The Repair and restoration of an early Racal RA17 . . . Page 14
Finally . . . Another photograph of my MK1, this time with the knobs in the right place.
Feb. 25 2009: The other day I became aware that something was not right with the RA17. Sensitivity and audio output were down and the AVC (AGC) was not functioning correctly. This condition would spontaneously clear after anything from 30 minutes to several hours. This afternoon I tackled the problem. Maybe I should have set about it clinically, however, after substituting several valves to no avail, I finally measured the HT voltage and found it to be only 80V. Interestingly, setting the main function switch to Stand-By resulted in the HT rising to 250V. Since the RA17 is essentially modular, I thought it would be simply a case of ‘lifting’ appropriate HT wires until the obvious excess loading was removed. However this exercise was inconclusive. Very likely what I should have done was ‘scope’ the HT line . . . But I didn’t. Anyway, with all avenues exhausted I finally replaced the GZ34 rectifier with my one and only spare . . . BINGO! Actually, looking down on the original GZ34, it was evident that one of the heaters was dimmer than the other. Schematically the GZ34 has a single 5V heater. However, due to its construction, mechanically it has to have two heaters which are obviously wired in parallel. I did find a data sheet which actually shows it as two parallel heaters. While I can see how one filament might fail outright, I’m not really sure why one might diminish in output. Whatever the reason, this was clearly the cause of the low HT voltage. Looking back at the various faults and seriously cooked bits and pieces that I encountered during the restoration work it is likely that the excess current incurred had over-stressed the valve to some extent.
The GZ34 is reckoned by many to be the finest general purpose valve rectifier ever manufactured. A search on Ebay will only highlight this. Those with the small brown base will set you back well over 100GBP! I don’t expect the replacement to fail in a hurry, so I shall shop around for a replacement so that I am not without a spare.
Thoughts on SSB reception:
A search of the internet will yield more than a few articles declaring that the RA17’s detector circuitry is fine for AM reception but lousy for SSB. Consequentially there are a couple of designs offering modifications to the detector circuitry. Some would have you believe that the RA17 was never intended for SSB reception, hence the need for the RA63. This is not the case (see here). Interestingly I have absolutely NO problems resolving SSB signals, strong or weak. My theory is that if you are using an RA17 full of leaky capacitors and out of spec. resistors, don’t expect it to work as well as it did when it left the factory in 1958. With this in mind it is worth noting that since the RA17L and subsequent models sported a different front end and an extended AGC line, these models are more likely to exhibit SSB reception issues since the AGC circuitry is particularly at risk regards ‘leaky’ capacitors
It proved somewhat tricky getting a good photograph of the low output heater filament without compromise, hence the dim shot on the left. Any more exposure would have rendered the two filaments over-exposed and thus indistinguishable. As can clearly be seen, the filament on the left is unmistakably dimmer.
