dropped a bill on a new oscilloscope

[shoggoth]

I dropped a bill on a new oscilloscope.

New to me.

And a $20 bill.

[img1024]http://i234.photobucket.com/albums/ee25 ... j3xiez.jpg[/img]

It's a 1951 Heathkit O-7 Oscilloscope. The guy who sold it had previously fired it up and got a horizontal line - I declined to try, didn't want an electrolytic shorting out and nuking the rectifier tubes. Opened it up, the 12AT7 vertical amplifier is cracked. Plus, a ton of bubbly wax and paper condensers, and 64 year old electrolytics.

I'll replace all the caps and the drifty carbon comp resistors. Might install modern BNC connectors for the probes as well, or maybe a pair of binding posts/banana jacks if the existing holes are spaced right.

Anyhow check out these little coil things in the upper right - I'm assuming these are old-school wirewound power resistors and not inductors?. The schematic just has caps & resistors there. The white thing is probably a resistor as well?

[img1024]http://i234.photobucket.com/albums/ee25 ... 2mp2cy.jpg[/img]

Anyhow, complete rebuild in order, then I'll have a cheapo tool to stare at signal shapes. Everything but the potentiometers is getting replaced.

I like how they did the sync circuit in the sweep generator, kind of clever. Maybe that's par for the course for oscilloscopes though, my first time looking at a schematic for one.

One thing I notice is that the metal chassis isn't grounded. I'll have to switch to a three-prong cord for that.

Might paint the case too. Maybe some flames or racing stripes, so it can measure really fast.

[xtian]

A labor of love! You go, guy.

[Structo]

Remember the old scopes use a CRT and as such have very high voltages inside. (flyback transformer?)

[shoggoth]

1300v on this model. It's more manageable than a typical CRT with 18kv, and all voltages are input at the socket on this particular oscilloscope tube, so no sliding screwdrivers under rubber caps to discharge. Instead, just short out some pins on the socket.

[shoggoth]

Oh duh, those little coils are inductors, regardless of what the schematic says. The scope's been modded a bit to improve performance. I'll do some modding too, the -1300v power supply isn't filtered very well.

[shoggoth]

Restoration complete, several months and a few hundred bucks later...

Had some brackets for a better scope tube and adapter plates with D-shaped holes made by eMachineShop.com. That was expensive. I was able to upgrade all the connectors to BNC connectors with the adapter plates though (they would have spun around in circular holes). The guy who built the thing had done some hand drilling of screw holes, I was able to precisely lines up all the holes in a CAD program, and it all fit together perfectly.

Anyhow here's some gut shots. I added a lot more filtering to the power supply and a bunch of capacitor multipliers to kill any ripple.

[img768]http://i234.photobucket.com/albums/ee25 ... p6fsqg.jpg[/img]

In the photo below you can see a red jumper wire. I was getting ripple that I just couldn't make go away on the first vertical amplifier. When I jumpered over the capacitor multiplier, it went away, so I cut the multiplier out of the circuit entirely. Nope, ripple came back. Replaced everything, ripple still there. I must have done something stupid under the board - but no way I wanted to disconnect everything to lift the board, so I just permanently installed the jumper. Anyhow that's why that section of the circuit board looks terrible.

[img:768:1024]http://i234.photobucket.com/albums/ee25 ... k9m1tv.jpg[/img]

There are two problems left. The vertical gain knob needs to be replaced, the trace jumps all over the place whenever I adjust it - and the lowest frequency horizontal sweep isn't very stable. It jumps around a bit. I expect it's modulating with 60hz AC, since I can get a test signal from the heater voltage displayed with no jumpiness at all. Maybe I'll look into it later, it's not a big deal.

Oh and the transformer vibrates like a bastard. Is that typical of 1951 equipment?

Here is a 1 kHz sine wave

[img:768:1024]http://i234.photobucket.com/albums/ee25 ... punh4g.jpg[/img]

And a 150 kHz sine wave - you can see the sweep trace from right to left, this oscilloscope doesn't have any blanking:

[img:768:1024]http://i234.photobucket.com/albums/ee25 ... hhdhqs.jpg[/img]

And a 150 kHz square wave:

[img:768:1024]http://i234.photobucket.com/albums/ee25 ... grc995.jpg[/img]

[xtian]

Pretty new face!

You SO crazy. Great work.

[Colossal]

Wow! It's beautiful! Great work and the price was right too.

[Structo]

Holy crap!

What a project.

So the $20 scope cost $400 to fix?

Just kidding, I can tell you have a passion for that kind of work and it shows.

[vibratoking]

I don't see a USB connector to store the traces.

Nice work!

[ToneMerc]

Very cool, lot's of assorted talent floating around these parts!

TM

[shoggoth]

So the $20 scope cost $400 to fix?

Just kidding, I can tell you have a passion for that kind of work and it shows.

It really did. I could've bought a brand new digital scope for what I sunk into it. On the other hand, analog is so much cooler. Circular analog screens even more so.