A 2 Part Article By Leighton Davies GW3FSP
PART 1 – Replacing the dial lamps in a VRC 353 VHF Transceiver with LEDs
Revised 28th Feb. 2014
After thinking about the chances of finding genuine replacement parts for the dial lamps I decided to modify the VRC 353 with the modern equivalent. This is a short description of what I did to replace the dial lamps with LED’s.
In this part the repair was carried out on a spare dial unit. In the second part the dial unit removal and replacement is discussed and I take the opportunity to turn off the 150Hz Tone.
The LED’s used to replace the dial lamps are standard White 3MM 4v @ 20Ma maximum rating. I got them from CPC P/No – SC11538.
After experimenting with 4 led’s in series I decided on a 2.2k series resistor giving me a current of 6 Ma , this gave more than enough brightness to illuminate the dial unit, ( having now fitted the modified unit I would however suggest a slightly higher resistance, say 2.8k. or even higher )
The lamp holder is a plastic fitting secured to the top of the dial unit with 2 screws which also secure the cable ties for the switch loom, there are 2 wires connected to the lamp holder, one is white with a brown trace connected to the left hand terminal ( positive ), the other is yellow with a brown trace ( negative) , the yellow/brown wire is connected to the right hand terminal of the lamp holder.
BEFORE you disconnect the wires check the polarity of the supply ! and note which wire going to the lamp holder is positive, then switch off and disconnect the supply !.
Disconnect both wires from the lamp holder, unscrew both screws and lift out the lamp holder, you will find the positive is to the left of the holder, you will need to extend this wire, I used a length of thin red flexible wire., sleeve the joint.
The flat edge of the lamp holder is towards the front of the chassis.
Unsolder and remove all 4 lamps, be carefull the plastic will melt very quickly.
I removed the 2 wires .
The leds I tested with my diode check meter to make sure I connected them correctly, with the led conducting the positive ends go to the resistor end of the lamp holder, I then fitted all 4 leds with the round end of the leds showing about 1/8th inch below the plastic.
I bent the led leads about 1/4 inch from the base of the led at 90 deg, more or less making a ‘ T ‘ shape , I cut the leads about 3/8 th inch from the bend, made a small loop in the ends of the leads and fitted them over the terminal posts, I started from the left hand end of the lamp holder.
Then soldered the left hand connection, fitted the next led, soldered the joining connection, and continued ’till I had all 4 leds soldered but leaving the right hand connection of the last led un-connected with the positive lead sticking straight up, I then cut the lead about 1/4 inch from the base of the led leaving that lead sticking up, I then looped the wires of the 2.2K resistor as close to the resistor as possible, trimmed the leads and fitted the resistor over the led lead and the connector post and soldered the resistor in place.
I then connected a 24v supply, positive to the outer end of the resistor, negative to the end led, it worked !. the current was just over 6.5Ma with more than enough brightness to illuminate the scale when fitted to my spare dial unit.
I now replaced the lamp holder, connect the negative to the left hand led post, the extended positive wire to the outer end of the resistor.
I have taken 4 photo’s of the dial unit and lamp holder, hopefully they will be good quality, I used my ‘new’ Samsung galaxy 7 gizmo.
Turning off the 150Hz Tone
As an extra little job, while I had the set out of the case I turned the 150Hz tone off, this is done by the 10 turn pot marked ‘T’ on the PCB retainer fitted on the left hand side of the set just below the TX turret assembly, there are 3 pots, the ‘T’ pot controls the 150Hz tone, The ‘D’ pot controls the data level and the ‘A’ pot controls the mic audio to the modulator card.
If the retainer plate is missing, the 3 pots are below the red warning sign, the ‘T’ pot is towards the front of the set, the middle pot is the data ‘D’ and the rearmost pot is the ‘A’ pot for audio.
Now back to replacing the lamp holder in the 353 VHF set….
On the terminal strip to the left of the synth module, the lamp holder wires go to tags 18 and 25. The white with brown trace on tag 18, the yellow with brown trace on tag 25.
The positive supply is on the left of the lamp holder this needs to be extended to the resistor on the right of the lamp holder. On my set the positive supply goes to tag 18, I measured full supply voltage.
One for the Pot
When I dismantle anything I use plastic tubs ( usually the tops from spray cans ) to keep the bits in, I suggest you do the same.
To start, the tag strip pcb is held with 3 screws, and one screw to the top pillar from the module to the right of the mode switch, remove the 3 screws and washers, also the 1 screw from the stud, put in a tub.
Remove the two studs from the left of the module and the one screw top right of the module, tub them.
Remove the mode switch knob ( set it at narrow first ) 3 screws, careful, the coupling will now fall out! Put these bits in a tub, you now have to remove the round ‘nut’ holding the switch in,
( I used a blunt 3/4 inch wood chisel as a screwdriver) put the ‘nut’ and washer in the knob tub.
To remove the lamp holder the 2 screws holding it to the top of the dial unit must now be removed. Unfortunately the left hand screw is under the chassis fitting for the tag strip removed earlier. The 4 screws holding the dial unit must be removed to allow access to this screw, there is a sealing gasket fitted in a machined groove in the dial unit, be careful not to damage it. Put the 4 screws and washers in a tub.
VERY gently, pull the module up and to the right, it will sit on the top of the chassis, move the tag strip left and then push the mode switch through the hole and to the left.
You will now have access to the lamp holder, there are several ties holding the loom together, these must be untied/cut and removed.
The dial unit will move just enough to allow removal of the 2 screws, do not lose them in the set! You will need to remove the cable clip on the left of the lamp holder, there may be other ties you may have to remove, GENTLY tease the wires up and over the top of the dial unit switches so you can see the whole of the lamp holder.
Slide the lamp holder to the left, you can now unsolder and remove the 2 wires to the lamp supply, remember to extend the positive wire!. the negative can be connected to the left hand led post.
( This is when I went for a coffee break ! )
After replacing the lamp holder and re-soldering the wires – the 2 screws ( with cable straps ) can be refitted.
Tease the wiring to the switches back into position gently! If you want to fit cable ties now is the time, you will need the smallest you can find.
Refit the 4 screws holding the dial unit to the front panel, make sure no wires are trapped.
Refit the module removed earlier, one screw and the two studs, short stud at the top.
Refit the mode switch, washer then the nut with the knob retaining ring, the large hole in the ring to the left. There is a lug on the knob which fits in there, the coupling goes in next, ( try a spot of grease to hold it in place). The slot in the mode switch needs to be horizontal ( narrow mode ) then fit the knob and tighten the 3 screws.
Next refit the tag strip, 3 screws for the pcb and one for the stud fixing.
(This is when I went for a break, 2 large coffee’s and a lump of fruit cake ! Yummy )
Powered up the set and – it worked!
This whole exercise took me 4 hours, including taking the photos with the Galaxy 7 and making various notes to get the photo’s in the right order, the modification to the lamp holder took about an hour including testing.