Gould 4072 display help! - UK Vintage Radio Repair and Restoration Discussion Forum

mole42uk · 27th Aug 2018, 5:23 pm

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.

buggies · 27th Aug 2018, 7:13 pm

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?

dave cox · 28th Aug 2018, 11:46 am

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

27th Aug 2018, 5:23 pm	#1
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	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. Attached Thumbnails __________________ Richard Index: recursive loop: see recursive loop

27th Aug 2018, 7:13 pm	#2
buggies Heptode Join Date: Jan 2009 Location: West Lothian, UK. Posts: 761	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? __________________ George

28th Aug 2018, 11:46 am	#3
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	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