How to debug TDS-640A CRT display?

[neil]

So I rolled the E-Bay "parts or repair" dice, and this time fortune smiled on me: I got a very clean "dead" TDS-640A at a good price.  The display does not show any activity (not even a flash) but with a VGA plugged in, I can see that the scope runs its full self-test with no errors. The inside is clean, with no signs of capacitor leakage even when examined under magnification.

I can see video signals at the test points on the main board (and also the VGA works) so I'm thinking that the problem is down on the display board or the CRT itself, although I see no evidence of burned or damaged components or traces.  If someone will tell me which are the filament pins, at least I can check that for continuity?

But with the display buried down below the main board, how do I debug it? Surely there's a way to make some voltage measurements with the scope powered up?  Will the SMPS be OK if I power it up with the main board disconnected and removed, and only the display loading it? Or do I need some sort of dummy load?

Actually, the same question applies to how to examine circuitry on the LVPS as well, since it's also buried below the main board.

[james_s]

I'm pretty sure the whole display assembly slides out after you take the faceplate off the scope. The tube will have the standard small neck monochrome pinout, I don't recall the filament pins offhand but you can see where they connect. When working on CRT displays that are dead the first thing I usually check is the horizontal output transistor followed by any other semiconductors in that area. The horizontal deflection and HV flyback transformer are part of the same resonant tank circuit and often the CRT heater and other circuits are powered by auxiliary windings on the flyback. If the horizontal deflection isn't working the whole monitor will act dead.

Careful with the CRT, those tubes are very hard to come by and they are used in a bunch of older high end Tek instruments. You don't want to damage it.

[pbarton]

I have no experience with the TDS-640A, however there is some help on the web.
Use 'Google translate' on this URL    https://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_modern/TDS640/TDS640.html

Apparently the type of monitor is used by Tek in all 5XX and 6XX series monochrome machines, they are identical.
There are schematics for the monitor here    https://www.tek.com/oscilloscope/tds520b-manual/tds520b-mod-cm-component-service-manual

If you remove the monitor from the 640A you can power it up, using 24V from a bench supply, for fault finding/repair.
Good Luck.

[neil]

Thanks to both of you for the replies.

I have the monitor out now, and I don't see anything obviously burned or damaged. I pulled the Horizontal Output transistor (MJE13007) and at least diode-mode multimeter testing seems normal for an NPN device. The CRT filament cold resistance is about 38 ohms, so that's not open.

I can do a little more checking on the board, but I suppose I'll have to power it up with a bench supply to proceed beyond that.

[pbarton]

It appears that the Tektronix 154-0959-00 B/W (Mono) CRT is used in all the TDS300, TDS400, TDS500 and TDS600 Series.
Once you get bench power applied, it’s easy to verify that the CRT heater is working, just turn your room lighting down and you should see an orange glow near the CRT octal connector.
There is a good description of the A20 Display Driver Board circuit within the ‘TDS 520 Component Service Manual’ you downloaded, starting at .pfd page 49 of 370 or Manual page 1-31.
There are a bunch of small electrolytic capacitors on the A20 board, with values between 1uF and 100uF that are probably past their sell by date.
Watch out for the 12KV anode supply.

[james_s]

The TDS300 and 400 series use a green phosphor tube while I believe the 500 and 600 use a white phosphor, otherwise they may well be the same.

[neil]

Yup, heater lights up. The 21volt rail is up. The HOT has not been reinstalled yet, but there is drive present  at the pad for the base lead, so I expect HV to come up when I put the HOT back. 

I thought I'd poke around a bit with a scope looking at outputs from the Horizontal and Vertical driver IC's before restoring HV. None of the electrolytics look swollen or are leaking, but I take your point about their age.

[neil]

I don't have a dedicated ESR meter, but a YT video by W2AEW shows how to use a scope and function generator to at least approximate the measurement, so I did that and found two caps with what I calculate to be high ESR.  I don't have anything on hand with a high enough voltage rating to replace them, so I'll have to wait for the small box from Mouser to arrive next week.

