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Vrc 353


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Hello all,


I've been given a VRC 353, which was so say working 2-3 years ago.


I've connected it up in the land rover but can't get anything out of it.


I have set it to OFF and the test switch to 28V but the meter shows nothing.


Do the 353 have an internal fuse as there isn't one on the front panel like my 321.


If not I will try and find a test set. Will this possibly find the problem? Or are they not as advanced as that?


Cheers in advance


Alex C

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Hi Alex, most probably of no help, but if you set the test switch to overide this will overide the tx if there is a fault on the heat sensor, which would fire it up regardless, but this may be just the transmit side and be independent to the receiver. Are you going to Malvern by the way, will email you as well regarding other items, all the best Ian. p.s will ask a friend about the 353, might be able to throw some light.;)

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I have never found a circuit diagram - the best technical description I have found free is:




Looking at that document para127 on page 44 of the pdf is the PSU description. It would appear that the meter should be connected when the radio is in 24V test position even if the set is otherwise off. Protection is electronic by a diode and relay for supply reversal (so supply is only connected to the electronics if it is right way round) and overvoltage protection is electronic. No fuse is mentioned.


I have copied the relevant paragraphs below. Summary is that the symptoms are consistent with a relay fault, blown diode, or thermal cutout fault. I will for my own sanity check that the meter reads with the set off when I am near my 353 later.





73 de G0OZS


127. The power supply unit is fundamentally a DC to DC converter controlled

by a square wave switching regulator. Control loops operate on the regulator

and converter to ensure adequate overload and over-voltage protection.


128. The voltage supply to the equipment, +24V nominaly is applied at (1a)PLI

from where it is passed via supply filter (le) to Meter Control Board (lb).

The negative is also routed directIy to (7g)X2 and the positive line to (1a)RLAl.


129. The Meter Control Board (lb) has a resistive network incorporating variable

component (1b)R I which enables the front panel test meter (1a)ME 1 to be centred

when the input voltage is at +24V d.c. (TEST switch 54 to 28V supply).


130. The positive line connnection into (lb) is routed via (1b)Dl to Power

Relay RLA. In the event of a reverse polarity supply connection into (lb), D l

prevents (1a)RLA from energising. The negative supply to ( 1a)RLA is routed

via (1a)THT (closed for temperatures LT 85 C) and wafer AF of POWER Switch

53. (1a)RLA will energise for correct polarity supplies in the range 21-33V d.c.

at all temperatures below 85 C when S3 is in any position except OFF. With

(1a)RLA energised, contact RLAl connects the positive polarity input line to

(7g)Xl. Both input lines are isolated from the chassis.

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Hi Again


I checked my 353 and the meter should read when the test switch is moved to the check 28V position and a supply is present, even if the power switch is at OFF.


The original field test kit was a metal box with a plate on top to which the insides of the 353 were attached and multiple plugs from the tester to the insides connected. I have one but it has broken connectors so is on the "to repair when I get round to it and find a donor 353 for the plugs" list and not immediately useful.


The base test set was I believe an early computerised automatic tester driven by paper tape, which was the donor for the Thorn 48V 50A bench power supplies that M&B Radio used to sell on e-Bay.


So I think it is a case of open up, keep one hand in pocket, and trace the supply through the set with a meter to find the fault. I would suggest checking supply polarity first as the diode would then prevent a reading, if I read the document I sent the link for earlier correctly.





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