Apologies for my lack of postings. Many things have happened in my life since my last post. My younger brother Iain passed away aged 51 on the 8th of March. I also retired from work at Reid Kerr College, Paisley, where I was an IT Support Engineer. Retirement doesn’t seem too bad, but of course I could get bored. Who knows?
This time, I have some information for those who own a RACAL RA1772 that’s ‘thrown in the towel’, like mine.
The RA1772 is an excellent radio, but the overly compact power supply lets it down a bit. The PSU is a linear type that’s crammed into a small space in the rear of he set. All linear PSU’s generate heat and this PSU is no exception. In a previous post, I described how the bridge rectifier D2 generates most of the heat and could be moved from the PSU area to the card cage where it can dissipate heat far more effectively.
This time, my RA1772 has failed again, for the third time since I bought it, quite a few years ago!
A quick ‘blip’ of the mains switch so I can flip through the various metered supply voltages shows only the -7V supply to be operational.
All of the positive voltages used in the set are regulated by 723 canned regulators and external pass transistors with the 723′s being fed from the +20V regulated line. If the +20V line fails, all the other positive supplies fall over like a stack of cards.
In this case, the electrolytic capacitor marked IC4 in the manual was found to be U/S. This is the reservoir capacitor for the +20V supply and is mounted in a capacitor clip on the rear apron of the radio.
Rather than just replace the faulty capacitor, I decided to replace all of the electrolytics used in the PSU, They have in all probability exceeded their ‘best before date‘ by a factor of 3 or 4!
In any radio of this age, it makes sense to replace the electrolytics in the PSU… and elsewhere too, if you have the patience. It’s especially the case for the RA1772 because you KNOW they have been ‘baked’ for many years by that mad hot PSU board.
Here’s another hint for faultfinding the RA1772, ALL of the various transformer secondaries are protected by simple ‘fusible links’ connected across pins on the PSU board. IF a fault develops, these links protect the transformer from being burned out by short circuits. ERGO if you need to disable a PSU line, you can simply unsolder ONE END of the wire link for that particular supply. The links are clearly marked on the PSU board… just pairs of PCB pins with ‘fuse wire’ soldered across them.
The links are marked LK1 through LK6. They protect the following trafo windings.
LK1 10V winding (for the -7V regulated line)
LK2 16.5V winding (for the +12V regulated line)
LK3 23V winding (for the +20V regulated line) … but see Note!
LK4 + LK5 (+ / – 60V line… used for the telegraph board if fitted… to drive a teletype)
LK6 10.5V winding … ( for the +5V (LOGIC) supply)
NOTE!! By simply unsoldering ONE END of a link, that supply line can be readily disabled for testing. When you are finished, just re-solder the link in place, taking care not to get any solder build-up on the fusible link. The fusible links are connected in series, between the mains transformer secondaries and the rectifiers on the PSU board.
Remember too, if you DISABLE the +20V oline, the other positive supply lines will all quit because there will be no +20Volt supply to the +5V (logic) and +12V voltage regulators on pins 7&8 of each. The -7V line is independent of these, so if the -7V line fails, then the rectifier, smoothing capacitor, regulator (ML1 on the PCB) or pass transistor may be faulty.
On the PSU board….
LK1 + D1 & ML1 plus ITR1 (on heatsink) are supplying the -7V line
LK6+ D2 & ML2 supply the 5V line, along with pass transistors TR1 & ITR2 (on heatsink)
LK2+ D3, ML3 TR2 and ITR3 (on heatsink) supply the +12V line.
LK3+ D4, ML4, and TR4 supply the +20V line.
LK5 & LK6 + D5 and IC6 supply a +80-0V -80V DC supply to the teleprinter circuitry (optional board )
ITR1, ITR2 and ITR3 are mounted on teh rear panel of the receiver (for heatsinking).
The ML1 through ML4 are metal canned regulators (Ua723). Made of ‘unobtanium’.
Until next time,
73, AL GM1SXX