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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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18th Apr 2014, 1:02 pm | #41 | |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
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Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.
Quote:
I'm pretty sure that what's going on is that the transformer is faulty and is losing most of the power fed in to it. Because the cathode voltage never reaches -1500V, TR816 is held off, TR811 is hard on, and the current through TR812/TR813 is limited by the fact that the current through R814 can't exceed the forward voltage drop of D811. That's why everything just sits there getting hot, and the emitter voltage of TR813 is stuck at about 4V or so. The situation when you disconnect R802 is exactly the same, as far as the feedback is concerned, as the -1500V output being too small. The pre-regulator and oscillator is working as hard as it can to try and bring the cathode voltage into spec, but the transformer is soaking up all their hard effort. Pulling P11 negative manually will start to switch off TR813, and its emitter voltage will probably fall a bit, but that voltage was already limited by the current limiting action from R814/D811/TR812 so the effect won't be dramatic. Chris
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18th Apr 2014, 1:48 pm | #42 |
Retired Dormant Member
Join Date: Apr 2014
Location: Bridgend, Wales, UK
Posts: 52
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Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.
Ok thanks chaps.
When, or if, I get the replacement EHT unit I will let you know how it goes. |
26th Apr 2014, 5:57 pm | #43 |
Retired Dormant Member
Join Date: Apr 2014
Location: Bridgend, Wales, UK
Posts: 52
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Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.
Received the EHT unit from Dickie this morning.
So after Lunch I got straight to it and removed the old unit, then soldered the replacement in and insulated the joints with two layers of heatshrink. Luckily the unit was identical from Dickie's OS3000A to my particular OS3000 so all the wires were the same colour so I didn't need to mess around and trace everything back. Powered up and got the trace straight away. Happy with that. Thanks for all the help in diagnosing the fault. Must remember to throw a 500mA fuse in there before I put it back together. Now that these things aren't quite as scary I might even see if I can get this thing cleaned up and restored properly. |