Luxor and Skantic tv''s from early 60''s, vert. sync problems

Many Luxor and Skantic tv's from about 1961 to 1965 had issues with unstable vertical sync, it went so far that those sets were nicknamed "Roll-O-matic" or something similar in the trade. Several service information papers were issued on that problem, i will try to make an useful abstract of them here.

The problem is restricted to only sets with a PCL85 valve in the vertical deflection and where the vertical oscillator is a blocking oscillator (a few low-cost or portable models used a R-C oscillator instead and those seem to have worked better), and it seems to be caused by the valve running too hot and causing grid current in the triode. Earlier sets (about 1958-59-60) with 1/2 ECC81 as vertical oscillator and PL84 for vertical output do not seem to have this problem although the circuitry is practically the same.

The following changes should remedy the problem:

1) The dropper resistor for the heater chain, R14 , 20 + 20 ohms , usually has a jumper across one half. The service manuals initially said that this jumper should be in place for 220 V mains and removed for 240 V mains. Later this was changed to say that the jumper should be removed in case of the least possibility of overvoltage on a 220 V mains, this means that it should always be removed for today's 230 V mains.

2) If the cathode resistor for the pentode part of PCL85, usually R4116 is 330 ohms, it should be replaced by 390 ohms 2 W.

3) Connect a 22 kohm resistor in series with the control grid of the triode in PCL85. Usually, the green lead between the PCB and the blocking transformer T-B 6. 

4) If a PCL85 valve once has shown grid current problems, it should be regarded as damaged. It is best to replace the valve even if the set seems to run well for the moment with the other changes applied. The successor PCL805 seems to be much more immune to that grid current problem.

Something that was never mentioned in the service information papers is that sync problems due to sync pulse clipping (overloading of the video amplifier or last IF stage) were common if the AGC (Max. Contrast) pot was wrongly adjusted or if some (notoriously unreliable) carbon composition resistors in the AGC or sync separator circuitry had drifted.

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