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468 Storage Mode Failure


kartng@...
 

Hi folks - quick intro since this is my first post. I'm a guy who wants to repair old computers/video came consoles, and about as green as one can be when it comes to using an oscilloscope. I'm not a total newbie when it comes to electronics, and can usually figure out which end of the soldering iron not to touch, but I recently spent all of $10 bucks on a Tectronix 468 as a "baby's first scope". The unit has the GPIB option and came with three OEM Tectronix probes. and in "non-store" mode, I have a bright, accurate trace displaying on the CRT. However, when I try to switch to storage mode (following the instructions in the Operator's Manual, which I got with the scope) to set the measurement up, the screen goes dark, no trace is displayed, and nothing is shown on the beam finder (except the line at the bottom that is always there). This is despite no error displayed in the Power On Self Test.

I know the 468 is prone to ROM Rot, and I think my example is ~35 years old. Would this failure mode match a bad rom? I would have thought that would throw an error code, but maybe I'm mistaken? I guess I should probably do the roms anyway, but anything else I should look for here? Bad ADC? Bad solder joints somewhere? Something incredibly stupid on my part? (Could a misconfigured switch on the front panel cause this? Would the GPIB board disable display of stored traces on the CRT since it thinks it is "sending" them somewhere?).

One further issue that I think is unrelated, but I'll bring up here just in case, is that the fan at the rear of the case is completely dead. I figure that is just age and plan to replace it with a comparably sized 12V fan, but if that points to anything else (dead rail from the PSU that might have also taken the storage side of the scope down but left the analog functionality intact?), or gives a clue as to what might have melted due to the fan failure, I'm all ears.

Thanks in advance to anyone who can offer any advice here, and take care!

-J


robeughaas@...
 

I've been going through the 468's in the vintageTEK Museum's boneyard, and of 8 instruments I've tested, only one had bad ROM's. Does your instrument do the "dancing zeros" in the numerical display and then go blank? If so, the ROM's are OK. Check the +5, + and -12 on the storage display board -- the one on top. If you have only the "save" button pushed when powering up the unit, do you see a dashed line near the center of the screen? If so, then the storage circuitry is largely working. Give more details on "line at the bottom...".

Do all the functions - sweep, vertical deflection -- work correctly in analog mode?


--
Bob Haas


Roy Thistle
 

On Mon, Oct 26, 2020 at 09:59 AM, <robeughaas@...> wrote:


"dancing zeros"
You can see the "dancing zeros", in the "penhouse" display, at 6:15, in the vid at
https://www.youtube.com/watch?v=YwSOfqli4j0


Roy Thistle
 

On Mon, Oct 26, 2020 at 09:59 AM, <robeughaas@...> wrote:


Do all the functions - sweep, vertical deflection -- work correctly in analog
mode?
For Kartng@
You can watch this guy give his the mostly analog portion of his 468 a workout here. https://www.youtube.com/watch?v=qzuYMOChwZA
Play along to confirm/test some of your scopes functions.


Michael W. Lynch <mlynch003@...>
 

I am following this conversation and working on my 468 as well. It may be beneficial to mention here that when isolating the analog section for testing, as outlined in the service manual; there is an error/omission in that procedure. When this procedure is performed as outlined, the analog section will not work as described. This may be well known, but bears repeating for anyone not familiar with the 468.

Thanks to Tom (KO4BB) for this old, but essential information. Link below describes in detail what is going on.

http://www.ko4bb.com/doku2015/doku.php?id=test_equipment:tek_468_repair_tips

For the record, I ran my jumper from the 5V Test Point on the interface board to the rearmost lead of A14R101 and the analog section then works as described.

Now on to troubleshooting the digital section.

Thanks to all who contribute to this great group!
--
Michael Lynch
Dardanelle, AR


Michael W. Lynch <mlynch003@...>
 

My analog section works in the "Bypass" mode. When I go back to the standard configuration, analog does not show a trace in "Non-Store", but the digital portion shows signs of life in the form of a mixed display. The display appears to be various overlapping "versions" of whatever signal is introduced into the inputs. Cursors can be seen and manipulated on screen. The display flashes rapidly but the image is clear and persistent enough to see those various traces on the CRT.

ROM Test at start is normal.

In the "Non-Store" mode, I can push the beam finder and i see 4 dots off to one side of the CRT, but no horizontal sweep. Sweep is normal and controlable in the "analog test" bypass mode.

I do not think that the ROM's are the problem as I have swapped them into a working 468 and they function correctly. I have also installed "known good" ROM's into this unit with no change.

I have a working 308 Signature Analyzer available and I have replacement ROM's as well as the SERVICE ROM. I have no experience with this sort of testing, but have fixed MANY TEK analog scopes.

I am going to work through the Tests outlined in the manual as best that I can.

Ideas?

--
Michael Lynch
Dardanelle, AR


kartng@...
 

Thanks to everyone for the input here. I think I've figured out that I have a much taller hill to climb here, as the analog side of my scope isn't working quite as well as I thought. Watching the videos Roy linked, I realized my traces in non-store mode are quite "fat" - I have a legible but too-fat at the tops/bottoms trace when in the 20mhz mode, and nothing but a mess of blurry bars when that's not switched on (measuring the built-in calibrator signal, as I don't have a function generator). So I've got work to do on the analog side - maybe the first thing I should do is the procedure above to isolate it from the digital side completely, and see if that improves things. Beyond that, I'm guessing I'd start with cleaning the attenuator switches, a'la the first video Roy linked?

Also, seating/reseating connections on the boards with the ROMs/ADC has produced some new behavior when I go into storage mode. Before, I would get nothing but a dark screen, though the "dashed line" would be present for just a second before disappearing . Now, I have the dashed line stable and staying on screen, but no trace, and no cursor bars (I could previously see the voltage cursor, but I don't see them now, though the 7-segment display still updates as I rotate the knob). I've had the "dancing zeros" from Day 1, and the POST always seems to complete successfully, so it looks like my ROMs are ok.

That said, I may have one new lead: I was poking around the A19 board (which I think gets power from the "main" power supply and regulates/distributes it?) and found that the 7952C voltage regulator, which google suggests to me should output between -4.9 and -5.2V, is outputting -0.50 volts (which I see both in circuit and on a breadboard). My +5V line seems fine, as I measured 5.02V at the test point at the top "storage display" board that Bob suggested. My +12 and -12 lines are a little out, however (11.8 on one, 12.2 on the other), so I'm thinking I should change all 3 regulators. I'm also thinking a 7905 is drop in sub for that 7952C, right?