Roberts RCM1 Display Fault.

[Joe]

I'm currently looking at a Roberts RCM1 clock-radio (serial number 402282 for those who are interested in such things).

It works and sounds very good but has an odd fault on the display: one of the centre segments doesn't go properly out making things like 10:30 look like 18:30.

   

Is there anything that's worth checking for a start or is it likely to be a duff driver IC?

Cheers

Joe

[Steve_P]

Are there a load of small capacitors on each line to the display? Change them!

Cheers,

Steve P.

[Joe]

I haven't actually had the back off yet and don't really know what's what without a circuit. I know it's in the 1981-2 R&TVS book but I don't have any that recent.

I'll have a poke about and report back.

Thanks

Joe

[Nick]

Its a VFD display. it normally has a grid per digit and an overall common cathode (the heater).

If a digit lights and goes off OK then the grid for that digit is ok and the overall grid supply is OK. If any digit lights then the cathode is OK and the overall cathode supply (and heater supply) is OK.

Normally these are multiplexed. i.e. the anodes which are coated in blue-green phosphorous are in parallel so you only need n + 7 control lines where n is the number of digits.

Here it looks like the digits are direct drive. very odd. and that one anode driver has gone AWOL. Or there is a possibility that the faulty anode has somehow broken free off the chain and is floating.

So could be a display or anode driver fault but not a PSU fail.

Note that due to their construction VFDs make reasonable audio triodes... they have a heater/cathode, a grid and an anode (even if the anode glows, it's still an anode!).

EDIT: AHA! Found the display schematic - its not multiplexed - its direct drive, single grid, individually switched anodes:
   
It has an AC grid with LDR control so its brighter in the day and dimmer at night (these are apparently prone to failure too). Although the schematic is difficult to read, I suspect that the chip is a National Semiconductor MM5316N - datasheet included below - they are on eBay for a few quid NOS, e.g. item #110998188239.

Looks like an output driver problem in the MM5316N... these are MOS devices and rather sensitive to static and prone to breakdown...

HTH

[Joe]

Thanks, Nick.

It's a set I'm looking at for my antique-dealing pal. The main fault is that the flex has been repaired with a chocolate block and PVC tape, which is at least easy to sort out.

It certainly isn't worth spending anything on, and I wouldn't trust my de-soldering skills to change the driver anyway. I'll have a look around just to make sure it isn't anything daft like a dry joint; I haven't properly tackled it yet as I've been doing a TR82CL and next in the queue is a clock.

In case I didn't make myself clear yesterday, the iffy element lights at full brilliance when it should; it just doesn't go fully out when it isn't required.

I've heard about using these things as triodes before. There are certainly plenty in a display so push-pull stereo should be possible.

Thanks for the help.

- Joe

[Mark Hennessy]

Could be a bit of leakage caused by dirt or aged glue. If the leakage is inside the driver, you might be able to bodge it by putting a resistor in parallel with the segment.

I have full schematics as .JPGs should you need them.

Mark

[Joe]

They'd be handy and appreciated, Mark, as I've now found that the tuning needle won't go all the way across and knowing how it should be strung can only help.

Cheers

Joe