8
geofy
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Post Re: 'Beat Note' interference on a Roberts R200

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Originally Posted by 'LIVEWIRE?' View Post
AF139s as substitutes for 127s(don't quote me on that!!)though an old Mullard data book does specify the 139 as 'a mixer/osc. for use up to 860Mhz'

If cricklewood are saying the AF139 is a sub for the AF127 then they are wrong even though it is for a mixer stage, it is a different type altogether. And going up to the UHF band is not really the best device for a MW LW set where the original passive components where designed around the AF117. Any of the others like AF125 126 should work better.

I only pointed out the leadout in case you where to fit the correct type.

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Old 20th Apr 2009, 11:29 am   #9
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Default Re: 'Beat Note' interference on a Roberts R200

Peter, in the end,no-I basically left it as it was. The set's owner mainly listens to a different station(Radio4 on LW I gather) so it won't be a problem for him.
Geof-I'll check the Cricklewood site in case I've inadvertently misinformed folks as to what they say
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Old 21st Apr 2009, 12:12 pm   #10
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Default Re: 'Beat Note' interference on a Roberts R200

I must issue an apology to Cricklewood Electronics- having checked their website again, I find that they DO NOT list the AF 139 as a substitute for the AF127. They sell the 139 for 1.15 each, inc VAT, whilst their price for AF127 is 5.75 inc VAT. Given this large difference in price I may have decided to go for the 139 without considering that it is a UHF device. Cricklewood are now listing AF117s, but I wonder if these would be reliable in the long term
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Old 21st Apr 2009, 3:31 pm   #11
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Post Re: 'Beat Note' interference on a Roberts R200

The AF200 AF201 might be a better type, being an alloy diffused junction instead of a Mesa and more suited to the transistor it is replacing. Fitting on the track side could have affected its operation as well, bit of a bodge there

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