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Old 14th Mar 2008, 10:23 pm   #1
adriand
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Default Roberts R600 problems

I've just acquired a filthy dirty but savable Roberts R600. All seems well with VHF but MW and LW have no audio at all. I've removed the chassis from the cabinet and had a look and gentle prod here and there but can see nothing obvious. I have read of headphone socket probs with this set and in fact when i move the socket wiring i get a crackle from the speaker. I guess im on the wrong track there though as i've got VHF ok. There is a soft hiss from the speaker at full volume.

Just to add i'm a complete novice so please go gently on me. Any ideas where to look next guys?

Adrian
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Old 15th Mar 2008, 12:44 am   #2
yestertech
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Default Re: Roberts R600 problems

HI Adrian
I have an annotated service sheet here which blames lack of AM only on failure of one of the transisitors ( AF115 ) within the IF module.
Sadly, replacing parts in the modules in this set is never easy and definitely not for the faint-hearted !
Worse still, the precise circuitry for the modules is not given anywhere in the manufacturers data ( that I can see... )
You may need some "hands-on" assistance from another forum member.
Andy
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Old 15th Mar 2008, 10:41 am   #3
adriand
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Default Re: Roberts R600 problems

Thanks Andy. Ah yes I guessed it might have been something more "complicated". I may just clean it up and leave it if its an involved repair. Unless anyone else can give advice or offers of restoration.

Adrian
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Old 15th Mar 2008, 12:41 pm   #4
geofy
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Post Re: Roberts R600 problems

Hi Adrian
If the FM is working then all the modules are working as the transistors are common as far as I.F. is concerned, though the AM stage may not be oscillating, so the transistor at the front end of the AM section may be faulty. But before that have a look and the waveband switches and the medium and long wave ferrite rod aerial and coils to see if there are any broken wires.

Geof
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Old 15th Mar 2008, 1:36 pm   #5
adriand
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Default Re: Roberts R600 problems

Ok Geof. Have had another look for broken wires to ferrite rod and elsewhere. All ok. Switches seem to work with a loud "clunk" from speaker when operated, but no audio. Does this point to the front end transistor you mentioned ?

Adrian
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Old 15th Mar 2008, 3:29 pm   #6
Darren-UK
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Default Re: Roberts R600 problems

Hi Adrian,

Nobody has yet suggested the old trick of tapping the modules. Easiest if you remove the chassis enough to access the IF module.

Having done this, connect a battery, switch on, select MW or LW and tune to where you know there's a station. You may be best selecting LW and tuning to BBC R4 at 1500m/198KHz. Turn up volume and ensure ferrite aerial points towards transmitter.

Next take a screwdriver and with the blunt end give the IF module a few sharp taps at various positions on its case. If you get an almighty crackle at the loudspeaker or a burst of audio at the 'speaker, your problem will be either a duff transistor inside the module (very likely) or a dry joint problem (less likely but still very possible). Be aware, however, that this dodge is not guaranteed to locate a duff transistor as you're tapping the module and not the transistor directly - but it very often does give the game away.

Try that but whilst holding the out-of-cabinet chassis make sure your fingers don't touch any part of the pcb that could cause loss of signal. Also take care not to damage any of the fine wires leading from the aerial coils.
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Old 15th Mar 2008, 5:18 pm   #7
geofy
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Post Re: Roberts R600 problems

Quote:
Originally Posted by darren-uk View Post
Hi Adrian,

Nobody has yet suggested the old trick of tapping the modules. Easiest if you remove the chassis enough to access the IF module.
Worth a try, if in doubt give it a clout. It is possible the first stage of the AM not oscillating, but a faulty wavechange switch could still be responsible, one section usually provides power to either the AM or FM osc section.

Geof
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Old 15th Mar 2008, 6:39 pm   #8
adriand
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Default Re: Roberts R600 problems

Thanks guys for the tips. I've already tried a few taps in likely places but still nothing doing. Will do a proper test on the switch when i've more time tomorrow and report.

Cheers
Adrian
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Old 15th Mar 2008, 7:16 pm   #9
Darren-UK
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Default Re: Roberts R600 problems

You could also have a look for a missing, broke, loose or obviously maladjusted slug in the AM oscillator can. Presumably you have the service data and this will show you the relevant can (I think it's actually located within the IF module IIRC).
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Old 15th Mar 2008, 7:52 pm   #10
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Default Re: Roberts R600 problems

Look at R11 as well if it does not work on MW and LW but is OK on VHF.Should be 330 Ohms.

Get a cheap meter from Maplin. Very handy.

Cheers,

Steve P.
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Old 15th Mar 2008, 11:48 pm   #11
Norman Raeburn
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Default Re: Roberts R600 problems

Hi Adrian, I live in Fife (near Leven) I am a TV engineer and could take a look at it for you as long as your not desperate for it. PM your details if you are interested. Norman
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Old 16th Mar 2008, 12:58 pm   #12
Tom_I
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Default Re: Roberts R600 problems

You can check if the AM oscillator is working without dismantling anything.

Get another radio which works, and tune it to a quiet area at the high-frequency end of the medium wave. Leave it switched on with the volume turned up.

Stand the R600 right next to it, switch on and tune it (the R600) up and down the medium waveband. If its oscillator is working, you will hear various whistling noises coming from the good radio. If there's nothing, then the AM oscillator/mixer transistor is the likely problem. As has been said, this lurks within the IF module (not the tuner module - that contains the FM front end).
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Old 16th Mar 2008, 2:52 pm   #13
adriand
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Default Re: Roberts R600 problems

Have just checked the oscillator with another radio and sadly all is silent. Although I do have a meter i dont have any service data sheets so can only "look" and do simple checks.
Therefore i've PM'd Norman who offered assistance and lives fairly nearby. Thanks for all your assistance guys in getting me this far. As a member of this forum only for about a week i'm overwhelmed by the amount of good advice and friendly help given.

I'll post the outcome as and when on this board.

Thanks
Adrian
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Old 4th Apr 2008, 2:27 am   #14
Roger Rabbit
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Default Re: Roberts R600 problems

First thing you need to try is the switches, get yourself a tin of Servisol and give em a spray. Don't be tempted to use wd40 the residue will cause problems. Check very carefully the soldering around the switch pins, very often there are dry joints here if the switches have had a hard time. If not the switches its the module, the wack treatment often works... but not always. The audio is obviously fine, the if module is dual freq..is the FM good or is it a bit deaf if the FM is fine then the af115 is dead in the module. If the FM is flaky check the AF117s You can heat these up with an iron to melt the whiskers, when it cools it will probably work, but be carefull. Bf450 transistors do work as replacements in the modules, watch the leadouts though! Will cost you 57p from Maplin to replace all three!!
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