Keyboard Shortcuts
ctrl + shift + ? :
Show all keyboard shortcuts
ctrl + g :
Navigate to a group
ctrl + shift + f :
Find
ctrl + / :
Quick actions
esc to dismiss
Likes
Search
Tek 485 Help
|
I have a Tek 485 that I have had and used lightly for 20+ years with no issues.? That is until last night.? I hadn't used it in couple of months. After turning it on I went to adjust the Intensity and it went totally bright and stuck there.? The Intensity flickers when the control is moved, but when left in any position, it's always stable on high.? A second symptom simultaneously appeared.? The retrace of the beam is not blanked.? So, both issues point to a problem with intensity modulation of the beam.?
?
I thought I had the service manual for this, but I am unable to find it.? I am sure the first thing I need to do is get a copy of this.? What is the best source for this?? I am a retired electronic engineer, so I have no problem digging into the scope.? But I only have one, so I hope I don't need another to troubleshoot this one.
?
Any suggestions on what areas of the circuitry to look at would be appreciated. |
|
Check ArTek manuals for an (excellent) PDF copy of the manual. ------------------------ DaveD KC0WJN ------------------------ On Tue, Dec 31, 2024 at 17:09 sbasprez via <beneckerus=[email protected]> wrote:
|
|
Also ------------------------ Dave (Nick) Daniel KC0WJN ------------------------ On Tue, Dec 31, 2024 at 17:12 Dave Daniel via <kc0wjn=[email protected]> wrote:
|
|
I have worked on several of these. Flaky or shorted tantalums are common in these scopes. Also look for precision resistors that raise in value and/or become intermittent. If you have another scope, probe all of the power supplies and look for ripple and noise. Try not to touch any of the tunnel diode triggering adjustments as proper adjustment is an iterative?process. Sam Sam Reaves
ARS W3OHM
Owner / Moderator of: LeCroy Owners Group on 开云体育 Sencore Owners Group on 开云体育 Sprint Layout Group on 开云体育?? Pulsonix EDA Group on 开云体育 LPKF Owners Group on 开云体育
Electronics and Mechanical Hardware Design Engineering Manager Staff Scientist Andritz Rolls Global Research Center (RETIRED) On Tue, Dec 31, 2024 at 5:42?PM sbasprez via <beneckerus=[email protected]> wrote:
|
|
Sam,
?
Thanks for your input.? From what I have been reading it almost sound like it would be good idea to shotgun the unit with new tantalums as a matter of course.? ?
?
After looking at the schematic and taking into account the simultaneous onset of multiple symptoms, I suspect that the wiper on the Intensity pot has gone open or high resistance.? I just need to open the case up and see if that is it.? I will report once I get in it.
?
Greg |
|
Covers for 485s the (original, not the TAS485s) are hard to find. It took me something like a decade to find one, and I bought that one based on the dimensions advertised and not identified specifically for a 485. I got lucky - it fit. Finding one will take a lot of patience. Covers for 465s and 475s are commonly available. I don't recall installing the cover as being necessary to remove the front panel. ------------------------ DaveD KC0WJN ------------------------ On Tue, Dec 31, 2024 at 21:47 sbasprez via <beneckerus=[email protected]> wrote:
|
|
The covers for the 465 and 475 are smaller vertically than those for the 485. They will not fit. I've owned all three 'scopes at one time or another. ------------------------ DaveD KC0WJN ------------------------ On Tue, Dec 31, 2024 at 21:59 sbasprez via <beneckerus=[email protected]> wrote:
|
|
On Tue, Dec 31, 2024 at 01:09 PM, sbasprez wrote:
?
I believe the DC restorer diodes (and maybe capacitors) are the common culprit for loss of intensity control. Assuming it's similar to my 475, I lost intensity control and retrace blanking due to leaky diodes. See page 38 of the troubleshooting guide, which has lots of helpful advice:
?
https://w140.com/tekwiki/images/5/5d/068-0315-00.pdf
?
Adam
? |
|
When I had to open my 485, I put it face down on a pillow on my lap to protect the knobs.
Not elegant, but it worked.
--John Gord
?
