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Old 19th Mar 2017, 7:00 pm   #1
Oliver35
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Default Homemade oscilloscope from spare parts..

I've been digging all my odds and ends out of the loft recently, and trying to find more accessible homes for them all, so that i don't have to spend ages up a ladder rummaging for something I know I've got, but can't remember where I last saw it. It's a small but eclectic collection, partly created by buying valves as a 'job lot' on eBay, just to get hold of one unit without paying inflated audio prices, and stashing away the rest.
Since it was raining yesterday, and I had an unusually free afternoon, I decided it would be fun to use up some of this stuff and build myself an all-valve oscilloscope, as one does. This was largely informed by a) ploughing my way through a dreadfully boring book I bought recently about Orford Ness, which made me think of Watson-Watt, which in turn made me think about early 'scopes, and b) by remembering that I've got a rather tidy 5UP1(F) tube sitting up in the loft that I otherwise probably wouldn't do anything with. I think I originally had an electrostatic TV in mind, but as it only cost me 8 I felt I ought to grab it, anyway.
Further rummaging then turned up two Admiralty pattern transformers, beautiful but both useless, and a rather grotty transformer that I found in a dustbin, which was less useless than the other two. This is designed to give 200-220v HT, with a separate winding for 12v, which was a bit awkward, but measuring the resistance of the windings indicated that the primary and secondary were very similar in construction. Turning it back to front and feeding mains into the 220v secondary raised the overall output only slightly against the stated figures, and meant I could take 20v from the 220-240 tap, connect the 220v tap to chassis, and get 220v between the 220v tap and the 0 tap. Still with me so far? The 20v is required because the only valve rectifiers I have spare are half-wave indirectly heated U31s, which require 26v for the heaters. I've wired the test chassis for two of them- I was originally going to set them up in a voltage doubler layout to get roughly 300vDC and 600vDC, but I chickened out after having a think about heater-cathode insulation, as both heaters will be wired in parallel, with one side tied to chassis.
As for valves for the timebase, I had a few thyratrons of various types floating around, but I don't really know much about them, and after a small amount of fiddling I decided to go back to a hard valve timebase. This has ended up being a couple of very flaky ARP34s that I found rolling around in the bottom of a box- I stripped them both and sprayed them with zinc primer, and set them up with one as a Transitron-Miller oscillator and the other as a phase reverser for X1 and X2. I'm not working to a given schematic, just boldly (foolhardily!) plucking circuitry from a book about the cathode ray tube by G. Parr and O.H.Davie, and an old book of television madness by the perennially baffling W.T.Cocking.
The supposedly variable capacitances are fixed at the moment, as I've had to order some wafer switches- I had absolutely nothing suitable- but a rough neon screwdriver test tells me that it is indeed working and I can vary the frequency with the charging resistance. rectified HT is just shy of 300v, around 295v. I'll dig my old Scopex 'scope out later and have a look at what sort of waveforms it's actually putting out, but it was a satisfying way to while away a rainy afternoon.
I don't know if there's anything unusual about the setup, others may have come up with it before me- but I knocked up a quick test bed by drilling holes into a sheet of plywood and screwing valveholders into it. The underside is covered in tinfoil glued on, and wherever an earth connection is to be made, I just bung a drawing pin in, tin the head and solder to that. So far it seems to work quite well.
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Old 19th Mar 2017, 8:46 pm   #2
Nymrod121
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Default Re: Homemade oscilloscope from spare parts..

"I don't know if there's anything unusual about the setup, others may have come up with it before me- but I knocked up a quick test bed by drilling holes into a sheet of plywood and screwing valveholders into it. The underside is covered in tinfoil glued on, and wherever an earth connection is to be made, I just bung a drawing pin in, tin the head and solder to that. So far it seems to work quite well."

Love it!

Looking forward to reading more on this ...
Best wishes
Guy
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Old 19th Mar 2017, 10:24 pm   #3
Oliver35
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Default Re: Homemade oscilloscope from spare parts..

Thanks! If successful, I will rebuild it all on a proper chassis, it's only a test rig. Hopefully the finished product will look neater! Just as a quickie, here are some waveforms.. I'm no expert on analysing waveforms (huge understatement alert) but they look fairly good to me. First photo I think is the oscillator output, second is the paraphase- IIRC. Maybe not very quick on the flyback there..

Oliver
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