Tektronix 2445 CRT/display and 87 Volts too high. - Page 1
Forum section: electronics / repair

author: Mr Lurpak
author_rank: Contributor
timestamp: March 05, 2025, 05:04:08 pm
content: I have a Tek 2445 where the RIFA caps blew. I do not know if there was issues before as I got it like this.1. I replaced the RIFA caps and a resistor.2. 87Volt and 42Volt was missing.3. I followed the schematics and checked all Transistor and diodes on the 87volt rail. I then replaced all electrolytes caps on this 87+42v rail.4. I now have ALL voltages present but the 87volt is HIGH as i 93volts - it this normal/tolerated value?Anyway, this is the history THE FAULT is that all logic, LED etc seems to be working - however, there is no clear beams/text on the CRT. I can see light as of VERY diffused beams (more than just out of focus) when I turn SEC/DIV and position for H/V doesn't make a difference.Any hints or similar fault fixed? is it the H/V deflection or where to begin as I cant see any clues on CRT.Cheers

author: Mr Lurpak
author_rank: Contributor
timestamp: March 05, 2025, 07:49:27 pm
content: And what specific would you be looking for and testing?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 06, 2025, 08:18:13 pm
content: HiNo it is not normal, 87V comes from a regulated supply so it must be 87VCheck voltage at both ends of R1220G Edmonds

author: Mr Lurpak
author_rank: Contributor
timestamp: March 07, 2025, 11:05:40 pm
content: Thanks, yes was thinking the same that I would focus on getting it to 87v. I had plans to replace the OPAMP and check resistors as all diodes and transistors are OK. On the 87/42 volt rail.cheers

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 08, 2025, 12:16:45 am
content: HiThe 87V supply is both constant voltage and current limited by Q1223, Have you checked the 10V reference voltage on pin 3 of OP-AMP. Does your schematic have the expected voltage levels shown?G Edmonds

author: Mr Lurpak
author_rank: Contributor
timestamp: March 09, 2025, 02:48:44 pm
content: Cheers.I checked all resistors and OK, so just replaced the OP-AMP U1281.I now got 87Volts - still no traces but getting a step closer. It seems like something like text, and beam find does show but VERY VERY faint. Could this be Focus circuit related or back to idea of Z-axis hybrid?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 09, 2025, 06:47:01 pm
content: HiNow is the time for the application of logic, not random component changes. The only down side is that it will mean working with voltages of some 2KV around the CRT, are you happy about this?G Edmonds

author: Mr Lurpak
author_rank: Contributor
timestamp: March 09, 2025, 09:21:09 pm
content: Yes, I am OK with High voltage and power supplies - do have (way long ago) experience in fixing CRT based televisions and monitors. That is not scaring me.I did have a look around on the high voltage board. The Q1851/52 and surrounding resistor test OK according to schematics. so so R1812/13. There seems to be 295 volts presents (300V according to schematics). However looking at pin 5 and 6 on the CRT which I would assume to be kind of symmetric with pin 12/13 the voltage are much over like 7-12 volt ish. Am i totally out on a wrong tangent?My problem is more of understanding what part of the circuits will be involved in having very unfocused traces. HV is present and when you power off the scope I see half a trace shortly.BTW changing the OPAMP was not random, I had verified ALL resistors, transistors, caps, diodes and I had 10V ref on pin 3 - so was only part left and it did get me back to 87V ;o)Cheers

author: Kwakerman
author_rank: Regular Contributor
timestamp: March 10, 2025, 02:22:43 pm
content: A couple of options to try:1. Your 'dim' crt may be due to the grid bias being set wrong, its the pot nearest the front on the A9 (HV) pcb on the left side. Insert a trimmer tool, remember its position then try tweaking it left & right to see if that improves things. If not put it back to where it was.2. It could be that the electron beam is being deflected away from the phosphor by stray voltages on the deflection plates, you can disconnect the deflection plate feeds by pulling the thin wire connectors from the crt pins (or desoldering the wire). These are NOT on the crt base but on the neck, you can see the horizontal ones in the slot in the main pcb next to U800. If you try this, ensure you have insulated the disconnected pins so as not to cause any damage. When you switch on be ready to reduce the intensity control or turn it off quickly as all the beam current will be going to one spot on the crt and you don't want to burn it. If you do get a bright spot somewhere near the middle of the screen then you know the beam is being deflected away. Try reconnecting either the horizontal or vertical plates to identify which set of plates cause it. I had to do this when trouble shooting a U800 failure as one of the U800 outputs was dead and so the other plate pulled the beam off screen.

author: Mr Lurpak
author_rank: Contributor
timestamp: March 10, 2025, 05:20:26 pm
content: Nice one will remember that trick.When I lift the two wires W918 and W919 I get a VERTICAL visual line approx mid CRT, and pressing beam find also a visual beam. This indicates a problem with the HORIZONTAL deflection right?The grid bias made no change, but helpful info for another time.

author: Kwakerman
author_rank: Regular Contributor
timestamp: March 10, 2025, 06:49:28 pm
content: Correct, horizontal issue, probably U800 as mine did the same. Try measuring the voltages on 918/919 and you will probably find one of them has nothing on it.

author: Mr Lurpak
author_rank: Contributor
timestamp: March 10, 2025, 09:53:13 pm
content: I am still not 100% sure it is the U800 need to do a little more digging/measurements. I had 75V on W919 and about 12-13volts on W918. However when I measure the output on pin 9 and 4 on U800 it is flat (DC). So checked the input (pin 16,18,20,21) and also flat (DC and not the pattern I expected from service manual), so I do believe (but no expert) that the fault could be before the H-amplifier as in maybe A/B sweep and triggers but need to set aside some time to check with a scope. Or maybe I am just naive and too optimistic haha.BTW does anyone know of copies replacements of the U800? I think I long time ago read about a guy that did a replacement circuit?Cheers,

author: Kwakerman
author_rank: Regular Contributor
timestamp: March 10, 2025, 11:05:03 pm
content: When that scope's U800 died I replaced it with a 'RU800' from 'Dr Ram' (if I remember correctly). I helped Ram with a bit of early debugging on the original V1 by doing a full calibration on the scope using the 'correct' TG501 / PG506 / SG503 / SG504 plug-ins that I have. I'll try and find some links for you. Ive used mine for about 4 years and 150 hours on it and it still works fine (I have 5 x 24x5A & B so rotate usage between them). I believe there is a V3 on the way.

author: Kwakerman
author_rank: Regular Contributor
timestamp: March 10, 2025, 11:07:31 pm
content: Send Ram a message via Youtube to see what the current status is.

author: tunk
author_rank: Super Contributor
timestamp: March 10, 2025, 11:39:54 pm
content: There's one for sale on ebay, $75 + shipping:https://www.ebay.com/itm/154246923795