Restoration Idea 4
Philco S8200 (formerly 7020) Oscilloscope
Restoration Date: October 2013
The style was unique and was based on the "Pocket Scope" by Waterman Products who made it for Philco. The Philco S8200 (originally model 7020) was sold for television servicing business in the early to mid 1950's. Picked this up on Ebay and was non-operating. The oscilloscope was previously worked on. Many of the large value capacitors were replaced and was sloppily installed. The person probably gave up because the oscilloscope did not work. As I worked on this, some interesting things "popped" up and are worth discussing about here.
I took a chance and turned it on before I worked on it. Result nothing on the CRT. This could be many things. The vertical and horizontal amplifier circuits were not working (which is relatively rare for basic 1950s oscilloscopes like this. The CRT has a non emitting cathode which I have seen happen in vintage TV's. The high and low voltage power supplies have bad electrolytic filter capacitors and associated circuits dealing with focus, beam and deflection were not functioning. I decided to go for the power supply first.
Finding the schematic was difficult. I knew from others that Waterman Products designed the oscilloscope, but this was built to Philco specifications. This model features a low voltage supply using a 6X4 full wave rectifier tube and a high voltage power supply using a 1V2 TV HV rectifier tube. I found a Waterman S14a oscilloscope on the internet that used this supply design and was wired similar to my S8200.
A. Power Supply Repair/Restore
The low voltage supply put out only 275 VDC (should be 350 VDC)
1. Patiently unsoldered the connections from the 3- section twist lock multiple electrolytic capacitor can and then tested it. All sections to this capacitor were bad when I tested on a simple capacitor checker.
2. Carefully removed the entire twist lock can (must be careful, patient and "be in a good mood"). Finally got it out, then cut the can open with a hack saw at the base. Pulled out the metal can cover and cut off and discarded the stuff inside but saving the base for a new set of capacitors.
3. Installed the new electrolytics on the old base.
4. Installed the base back into the oscilloscope chassis and connected all the wires back to the correct places.
5. In the photo on the right. The new capacitors can be seen in the lower middle part of the photo.
6. I also replaced some resistors that were out of tolerance or incorrect values (this was previously worked on before I got it.
7. Turned it on, nothing showing on the cathode ray tube. However the power supply was producing the correct voltages as stated on the schematic. 350 VDC out of the 6X4 cathode output and -590 VDC from the 1V2 filament.
8. At this point I needed to get a correct schematic of this model. Easier said then done. Got lucky, VintageManuals Inc- Batesville, IN had one and I ordered a reprint.
B. CRT Bad?
If the CRT was bad, nothing can be seen. This is rare because unlike TV, oscilloscopes are used much less so less wear on it, unless the bad power supply cause components to fail, damaging the filament and cathode. I manage to order one for a reasonable price, if the CRT was good, I now have a spare which would be even better!1. First I decided to check the CRT filament and search for any CRT pins that need to be reheated as over time, the solder may shrink and separate from the wire leads from the CRT. Removing the CRT from the socket and shield was very difficult. I had to find a way to remove the socket securing screws from the CRT shield housing. Needed to loosen the shield mount to rotate the shield then get a short cross head screw driver and slowly remove it without stripping the screw head (60 year old hardware).
2. Next I had to remove the CRT from the socket. After 60 years it was really stuck. It took a lot of slow and careful twisting and prying to finally get it out. Took a long time.
3. Tested the filament of the CRT with an ohmmeter and applying voltage. The filaments was working great. All that work just to find the filament was good. However, the sometimes the CRT would have a rotted cathode. This can happen if there was excess cathode current caused by a bad capacitor or resistor. The new CRT I bought may be the only solution. While I was waiting for it to come, I decided to check over the components to see if they were good and add new ones as needed.
4. Who ever worked on it was either in a hurry or suspected that the original paper capacitors tend to fail or soon fail. The person replaced as many as could be done easily. The work was sloppily done. There was one large 0.05 uf 1600 V original paper capacitor left for the deflection circuit. I decided to replace it and check the associated resistors in that area.
5. After patiently removing the old capacitor, I checked it and it was still good. Since the large capacitor was covering up the resistors, this would be a good time to checked them. The first one I checked with the CRT cathode bias resistor. It was marked as 47,000 ohms +/- 20% tolerance. I measured it at it was open.
6. If the cathode bias resistor was open, the CRT would not work since there would be no electron emission. I replaced it with a much better and reliable modern one.7. All the other resistors checked good and in tolerance. I put the new capacitor in of equal value and operating voltage (of course a modern one that is reliable).
8. Decided to turn it on and it was working. My first visual was just a zero signal green faint horizontal trace.
9. After a few internal and external adjustments, it was acting like a basic oscilloscope again. I put a 1 KHz sine wave in, and I got a nice solidly stable trace.
10. Conclusion: A low quality 15 cent resistor prevented the oscilloscope from functioning. I wonder how long it has been that way?
Somewhere between 1953 and 2013.
C. Finishing it up
With the correct schematic found out that the wrong value capacitor was used for one of the range steps for the sweep control. The person installed a 0.0025 uf when it should be 0.02 uf. This would change the sweep width across the CRT. The correct value solved the problem. Replaced and reinstalled all the capacitors that were replaced or checked bad before so it will look more professional. All other resistors checked and replace if out of tolerance. Reinstalled the can covering on the power supply electrolytic. Put everything back in and did some cleanup and adjustments.
Removal of Deflection Paper Capacitor and defective (open circuit) 47000 ohm CRT cathode bias resistor.Middle Right: Installation of new high reliability PTFE capacitor for deflection and precision metal film heftier resistor for CRT cathode bias.
Bottom Right: The original CRT showing a sine wave. CRT was never bad.
Finished Vertical and Power Supply side
Finished horizontal side
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