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Tektronix 7603 Oscilloscope - Repair Log

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sean0118:
Following on from the success of the Tek 7623A repair I thought I'd have a look at this 7603. This one came with a 7D01 Logic Analyzer plugin installed.

This thing is super dirty and crusty in some places inside, so I think one of the first things I'll do is clean it up a bit. I checked the voltage rails and they were out by maybe 1% but trimming the -50V brought them back within range.

Although, it seems this scope does have some major issues. The trace is doing some very strange things with and without plugins installed. Turning the Intensity knob has no effect, but turning up the Readout Intensity knob causes a strange pattern I don't like the look of, if you know what causes it let me know.  ???





With no plugins:



With plugins from 7623A installed:



With the Readout Intensity turned up:









Board locations:



Power rail test point Locations:



A5 Vertical Amplifier:



A6 Horizontal Amplifier:





A8 Z-Axis Amplifier



A3 Readout Board

sean0118:
Okay, I made a little bit of progress. The vertical amplifier outputs are way off from what I would expect. It's not a square wave, and it's way too slow (period should be ~1ms). The period doesn't change when I change the time base div either.

But, turns out there's massive 100Hz ripple on the -15V! it's pretty much dropping out to 0V then going to -20V. The multimeter was measuring about 14.8V which I thought was good enough, just shows I should have connected a scope to the power rails earlier.

I suspect the main bulk storage capacitor for the -15V may have gone bad, which would explain why it's 100Hz (rectified from 50Hz). But I guess there could be a faulty component pulling down the -15V too.

Vert Amp Outputs:





-15V rail:

wn1fju:
I've been burned many times by voltmeter readings when checking power supplies, since the voltmeter will just give you an average value.  I always look at the power rails with a scope even if the voltmeter readings seem perfect.  In fact, even if a piece appears to be working correctly, it's always a good idea to consult the oscilloscope.

bd139:
This will be a fun one. Whole power supply probably needs recapping. Then there’s the game of hunt the tantalum capacitor to play when it still doesn’t work. This journey is well worth it though. Good luck.  :-+

There’s a good thread here regarding the power supply and using adapter boards for the capacitor https://www.eevblog.com/forum/testgear/tektronix-76037613-repairs/

David Hess:
My first suggestion was going to be massive ripple from the power supply because of worn out input capacitors; I have seen it before.

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