Re: Strange Tek2440 issue - SOLVED!

Vince Vielhaber

Ok, the problem has been solved!

As Siggi said, "this makes no sense". He was right, it made no sense. On the inverting input of U870A is the 4.99V reference. It measured 5.14V and the non-inverting input measured 5.15V (Fluke DVM). The theory of operation mentions that the 5V regulated supply is the same as the 8V regulated supply. So I made some comparisons and saw that on the 8V there was a 0.10V difference between the inverting and non-inverting, not so with the 5V.

As I mentioned before, I lifted one lead of both resistors (and a couple of others) and they were all within spec. Strangely enough, it took another 6.8K across R875 (a 6.19K) to drop pin 2 (inv) on U870 to 0.10V less than pin 3. Measuring R875 again (with a lead lifted) it was giving a new value.

Both resistors (R875 and R877) are now on order.


On 09/08/2020 11:50 PM, Vince Vielhaber wrote:
Ok, here's where I'm at.

ALL of the electrolytics, except the two big ones in the back, have been
replaced. I tested them and they were all good with very low ESR so
I'll toss them in a box for now.

Q870 and Q879 have been replaced. Had to order these.

All resistors in the 5V Regulator have been checked and are all well
within spec.

The few small caps (.001uf) were also checked and are right on.

Problem still exists.

Before I was toying around with the idea of hanging an external 5V
supply on it. I did try that last week and it did NOT do it and
according to the meter on the supply, it wasn't really drawing much
current from it - less than 100ma.

Tomorrow I'll go thru the supply again and take more measurements, there
were a few points I was looking at and couldn't remember if I checked
them and if so what they were. So tomorrow I'll check them and take
pictures and better notes.

That's it for tonite.


On 09/02/2020 02:50 PM, Siggi wrote:
On Wed, Sep 2, 2020 at 2:07 PM Vince Vielhaber <vev@...> 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.
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
designations are.
Even so, it still doesn't make sense, as the voltage-control op-amp
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
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 &
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.
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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