Sunday, March 23, 2014

1942 Philips GM 3155B Kathograph II Oscilloscope

     About 35 years ago, an older gentleman I worked with gave me this Philips oscilloscope.  It was given to him many years before, and that man told him that it came off a German U-Boat.  It is possible, but I can't prove it.  Apparently a GI brought it home as a war souvenir.  This sat among my collection of radios until today, when I decided to see if it worked.  Why did I wait 35 years?  Because I was happy to have it on my shelf, and did not want to risk blowing the rare Valvo tubes.  But  now I need to sell it and other things in my collection, and I felt that if it were operable, I could get a higher price.
     I first had to tackle a problem that I have known about for 35 years, and was one of the reasons I simply put it on the shelf. Someone in the past tried to repair it and made a mess of things.

The Y gain control potentiometer failed, and someone decided to "fix" it with a volume control from an old radio. Trouble is, the volume control had a SPST switch at the full CCW position, while the original had a SPDT switch to change the range of the gain control.  To solve that problem, the repairman put a toggle switch inside the cabinet.  I knew it couldn't be wired right because he had the the wire from the braided shield connected to the switch, when it must go to ground.
At that point, I did not have schematics, so I took my best guess. I also rooted through my collection of potentiometers and found one with a SPDT switch. It was a 2 gang pot, but I will just use one.  I later found out it is the exact value I need, 500k ohms.

I connected the wires, and was ready to try it out.  All I needed was a power cord.  I did not want to solder wires to the mains socket as was done before, so I went back to my supply of electrical hardware. It turns out that an American appliance connector is a close fit for this vintage European connector.

This oscilloscope is very versatile, and can be used nearly anywhere in the world, thanks to it's multi-tap transformer and voltage selector switch.Both sides of the mains are fused. 

I verified I had the right voltage, and plugged it in.
Nothing. No lights, no hum, not even any smoke.
I discovered that the power switch is bad.  Both sides of the mains are switched, and both switches were stuck open.  I soldered jumpers across the switch, for I want to leave the original switch intact.  This way everything is original except for the Y gain control.
I tried it again.  I should mention that I don't just plug it in, rather I raise the voltage slowly using a variable transformer.  This way I can catch a little problem before it becomes a big one.
 Amazingly, all the tubes lighted up, and after perhaps 50 years, the oscilloscope came to life again! Soon I had a trace on the CRT.
I did not know it, but I was soon in for a shock!  A 610 volt shock, to be exact.  See the 4 filter capacitors in a row? Unlike what is commonly found, where the metal cans are grounded to the chassis, these are on insulated bases, and the can voltage is very high!
From left to right, the first one is at chassis ground, no surprise there. However, the second one is at +205 volts. The third one is at -405 volts!  Between capacitors 2 and 3 is a potential of 610 volts!  Cap #4 is "only" -262 volts.  So, if you ever encounter one of these oscilloscopes, do not touch the capacitors!

The two clear glass tubes are 1876 rectifiers, and the gold tube is a 4673 pentode for the Y amplifier.

 Impressively, all of these high voltage capacitors appear to be good.

A view inside the cabinet.
That little rectangle of paper is very important!
That is all that is preventing the capacitors,
and their high voltages, from touching the metal cabinet.

Here is a view of the other side:  The clear tube is a 4890 triode oscillator, and the shielded tube is a 4673 pentode amplifier for the X axis.

Underneath the chassis, it looks like the day it was manufactured, 72 years ago.

 A side view of the cabinet.  The 72 year old decals are still in fair condition:

Finally, here is it in operation for the first time in 50 years.  Most of the controls work well, and the trigger is surprisingly stable.

The trigger is disabled by pulling this jumper out:

 This simple scope does not have centering controls, and I did not see any trimmers inside.

 I have to translate the German language instruction manual to see if there is a way to center the image.  Unfortunately, two of the knobs were missing, and I had to substitute the ones seen in the first picture.

The Kathograph has found a new home at Volker K's Oscilloscope Museum.
Should you have any questions about an old oscilloscope, I highly recommend his website.

1 comment:

  1. Thanks for sharing your experience. I found one of these and started working on it last night. I thought there was something dreadfully wrong with it when I felt the high voltage on the exterior of the filter caps. I was also shocked when the green trace appeared on the CRT. Its amazing how these things work after so long! More to be done on mine as the y amplifier isn't working well. The photos you posted are helpful. Mine isn't so clean so I couldn't see the values on the capacitors.