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A tale of two Tektronix scopes

Hello all,

I have two scopes that are giving me fits. A 2445 and a 2465 DMS. They both have the same failure mode - F1330 on the power supply which supplies

-15 UNREG to the A9 high voltage board keeps opening up. I have isolated the problem to the A9 board. I disconnected the -15 UNREG to the A9 board and supplied -15 volts to it with an external power supply current limited to 500mA. I Then checked the feedback circuit for proper operation which uses a voltage divider from -1900 volts to U1956A using another external power supply injected at R1945 and R1944 and verified that Q1981 and U1956 are functioning correctly. There are no shorted semiconductors on the A9 board. The only thing that I can see that is left is the operation of U1830 high voltage module or the CRT. If I disconnect the CRT the problem persists.

I guess one question is whether it is a valid test to disconnect the CRT. On a working scope would this cause the A9 board to misbehave and blow F1330?

Has anyone seen this failure mode and figured out what the fault was? Is this a common failure mode in the 24x5 scopes?

Many thanks for any advice.

Reply to
JW
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JW wrote in news: snipped-for-privacy@4ax.com:

A bad CRT -could-,but that's a long shot.Rare.

the HV multiplier may be bad. "common";no. it happens,yes. You can disconnect the XFMR feed to the multiplier and see if your current stays within limits. Sphere in Canada may have one,or you may have to cannabalize one from another 24XX scope,or build one.

--
Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

Would you know if the A9 board can run properly with the CRT disconnected? Is it a valid test for ruling out the CRT?

Ah, I think I see what you mean - disconnect everything between the transformer and the U1830 high voltage module EXCEPT what's needed to drive the -1900 volts, right?

That sounds like a good plan...

Thanks for your help Jim!

Reply to
JW

JW wrote in news: snipped-for-privacy@4ax.com:

yes,it can.

Just unplugging the CRT anode lead is sufficient. If your HV multiplier is OK,the CRT anode plug tip will have a considerable charge on it,so ground it to insure it's discharged,and DONT let it swing across any PCBs!!

Yes.Usually,there's a single wire to a terminal on the side of the HV multiplier,and sometimes (depending on the particular scope circuit)there's a HV diode from that terminal to the PCB for the cathode voltage. You can lift the wire and diode off and leave those connected so the -1900V is there for feedback regulation.

You're welcome.

Did you download the schematics from the BAMA website? Having the HV circuit diagram will be very helpful.

--
Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

No discharge was seen when I disconnected it..

Yup.

I finally picked up another 2445 that had a defective A1 board so I'd have a donor for the A9 board. I swapped the A9 board into the 2465 DMS unit and it ran fine. (Man is that thing a bitch to put back together!) So, I think the problem may be in the multiplier. Now I just need to find another donor... Thanks again for your help, Jim.

From what I've read of your posts over the last few years or so, you seem to be the expert on Tektronix stuff - I guess it helps that you used to work for them :) Perhaps I can pick your brain on another one I'm trying to fix. It's a 2467B; the readout display is working OK, but no trace can be seen unless you turn the intensity knob a bit - then the trace can be seen for a second or two and then it fades out. It doesn't matter whether you turn it clockwise or counter-clockwise. If I constantly twiddle the knob over a 20 degree arc the trace stays on the screen. Weird. I was thinking maybe the Z-axis circuitry, but would this suggest something else to you by any chance?

Once again, many thanks for your help.

Reply to
JW

JW wrote in news: snipped-for-privacy@4ax.com:

2467B has an image-intensifier CRT(photomultiplier tube),and there's a protection circuit that dims the beam if set at certain conditions to protect the phosphor from burning. I can't recall if there's a way of setting the time before the circuit dims the trace;you'll have to check the manual for that.(I have no manuals) There may be a problem with that circuit or the CRT bias is set wrong.(or the CRT is nearing it's end.)

Unfortunately,I have very little experience with the 2467 series("Brighteye" scopes).I didn't see many of them in the shop.

--
Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

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