[ronbryan]

Alan brought the S51A round to me a couple of weeks ago and essentially I found that PJL's suggestion in post #10 was correct. The pcb had become carbonised between two traces, one of which carried 330V HT and the other being the grid pin 7 of V7B, the X deflection amplifier. The tracking between these two traces seems to have been caused by a deteriorating Hellerman type rubber sleeve on a wiring bundle, as this was directly in contact with the traces. There was also some sticky chemical compound on the pcb and on some solder side mounted electrolytics, which may have contributed to the failure. +100V on the ECC88 grid pin 7 was enough to see off the cathode resistors and my spare ECC88, which went grid-cathode short while I was fault chasing.

The two tracks were cut either side of the tracking area and linked out with ptfe covered solid core wire and the gunge cleaned off the pcb with RS pcb cleaner and cotton buds. It's not that easy to get to the circuit board on the S51A as it is rivetted to the box-shaped chassis and the tube position restricts access. I changed all three cathode resistors and fitted a replacement V7 ECC88, and then the X deflection amplifier behaved normally.

There was no sweep from the time-base at this stage and having confirmed that the trigger circuitry was working correctly, I found that V5A, another ECC88 in the time-base keying circuit had a grid-anode short. Substituting that valve and cleaning the time-base double pole on/off switch got the time-base going.

The 'stability' control did not have enough adjustment range to cut off the sweep and a check on the -1000V cathode supply which feeds the control showed only -700V. The K8/50 selenium rectifier MR4 was changed for a BY8410 HV diode plus a 12k series resistor. That restored correct operation, allowing the control to work in the normal manner.

The 'Y' attenuator switch needed cleaning, also the AC/DC input selector. The 'Y' shift control was not centred properly (i.e provided more 'up' than 'down') and an improvement was made by changing the 6k8 resistor in the shift pot divider chain as the original had gone up to 8k5.

The valve bases and valve pins were cleaned with DeoxIT and then 'X' and 'Y' amplifier gains were set up using the appropriate preset pots and also the trigger sensitivity preset was adjusted. The scope produced a decently focused trace and triggered well. I had some difficulty seeing the ruled vertical lines on the (non-illuminated) graticule but apart from that, it performed nicely.

Ron