Hello all,

I bought this oscilloscope last weekend, it's the first one I've
owned. I have a basic understanding of electronics, my interest
mostly relates to keeping older stereo gear running, my own and the
stuff on permanant loan to family. So I usually am only opening
things up to change lamps and clean the pots. Anyway, I wanted to try
to gain a deeper understanding of how circuits work so I decided to
keep my eye out for a scope and now here I am.

The scope seemed to work fine with one exception: The intensity
control for 'A' (it has seperate controls for 'A' and 'B') seemed
wonky. When A was turned all the way down, there was still a trace
visible, which didn't seem right. As you turned it up it would stay
at the same intensity and then at about 40% it would suddenly get much
brighter.

I did a web search and found a couple of people who had issues with
the intensity control on the 22xx series of scopes (though none of
them had the same symptoms as mine), the problems ranged from a
defective/dirty pot, to a broken wire, to an open resistor, to a
problem IC (LM324). I also learned that this scope actually does a
POST when you turn it on and it will throw up an error message if
anything is majorly wrong. I also read the advice to keep a fan
blowing on it while the cover is off, which I have been doing.

I found a copy of the schematic (I have linked to excerpts here, the
whole service manual is 121 megs) and decided to do a cursory
examination; this also would give me a chance to see if the inside was
filthy which I was planning on checking at some point (actually, it
was very clean).

http://i.imgur.com/fu46CN4.jpg (intensity controls)

Well my problem wasn't a wire since the pot was soldered directly to
the CRT control board. I checked voltages at the 'A' control pot,
fine. R505 was nearby and was fine. It took a while to find LM324,
it was a mile away on the main board, half of it was poking out from
under a plastic cover that covered the high voltage section:

http://i.imgur.com/NnnRNvI.jpg (hv cover)

http://i.imgur.com/oCOyLOz.jpg (hv cover removed)

So I checked all the resistors in the above schematic associated with
'A' and they were all okay. However I did notice a resistor that was
burnt. Checking the board layout, it was R1027. It was at this point
I noticed the board layout in the service manual was different in
places than my actual unit.

http://i.imgur.com/abfYQAN.jpg (board layout)

This resistor is on this schematic:

http://i.imgur.com/AzCwCjt.jpg (power)

It's listed as a 0.2 W he

http://i.imgur.com/YYGMdZ0.jpg (parts list)

On paper it is supposed to be 10 ohms, it measured at 27. I check
R1026 to see if it matches the schematic and it is 10 ohms and it
looks fine (it's right next to the two 10uf caps in the picture). To
see what effect replacing this resistor would have, I put a couple of
resistors in parallel until I had 10 ohms total. The only effect
adding and taking away these resistors had was moving the trace
slightly up and down on the screen.

Now here's the thing - at some point during all this the intensity
problem seemed to fix itself. The trace would now go completely off
with the dial turned down all the way and it would get progressively
brighter as the control was turned. Okay, I was happy to leave it
alone except that I figured I should replace the resistor. I assumed
it got too hot being under that shroud. I took out the old resistor
(which flaked apart) and cleaned the area - the trace at one end was
blackened. I don't think I'm the one that caused this but I'm not
sure. The old resistor was too burnt to tell if it was original, it
was the same size as the others though.

Okay, now here's the actual problem - I replaced the resistor with a
0.5W one for good measure (the blue one just to the right of the
LM324):

http://i.imgur.com/oCOyLOz.jpg (hv cover removed)

I turn the unit on and it doesn't complain. The controls are still
working fine. I check the new resistor and it's very hot. Measure
the voltage across it: 4.6v. If my math is correct, that's 2 watts.
Turn it back off and let it cool. Back on to check voltages:
Incoming voltage to new resistor is +7.4v to ground. LM324 pin 4 to
ground is 2.7v. Voltage pin 4 to pin 11 (across IC) is 10 volts.
Voltage pin 11 to ground is -7.43 volts. Voltage after R1026 is -7.46
to ground. I desolder one end of R1026 and verify it is 10 ohms.
C1006 (the yellow guy near pin 7 of the IC) measures 47 ohms with a
multimeter. Shouldn't it read open? For reference, C1005 measures
about 100 ohms.

I'd just like to have an idea of what the problem might be before I
change any more components. Now I'm half wondering if it's the IC and
the fact that it's facing up and has a fan on it is allowing it to
work properly. Anyway, thanks for reading my long story and I'd
appreciate any suggestions you have.

This is where I downloaded the service manual:

http://elektrotanya.com/tektronix_22.../download.html



















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