UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 18th Apr 2014, 1:02 pm   #41
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Quote:
Originally Posted by TonyPhs View Post
Lifted P11 and fed a negative voltage from a pot and -15v rail into it. I measured the voltage across the pot as -13.5v but as soon as I attached it to Point 11 the voltage I read dropped to 1.3v. As I turned the pot I could reduce the voltage down below 1.3v but not up beyond it. Turning the pot did have a small effect on the waveform amplitude coming off the TR815 collector but no gain discernible cathode voltage output.

One slight difference I noted is that with P11 connected as normal I was getting 0v on TR811 collector, while with the negative voltage fed in I was getting 19v at the same location.
That's exactly what I'd expect. P11 won't go above 1.3V because it's limited by the forward voltage drop of TR811/D812. Pulling it negative will, if you pull hard enough, simulate the effect of the -1500V cathode voltage being too large, and start to switch on TR816, which will start to switch off TR811, and thus start to switch off TR812/TR813 which should reduce the regulator's output voltage.

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
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline  
Old 18th Apr 2014, 1:48 pm   #42
TonyPhs
Retired Dormant Member
 
Join Date: Apr 2014
Location: Bridgend, Wales, UK
Posts: 52
Default 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.
TonyPhs is offline  
Old 26th Apr 2014, 5:57 pm   #43
TonyPhs
Retired Dormant Member
 
Join Date: Apr 2014
Location: Bridgend, Wales, UK
Posts: 52
Default 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.
Attached Thumbnails
Click image for larger version

Name:	eht99.jpg
Views:	104
Size:	63.7 KB
ID:	92015   Click image for larger version

Name:	scope-working-1024.jpg
Views:	147
Size:	96.8 KB
ID:	92016  
TonyPhs is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:50 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.