Re: 475A repair

ehsjr
 

On 1/4/2019 1:30 PM, satbeginner wrote:
Hi all,
for nostalgic reasons I wanted a 475A, just because it was the very first scope I ever touched in 1979.
My mentor at my first job made sure I would understood what a scope was, so he made me try, use and understand all features of this scope like delayed sweep, single sweep, add and subtract channels, etc.
Just before Christmas I found one that I could have shipped one in reasonable condition to me in Spain.
It was shown with two traces, so I felt confident about a possible repair.
When it arrived it was quite dirty, also it was very difficult to remove the cover, because it had several (small) dents in it, also the legs are crumbling already.
The surprise however was that the inside was remarkably clean :-)
No black soot near the high voltage cable, no dust, really clean actually.
There were signs of previous work, the cover of the channel two attenuator was missing, and also the cover over the high voltage board was removed and missing.
But still, when I turned it on, the traces appeared, like in the pictures, so not bad for a start.
I started cleaning the outside and I also cleaned all switches using alcohol and a brush, as described in the manual.
I also flushed the potmeters with alcohol, because they were very, very "touchy", just by looking at them traces would go everywhere.
I still need to lubricate them, but not quite sure what to use?
All this helped, and the traces became quite stable, but....
Now I could start testing the basic functionality, and it turned out that the delayed timebase was not working.
Tempted to go directly into that part of the scope, I did the smart thing (I think) , and started with the power supply voltages and ripple check.
Aha!! Funny thing number one!!
The -15V was down to -1,9V, while the rest of the voltages were very near, 50V was 49,9V , -8V was -7,98V, etc.
But: the scope (more-or-less) was working anyhow, regardless of the missing -15V??
Could this be a tantalum capacitor?? I checked the resistance as specified in the manual, and no, no shorts.
So I checked the transistors in that part of the power supply, and they were all OK, it turned out to be the relevant half of U1464, so U1464B.
I replaced it with a LM358 and behold, the -15V came back as -14,99V :-)
What about the delayed timebase? Hmmm, still not working.
Checking some stuff in that area revealed more broken parts:
The second broken part:
FET U930 had became a resistor, so I replaced that with a 2N5485 I had lying around.
And third:
Dual OpAmp U930 was gone as well, and yes, another LM358 brought the delayed sweep back to life. :-)
I will test further, and if and when everything looks OK, I will calibrate it.
Some pictures of my new scope can be found here: https://groups.io/g/TekScopes/album?id=81177
To be continued,
Leo
Nice. Sounds like you've got a good one. :-)

Ed

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