Tektronix 2465B oscilloscope teardown

[xardomain]

>I would try first with the DS1225 NVRAM
Yes, good advice, I really want that eprom programmer, always wanted one since long time...

I still remember a friend of mine, hanging eproms in a glass of water (scuba eprom erasing) in front of the tanning lamp stolen from his mother to erase eproms without cooking them...LOL so '80...he was able to program them but due to some timing error, he "consumed" a fair share of TTL buffers in the process. Good times with a C64...

>I don't think there is any way to verify the checksum of the data outside of the scope.
I am not too sure that the scope could tell if an image is good or not, except if the values are out of range for the variables stored and/or they conflict with installed options(not even sure about this).

I mean suppose you have a calibration value of 0x03(in a position where good values are from 0x01 to 0x0f), and in reality it should be 0x02 (only one bit is different) how could the scope know that is not the right value if the value is within acceptable calibration data for that value?

It could only know if the checksum itself is inside the eprom(easy answer).

If the scope is able to detect a change in value that is still "legal" would imply that the checksum is inside the scope but then it will not accept any other eprom and swapping images from one scope to another would be way more difficult). No certificates/blockchain and other exotic stuff back then I hope.

Or am I missing something?

Giuseppe Marullo
IW2JWW - JN45RQ

[tonedeak99]

Ok, I have some ideas for you to check for the dimming.  It could be the intensity pot input which goes to the CPU, something in the CRT circuitry, or something in between which includes the DAC and display sequencer.

I would suggest looking at two different points to figure out which section to concentrate on.

1) The first is to verify the INTENSITY input control  This is fairly easy and you can do it from the front panel.  Get into Exerciser 01 and turn the INTENSITY knob so that the INTENSITY knob status line is displayed (should start with "17").  See table 6-9 for the meaning of these fields, but what you're looking for is any variation in the displayed numbers as the dimming is occurring.  If there is, the CPU thinks it's reading changes coming from the INTENSITY pot and the problem is on that side.  Hopefully the problem with dimming will not get in the way of seeing the status display.

I'd suggest reading the section on DIAGNOSTIC ROUTINES starting on 6-10 through (at least) Exerciser 01 to learn how to operate the diagnostic menus.


2) The second place to check is after the CPU, DAC, and display sequencer to see if the CRT is being instructed to dim.

Set up the scope for the test waveforms for the A1 board, which immediately precedes the A1<6> schematics in the manual.  Make sure the readout is completely off.  There is a very small range at 12:00 that this occurs.  If it's not off you'll see lots of junk on the test waveforms as the beam turns on and off when the CRT makes dots on the screen to form the characters.

Check signal VZ on the anode of CR966 (diode further from back edge), schematic <6> loc 8L, waveform [65].  The upper level of VZ is the brightness.  Watch this to see if it correlates to the dimming.

Check signal VQ on the anode CR972 (diode closer to back edge), schematic <6> loc 8L, waveform [66].  This is related to the brightness, but is probably not the cause.  Check for correlation here.


3) (Ok, so I added a third test..)  If you find that VZ or VQ correlates to the dimming, you can take a look at the output of the DAC on the cathode of CR354.  It has a range of about 0 to 2.5V which you should be able to verify as you turn the INTENSITY knob.  The DAC and associated sample and hold circuitry are a known source of problems.  You could even do this test before #1 or #2 since it's easy and targets a known problem area.

Apologies Mark that it took me so long to get to this project.  I was finally able to be lucky enough to test #2 and #3 while the dimming and even complete disappearance of the trace and readout occurred.  Actually, this was the first time the readout disappeared together with the trace.  So I had a completely blank screen when I tested.  Unfortunately, the waveforms were as expected in the manual for 65 and 66 (They even changed in amplitude with the intensity knob while the trace was missing).  Also, the voltage on CR354 stayed constant during the dimming at around 1.5V (But changed in voltage with the intensity knob while the trace was missing).  The intensity knob was in the middle when it went blank. I then turned it all the way up and the trace reappeared at full brightness.  I then turned it all the way off and then back all the midpoint and it wouldn't come back.  I then turned it all the way up and it still wouldn't come back.  I then tried the beam finder and it also didn't produce anything.  I tried turning the intensity knob fully CW and CCW a couple of times and still nothing.  I finally had to reboot it to make it come back. 

I haven't tried reseating the z-axis hybrid since waveforms 65 and 66 both look good, and therefore I don't think that hybrid is the problem.  Not sure where else to look.

UPDATE:  I had another strange trace issue occur on a separate occasion.  This time I had 2 traces being shown and only 1 disappeared.  The readout was still showing fine.  I then tried to move the visible trace with the vertical and horizontal knobs and it didn't move vertically, only horizontally.  I then turned the readout intensity knob to off and that made the second trace re-appear and allowed them both to be moved with the vertical knobs.

[faraday]

I tested mine with fast rise time pulse generator not a bad performance for 400MHz scope

[Sqwarky]

Hello Gang,

I was given a non-functioning 2465B and upon inspection of the power and inverter boards, found a dead Rifa cap and a couple of other electrolytics with shifted capacitance and high ESR.

I decided to recap both boards in accordance with relevant info contained in this thread and unit observations, then went to plug it all back together some days later I realised that I hadn't documented a connection from the digital board to A5.  Attached photo's show the connection that is under a ribbon cable. I'm presuming it goes onto J305 but the board I have (double the length of the documentation with SMD's) isn't represented on any SMs I've downloaded from the net and J305 doesn't seem to exist on the interconnects diagram either.

Would anybody have some insights about how it's connected and orientation? Cheers, P.

Cheers, P.

[tggzzz]

I always take photos and make a crude sketch!

What can you see in the service manual?

Tek's normal practice is to have a bas-relief triangle moulded onto the flying socket, and a matching triangle visible on the PCB.

[AMR Labs]

Was going through my pictures from when I added the CTT option board to my 2465B, and this looks like its the same connector. Hope that helps.

[Sqwarky]

Looks like it AMR Labs even though my connector lacks the identification.

Do you have the manual for the CTT card? Cheers, P.

[MarkL]

I don't see J305 in the standard scope service manual either, but it does show up in the 24X5B/2467B Options Service Manual (pub #070-6864-02).

J305 carries +5VD on both wires from the A3 inverter board to the A26 TV/CTT option board.  So in this case orientation doesn't matter.

A couple of excerpts from the manual attached.  J305 is at grid location 8A in the schematic.

[MarkL]

If you want the whole thing, the 24X5B/2467B Options Service Manual is on TekWiki:

  https://w140.com/tekwiki/images/1/17/070-6864-02.pdf