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Tektronix 7603 Oscilloscope - Repair Log

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sean0118:

--- Quote from: David Hess on January 10, 2022, 02:38:08 am ---The DC restorer which levels shifts from low to high voltage in the z-axis circuit path could be leaking before failure.  You might inspect it in a dark room and see something.

--- End quote ---

Thanks, that seems quite possible, is the DC restorer in the HV module?

Since posting I have probed the Z-Axis test point with a scope (see pic below), I was trying to follow the 'Adjust CRT Grid Bias' instruction in the manual (pg 3-6). It seems there's meant to be a voltage offset at that point which is varied by the trace intensity control, however I only measure a constant 0V. But, varying the readout intensity does produce voltage spikes at that test point.

I think this might point to an issue on the Z-axis amp board? As a fault in the HV module would load down the line and effect the readout intensity too?

I've also ruled out a few other things, the plug-in modules work in the other scope, the intensity trim pot seems to be good (resistance measured at end of cable) and the CRT grid bias adjustment still varies the intensity.

sean0118:
I can't catch a break with this scope, I accidentally left it plugged in over night, but with the front switch off. Went to turn it on but the mains fuse was blown, put a new fuse in (bad idea I know) and it blew that one in a few seconds...   How is it possible for one scope to fail in so many ways?  :D


edit: One of the +18V rectifier filter capacitors is short, either C813 or C814

sean0118:
Okay, I got it powering up again, it was actually rectifier CR811 that was shorted, but I had already cut out C813 and C814 so replaced them too. They probably needed replacing anyway.

CR811 seems to be a D-72 package, I had some similar spares that were very close but 1mm too high. It gets sandwiched between the PCB and enclosure so the extra 1mm meant they wouldn't fit. I eventually found a suitable replacement from Vishay which is in the list below. I was under the threshold for free shipping so I ordered long life Kemet caps for C813 and C814, I also ordered a 15,000uF replacement for C811.


Vishay   VS-KBPC602PBF   200V   6A
KEMET   ALC70A153CD040   40V   15,000µF
KEMET   ALC80A103CD040   40V   10,000µF
SIBA   70-065-65/1.6A   Time Delay   1.6A

It powers up now but I still need to fix this z-axis / trace intensity issue...     ::)




sean0118:
Okay sorry to bump the thread again but I think I found the cause of the z-axis issue...   U99C is dead!

It's part of a CA3046 transistor array, the other transistors in the array test okay (just measuring BE, BC diode drops with a multimeter).

The exact same IC failed in my 7623A, maybe it's a common issue?
https://www.eevblog.com/forum/repair/tektronix-7623a-oscilloscope-repair-log/

sean0118:
Okay, replacing U99C fixed the intensity issue, well it still seems a bit unstable but the intensity knob does work now.

But I left it running for an hour and something else has failed... this is fault #7, must be close to a record...

The readout display has now collapsed vertically, and it moves with the trace position (I don't think it's meant to do that?). If I feed in the 4V ref the trace is the correct amplitude but I can't get it to trigger.  >:(

No Input:


4V 1kHz in (1ms/Div):

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