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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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27th Aug 2018, 5:23 pm | #1 |
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
Posts: 2,613
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Gould 4072 display help!
I thought I'd start a new thread about the Gould 4072 / 4074 'scope I'm working on since I think I have the power supply working now. I'd appreciate any ideas about this new fault.
I have attached a part of the circuit diagram for the pre-amplifier board. During extended testing after sorting the power supply I noticed the smell of something hotter than is should be and started to investigate. There was a 'pop' and the screen went blank. The encircled resistor, a 100R surface mount type, had gone open circuit. I did some basic tests around the component and, finding nothing of note, replaced it. The new part lasted about 30 seconds and popped again. I have checked a few other voltages such as the TRACE/ALPHA line at PLNA22 which switches correctly. The -12V supply is okay although the +12 is nearer 11V so maybe the regulator isn't behaving. I am beginning despair that I'll ever get this 'scope reliable.
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Richard Index: recursive loop: see recursive loop |
27th Aug 2018, 7:13 pm | #2 |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 762
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Re: Gould 4072 display help!
Never did like circuits with no component values...
Perhaps tack on a couple of wires to a higher value resistor (10k?) which may give you a chance to track where the current is going without it fusing. Is that 270v on the right-hand end? C835 breaking down?
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George |
28th Aug 2018, 11:46 am | #3 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,062
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Re: Gould 4072 display help!
Q802 is a cascode, so should be soaking up most of the voltage here as its emitter is pinned to +11.something volts ! Might be worth a quick check that its still a transistor!
Q801 looks a bit strange, it will need something to yank the emitter toward -ve to turn on (via R825) also reverse biasing D804. The current through the 100R should go through R970, R825 or R804 but I suspect R804 is too high a value. R825 could be isolated by pulling the connector (maybe). Or that cap as buggies says dc |