Heathkit O-11 Oscilloscope Repair

[Yoa01]

Hi All!

So I recently got a 1957 Heathkit O-11 Oscilloscope off of eBay. My oh my, this is a project, but I'll start with this:

First thing I found is that they don't ship well. After removing the casette tape and book on picking up women from inside the cabinet, I found that a resistor had half-way fallen out, a capacitor was totally out, and a tube had fallen out of its socket. Well, I replaced the parts (and by that I mean literally, I re-placed the original parts) and now I have an interesting problem: I turn it on, and I'm supposed to get a green dot on the screen after it warms up. Though nothing explodes and all the tubes illuminate (except one which looks like it wants to glow but doesn't), after waiting half an hour I still get no dot, even with adjusting the display controls.

Before it shipped, the seller had tested it and it did light up (well, according to the auction page) I'm assuming something went awry during shipping, but I can't see anything out of the ordinary.

This is really I guess more of a log than anything, but I suppose, though futile, I can ask: does anyone happen to know what might be the issue? Like is there some certain part on these that always seems to go?

 

[KrisBlueNZ]

If one of the tubes isn't glowing, the most likely cause is that it's lost its vacuum because the glass is cracked somewhere.

Compare it to a similar tube and see if you can find a round area on the inside of the glass that's shiny on the good tube, but grainy and white on the faulty one; that's another indication that the vacuum has been lost.

I'd be keen to try to help with the scope. Can you post links to a user manual, service manual, schematic diagram, etc?

 

[Yoa01]

There's oddly no difference aside from one thing: the top silvery part (I've read that's where most of the voltages are held) appears to be worn away from the inside. However, some of the other tubes also have this and they work fine.

I would love to, but I eventually just bought the manual (with schematics and such) because it couldn't be found online after several hours of searching. Being still under copyright, it's not legal to post it online, from what I've read.

However, the schematic is here: http://www.rsp-italy.it/Electronics/Kits/_contents/Heathkit/Kits/Heathkit O-11 schem.jpg (I double checked it, it matches the original). The bad tube is V9/IV2.
And, a partial manual missing assembly instruction and a couple other pages is here: http://www.mediafire.com/view/?6375rg488zbkxed

Thanks.

 

[(*steve*)]

Are you sure that resistor and capacitor aren't supposed to be soldered in place?

At this age, the capacitors are possibly quite suspect. I wonder if its old enough to have waxed paper capacitors?

 

[Yoa01]

They are. Firstly they are in the schematic connected to other parts (always a big hint) and second they had solder on them and holes where they should go. Do unconnected parts really have any effect?

I believe they may be. The manual mentions ceramic, molded mica and molded paper capacitors, but does not identify each cap by type. They don't appear damaged in any way, though I think only electrolytics can be visibly bad. Correct me if I'm wrong. Is there a way to test them, preferably in place?

 

[(*steve*)]

Anything paper or electrolytic should be replaced.

Unconnected parts really do have an effect. Disconnect your car battery and see how well it starts (as an example).

 

[Yoa01]

... Well, that's certainly not what I wanted to hear Er, read. Still, I guess you're right. Now, the question: do I want to go through and replace ALL of the caps in this thing, or just relist it on eBay and get something newer.

It's just strange because it was working for the seller. Then again, the seller could have shopped it or whatever. *sigh*

Right, right, I see. I was thinking something else.

 

[KrisBlueNZ]

V9 is the rectifier for the negative rail for the CRT cathode. It does have a separate filament power supply, which could have become disconnected I suppose, but most likely it's cracked. It's just a diode so you could replace it with any diode that's rated for a few kilovolts. Some adjustments would be needed because a silicon diode doesn't drop much voltage compared to a tube diode.

Thanks for posting the schematic. You're right that it's illegal to post copyrighted material but in this case no one is disadvantaged by you doing so, so no one has any reason to complain.

In this case I think you've bitten off more than you can chew, and even if you get the thing working, it probably won't be reliable, it's only a single-trace scope, and its bandwidth is probably only a few MHz, so it will be of limited use in any case.

 

[Yoa01]

If I had a diode like that I'd feel like a king. haha

So, selling it back somewhere is probably the best answer considering I'm still a noob. If nothing else it's vintage and looks cool, so I can probably sell it to some local antiques place for like $30. Maybe.

I'm building audio synthesizers, so I don't really need even a 1MHz 'scope. This one is rated at 60MHz. I probably only need one trace, but I suppose two would be useful for measuring phasing. Any (preferrably <$100) oscilloscopes that you recommend?

 

[KrisBlueNZ]

You could TRY to sell it to an antiques dealer. Good luck!

Is it really a 60 MHz scope? I doubt it, based on the schematic diagram. Perhaps the eBay seller was being a bit... creative. That design has many weaknesses, and I expect poor bandwidth would be one of them.

Dual trace is very useful, not just for checking phase relationships.

No, I can't suggest any oscilloscopes, sorry.

 

[Yoa01]

I'll see what I can do.

No, the manual itself says it Lest I was reading it wrong.

Really? Sorry, I'm used to virtual oscilloscopes (aka the one in LTSpice) and I can see some minor phasing with that. Then again, that's a simulation and is really only showing mathematical equivalents, so...

 

[duke37]

I see the O-11 is AC coupled only. This is a distinct disadvantage, it does not allow you to see how audio circuits are clipping which will occur if DC biassing is wrong.

 

[Yoa01]

So generally, it seems I should consult you guys before buying new (or old?) electronics equipment

 

[nepow]

I remember using one of these scopes during my early days as a radio and tv bod more years ago as I care to remember!! I agree with all the replies here... it has it's limitations. If you are aware of its hazardous voltage points around the circuit and can use a multimeter then it would make a challenging project to restore it to working order again, but be very careful!!
As with most of the heathkit equipment you had the choice of purchasing ready built or self assembly. Those were the days.