Re: Strange Tek2440 issue

Vince Vielhaber

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.


On 09/01/2020 08:55 PM, Siggi wrote:
On Tue, Sep 1, 2020 at 8:15 PM Vince Vielhaber <vev@...> wrote:

Ok, Q879 collector is solid. When things go sour, it stays. The base
voltage is moving, as is the emitter. So it's being told to compensate
for the change. However, under normal conditions, the collector voltage
is lower by a few hundred mv than the schematic calls for while the base
and emitter are within a few mv.

How are you measuring the collector voltage?
The switching frequency is ~25kHz, and it's going to be ugly as heck if the
capacitor is out, so the instrument you're using to measure this and its
bandwidth are quite important. The average or RMS voltage there just isn't
super important :).
What happens if you tack on extra capacitance from Q879 collector to
ground? You probably want a capacitor that's fairly low ESR, the ripple
current is going to be something wicked if the average current is 3A (see

Under normal conditions there's a
300mv drop across the shunt.
That looks sane according to the voltages annotated on the schematic.
That's a 3A nominal load, though.
U870A/B are the current/voltage controls, respectively, or-ed together
through the diodes. You can tell which one is in control by the voltage out
of pin 1/7.

Going to take a closer look at Q879's feed.
Have fun!

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