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Tektronix 7603 Oscilloscope - Repair Log

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--- 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.

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

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...     ::)

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?

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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