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Old 4th May 2020, 9:05 am   #1
John G8MWF
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Default Roberts Portable RP4 RMB Frame Aerial

I am just in the process of trying to restore my first portable valve radio.

This is the Roberts Portable RP4 RMB and according to Trader sheet 1037 it has a frame aerial - the one that I have has a ferrite rod and some other minor differences to the information in the trader sheet.

The radio appears to have been worked on for repairs but I don't think it has been modified?

Was there a version of this radio that had a ferrite rod aerial factory fitted or is this a 'nicely carried out' modification?

Cheers


John
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Old 4th May 2020, 6:28 pm   #2
music-centre
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Default Re: Roberts Portable RP4 RMB Frame Aerial

Hello John,
The RP4 and RMB were 2 different models with the RP4 being battery only.
What you have is an RMB where the later ones were fitted with a ferrite aerial.
Steve.
https://www.radiomuseum.org/r/roberts_rmb.html
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Old 4th May 2020, 6:35 pm   #3
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Default Re: Roberts Portable RP4 RMB Frame Aerial

A real proper ROBERTS. They work very well. J.
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Old 4th May 2020, 7:52 pm   #4
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Default Re: Roberts Portable RP4 RMB Frame Aerial

As Steve says, an RMB. It and the RP4 were launched at the same time, withdrawn at the same time, and share the same serial number sequence - beginning around 41000, ending at 62000 or so - with the more expensive RMB having the option of mains operation. The ferrite rod aerial is standard on later production examples of both models.

Paul
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Old 5th May 2020, 12:00 am   #5
John G8MWF
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Default Re: Roberts Portable RP4 RMB Frame Aerial

Phew! That is what I was after - thank you. I really thought that someone had modified the set in the past.

Trader sheet 1037 does not show the ferrite aerial option and there are minor differences in the layout. It does mention both the RP4 and the RMB models.

The electrolytic capacitors are all bulging at the cap ends, LT & HT, and I am just waiting for some replacements to arrive in the post before I can fit them and power up.

I did manage to power the heaters up from a 1.5V power supply and they drew around 135mA in total. It surprised me how cool they remained and that there was no visual indication they were powered up.

The case has seen a few too many 'picnics' in its time and I am not sure if I should just leave it like it or not.

Thanks again for all of your help - appreciated.



Almost forgot the serial number is 70674 so it looks to be a later model?
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Old 5th May 2020, 1:27 am   #6
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Default Re: Roberts Portable RP4 RMB Frame Aerial

Quote:
Originally Posted by John G8MWF View Post
Almost forgot the serial number is 70674 so it looks to be a later model?
Hello John,

That's so far from the serial numbers on file at the Roberts group - 51 RP4s and RMBs there, lowest 41264 and highest 62040 - that my first guess has to be you're reading something else entirely, possibly the number on the licence plate? That has nothing to do with the manufacturer. In the RMB the serial number is on the control panel, an unobtrusive five digits just in front of the wavechange knob.

These generations of battery valves do surprise anyone who's accustomed to mains types, they're designed to achieve such low consumption that there's little or nothing of visible filament glow and hardly any perceptible warming of the envelope. Heater voltage is a bit critical, so it's well to check it's not excessive when running such sets from the mains.

Paul

Last edited by Paul_RK; 5th May 2020 at 1:37 am.
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Old 5th May 2020, 8:48 am   #7
John G8MWF
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Default Re: Roberts Portable RP4 RMB Frame Aerial

Hello Paul.
You are correct - I was reading the wrong number. It should be 60895.

All noted about the heater voltage. My idea is remove all of the valves on the initial PSU test and to put a 10 ohm load in place for the heaters.

This will allow me to monitor the heater voltage and make sure that I do not damage the valves. The only problem with that method is, as I do not know the nominal HT current, that there will be no load on the HT circuit so it should be higher than normal and not exceed the replacement capacitors 160V working limit.

I also intend to change 'that' 0.01uF capacitor and I have only found one other electrolytic to be replaced, 20uF, that is not on the PSU.
I am going to use a 47uF for the 20uF as that is all I have to hand at the moment.

When I initially looked at the radio most of the valves were covered in what looked like very old crinkly sellotape that had faded cardboard with writing on it. So they obviously did not expect any heat issues!
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Old 24th May 2020, 12:33 am   #8
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Default Re: Roberts Portable RP4 RMB Frame Aerial

I would very strongly advise anyone working on mains/battery valve portables to do the initial testing and fault-finding using batteries. It is much easier (not to mention safer) and the LT and HT currents and voltages can be conveniently measured, plus you will know when the set is working. The last step is to repair and adjust the mains PSU.
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Old 25th May 2020, 12:43 am   #9
John G8MWF
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Default Re: Roberts Portable RP4 RMB Frame Aerial

Quote:
Originally Posted by Phil G4SPZ View Post
I would very strongly advise anyone working on mains/battery valve portables to do the initial testing and fault-finding using batteries. It is much easier (not to mention safer) and the LT and HT currents and voltages can be conveniently measured, plus you will know when the set is working. The last step is to repair and adjust the mains PSU.
Noted!
This was the first method I used as the 3 electrolytic cans were leaking and I had to wait for replacements before I could work on the mains PSU.

I was very surprised to find how cool the DAF valves run and it was only when I got a station tuned in that I realized they were actually working!

Thanks for the advise and I am sure others will put it to good use as well.
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Old 25th May 2020, 10:43 am   #10
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Default Re: Roberts Portable RP4 RMB Frame Aerial

Yes the DAF valve is the coolest of the four with only about a quarter of a mA of IK.
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Old 25th May 2020, 7:41 pm   #11
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Default Re: Roberts Portable RP4 RMB Frame Aerial

When repairing valve portables I have put a 3 diodes in series across the heater supply so if the volts go too high it does not result in 4 valves dying !
Cheers
Peter
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Old 26th May 2020, 10:12 am   #12
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Default Re: Roberts Portable RP4 RMB Frame Aerial

That's fine for a battery set with parallel filaments but would spoil reception a bit on a series filament mains/battery set. A 1W 7.5V zener diode could be used there. To similar effect if it's the wrong way round!
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