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Old 22nd Jun 2004, 12:13 am   #1
Hybrid tellies
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Default Howling 'Realistic' pocket radio.

Last year at the local village fete for 2 I bought a little Realistic 12-9314 which is a small pocket 3 waveband radio covering LW MW & FM. I gave it a clean up put some batteries in it and it worked ok on all 3 wavebands.
So I took it to work wher it worked ok on stations like Radio 2, Radio Bristol and if you put it by a window Classic Gold and R4 LW for the Cricket.
The other night I was working late so decided to listen to the England game on R5L but all I got was a loud howling noise.Thinking interference I picked the set up and my suspisions were aroused. As I touched the cabinet the pitch of the howling changed.It was self oscilating but only on 909Khz, perfect on all the other AM stations. It almost ended up in the bin there and then
So I took it home and have swapped it over with another old radio. As for the Realistic I still have n't got round to looking at it and I am not sure if I will bother
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Old 22nd Jun 2004, 4:42 am   #2
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Default Re: Murphys Law

Hmm... second harmonic of 455kHz i.f. getting through? Recommended action probably disposal to a district where R5 is better on 693kHz... :P

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Old 22nd Jun 2004, 8:39 am   #3
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Default Re: Murphys Law

Quote:
The other night I was working late so decided to listen to the England game on R5L but all I got was a loud howling noise.
Are you sure that is not what it was meant to sound like? :
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Old 22nd Jun 2004, 9:50 am   #4
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Default Re: Murphys Law

I'd probably be inclined to detune one of the IF trannies slightly - I haven't seen the circuit diagram for this set but occasionally the IFs were staggered either side of the IF frequency, and if they peak instability is possible. It might reduce the sensitivity of the set a little, but could be better than throwing it out!
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Old 22nd Jun 2004, 12:04 pm   #5
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Default Re: Murphys Law

I think Paul_RK could be close to the answer. Second channel 455k is 910k and is close enough to cause a strong 'beat' with 909k. A well designed set will not suffer problem because the second channel image should be well down into the noise.

This sort of problem can be caused by a poorly aligned aerial circuit. Just check to see if the MW coil is tight on the ferrite rod. If it's loose, it may have moved. If the latter then you need to tune the set to a weak station at the low (550khz) end of MW and slide the coil along the rod for maximum signal. Fix it in that position and then try the tuning again. You may find the whistle has gone.

If of course everything seems OK then Quantums suggestions are best.

I remember this was a problem with early trannie sets from the Far East using 455k as IF.

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Old 22nd Jun 2004, 12:24 pm   #6
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Default Re: Murphys Law

Quote:
Recommended action probably disposal to a district where R5 is better on 693kHz... :P
693kHz is a big transmitter in the midlands (at Droitwich IIRC) so should get to Bristol, especially after dark.

Doesn't help with the 'Grrrr' factor though

Best regards, Paul

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Old 23rd Jun 2004, 1:00 am   #7
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Default Re: Murphys Law

Thanks for the suggestions. Yes Paul I did go Grrr, very loudly. I did use 693Khz from Droitwich but this signal in our office is much weaker and reqires that you sit by the window to hear anything.
I bought this cheap little radio possibly from the 1980's because of its small size so it could fit in my limited office storage unit at work and of course because it was only 2.
You have all persuaded me not to bin it but to take a look. I have also noticed the battery terminals are rather corroded which explains the other fault of intermittent cutting out. I shall let you all know in the not so distant future what I find.
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Old 26th Jun 2004, 11:53 pm   #8
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Default Re: Murphys Law

As promised my report back on this small radio.
First the intermittent cutting out was down to some old corrosion on the battery terminals. Now all clean and shiney.
The interference on 909Khz was in the end down to a tweek or slight re alignment of the RF/IF stages, which I did by ear. Well I could n't be bothered to get my signal generator out for this job
Now all the stations on all 3 wave bands are reasonably clear and much closer to where they should be on the dial than when I started out.
Not the nicest of jobs I have undertaken but at least you can take the set apart easily. I reckon it dates back to the 1980's. Some of the newer sets that I have seen are far worse.
Now I can take the Realistic back into the office for very occasional use and bring my Bush Sandpiper back home. After all if the Realistic gets nicked thats the thief's bad luck . This would be a different matter with my Bush Sandpiper of course.
Many thanks for the helpful suggestions.
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Old 27th Jun 2004, 1:27 pm   #9
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Default Re: Murphys Law

It was probably second channel interference. This can happen when the aerial circuit is off resonance and the 'peak' is in the wrong place. This would allow a strong signal on (say) 1.9MHz to get through and beat with a wanted signal on 300 meters (1MHz).

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Old 27th Jun 2004, 9:29 pm   #10
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Default Re: Murphys Law

I would agree with you on the 2nd channel interference but there are now signals around 1.9Mhz to cause this. No this is just pure poor design and is probably due to radiation from the local oscilator. As you said yourself 2x455 = 910Khz. And the annoying beat note was about 1Khz as well.
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