In the meanwhile, here's another question: what's the best way to clean the conductive pad on the back side of the rubber button strip? Apparently the fan sucked air directly over the Op/Stby pad for the last 23 years, because it's nonfunctional and a light grey color from all of the fine white dust on it - all of the other pads are clean, black, and work fine. The scope is in pieces right now, so I can't just press another pad onto that printed switch to see if it will work (something I should have done before. Sigh.)

[pbarton]

Well, I would use what we call a ‘rubber’ in the UK, but I’m told that this word means something else in the USA, and I should use the word ‘eraser’ to avoid confusion.
However, I see that there is a product on Amazon called ‘Keypad Fix’.

[neil]

Lol. Yes, "eraser" would be a better choice for many reasons.

As Oscar Wilde wrote:
   "We have really everything in common with America nowadays, except, of course, language."

What I was wondering was if the carbon was just a thin surface coating that could be damaged easily by rubbing or detergents, or if it was a solid piece that would tolerate a little moisture and abrasion to clean it. 

It looks like the "renewal" kits are coatings applied to the top.

[james_s]

No, don't use an eraser or anything abrasive, you'll just wear down the conductive rubber. Wash the rubber strip with warm water and a dab of liquid dish soap. I have to do that occasionally with TV remotes too. I think silicone oil leaches out of the rubber and the buttons have to be pressed harder than harder. These Tek scopes have the same style of conductive rubber pads.

[humgar]

If the board looks fresh without capacitor leak and there's no display at all, the flyback transformer might be busted. It's a common problem. I had mono CRT board that looks dirty with lots of hours on them still working strong yet many like-new CRT driver boards in TDS 620A that the flyback transformer fails after leaving the unit on for 1 day.

https://wonghoi.humgar.com/blog/2017/12/10/tds-500800-series-monochrome-crt-driver-repair/

[tooki]

In the meanwhile, here's another question: what's the best way to clean the conductive pad on the back side of the rubber button strip? Apparently the fan sucked air directly over the Op/Stby pad for the last 23 years, because it's nonfunctional and a light grey color from all of the fine white dust on it - all of the other pads are clean, black, and work fine. The scope is in pieces right now, so I can't just press another pad onto that printed switch to see if it will work (something I should have done before. Sigh.)

Either use IPA on a cotton swab or kimwipe, or if it’s easy enough to remove, wash with dish liquid as james_s said. I’ve done the latter many times for remote controls, and I’ve done the former on my Keithley 197s.

Remember to use some IPA to clean the PCB contacts, too. (If they’re carbon coated, be gentle.)

[neil]

Humgar - well, if it comes to the flyback being gone, then I think I'm going to seriously look into swapping in an LCD monitor running off of the VGA jack. I'd like to restore the scope if I can, but it's not like it's an endangered species.  I should have the new caps in a few days, and will be able to test it then.

All - I washed the rubber key-pad strip, as suggested. Seems to have cleaned up nicely. 

While in that area, I noticed something about the glass plate (parts list calls it the "CRT filter") that sits in front of the tube. The outer face has a slightly mottled appearance, as though something spilled on it, or perhaps someone tried to clean it with something caustic and damaged a coating?  I'd like to do something about it. Any way to repair, or replace it?

[pbarton]

The Tektronix Nucolor system display.
Scroll down this page to see the repair, its a bit messy!
http://jvgavila.com/tds524a.htm#sec4

[shakalnokturn]

TDS640A is monochrome. The anti-glare filter must have been splashed with solvent.
Either a case of replacing the filter or taking all the coating off.

[neil]

I replaced a couple of caps on the CRT board with bad ESR's, and now have traded one problem for another - while the monitor was stone dead previously, with no evidence of any HV, it now has plenty of HV. Enough, in fact, to burn through the rubber anode cap with an arc to the surrounding chassis frame (see photo's). Suggestions?

[dogbert]

Check for all electrolytic leaks. Did you smell anything funny when you desolder/solder around the bad capacitors? You might need to wash the board regardless of clearances.

Given you can get one on eBay now for $160 or less. I don't think it's worth the hassle repairing it:
https://www.ebay.com/itm/TEKTRONIX-TDS640A-DIGITAL-OSCILLOSCOPE-W-OPTIONS-13-1F-2F/263269023485?hash=item3d4c0e7efd:g:mSIAAOSwb8BcAaMp

Just move your front bezel to the one you got from eBay. Problem solved!

