Roberts RM33 FM band extension??

[Robsradio]

Hello,
I’ve been to the car boot again and have added another Roberts to the collection, an RM33 in a teak case, all works well just needs a clean and waxing.
A question though, I thought I would tune to my local station on 105.2 MHz only to find the FM band scale stops at around 104 MHz I would have though by the time this was made in the early 80s all FM scales would have been up to 108?
Do you think the FM osc would adjust that bit further to 105.2 MHz?
Thanks
Rob

[paulsherwin]

You should be able to tweak the oscillator trimmer enough to shift the tuning by a couple of MHz. How much you can get away with depends on where the BBC R2 signal is in your locality (assuming you want to listen to it.) Obviously the tuning scale will no longer give the correct frequency indication.

The RM33 was made between 1980 and 1985, and was one of the last models sold with restricted VHF coverage.

[Chris55000]

The Roberts RM33 is a varicap–tuned set for the VHF band with a conventional two–gang capacitor for LW and MW, so it's easy to attempt a little experimentation!

Measure the voltage across the bank of tuning potentiometers on the Preset Tuning Board, and note the voltage obtained, and then adjust R6 and R13 in turn to give you about 0.75 to 1V more than the previously noted figure.

This should allow you to increase the oscillator frequency enough at the higher end of the F.M. band to bring your desired local station in!

Chris Williams

[paulsherwin]

You may find one of the presets will tune up to 105.2 despite what it says on the tuning scale.

[Robsradio]

Thanks gents for your help, Chris I will undertake your suggestion, I just need to find a bit f spare time!
Thanks all

[G6Tanuki]

Most \roberts radios with emasculated FM coverage can be 'tweaked' - my Roberts Radio-of-the-day is a R707 where I have happily sacrificed the BBC-national stations in order to receive some of the more-listenworthy stations above 100MHz.

Tweaking does of course render the frequency-scales somewhat deranged; quite a few Roberts radios have little red 'sliders' on the tuning-scale you can use either to identify your favourite stations, or to mark-out the Megahertz on your 're-optimised' tuning-scale.

In times-past it was not unusual to reverse-engineer an additional 1.5V battery into the varicap tuning-network to stretch the HF coverage; this worked both for FM radios and the first-generation of varicap-tuned MW radios using the likes of the MVAM115 diode-package, which otherwise had a somewhat-cramped dial at the HF end [where Luxembourg and a few free-radio stations hung out]

[Robsradio]

Originally Posted by G6Tanuki

Most \roberts radios with emasculated FM coverage can be 'tweaked' - my Roberts Radio-of-the-day is a R707 where I have happily sacrificed the BBC-national stations in order to receive some of the more-listenworthy stations above 100MHz.

Tweaking does of course render the frequency-scales somewhat deranged; quite a few Roberts radios have little red 'sliders' on the tuning-scale you can use either to identify your favourite stations, or to mark-out the Megahertz on your 're-optimised' tuning-scale.

In times-past it was not unusual to reverse-engineer an additional 1.5V battery into the varicap tuning-network to stretch the HF coverage; this worked both for FM radios and the first-generation of varicap-tuned MW radios using the likes of the MVAM115 diode-package, which otherwise had a somewhat-cramped dial at the HF end [where Luxembourg and a few free-radio stations hung out]

Many thanks
Rob

[Robsradio]

It’s operating life with me has been short... it worked great, and sounded good, so left it on for a few hours, volume turned fully down, (as TV was being watched). Turned up volume in an ad break, audio is awful and distorted on all bands and stations, thought it started out too well. I just hope it’s not one of the ICs.

[Reelman]

Maybe flat battery?

Peter

[Robsradio]

Originally Posted by Reelman

Maybe flat battery?

Peter

Sadly not, it a mains set.

[Robsradio]

Looks as if it is the speaker, I don’t know what has changed since it has been on at zero volume, but if I apply light pressure to the speaker connector is sound fine, but it’s not an electrical connection problem, it’s as if something is impeding the movement of the cone. Will need to investigate further.

[Robsradio]

Well as thought, the cone movement was being restricted by a very tight flexi wire from the speaker connector to the cone. Only one wire was tight, the otherside was fine. I bent the bracket slightly to give some slack as they were riveted so wasn't able to unsolder and reposition to give some slack. Why this suddenly started to give a problem when it had been working fine, who knows?
Also found one of the preset pots had never had solder on it from new!
One question if anyone here has this radio, should the AFC switch latch, or is it momentary? It looks as if there is a groove in the switch where a latching wire may have been, but not sure.
Thanks
Rob

[Paul_RK]

Mine doesn't latch. The idea must be that holding the button down kills the AFC to facilitate accurate setting of the presets, then in operation it's always there to compensate for any drift.

Paul

[Robsradio]

Thanks Paul, that makes sense.
Appreciated Rob