Re: Strange Tek2440 issue

Vince Vielhaber

On 09/02/2020 02:50 PM, Siggi wrote:
On Wed, Sep 2, 2020 at 2:07 PM Vince Vielhaber <> wrote:

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:

Uh, this makes no sense to me.
The base of Q870 should be pulled toward +15V unreg through R864(? hard to
read on the schematic), and the two op amps shunt it towards ground when
they detect too high voltage or too high current, respectively. Whichever
of pins 1/7 is lower is the op-amp in control of the output.

... time passes ...

Yeah, still doesn't make sense to me. I'm going to guess that either the
op-amps are (pins 1/7 are) reversed on the schematic, or that your channel
designations are.
Double checked, followed the foil, Channel 2 is definitely on pin 1.

I got the power supply board out right now so it's definitely going to see some new caps. I'll also take a real close look at Q870 and Q879.

I measured R478 with a 4 wire ohmmeter and it's 10.01. Haven't checked any of the other ... yet.

I did look at the +8 and _15v rails as well as the 5V unreg and the +10 ref. All were ok with both the voltmeter and the scope. While the board's out I'm also going to track down U870's inputs to see if something's backward in the schematic.

More later!

Even so, it still doesn't make sense, as the voltage-control op-amp starts
dialing more voltage (Channel 2 normal at 5.54V rises to 16.94V), while the
current limit op-amp goes on the limit (Channel 1 normal at 16.76V dips to
6.16V). What doesn't make sense here is that the current control op-amp is
dialing a voltage that exceeds the voltage control's usual/normal voltage.
I haven't done the maths, but as there's a bias on the current limit
op-amp's feedback input, it might be that it's simply kicking in because
the output voltage has sagged.

I think you need to look further afield, how's the +8V rail doing,
the +-15V rails, raw and regulated? How is the -5V rail doing? Any other
rails you can find...

If you can't find a bad rail, start looking at the biasing for Q870 & Q879,
have the resistors drifted? If R864(?) or R478 have drifted significantly
upward, that'd explain why the transistors can't meet the current demand.
If the biasing is OK, you need to look at the transistors themselves. If
their gain is down, that'd also explain things. You can measure the base
current to Q879 indirectly by the voltage drop over its base resistor. Even
looking at what's happening on Q870's emitter under collapse - is that
transistor going to saturation, indicating that Q879 is dozing off on the

Good luck!


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