[fzabkar]

I haven't looked at the circuit closely, but I can't see any EHT regulation circuitry. Based on my experience with TV line stages, I would check the tuning diode and capacitor (CR205 and C208). If C208 is O/C, then the flyback pulse will be very large.

[pbarton]

I would be tempted to squeeze a small sheet of Perspex between the anode cap and the frame. If you need more space you could cut the top of the anode cap off, as its not doing anything (its already compromised/conducting). Don't have any Perspex sheet? Then try a plastic milk bottle cap or similar.

[neil]

fzabkar - I checked those components. The diode is OK as is C208. Oddly, though, the value of C208 (marked on the cap, and measured out of circuit, and also in the A20 board parts list) is .0047uf, although on the schematic it is shown as .0068uf. I don't know if the difference would matter to my arcing problem?

pbarton - I have some 1/8" acrylic (Perspex in your language) sheet lying around, so I think I'll cut one or two insulating inserts out of it to fit the monitor recess. Might add a layer of silicone rubber as well. There's not much clearance in there, so the rubber dome of the anode cap will have to go; as you pointed out, it's now a carbonized conductor anyway.

[james_s]

It's possible that there's nothing wrong with the HV and that the rubber cap has simply deteriorated. Not super common but it's not particularly rare either, especially if grounded metal is very nearby. I assume you do not have a HV probe, can you measure the voltage from any of the supplies derived from aux windings on the flyback? Most monitors have at least two other voltages from that, and if one is excessive they will all be excessive.

[neil]

james_s -  There's a 50 or 60 year-old Eico HV probe that's been following me around, but I think I'll pass on trying to use it here. What is shown on the schematic as a 50v. rail measured 59.9v.  Problematic?

pbarton - No arcing with a layer of 1/8" acrylic together with silicone rubber sheeting. When the screen mounts are fully tightened, the rubber cap is actually pressed against the side wall of the housing, which seems a poor design.

So now I have HV without the fireworks, and I was able to let the scope boot fully.  I found that by reversing the scope (rear facing me) I could put the monitor assembly next to it facing out, raised on a box, and connect it to the main board (I added a ground wire for safety). 

Annnndddddd.... the monitor shows a confused screen. Here are simultaneous screenshots of the monitor and a VGA for comparison (the CRT intensity top-to-bottom is even, despite what the photo looks like).

I'm not familiar with CRT work, so I continue to welcome any suggestions.

[james_s]

That looks like it has lost hsync, is there an adjustment pot for that? It may be labeled h-hold, if you can't get it to lock by adjusting that then there may be a fault remaining. Even if it won't lock you should be able to get a recognizable picture by tweaking that.

I always liked working on crt displays, they're a lot more interesting than most modern displays, they have a nice look too IMO.

[neil]

The scope is alive and well!

The horizontal hold adjustment was extremely twitchy, and would not hold when the machine was powered off and then rebooted.  I found R311, a 39K 1/4W resistor had opened, thus failing to deliver a sample of the sawtooth to the PLL in the MC1391P horizontal support chip. Since the loop couldn't lock, the horizontal oscillator was unstable. Replacing the resistor yielded a nice, stable, easily-adjusted  image.

Additionally, the image was a bit narrow, but the slug in the horizontal width coil was already broken and appeared to have some sort of thread-locker on it as well, so I didn't want to fracture it further by trying to turn it. I added a little extra capacitance in parallel with the horizontal width capacitor C320 and the raster now fills the screen.

The best documentation I found for the A20 Monochrome monitor is at the end of this manual (it doesn't quite match my board 100%, but is very close):

      www.tek.com/oscilloscope/tds520b-manual/tds520b-mod-cm-component-service-manual

The acrylic sheet plus a layer of silicone rubber seems to have resolved the anode-cap arcing issue.

In the end I did nothing with the splotchy anti-glare coating. But since it was noticeable by reflection more than by transmission, I reversed the glass, putting the bad face toward the CRT. There seems to be a similar, undamaged coating on the other side of the glass. The damage is really not visible.

So I am a very happy camper. Thank you so much to all who responded in this thread!