A tale of two Tektronix scopes

[JW]

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.

 

[Jim Yanik]

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?

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

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.

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.

 

[JW]

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

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

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.

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...

Sphere in Canada may have one,or you may have to cannabalize one from
another 24XX scope,or build one.

Thanks for your help Jim!

 

[Jim Yanik]

Would you know if the A9 board can run properly with the CRT
disconnected?

yes,it can.

Is it a valid test for ruling out the CRT?

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!!

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?

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.

That sounds like a good plan...


Thanks for your help Jim!

You're welcome.

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

 

[JW]

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!!

No discharge was seen when I disconnected it..

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?
Yup.

Having the HV circuit diagram will be very helpful.

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.

 

[Jim Yanik]

No discharge was seen when I disconnected it..


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.

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.