Repairing a Tektronix 2445A oscilloscope
I bought a Tektronix 2445A oscilloscope from eBay. It was a very good deal for a 4-channel 150 MHz oscilloscope. Maybe too good a deal, or so it seemed for a while...
The seller used the usual eBay boilerplate disclaimer for electronic gear — they had turned it on, no smoke came out, it lit up, but they "were unable to test further". It was guaranteed not to be "DOA" ("Dead On Arrival"), which probably means only that smoke shouldn't appear and it should "light up" when you turn it on, and nothing more.
The problem was that the scope would not focus! The very best, with the focus control turned to the end of its range, was as pictured. The trace was at least 1.5 cm wide, and the on-screen indication of vertical scale and horizontal timing was nothing but pale green smudges instead of readable characters.
I had not placed a bid on the item until after I had
found a PDF file of a service manual.
Places to find service manuals include
Those were discovered by
for something like:
tektronix 2445 "service manual" filetype:pdf
So, I had a service manual and it was time to get to work...
The alignment procedure includes setting the trace focus as sharply as possible, and then using the astigmatism control (a small screwdriver-set pot below the display) to further refine the setting. Yes, the astigmatism control had some effect, but it seemed as if I needed another 90-180 degrees of rotation on the focus control to get the job done.
|YOU SHOULD NOT READ BEYOND THIS POINT.|
|ABOVE ALL, YOU SHOULD NOT DO WHAT I DID.|
|IT IS VERY VERY DANGEROUS AND YOUR LAWYER WOULD NOT APPROVE.|
|BUT IT DID FIX MY OSCILLOSCOPE....|
I needed some Torx drivers — T-20, T-15, and T-10 got me through the entire project.
I removed the six Torx head screws and
then removed the rear panel.
I tipped the unit up onto its back end, and very carefully lifted the oscilloscope out of its case.
I was looking for the high voltage power supply. An aluminum cover panel must be removed to expose it. This requires two sizes of Torx drivers.
At left: The high voltage power supply at top, under the aluminum cover.
At right: After removing that cover — four T-10 screws and one T-15.
Tracing the control voltage from the wiper on the focus control into the high voltage power supply board, I found that the control had no easily measureable effect on the output of focus amplifier transistor Q1852. The output focus voltage was fixed at approximately -300 V DC. Further testing showed that the precision 332kΩ resistor R1854 had become an open circuit and now was a fairly precise infinite impedance....
With R1854 opened — effectively removed — Q1852 was no longer correctly biased. And so that transistor was not controlling the output voltage as designed. Here you see the circuit biasing Q1852, click on the picture or here for the full page from the service manual.
Above is the view from the opposite side. R1854 is in the row of resistors between the white-topped transformer and the red potted HV circuitry.
I did not have a 332kΩ resistor handy, but I did have two 680kΩ resistors that I could stack in parallel. The original quarter-watt resistor was asked to dissipate about 0.19W continuously, and this was probably the cause of its failure. Since I would be using two quarter-watt resistors in parallel, I should have much more thermal leeway.
You can see my repair here — the pair of 680kΩ resistors is stacked vertically in the original R1854 position. The repair sequence was:
And that fixed it! A friend later told me that he had had a nearly identical experience on a different piece of test gear. A precision resistor had failed to an open circuit and disabled the instrument's display. It isn't very clear why the design uses a precision resistance (in a circuit with multiple potentiometers!) instead of a more rugged device.
Vinny, N2LWN, email@example.com, found my page through Google and sent me the following:
Does anyone have any idea about how to fix that? If so, please contact Vinny, and also copy me so I can add it to this page!
This is from Jeremy Linton, and it sounds like the fix to at least part of Vinny's problem described above....
Someone posted to an electronics forum:
The conclusion — "Press A/B Trig to Clear" comes up when any of the power-up tests fails. Routine Test #05 is done on the main board, code #44 means that the trigger level is just a little too high.
Here is the sequence of power-on self tests and their meaning, from Chapter 6 of the Service Manual.
|01||Interrupt Request||01||Interrupt request is missing or has wrong period.|
Trigger COUPLING up
Trigger COUPLING down
CH 1 Coupling down
CH 1 Coupling up
CH 2 Coupling down
CH 2 Coupling up
Trigger SOURCE down
Trigger SOURCE up
Trigger MODE down
Trigger MODE up
A/B TRIG select
Shift register failure
Readout RAM failure
Read parity error (bit 0 set)
Bad read after write (bit 3 set)
Bad checksum (bit 4 set)
AUTO LVL failed to trigger
Negative level not negative enough
Negative level too negative
Positive level not positive enough
Positive level too positive
As someone suggested, failure in the voltage test (05 XX) indicates that the voltages are outside the specified ranges. Check the power supply voltages at the empty 14 pin DIP socket near the sequencer chip, as in the service manual. If a voltage is low, suspect the electrolytic capacitors on the power supply.
Then there was speculation on the longevity of the unit — Tektronix said that it was built for 2,000 hours of operation. As for finding the number of hours of operation so far, see Appendix A of the Operator's Manual. A simplified version is:
As for the tests, EXER 05 displays operating time
and power cycles:
> HRS ON nnnn OFF/ON CYCLES mmmm
where nnnn is the accumulated number of operating hours, and mmmm is the accumulated number of power on/off cycles.
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