Tek 7854 with shadowy CRT (gassy/leaky CRT or anode HV trouble) - Page 1
Forum section: electronics / repair

author: MadTux
author_rank: Frequent Contributor
timestamp: December 21, 2014, 09:10:47 pm
content: Hi guys,I just repaired the second Tek 7854 that has been sitting under my table for a while now. The previous owner tried to to repair it and probably exchanged the position of a few mini-coax wires which resulted in no beam on screen. So it ended upon ebay and I bought it for cheap. Since it wasn't working properly and I had no time to check everything, it ended beneath my lab table.Recently I bought another supposedly dead 7854 for really cheap (250$ incl shipping from US to EU and with 7D14 counter). It arrived in great condition, worked beautifully, but was covered in a thick filthy nicotine-tar layer. So I disassembled it for cleaning and used it as reference to check the wiring in my other dead 7854. The position of some coax wire on the bottom of the digital mainboard were exchanged. Put them back in order and had another nearly fine 7854 in my Tek 7k collection.Only problem are the shadows in the tube. Anyone ever seen something like that before? Position of shadow relative to the trace changes if the trace is moved up or down,with no shadow (shadow on trace), if the trace is centered on screen. The more vertical deflection, the greater the vertical deflection of the shadow.This makes me think that it might be caused by unstable anode HV voltage, since a beam with less acceleration is deflected more. But I've also read that CRTs with gas inside show similar symptoms.Just want to know, before I start digging into the HV section behind the CRT, whether someone has seen something similar and can guide me into the right direction. Deflection seems OK, no noise on it, since the display captured from deflection plates in X/Y mode on my 7904 is completely fine.

author: edavid
author_rank: Super Contributor
timestamp: December 21, 2014, 09:26:12 pm
content: It's normal. The shadows are caused by the scan expansion mesh in the CRT.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 21, 2014, 10:28:50 pm
content: Sure that's the scan expansion mesh??My other scopes have no such shadows. It's still usable, but it nevertheless looks bad and I prefer analog scopes with sharp trace, since glitches and modulation are much easier to see on a sharp trace without such shadows around.Anyone an idea how to measure anode voltage. Have a DIY 1Gohm (100x 10M resistors in series) voltage divider lying around that might be used for that. Would be a 1000/1 divider on a 1M scope input, which is fine. Problem is how to connect it onto the anode cable, since it goes directly from the multiplier on the back to the anode on the front.

author: tautech
author_rank: Super Contributor
timestamp: December 21, 2014, 10:45:22 pm
content: Quote from: MadTux on December 21, 2014, 10:28:50 pmIt would annoy me too. IMO its not quite right.I would suspect the HV DC restorer (caps or diodes)and the effect its improper operation might have on the PDA and the CRT base voltages. Hunch only?Even to check the ripple on the HV is next to impossible with the voltages present.The Tek troubleshooting guide describes a method to isolate a faulty restorer cap.The HV diodes normally have a high Vf so don't let that confuse you.The Tek troubleshooting guide link is in the first post in this board.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 21, 2014, 10:56:13 pm
content: does the CRT have manual focusing rings at the rear ? ( i am thinking in terms of those old TV that have rings to correct focusing)

author: edavid
author_rank: Super Contributor
timestamp: December 21, 2014, 11:17:13 pm
content: Quote from: MadTux on December 21, 2014, 10:28:50 pmYes, I'm sure... I have 4 7854s

author: JuiceKing
author_rank: Regular Contributor
timestamp: December 22, 2014, 12:28:49 am
content: Quote from: edavid on December 21, 2014, 11:17:13 pmSame in my 7854 here. Turn down the intensity until the halos vanish.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 22, 2014, 12:58:52 am
content: Jep, seems like david and JuiceKing are right, 7854s have quite noticeable halos. 7904As don't. Group picture as evidence, 7854 with IMO worst halo is in the middle.Since I've been using that 7904A for a long time (since 2004, second scope I've bought and by far my favorite everyday scope) so I thought every scope must be free of halos.

author: rf-loop
author_rank: Super Contributor
timestamp: December 22, 2014, 07:06:51 am
content: Quote from: JuiceKing on December 22, 2014, 12:28:49 amJust for more evidence - too much brightness (as user error) and same here.And this is nothing if compare to HP 17xx these models what have fast draw memory CRT. (note: 1741 do not have this complex fast write memory CRT)