Re: Strange Tek2440 issue

Vince Vielhaber

I had actually done that last nite, but didn't have the parts layout handy to see which probe was on which pin (I had them on the diodes>.

Here they are with voltage measurements:


On 09/02/2020 01:25 PM, Siggi wrote:
On Tue, Sep 1, 2020 at 10:06 PM Vince Vielhaber <vev@...> wrote:

Both the voltmeter and the 2430A. There's not much garbage on it but I
did hang another cap on it and it made no difference.
Here is a shot of both sides of R873 on the 2430A. At power off, both
traces are together. The top one is the at the junction of R873, R876,
Q879 E. The lower one is on the C494 side. This was caught just when
the sweep was turned to 100ns/div. I quickly hit Save when it happened.

I guess next up is to capture U870 pins 1 & 7, see what's up there when
your rail collapses.
On one hand, you could be hitting the current limit, which could be because
of actual overcurrent, or because the resistor network on U870A pin 2 has
drifted. Also the -5V rail could be compromised, as the current limiter is
referenced to that. Also check all the other rails, as the +5 needs the +8
to be up and sound as that's where Q870 draws from.

On the other hand it could be that Q870/Q879 have degraded and can no
longer do the lifting required.

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