24-05-2016, 12:03 PM
So there was this 14 year old kid, into electronics in as best a way as possible. Just out of hospital after having his toes straightened with steel pins and feeling very miserable with himself. Then one afternoon an uncle popped by, the one who was colourblind but repaired televisions and radios for a living (don't even ask how he read the bands on resistors let alone cable colours) and told his father his works were chucking out a late 60's S43 scope and would the kid like it?
Talk about biting the man's hands off.
Loving care on that scope, the dirt was removed and the horrible flaking off green paint was cleaned back and the whole lot resprayed to something a bit more tidy, ok not original in colour but who's a purist when your aged 14?
Forward fast 34 years and I decided to brush off the old S43 and give her a check over. She was working when I put her into storage ten years previously, just the calibration port was playing up but ten years on the shelf is a long time. First job was to get the capacitor tester out and check all the cans. Surprisingsly all were in tolerance, some looked as if they had fallen out of the manf stores just yesterday. I decided not to touch them and take a chance. Checking around, all other caps were in good condition, even those nasty TCC elkomold ones which are usually the first to go dry.
However, and a big however at that, the carbon resistors were subject to drift. The service manual states a +-10% tolerance which is pretty abysmal even for a new part. From experience the high the ohm the more the drift and this scope was no exception with for example the 680k resistor in the EHT tripler stage (selenium diodes no less) reading a near astounding 800k in value. In all, about three high ohm resistors on the HT and EHT side needed to be replaced which was a bonus.
As for the calibration port, which never worked. The circuit diag showed a feed from the 150v HT supply going through a 0.1uf cap and then a 220k resistor. The 0.1uf cap being near to the transformer was poor and the 220k resistor was somewhere up in the high megaohms. Both were replaced, the rest of the parts in the circuit to condition the sine wave into a square wave were a little out in value but within tolerance. A gentle power up rewarded with a working screen and the calibration port now working. Next job is to calibrate the port by adjustment to 1 volt peak to peak.
She still has a little vice, the x trace tends to drift as she warms up until a loose working temp is achieved, but for a 50 year old scope I can't really complain, as I often now have days when I drift the same way.
Talk about biting the man's hands off.
Loving care on that scope, the dirt was removed and the horrible flaking off green paint was cleaned back and the whole lot resprayed to something a bit more tidy, ok not original in colour but who's a purist when your aged 14?
Forward fast 34 years and I decided to brush off the old S43 and give her a check over. She was working when I put her into storage ten years previously, just the calibration port was playing up but ten years on the shelf is a long time. First job was to get the capacitor tester out and check all the cans. Surprisingsly all were in tolerance, some looked as if they had fallen out of the manf stores just yesterday. I decided not to touch them and take a chance. Checking around, all other caps were in good condition, even those nasty TCC elkomold ones which are usually the first to go dry.
However, and a big however at that, the carbon resistors were subject to drift. The service manual states a +-10% tolerance which is pretty abysmal even for a new part. From experience the high the ohm the more the drift and this scope was no exception with for example the 680k resistor in the EHT tripler stage (selenium diodes no less) reading a near astounding 800k in value. In all, about three high ohm resistors on the HT and EHT side needed to be replaced which was a bonus.
As for the calibration port, which never worked. The circuit diag showed a feed from the 150v HT supply going through a 0.1uf cap and then a 220k resistor. The 0.1uf cap being near to the transformer was poor and the 220k resistor was somewhere up in the high megaohms. Both were replaced, the rest of the parts in the circuit to condition the sine wave into a square wave were a little out in value but within tolerance. A gentle power up rewarded with a working screen and the calibration port now working. Next job is to calibrate the port by adjustment to 1 volt peak to peak.
She still has a little vice, the x trace tends to drift as she warms up until a loose working temp is achieved, but for a 50 year old scope I can't really complain, as I often now have days when I drift the same way.