Cwmbran and District Amateur Radio Society
The “Lady Margaret”
Thanks to GW0OAJ for this article
Richard, GW0DHA, presented with a Vidor 429 Set, the "Lady Margaret"
model. The task was to get it working:
The set uses 4 @ D96 series valves. The B7G based D96 series were the last word in valve development before all thermionic work ceased with the introduction of semiconductors. The heaters, for example, are 1.5v at 25mA compared with the more normal 6.3v at 300mA of other B7G series. Likewise, HT is 90 v at 0.7 mA and can be run as low as 24v, compared to the more usual 180v - 250v and 50 mA:
Armed with the, "usual stuff," including dusting brushes, spray cleaners and hypo needles full of suitable fluids (for radio servicing, steady on), the first task was to open the set for inspection:
Inspection showed no signs of corrosion, vandalism, fire or what have you. Bench supply power was then applied. At 1.5v heater the Ih was 125mA; correct because the output valve has a double heater wired in parallel. The HT of 90v drew a little more current than expected at 7mA; within expected tolerances! Subsequently, replacement batteries were made up of a "D" cell in a clip holder as LT for heaters and a taped together block of 10 PP3's to give the HT.
Quickly, loud crackling became apparent with adjustment of the controls and plain physical movement of the set. Investigation revealed several; faults. The tuning capacitor plates had moved into very light touching contact at parts of the range. This is probably because they are in a vulnerable position when the set is opened for battery changing. The plate position was adjusted:
After adjustment, intermittent crackling on set movement remained. The automatic "ON/OFF" switch that actuates with the lid was suspected. It was stripped and examined then spray cleaned and re-assembled. Crackling remained and this was traced to a faulty V4 valve base. Over time one of the pin grippers in the base had failed and was only giving light touching contact. The larger capacitors were also tested for their ESR, given the age of the set: they all passed but a couple were changed on principle!
The sets were built using mainly point-to-point wiring. This was a low tech system of building that was done for low cost and great saving of design layout. On a simple set it was manageable but on complex sets was a horror; with ageing it becomes a nightmare to service. Compared to proper tag-boarded construction, it is very prone to fatigue failure of unsupported components. As a by-the-bye, on aircraft and other equipments with serious consequences for failure, it became illegal to use from circa the late 1940's. A substitute new old stock base was fitted in-situ for minimum disturbance; a bit of a pig because the bases were rivet assembled and not bolted:
Job done; the set was a worker. Re-alignment was conducted and the performance was good. Ominously, but in hindsight, very slight doubts about an element of crackling remained. The set was brought in on Club Night to return to Richard. On testing in front of Mark Gray's eyes the set failed! Swift checking found that V3 had a failed base too. Back to the old drawing board; bah!
This time the set had to be dis-assembled. Two new valve bases were re-fitted and the opportunity was taken to re-wire some of the interconnections. Start to finish this time, including re re-alignment, took just over 3 hours :) the set now works very well:
A thoroughly enjoyable task to resurrect a set that was the final valve portable set before all was swept before the semiconductor revolution. It is further recommended, for those with no valve experience to, "have a go," with the kind of voltages that are well behaved if you touch something live :D
As a further thought. A line-up of 4 @ D96 series might make a very nice little retro Top Band / 80m set; especially if modern Xtal ladder filtering is used.