TEK 485 - need help


ehsjr@...
 

Intensity way too bright & no control of it. Details below.

A while back I posted on this scope. It completely died, but I found
and fixed that problem. It was caused by a leaky .001 uf cap in the
cathode regulator. (C1611) Replaced the cap, & it powered up - but
I have no control over the intensity. It is far too bright. All voltages are ok,
and it displays the proper waveforms. With no input and no trigger,
it creates a VERY bright spot on the left of the screen. Documentation
tells me the grid is supposed to be more negative than the cathode - but
does not give me a voltage. I believe my problem is in that area, but I
don't know how to test. I have (repeatedly) tested every diode in the
grid and focus DC restorer circuits, and checked every cap for leakage.

Can I connect one lead to the cathode and the other to the grid with
an 11 meg input on my DMM, or does that load down the grid too
much ? And, if I can, how much difference should I see? My
"HV probe" is a home made ~ 100 meg string of ten 10 meg resistors.
With it, I see the same voltage on the grid or cathode with respect to
ground. Varying grid bias has no effect on the intensity, nor does
varying the intensity control change the intensity on the screen. However,
varying the intensity control does change the amplitude of the signal
at the anode of CR1660, while varying the grid bias has no effect
on that amplitude. (As seeen with a separate 475 scope.)

I'm lost. Any help would be appreciated, especially what to look for
at 1653 and TP 1665.

Thanks,
Ed


Jeff W <vwthingy@...>
 

You stated that you checked, (repeatedly) the DC restorer diodes.
How did you check them?? They can only be tested with a curve tracer
or some other method that applies a few hundred volts to them, as the
common failure mode is breakdown under high voltage. (not shorting,
as simple diodes/rectifiers tend to exhibit)

DC restorer diodes on the 485 were one of the most common failure
items.....

Good luck, the 485 can be a bear to troubleshoot.

Jeff


ehsjr@...
 

Jeff W wrote:

You stated that you checked, (repeatedly) the DC restorer diodes. How did you check them?? They can only be tested with a curve tracer or some other method that applies a few hundred volts to them, as the common failure mode is breakdown under high voltage. (not shorting, as simple diodes/rectifiers tend to exhibit)

DC restorer diodes on the 485 were one of the most common failure items.....

Good luck, the 485 can be a bear to troubleshoot.

Jeff

Jeff,

Thanks!

What a great group!! I had numerous responses in the thread and
via E mail. Thanks to you all: Anand, Dennis, Rolynn, Deane,
David, Jeff.

From the many responses directing attention to those diodes,
that must be the source of my problem. I'll order a bunch
and replace the ones in the scope. My testing was with
a DVM - didn't know I needed to use higher voltage. Nice
to learn something new - and especially to have hope that I
can fix the thing.

Ed


Denis Cobley <denis.cobley@...>
 

Best way to test them is to replace all of them.
They are often found faulty after almost any test shows them good(learned
this the hard way).
Regards
Denis Cobley
www.newteksupport.com

----- Original Message -----
From: "Jeff W" <vwthingy@...>
To: <TekScopes@...>
Sent: Saturday, August 21, 2004 11:41 AM
Subject: [TekScopes] Re: TEK 485 - need help



You stated that you checked, (repeatedly) the DC restorer diodes.
How did you check them?? They can only be tested with a curve tracer
or some other method that applies a few hundred volts to them, as the
common failure mode is breakdown under high voltage. (not shorting,
as simple diodes/rectifiers tend to exhibit)

DC restorer diodes on the 485 were one of the most common failure
items.....

Good luck, the 485 can be a bear to troubleshoot.

Jeff





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