Redifon R551N Receiver

I’d looked for a Redifon R551N receiver for some time, not actively, just a case of keeping an ear to the ground so to speak.  When one finally did turn up, it was complete with the ARU18 ‘Antenna Common Adapter’ and ARU11 ‘Synthesiser’.  The set didn’t work but was in astoundingly good condition.  When I took the bottom cover off I was delighted

to find ‘HMS Charybdis’ lightly engraved underneath…I must assume it came from that ship, and you can find more out the HMS Charybdis here.

Most of the Redifon R551N’s I’ve seen have been well hacked about and often corroded, with bits missing and one set had the top of the front panel sawn off, because the owner had neither of the optional modules…you have to wonder don’t you.  Many of them also have older type 1960’s capacitors which are leaking and causing problems.

Mine seems to be rather late one, many of the components are dated 1973 which would suggest a manufacturing date of maybe 1974.  Better still, the radio seems to have been back for refurbishment at some point because the reservoir capacitors are dated 1987.  The smaller electrolytics all seem to be the Philips translucent blue ones…I’ve never replaced one of those.  No corrosion either and the set generally looks very little used.

My Redifon R551N receiver powered up but sounded awful, the ARU11 synthesiser didn’t work at all and the ARU18 Antenna Common Adapter did nothing.  I managed to track down the manual for the main receiver quite easily, the ARU18 was a little harder…but there was no ARU11 manual anywhere.  I put a few feelers out to people who are interested in these sets and managed to get a copy.

I began the various checks in the instruction manual and found the 5v rail low at about 3.5v.  There was no fault and the preset to set the 5v had marks on it indicating it had been ‘adjusted’.  I reset the voltage to 5v and checked everything else…the receiver now worked great and was stable.

The ARU18 Antenna Common Adapter didn’t work because it is not what it seems…there were two cables to install on the back of the radio…see the photograph.  Finally the ARU11 which I actually fixed before receiving the manual…there was evidence someone else had tried to fix it too.  There was no 600KHz coming from the synthesiser, although the divided square wave was coming from the divider board and into the filter / pulse shaper, which has a narrow pass band making the square wave in to a sine wave.

It turned out that C32 on the 600KHz filter board, a 0.1uF capacitor, was open circuit.  Once replaced the synthesiser worked perfectly.  I’ve checked the radio out fully and frankly, I’m really impressed with it.  If you get the chance of a good one, snap it up before someone else does as there are not many about.

I’ve posted a copy of the three main manuals for my set on my ‘Downloads’ page.

Phil Moss wrote an excellent article in Radio Bygones, sadly now out of print, on his experiences with a Redifon R551N.  Well worth a read if you have, or are thinking about getting, one of these wonderful receivers.  With Phil’s kind permission, you can download a copy of the article here.

A further article of note is ‘Fault Finding Guide for the Redifon R551N’ by Jan Verduyn G5BBL.  There’s enough information in Jan’s article to get most receivers up and running quickly…and if you have a tricky fault, you will know where to look.  You can download a copy of this too…here.