[Catkins]

Last year I found it increasingly difficult to complete this thread, due to lack of time. The company at which I worked got taken over by another company, and under new management, my workload doubled and steadily increased throughout the year to an impossible level. Things came to a head just before Christmas, and I resigned. The upshot is I now have some time to complete this thread, apologies for the length of time it has taken.

Reviewing the thread I left off at about May 2018, I'm going to fast-forward over the 4 or so months to September 2018, at which point I had put the set back together. Obviously once the set was back together, it was time to turn it on and test it. But, I was aware I had completely rewired it, and I first wanted to do some sanity tests before turning it on. So I checked the HT resistance to earth in various parts of the circuit, which should show up major mistakes. One testing issue here which I hit is there is often a legitimate low HT resistance to earth in one part of the circuit, which can mask out unexpected low resistances elsewhere. But, on this set I discovered I could isolate various parts of the circuit by unplugging the loudspeaker (with HT going through the field coil, as it is one of Murphy's huge mains energised loudspeakers), and the PSU (power supply unit).

Overall HT resistance to earth was OK, but unplugging in various combinations, showed there was an unexpectedly low resistance to earth in the timebase circuit. Checking of that circuit showed I had incorrectly connected two wires to the occasional controls (line/frame hold, height/width etc). Some of the occasional controls are in the cathode circuits, and some are in the anode circuits, and the miswire effectively connected HT to the cathode circuit of one of the line/frame output valves, which could have led to some unpleasant results if not caught early.

Once that was fixed, the HT resistance was as expected, and I could not discover any more faults. Time to turn it on (without a CRT and EHT transformer disconnected, no point in having lethal voltages present when I'm concentrating on the rest of the circuit first and may want to do some live tests). Again taking no chances, I connected up four multi-meters to monitor the HT at various points, and slowly brought up the voltage on a variac. After a couple of hours, I was at max voltage without hitting any problems, and I could hear the line whistle from the LOPT.

Time to inject a signal, with an oscilloscope connected to the video detector diode (because as yet there was no CRT). I got sound on the loudspeaker, but, disappointingly no signal on the video detector diode. Working backwards from the video IF stages (3 stages) with the oscilloscope, I discovered the fault lay with the first IF stage. Visually checking the circuit showed a short circuit on the valve base due to wires touching. Once that was fixed, I got a video signal on the video diode (Photo 1).

The next weekend, I connected up EHT and put in a low-emission CRM 92 CRT. This was obviously in case there were faults with the deflection which might damage the phosphor, no point in damaging a good tube. The result was very disappointing. Non-existent focus, which obviously made it difficult to see if there was any raster or picture, but, it did show line and frame deflection was working (at least to some extent).

Doing some voltage measurements showed there was voltage on the focus coil, but no voltage drop across it, when there should have been about 10 volts. The focus coil is bypassed by a 100 uf capacitor (in the refurbishment of 1946 this was changed to two 50 uf capacitors, which I restuffed). One of the 50 uf capacitors had exploded and was now a short-circuit (Photo 2).

At that point I could not discover why the capacitor had failed, I checked the focus circuit and time-base HT circuit and found no problems. Later I realised I'd done a stupid thing when testing the set for sound when incomplete about 6 months before (see postscript).

Discovering no underlying fault, I restuffed both capacitors (the other, though not exploded, had gone high). At this point I got a picture (Photo 3). This was at max focus. Obviously the focus coil was positioned incorrectly. But importantly, the question was did my modifications (to the mounting to accommodate the longer CRM92) allow the focus coil to be positioned to allow focus (and centring) to be obtained? It would be embarrassing if it didn't, but, happily the answer was Yes.

With a better tube, and after a couple of hours adjusting the focus coil positioning, gain, fine tuning, and the various other controls, I produced a much better picture (Photo 4). Which I thought was very good progress.

Postscript: The cause of the exploding capacitor became obvious later. I had tested sound on the set about six months before when I thought the time-base circuit was completely disconnected and inactive and so it would be harmless. But, it wasn't, there was still a circuit from the time-base HT to earth, and with the focus coil disconnected, the voltage across the capacitors would have been much higher than intended.