Tektronix 2440 Oscilloscope Repaired- Part 3
Here is the part 2 article:
Today I started with Calibrating the 500MS/s Tektronix 2440 Memory Oscilloscope. Something that was easier then I first anticipated. Mainly because it is a pleasure to work with this (and probably any other Tektronix Oscilloscope) really fine assembled Device. And the beautifull way in which every piece of hardware is documented, and can be found when any repair is neccessary.
I began reading in the Tek Service Manual thinking it would take a lot of work and time to Calibrate or even to understand the Diagnostic Messages of this 2440 Scope. But it wasn’t. But there are also several ways to do the Calibrating thing.
First there is the Automatic SELF CALIBRATION which is the easy way. And that requires no external test equipment. The Built-in computer firmware in this case is used for calculating the over 100 Voltages, that control Gain, Offset, and so on. Second there is the external calibration method. Jumper J156 has internally to be removed. And external equipment is needed to Calibrate all electronic circuits.
Of course I tried the easy method, and running SELF CAL showed up on screen. But to start this I had to press the Button MENU OFF/EXTENDED FUNCTIONS (once or twices depending what is presently displayed on screen). And after this Extended Functions Menu is displayed, I pressed the menu button CAL/DIAG.
On previous photo the first visible Screen was seen that presented itself after the Bad PMI DAC10/U250 horizontal deflection chip was replaced by a new chip. And the internal Built-in Diagnostic Start-up Test had finished after Powering Up.
It sadly showed not less then 6 FAIL messages. And every message can be viewed into deeper for more info. But the FAILS preventing me from using any of the normal Oscilloscope functions.
After with some playing around with the Screen Buttons I understood that SYS_RAM U664 (a Non volatile 32k x 8 RAM) probably was partly defect. Cost on Ebay about $30 dollar. Okay since it was not my Scope my friend has to decide it was worth the Gamble.
I also menaged to reduce the Fail count to 5 after a few Startups which was good.
I found out that SYS-RAM component U664 in the schematics was only partly bad (a DS1230 that is called a 2374 chip on the System Processor Board [A12 Board]). As can be seen on next photo. And on the following photo you can see deeper into which part of the U664 SRAM Failed in the test.
But I still wasn’t able to get into the Normal Scope Control Mode probably because of the too many FAIL messages.
After reading the Service Manual on how to startup the ‘Automated Self Cal’ it finally started running. And this display was shown every time I started up the Scope again.
And it got automatically the FAIL count down to only 4 FAILs left. See next Photo.
But probably FAIL line 3000 means that the Nonvolatile SRAM chip (28 pins) has to be replaced before the 3 following FAIL Messages will be dissolved. Or at least it is the best action without having to Re-Calibrate the Scope’s CCD-Modules etc. manually afterwards. And the 2440 oscilloscope probably sometimes Freezed in executing all its functions because of this bad SRAM. But that is not proven yet.
But as I found out further on in this article, the SELF CAL function really works in Helping to restore the Scope’s old functions.
And as can be seen on previous photo the AUTOSETUP is working which Helps to gain control over the front buttons on operating the Scope.
Not soon after the Scope Self Calibration began to showing its great work, the old Wave Signal became visible on the CRT screen. See photo below.
Next Photos show that the 2440 Memory Oscilloscope really is working after the Built-in Self Calibration function.
And last but not Least here is a photo of the 50 Hz Sinus picked up by the Probe. 2mV/div.
So probably all scopes on Ebay with 6 Fails or less can be repaired or Fixed. Especially since this repaired 2440 Oscilloscope was completely dead on arrival (DOA). And now is totally back to life!
Albert van Bemmelen, Weert, The Netherlands.
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