[Premium-Rx] RA1792 displays

[David Bobbett]

Hi France,

Nope, nothing anywhere near as sophisticated as that for the 
backlighting! Each display is lit using 16 series/parallel wire-ended 
filament bulbs mounted on a small PCB which is then screwed into two 
mounting bushes set in the PCB behind the front panel. Each bush acts as 
the connection to feed about 9v AC from the PSU and the brightness of 
the display can be varied by changing the two wirewound resistors which 
you will find on the front vertical face of the PSU. The bulbs are 5v 
60mA T1 wire-ended types and are still available.

In my case, I converted an early non-backlit 1792 to a backlit version 
as I had some spare front panel and backlighting PCBs left over from the 
junk pile. The PSU is the same in either version and only needs a bit of 
re-wiring and the resistors to get it to work -  some mods  to front 
panel PCB are needed, but that's another topic.

A white plastic diffuser is stuck to the rear of each LCD using two thin 
strips of double sided sticky tape (although this has usually come loose 
over the years) unless some genius has epoxy glued it back on as I found 
with a couple of 1792s which I refurbished. The epoxy also ruins the LCD 
over time, so it was not a good plan, although getting some replacement 
diffusers cut shouldn't be a problem if this has happened.

Easiest method seems to be: sit radio with front panel uppermost, remove 
knobs, front panel screws and the front panel itself; you don't have to 
remove the inner black front panel. Gently lever out the old LCDs by  
inserting a screwdriver under the top left hand corner and lifting the 
display slightly, then do the same to the bottom left hand corner, top 
right hand corner and bottom right hand corner. The objective is to just 
loosen the display at this stage, then work around until the display 
comes free. You will then probably find the diffuser sitting on top of 
the backlighting pcb. If you've got good diffusers, clean them and 
remove any tape residue with iso-propyl alcohol (propanol) and put to 
one side.

Undo the cross-head screws which secure the backlighting PCBs and fire 
up the soldering iron. Although it is a chore, I would recommend 
replacing all the backlight bulbs because a) you will get more even 
illumination and b) the old bulbs tend to suffer from wire oxidation 
which causes intermittent bulb failure. When you have finished replacing 
all 32 bulbs it is worth soak-testing the boards for 24 hours by fitting 
them without the LCDs in place - you need to avoid having to remove and 
refit the new LCDs if you can as their pins are a bit fragile.

The new LCDs have a tough clear plastic tape over the rear of the 
display, remove this by gently pulling slowly upwards at 90 degrees to 
the back of the display, reduce the strain on the LCD by placing a 
finger just in front of the line where the tape pulls away the display - 
you have to be rather careful! Once that layer is removed, do the same 
with the backing for the silvered finish and very, very gently rub off 
the remaining silver layer using cotton buds and a dish of methylated 
spirits. I put a pad of kitchen paper in the bottom of the dish to avoid 
damaging the front face of the LCD and this last process took me about 
an hour for each display. Don't be tempted to press too hard though, as 
you can mark the back of the LCD that way. At the risk of stating the 
screamingly obvious, no smoking or naked lights and make sure that you 
have plenty of ventilation when working with methylated spirits.

Assuming that you have finished this process and still have your sanity, 
its time to fix the diffuser to the new display. The method I used was 
to carefully centre the diffuser against the back of the LCD and secure 
it using clothes pegs - use a piece of folded paper or card to protect 
the front face from the pegs. I then ran a thin bead of hot-melt glue 
along each end of the assembly where the diffuser meets the LCD; you 
don't need very much and hot-glue is fairly inert so if you muck the 
alignment, the glue can be scored with a very sharp knife when cool to 
break the seal and have another go.

The LCD sockets on the main chassis are actually two or three strips of 
DIL socket material sat in a row, so it is wise to check that the new 
display legs will line up correctly, a certain amount of judicious 
socket bending may be required before the LCD will fit easily. A quick 
squirt of switch lubricant/cleaner should be applied to the sockets and 
the LCD pins (removing any excess of course) to help things along before 
installing the displays. Check that the LCD is the correct one and the 
right way up before inserting the new display. All that is left to do 
now is to switch on the radio, check the display, remove the protective 
film from the front of the LCDs and tell yourself that you never want to 
do that again! If you are like me, you will probably want to clean and 
wax the front panel before refitting it, but don't forget to clean the 
rear of  the yellow filters before re-assembling the radio.

As for choosing which display upgrade to go for, I did see the retro-fit 
kit which Hienz was kind enough to mention, but at 200 Euros (vs a 50 
euros for the LCDs I used) I thought it was overpriced when you consider 
that a 1792 sells for between £200 and £450. The kit is only a pair of 
industry standard 2 line by 24 character displays and a CPU, so that 
strikes me as being expensive - but perhaps I am just mean! Also, I 
didn't like the idea of having another microprocessor and its crystal 
oscillator installed inside the radio as I would be concerned about the 
spurii from such an arrangement. On the positive side, it looks as 
though the kit already has backlighting and should be quick and easy to 
fit, so I suppose it is a question of which you prefer.

Apologies for the 'War & Peace' posting, but hopefully this info will be 
of use to fellow 1792 users.


73 de David, G4IRQ






On 11/08/2010 09:23, fantelme2 at comcast.net wrote:
> Hi David,
>
> . . . I presume that the backlighting is done with  electroluminescent 
> panels, not having dismantled the front panel to actually take a look. 
> At the risk of being a nuisance, do you know if there are replacement 
> backlighting panels available?
>
> Thanks once again for your time
>
> 73
>
> -France K6QO/ZS6GO-
>