Re: 485 No Trace - HV Resistor

Robert Simpson

Is there a good way to test a thin film HV control resistor set while in circuit?

Finally getting back to my 485 repair after fixing a major leak in the primary water line to our house under the concrete driveway. Working on the 485 is much more fun, and also some progress today.

I decided to check all the diodes in the restorer circuit area. (I used a simple single trace set up, 12V AC through a resistor, with scope in XY mode). They were all good until I got to the last diode to test. It was bad. (see photos in "485 no trace" folder). After replacement, the HV is now stable although too high. Originally the HV jumped around a 300-400V range. Now its stays steady at 3350V. Should be 3KV +-50.

The manual indicates the thin film resistor set controls the HV, (similar to 7603's I have worked on). Is there a way to test these resistors in circuit? I do not want to unsolder it unless I am sure it is bad.


--- In, "Robert" <go_boating_fast@...> wrote:

Thanks for the tip. It will help. I just bought up that part of the cicuit to look at. In fact I am just printing out the layout for note takeing.
Note, nothing on beam finder either.

--- In, David wrote:

Are the trigger and sweep occuring? The A Gate signal signal from
schematic 6 triggers flip-flop U1535 which controls pin 15 of U1560
and should be doing something.

In chop mode, U1585 generates the chop timing which should show up at
pin 4 which blanks the CRT and pin 13 which toggles the displayed
channel. I doubt the origin of the chop timing is the problem but it
is easy to understand and check.

On Fri, 01 Feb 2013 00:42:53 -0000, "Robert"

The difference at Pin 5 was just a front panel button. When set the same, both scopes have the same value. Although Pin 15 comes from the delayed sweep control, both scopes are set the same. Now trying to figure out how that circuit works.

--- In, "Robert" wrote:

Funny, as I was thinking about this while falling asleep, the thought of a stuck front panel switch came to mind, like being in single trace mode or something. It will take me some serious manual reading to figure out the switch/signal circuit effects.
--- In, David wrote:

I suppose that is good news since U1560 would probably be difficult to
replace. On the other hand, that just moves the problem into the
inscrutable oscilloscope logic.

I would take a close look at the horizontal and vertical mode switches
because they are subject to mechanical failure and those pullup
resistors. Those are easy to check.

Sometimes a pushbutton switch can conduct a static discharge into the
circuits and damage them so I would check any logic ICs directly
connected to them like U1530, U1535, and U1580.

On Thu, 31 Jan 2013 05:39:01 -0000, "Robert"

Quick test: I tried IC U1560 in the working scope and got a trace, so it looks like some input is keeping it in blanking mode. On to more circuit analysis and tracing.


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