On Tue, Dec 31, 2024 at 06:47 PM, sbasprez wrote:
|
开云体育I am not familiar with the 485, but surely with an uncontrolled brightness fault, the first place to start is to measure the CRT grid to cathode voltage. Many scopes have a 100v Zener diode sitting across the two, which often goes S/C and gives uncontrolled brightness, plus obviously lack of F/B blanking. Les.
|
|
On Wed, 1 Jan 2025 at 01:26, Sam Reaves via <sam.reaves=[email protected]> wrote:
There are too many 15V tant beads across the 13V PSU line. That PSU line is "hidden" in that it is not on the PSU schematic, but is on the interconnection schematic 17, and the caps are scattered "randomly" across several schematics. So, make sure you find c861 c911 c965 ?c2101 (possibly c844) and ensure they are at least 25V (there may be others). When C911 blows short circuit, it might take out resistor r965. I would not replace the axial tants, unless they are visibly causing a problem. However, I doubt those tants are the cause of your problem. I have had a similar problem to yours with a 465; but the brightness was uncontrollably bright and the CRT was doing a reasonable impression of a torch. Using only a voltmeter, I eventually traced that to the grid being detached inside the CRT. Since you can control the brightness, I doubt that is your problem! The key to understanding how the brightness is controlled is to realise that the grid-kathode voltage Vgk ?is modulated by the?A trace brightness control, B?trace brightness control, the retrace blanking. That modulation is in the range 0-120V, visible at TP1781 z out dc. The AC component of that modulated voltage is then 1:1 shifted up to the CRT cathode voltage. The shifting components frequently fail, but I suspect they are not at fault. So to test with only a voltmeter, I would use it to measure the (mean) voltage at TP1781. You should be able to see that voltage changing with the A&B brightness controls, the proportion of scan time for the B brightness, and the proportion of time spent blanking. The last can be varied using the trigger holdoff control. If one of the front panel controls doesn't change that voltage, then it might indirectly indicate the source of the problem.? You can also, very carefully, measure the Vgk on the CRT. It should be exactly the same as on TP1781. If not, the level shifting components are suspect. ("Carefully", since Vg and Vk are at -2450V!) However, firstly you should RTFM to see how the blanking voltage is adjusted :) |
|
Those are nice 'scopes!? Well worth repairing.
You probably need to check the CRT circuit, especially the DC restorer circuitry (diodes and capacitors) that handles the control grid, and possibly the Z axis amplifier inputs and controls.. De-focus the beam to prevent screen damage for now.? One extreme setting will be more de-focused than the other, that is normal and best to use that.? Dont run it too long like that, the beam current will be high, and among other things, stress the PDA multiplier. The 485 is an "interesting" unit to work on.? You will need the service manual, if just to find out how to dismantle it for access, and especially re-assemble it later. The circuitry "theory of operation" sections are very good..? Always well worth reading before diving in. Happy New Year. Dave B.? Ex Tek UK (when those were current products!) -- Created on and sent from a Unix like PC running and using open source software: |
|
Thank you all for the input.? I think this will be a long-term project to figure out the problem.
?
I designed a 3d printed spacer to allow me to place the scope face down.? I opened it up this morning and see it's going to be a challenge to do any disassembly.? So I took a look at the mechanical drawings in the manual that has been given in some of the links above, and the drawing after page 9-9 is definitely not of the 485.? It shows a totally different face plate.
?
I can check low voltage stuff, but I don't have the tools to check high voltages, so I hope I don't have to go there. |
|
Well, after my last post stating that the drawing in the manual was wrong, I put on my new glasses and realized my error.? After having this scope all these years I had it fixed in my mind that it was a 485.? With blurry glasses that's what I saw on the face plate, I thought.? Well, the new glasses let me see the front panel clearly and I see it is a 465.? Boy, do I feel dumb.? Since much of the advice given above was from 465 users, I can certainly use it to aid my trouble shooting.? I imagine I can find the 465 manual at the same sites previously referenced for the 485.
?
I do apologize for my faux pas.
?
Greg |
Sam Reaves
to navigate to use esc to dismiss