Tek 2465B with blank screen

[tekky]

Tek 2465B with blank screen

Previously done:  DallasNV replaced with Fram.

Scope was working until one day the screen remained blank. I did a recap from a kit but the problem remains.
It powers up with all leds on, then steps thru leds: ch1,ch2,ch3,ch4,add,invert,vert,add ;stuck here
No text or trace appears on screen, except beam find shows some short traces.
Able to enter normal mode by pressing A/B trigger button.
Able to enter diagnostic monitor and manually run tests, but no display.

The nine J119 reference voltages look good, but I found the front panel/DAQ outputs to be low (+0.225v,-0.250v instead of +1.36v,-1.25v). I was unable to make the suggested DAQ Ref adjustment because the voltages only vary a little. Not sure I am performing the adjustment correctly. Replacing DAQ Ref pot R2010 and cleaning the area did not fix it. While disconnected, the Ref input circuit seemed ok.

Jumpering J504 for a fixed -1.25v reference artificially restored the +1.36v,-1.25v refs, so downstream circuits are probably ok. This also brought text back to the screen and allowed me to see the boot error as "Test 05 Code 24". The service manual says Test 05 checks "Line Trigger source, A Trigger generation, and Control DAQ". Troubleshooting flowcharts say to see the Options service manual about Code 24, but I see no mention of it there.

I'd like that code explained before I decide its the DAQ.

[med6753]

Make sure you have the correct options service manual that match the options on your scope. On the older 2465 (no suffix) there were 3 separate options service manuals. Not sure if the 2465B is the same.   

[tekky]

Ah! The Options Manual I was able to download is for the 2465A. Not yet finding one for my 2465B (B050000 & above).

Make sure you have the correct options service manual that match the options on your scope. On the older 2465 (no suffix) there were 3 separate options service manuals. Not sure if the 2465B is the same.   

[tekky]

It turns out Test 05 Code 24 is explained in the scope service manual Table 6-4. It is two errors under Main Board routine, "Positive level not positive enough" and "Negative level too negative".
Without further explanation I'd have to believe this is the 1.36v,-1.25v the DAQ is supposed to produce, so this points to a failure in the setting of these voltages (Muxes/DAQ) or in the way the voltages are measured (Mux/comparator feedback).  It would be nice if there was a way to statically set and hold specific DAQ values to verify the DAQ itself.

[Testtech]

Since your serial number is above B05000 check the control board for corrosion from leaky SMD caps. Repair as required. This should get it going. Recal after.

[SoundTech-LG]

Any of the resistors in the bridge network adjusted by the pot (also suspect) can and will be likely cause of not able to set the reference voltage for the DAC. All of this from leaking electrolyte. You may have to remove them one at a time to measure, out of the bridge.

[tekky]

I had previously checked out that bridge network when I replace the pot, so I removed the DAQ. I didn't really see anything suspicious, but back when the scope was still working I had really cleaned that area and had found a bit of gunk between pins 19,20. Since I had a replacement DAQ chip I put that on and rung out all the traces (should have done that first). What I found was pin19 (/IOUT) was open. This pin normally connects by a short trace to the only via under that chip. My guess is that trace was either corroded and/or wiped away when I scrubbed the pads. A short jumper later and the scope powers up normally. The old DAQ chip is probably fine.

Attached is a sim of that bridge network to verify voltages (DAQ removed). My measurements matched that closely.

[SoundTech-LG]

Once again proving that the electrolyte leakage is insidious and results in hidden problems in that whole area of the board!

Good work, and persistence! No pun intended.