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Old 1st Jan 2013, 1:35 pm   #1
sobell1980
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Default Help needed on Roberts RMB alignment

Hi all,

I finally got round to having a go at the above model, and after getting the low tension to valves sorted I was receiving lots of MW stations but nothing on LW. I could faintly hear Absolute Radio mirrored when switching wavebands. Foolishly I tried adjusting the LW aerial trimmer C33 marking its position so if it didn't work I knew the original position.

Now I've lost all stations on all wavebands since this adjustment. What an idiot. I don't own a signal generator and made a fatal error. I should have known better I know. If anyone could help that would be great. Looking at C33 I cannot see how adjusting it would harm MW?

Many thanks,
Dave.
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Old 2nd Jan 2013, 8:04 am   #2
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Default Re: Help needed on Roberts RMB alignment

Sorry, many thanks.
Dave.
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Old 2nd Jan 2013, 5:22 pm   #3
Pete_kaye
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Default Re: Help needed on Roberts RMB alignment

With mine (discussed elsewhere) it was quite easy to knock valve 3 and lose everything - especially if out of the case and valves upside down. Perhaps that is what you have done.
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Old 2nd Jan 2013, 5:32 pm   #4
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Default Re: Help needed on Roberts RMB alignment

Also, the wavechange switch can be unreliable and give the kind of symptoms you're experiencing. Check all the contacts are actually making using a continuity tester, and try pulling/pushing the knob and twisting it so the switch is held slightly off its rest positions.

Nick.
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Old 2nd Jan 2013, 6:34 pm   #5
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Default Re: Help needed on Roberts RMB alignment

Thanks guys!
So adjusting the L.W aerial trimmer would not have caused this problem of me loosing all wavebands?
I will check these items and report back.
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Old 2nd Jan 2013, 9:54 pm   #6
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Default Re: Help needed on Roberts RMB alignment

Good news! Fiddling with valve 3 it bursts into life then cuts out and if you can get the angle right of the valve it will stay working. It is strange as it is the tightest fitting valve in the set! Yet just drop it gently onto the contacts it works, push it home and it stops. What is the best way to tackle this? Could it be loose connections inside the valve itself or just the pin to socket contacts?

Regards,
Dave.
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Old 2nd Jan 2013, 10:13 pm   #7
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Default Re: Help needed on Roberts RMB alignment

Well done Dave and Pete! Can you find which of the contacts is the defective one, maybe by poking with a cocktail stick from the tag side?

N.
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Old 4th Jan 2013, 8:41 am   #8
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Default Re: Help needed on Roberts RMB alignment

It is very difficult to find which pin, or pins are causing the issue. Would it be easier to remove the valve socket and try and tighten the socket connections? Having never done this before is this an easy task to perform or am I better trying to squeeze the socket connections tighter onto the valve pins in situ? Are the valve pins known for coming loose on the actual valve, or is more likely to be the socket causing an issue.

Thanks again for your replies. Also could anyone give me a description of what happens at valve 3? What is its purpose and how does it function? I'm a little confused by the Trader sheet's description.
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Old 4th Jan 2013, 12:52 pm   #9
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Default Re: Help needed on Roberts RMB alignment

Quote:
Originally Posted by sobell1980 View Post
Would it be easier to remove the valve socket and try and tighten the socket connections? Having never done this before is this an easy task to perform or am I better trying to squeeze the socket connections tighter onto the valve pins in situ? Are the valve pins known for coming loose on the actual valve, or is more likely to be the socket causing an issue.
I'd try and do it in situ. It will almost certainly be the socket/valve holder rather than the valve itself, but nothing's impossible.

Have you tried gently scraping the valve's pins with a Stanley blade until they're nice and shiny?

If you can locate which tag in the valve holder is to blame, it's sometimes possible to steal an unused one from another valve holder to replace it, if you can't retension it. Much easier than changing the whole thing.

Nick.
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Old 4th Jan 2013, 1:22 pm   #10
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Default Re: Help needed on Roberts RMB alignment

Quote:
Originally Posted by sobell1980 View Post
Also could anyone give me a description of what happens at valve 3? What is its purpose and how does it function? I'm a little confused by the Trader sheet's description.
The circuit split over two pages doesn't help here.

V3 is really two valves- a diode which acts as a half wave rectifier which outputs the negative half cycles of the IF signal onto C14 and the volume control and a pentode which is the first audio amplifier stage. C14 smooths the IF ripple but is too small to smooth out the audio frequency signals also present in the mix. The dc component of the signal is then blocked by C15 leaving just the audio ac part fed to the grid of the pentode section of V3 which is the audio preamplifier stage. The signal at C14 is also fed via R5 and C6 which filter out the audio component leaving just the negative dc part proportional to the IF signal level which is fed back to the frequency changer (V1) and IF (V2) grids to provide automatic gain control.

The AGC works because the two valves are designed to have gain which decreases as the grid is made more negative- the more signal in the front end, the greater the negative voltage fed back and the less the gain. The upshot is that within limits, the signal level at the detector diode in V3 is held fairly constant.

HTH....
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Old 4th Jan 2013, 2:18 pm   #11
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Default Re: Help needed on Roberts RMB alignment

Corrosion, weak spring tension and even breakage of valve base contacts are frequent causes of problems in battery sets. I suppose that this could be due to some corrosive fumes produced by the batteries but may just be due to poor storage conditions or poor quality materials.
Nick's advice is sound; try to retension the contacts in-situ with a sharp tool, but be gentle as it is easy to break the contacts.

John
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Old 5th Jan 2013, 8:01 am   #12
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Default Re: Help needed on Roberts RMB alignment

Thank you for all your help.

I will spend some time this weekend on it. Thanks for the description of valve 3. I'm still fairly fresh at this and doing lots of revision from a book that one of the other members has written. So valve 3 deals with the negative cycles as described, what happens to the positive cycles? How is the signal kept in time or together when it's divided and split around the circuit? Maybe I need to get back to the books!

Also I'm still struggling to get my head around AGC? I have read different conflicting advice and its purpose on here.

Many thanks,
Dave.
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Old 6th Jan 2013, 9:02 am   #13
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Default Re: Help needed on Roberts RMB alignment

Going to have a go at repairing valve 3 socket today! Will let you know the outcome later.

Thank you all for your sound advice and help with this matter. If anyone could get back to me on the couple of questions I posted above about AGC, and what occurs with the positive cycles from the IF at this point as I'm still a little unsure.

Once again many thanks for all your help and time.

Regards,
Dave.
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