"Peter" wrote in message
news:JOyed.304002$D%.257789@attbi_s51...
Here is a copy of an email I sent out:
I have removed the cover from the scope to check out U800. With the unit
lying on its left side so the front is to the left and the back is the
right
and I'm facing the bottom, the IC (U800) was on the bottom of the board on
the Y axis and about the middle of the X axis. I turned the scope on and
waited for about the 3 min before the screen shuts off and as usual, the
same results; no picture after the 3 min. So I did some fiddling around
and
found that the 'beam find' still works because it puts a line about 2
divisions across nice and dark with a few dots around it most likely being
the digital read outs of the amplitude and time base that are normally
shown
on the screen. I shut it off to see how hot the IC gets and it did not
feel
that hot at all, it actually felt like it was running pretty cool. It was
only one for about 4min total though.
I turned the scope back on and still no picture which is a bit unusual,
usually I get something for about 10-30 seconds, but not a major problem.
I
sprayed the chip with some freeze spray and still no picture, I shut it
off
and back on, but still nothing. I sprayed it again, but still nothing when
I
turned it back on. I believe I repeated this about 4-5 times and only got
the picture to come on once for about 30 seconds. The IC was pretty cool
the
entire time from the spray. I noticed that after I turned it back on and
needed to push the 'trig a/b' button because of the error I get at turn on
(not a big deal to me right now), that the controls all work fine.
You are able to switch between AC and DC coupling, change channels, etc...
Basically the entire logic of the scope works fine and there is a glow on
the screen like some sort of picture is present. It's almost like there is
a
bright line off to the side reflecting onto the screen, but you can't
position it onto the screen so it just glows on the side.
I found a site that sells Tek parts located in Canada, but they are
currently sold out. I'm sure I can find someone that may sell this part.
But
right now my problem is, I'm not sure if that's the actual problem or not.
I
don't have a service manual for this scope nor do I know what signals I
should get in/out of this chip. Do you have anymore input?
The site I saw with the part I have (Maxim 155-0241-02) is:
http://www.sphere.bc.ca/test/tek-parts/tekparts3.html
Thanks
Peter
Having just gone through this exact fault finding procedure today, here's
what to check:
U735 (14 pin DIL package located to the left of U800) pin 3 should have a
ramp waveform between -1.3V and +1.3V on it, the ramp should be linear and
there is a delay between each ramp determined by the timebase holdoff /delay
settings. If this is OK, check the 2 outputs of U800 (pin 9 and the RHS of
R804 which connects to the - Y plate of the tube), these should be ramps
similar to the signal on U735 pin 3, but going between +28V and +46V, the
ramp on pin 9 is + going and the ramp on R804 is negative going (the inverse
of the pin 9 signal). In my case, the signal on U735 was correct, but the 2
outputs from U800 were correct for a few seconds then become very low in
amplitude and distorted. Check also for +87V on pin 7 of U800. U800 on my
scope ran only slightly warm but is almost certainly faulty.
I earthed the scope (the one displaying the waveforms!) and the meter to pin
7 of J119, the empty 14 pin DIL socket just above the pushrod for the power
switch, which is the PSU diagnostic socket.
Martin
--
martindot herewhybrowat herentlworlddot herecom