My newest oldest oscilloscope - GEC M861B Miniscope

[dazz1]

Hi
My newest scope is also the oldest thing I own.  It is a late 1940's, early 1950's, GEC M861B Miniscope.  A marvel of miniaturization at a time when test equipment weighed as much as a sack of coal and used enough power to heat a small orphanage. 

My research reveals that this model of scope is the direct descendant of a scope (model M861A?) developed to fault-find clandestine radios in WW2 enemy occupied territory.  The post war version was sold to traveling TV repair men (probably not many TV repair ladies at that time).

It's military origins can be seen in the knobs.  This style was used on aircraft to allow crew to turn the knobs while wearing thick gloves. 

It features an astonishing 70kc/s bandwidth with a 1 1/2 inch CRT and a weight of just 7 1/2 pounds. 

The plan is to restore it to working condition.  I have not attempted to apply power yet because it has probably been in storage for decades.  The electrolytic caps won't be caps anymore. 

If anyone has a spare M862B Double Beam Unit, or a M863B IF Alignment Unit lying around, I'd be a keen buyer.

[tautech]

Oh so you did buy it !   

Next you'll be joining the TEA thread. 

[dazz1]

Oh so you did buy it !   

I couldn't resist.   

[adinsen]

What a beauty, congrats. I love the carrying case 

[dazz1]

...
Next you'll be joining the TEA thread. 

Maybe an admin should/could shift this post to the TEA thread?

[tautech]

...
Next you'll be joining the TEA thread. 

Maybe an admin should/could shift this post to the TEA thread?

Or you can post there with a link to here.

Slightly more complex is Quoting your first post and Copy all content into a new Reply/post in the TEA thread.
You could add the images by way of uploading them again or copying their links and pasting them between Image flags like this:



Quote this post to examine the syntax used. 

[dazz1]

I took the Miniscope to the local technical institute where I used a variac to gradually build up the supply voltage to 240VAC to reform the caps, and see if any magic smoke was waiting to escape.  I was working in a class with 3rd year apprentices and the instructor asked me to give a mini-presentation on what I was doing.  They were all interested. I forgot to take photos. 

Over a period of about 20 minutes, the voltage was gently eased up and absolutely nothing happened.  I had the variac output in series with a 18R 10W resistor and an underated fuse.  Only 200mA.  At full supply voltage it drew about 130W which is too high.

The critical part is the transformer.  This is sealed in a metal can, probably with transformer oil for cooling.  If this blows, the Miniscope would probably be unfixable.

One of the potentiometers is seized solid and two others have high friction.  They are 1/4" SS shafts with no flat.  I suspect this was done to prevent damage by over turning the knobs.  The set screws are BSW 1/8, 40TPI, but one screw was 38TPI and that was enough to strip the thread in the Bakelite knob.  I am currently making some replacement set screws (can't buy them here). And I need to repair the striped thread.

Now that I know it can take 240VAC, I plan to run it at about 115VAC to go through and check that individual components are doing their job.  Before I do that, I need to reverse engineer a circuit diagram.  That will take a while because of the way the components are packed in.

[tautech]

The critical part is the transformer.  This is sealed in a metal can, probably with transformer oil for cooling.  If this blows, the Miniscope would probably be unfixable.

One of the potentiometers is seized solid and two others have high friction.  They are 1/4" SS shafts with no flat.  I suspect this was done to prevent damage by over turning the knobs.  The set screws are BSW 1/8, 40TPI, but one screw was 38TPI and that was enough to strip the thread in the Bakelite knob.  I am currently making some replacement set screws (can't buy them here). And I need to repair the striped thread.

Now that I know it can take 240VAC, I plan to run it at about 115VAC to go through and check that individual components are doing their job.  Before I do that, I need to reverse engineer a circuit diagram.  That will take a while because of the way the components are packed in.

Pictures of progress or it's all lies.