New Project- Ancient Oscilloscope

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New Project- Ancient Oscilloscope

Postby Lawnboy » Sat Apr 11, 2009 3:58 am

i found this old oscilloscope that has been sitting as a display piece in a friend's store for about 10 years. id like to see if i can get it working but judging by the electronics that may not happen. i have tried plugging it in, but nothing happens. no mains hum, no glowing tubes, nothing. i dont know where to go next- should i try troubleshooting it, or just use it for parts? it has the label "DeVry Institute", and a little research reveals that it was most likely a kit that someone built.
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Lawnboy
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Postby Dave Moll » Sat Apr 11, 2009 5:10 am

T'would seem a shame to break it for spares without even trying to get it working.

As you've already applied power to it, I presume that no further damage would be done by doing so again and seeing how far the electricity is managing to get. Being particularly careful of any EHT, what's going in and coming out of that mains transformer to start with?
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Postby Klaas Robers » Sat Apr 11, 2009 8:33 pm

This is a tube voltmeter (right) and an oscilloscope in one.

Please first make a list of the tubes and the position where they are now.

Then remove the rectifier tubes (the two large ones next to the power transformer) and first test the primary and the filament circuits. Then the tubes should glow.

If they don't there are only a few causes possible:
1. No mains voltage, power cord broken, plug not making contact, measure where the power cord enters the chassis (right back)
2. Fuse in the mains circuit blown (this always has a deeper reason) (I haven't seen one yet in your nice photos.)
3. Mains switch not working.
4. Transformer burnt out (you can smell this).

So: measure the voltage going to the transformer, if present:
measure the voltages coming out of the transformer. The twisted yellow and black wires are the filament voltage, going to all small tubes, should be 6,3 volt AC.
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Postby dominicbeesley » Sun Apr 12, 2009 10:34 pm

I've had scopes die (Philips) when their thermal fuses break. This a spring held together with a blob of low melting point solder shoved into the windings of the transformer.

After a while these can give up due to boredom rather than their intended function of detecting overheating transormer so if the transformer is out then it may be fixable!

A 'scope and a valve voltmeter in one is a brilliant bit of kit to have. I only just got a valve voltmeter and it is great for measuring high impedance circuits...

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Postby Lawnboy » Tue Apr 14, 2009 4:56 am

i brought it to the shop where i work this morning and messed with it to no avail. it seems to me that the transformer is dead- there is power getting to it, but nothing coming out. there is no burnt smell, nor any signs of overheating. i removed the covers in hopes that there was a fuse inside, but it is pretty solid and filled with a silicone-like substance. i did not try to completely remove the cover for fear that it may pull the wires out. there is no writing on the transformer other than the numbers "7-41" stamped with ink on the top.

another thing i noticed is that my multimeter will register 12 or 13 volts AC when one of the leads is placed on the chassis, or any wire terminal when the unit is plugged in. could this be related to the transformer at all?
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Postby Klaas Robers » Tue Apr 14, 2009 11:09 pm

Ah, you have a multimeter. Can you measur the resistance between the two black wires that come from the transformer through the latge hole and go to connectionstrip TS-C to the lugs number 1 and 4? You should measure a resistance of less than 100 ohms.....

Oh, I suddenly get a very bad idea. I remember that the set comes from some american company. Isn't it DeVRY Chicago Toronto? It might be that it is a 115 volt transformer, and if this has been connected to the European 230V power grid.....

Because, can you measure the 230V at the lugs 1 and 4 if the plug is in the wallsocket? Be careful !! Dangerous voltages.!
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Postby Dave Moll » Wed Apr 15, 2009 3:58 am

I'm assuming that, as Lawnboy is in the USA, the 'scope is being fed with 110V. Unless it has previously spent time in a 230V part of the world, it is surely unlikely to have been fried in this way. This is, of course, no consolation if the transformer has burnt out.
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Postby Lawnboy » Wed Apr 15, 2009 5:59 am

i measured the resistance between several pairs of wires including the black pair and they all had a resistance of less than 10 ohms, which is a good sign. i am assuming that it operates on our 115 volt supply, there is nothing that indicates it is made for the European power grid. although the power cord is unusually small, typically the type you would expect to see on a lamp or small radio rather than an oscilloscope. but it looks home-built, so you never know what to expect. here also is a slightly better picture of the wiring from my cell phone.
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Postby dominicbeesley » Wed Apr 15, 2009 10:54 pm

Just a quick one - I'm sure you've probably checked this - the green / blue twisted pair at the top of the picture looks to go off to a front panel switch have you checked that the switch is working?

with the power out you should be able to measure a low resistance across the power plug when its switched on - if not temporarily move the top brown wire to the next tag up to bypass the switch...

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Postby Lawnboy » Thu Apr 16, 2009 7:00 am

i got it working. the green and blue wires were wired into a switch in the front. i never realized it because the power switch was built into the "ohms adjust" knob in the top right corner of the faceplate, but wasnt labeled. at first glace it all seems to be working properly, all the tubes glow, and a ragged green line appears on the CRT. i cant check its measuring capabilities until i get a pair of probes, which i understand are a little more complicated than just a wire with clips on the ends.
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