Crypton oscilloscope circuit operation explanation. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

ms660 · 13th Oct 2020, 10:47 am

Brimar D21-102 CRT in here:

https://frank.pocnet.net/other/Brima...ion_Issue3.pdf

Lawrence.

Radio Wrangler · 13th Oct 2020, 11:55 am

Thanks, Lawrence

That says it's a PDA tube and it looks like it's for running with -3kV on the cathode and +3kV on A4 which will put A3 and the plates roughly at 0v.

David

ScottyH · 13th Oct 2020, 6:45 pm

Thank you.
Now I have to find where my 3KV has gone!

The heater voltage is present on the CRT, glows ok, and it tested at about 3.8V within 10 seconds with a 6V tester, so far so good.

There is a large power supply box lower down in the cabinet, mains (230VAC) goes straight through it, but there is a another transformer in there I think.
I would be suprised if it had an HV winding, and the designers wired it up to the 'scope, but I'll check anyway.

Malcolm G6ANZ · 13th Oct 2020, 7:03 pm

Have a care when measuring the heater volts. The cathode is sitting at -1-2 kV and for insulation the heater is usually floating at this potential. If the tube is glowing then the volts are OK. You can measure the -kV with a resistive divider (10M / 1M) make sure that the resistors are rated to take the voltage.

ScottyH · 13th Oct 2020, 7:44 pm

Will do.

Diabolical Artificer · 15th Oct 2020, 6:54 am

I'm learning loads about CRT's here, great thread.

Andy.

ScottyH · 15th Oct 2020, 7:38 pm

I'm (re) learning some of this stuff from this, and other threads, but quite a bit is new to me.

This is a great resource, and again, thank you to all who have contributed so far.

I'm taking a short break from this (components on the way) and the fact I have some actual work to do now we are back to servicing aircraft.

I will continue to keep updates rolling in.

Scott.

ScottyH · 3rd Nov 2020, 5:16 am

Update time.

I have fought all the pressing jobs to a standstill, and re-visited this little puzzle.

First of all, I have to apologise for the poor rendering of the schematic at the start of this post. It was not complete and I had missed a few things as well as making a mistake or two...

Anyhow.

I think I can see how it works now (or at least how it is meant to work)

What I can see is each of the two EY86 rectifiers feeding different parts of the CRT.
The A4 button from one EY86 and one of the .15uF 3.5KV caps, and the cathode, A2 and grid via the capacitor/resistor/capacitor filter and suitable resistor strings.

The high voltage winding on the power transformer measures 5K ohms or so.

I have no way of testing the two tubes, and am currently trying to find replacements in case they are faulty.

Attached is the updated (although not totally) complete schematic.

Scott.

13th Oct 2020, 10:47 am	#21
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Crypton oscilloscope circuit operation explanation. Brimar D21-102 CRT in here: https://frank.pocnet.net/other/Brima...ion_Issue3.pdf Lawrence.

13th Oct 2020, 11:55 am	#22
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Crypton oscilloscope circuit operation explanation. Thanks, Lawrence That says it's a PDA tube and it looks like it's for running with -3kV on the cathode and +3kV on A4 which will put A3 and the plates roughly at 0v. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

13th Oct 2020, 6:45 pm	#23
ScottyH Triode Join Date: Sep 2020 Location: Auckland, New Zealand Posts: 29	Re: Crypton oscilloscope circuit operation explanation. Thank you. Now I have to find where my 3KV has gone! The heater voltage is present on the CRT, glows ok, and it tested at about 3.8V within 10 seconds with a 6V tester, so far so good. There is a large power supply box lower down in the cabinet, mains (230VAC) goes straight through it, but there is a another transformer in there I think. I would be suprised if it had an HV winding, and the designers wired it up to the 'scope, but I'll check anyway.

13th Oct 2020, 7:03 pm	#24
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Re: Crypton oscilloscope circuit operation explanation. Have a care when measuring the heater volts. The cathode is sitting at -1-2 kV and for insulation the heater is usually floating at this potential. If the tube is glowing then the volts are OK. You can measure the -kV with a resistive divider (10M / 1M) make sure that the resistors are rated to take the voltage.

13th Oct 2020, 7:44 pm	#25
ScottyH Triode Join Date: Sep 2020 Location: Auckland, New Zealand Posts: 29	Re: Crypton oscilloscope circuit operation explanation. Will do.

15th Oct 2020, 6:54 am	#26
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Crypton oscilloscope circuit operation explanation. I'm learning loads about CRT's here, great thread. Andy. __________________ Curiosity hasn't killed this cat...so far.

15th Oct 2020, 7:38 pm	#27
ScottyH Triode Join Date: Sep 2020 Location: Auckland, New Zealand Posts: 29	Re: Crypton oscilloscope circuit operation explanation. I'm (re) learning some of this stuff from this, and other threads, but quite a bit is new to me. This is a great resource, and again, thank you to all who have contributed so far. I'm taking a short break from this (components on the way) and the fact I have some actual work to do now we are back to servicing aircraft. I will continue to keep updates rolling in. Scott.

3rd Nov 2020, 5:16 am	#28
ScottyH Triode Join Date: Sep 2020 Location: Auckland, New Zealand Posts: 29	Re: Crypton oscilloscope circuit operation explanation. Update time. I have fought all the pressing jobs to a standstill, and re-visited this little puzzle. First of all, I have to apologise for the poor rendering of the schematic at the start of this post. It was not complete and I had missed a few things as well as making a mistake or two... Anyhow. I think I can see how it works now (or at least how it is meant to work) What I can see is each of the two EY86 rectifiers feeding different parts of the CRT. The A4 button from one EY86 and one of the .15uF 3.5KV caps, and the cathode, A2 and grid via the capacitor/resistor/capacitor filter and suitable resistor strings. The high voltage winding on the power transformer measures 5K ohms or so. I have no way of testing the two tubes, and am currently trying to find replacements in case they are faulty. Attached is the updated (although not totally) complete schematic. Scott. Attached Thumbnails