Re: 2235 Trigger on one channel

Kevin Clark

Hi David

I have checked the resistors in the feed to the 3102E chips as suggested with
the following results. I checked them in circuit on the basis that I'm looking
for a difference between identical circuits with one circuit that's working and
one that has a problem, I hope this logic is ok and a sound basis?

So:-

Res Designation Ch1 (design value) = In circuit value, Res Designation Ch2
(design value) = In circuit value

R314 (576Ohm) = 424Ohm, R339 (576Ohm) = 424Ohm
R315 (576Ohm) = 424Ohm, R340 (576Ohm) = 427Ohm
R317 (1.82KOhm) = 1.082KOhm, R342 (1.82KOhm) = 1.084KOhm
R318 (1.00KOhm) = 779Ohm, R343 (1.00KOhm) = 777Ohm
R319 (1.58KOhm) = 1.024KOhm, R344 (576Ohm) = 1.026KOhm

I don't see any real difference between the two circuits to indicate a problem
with the resistors on channel 1, I am assuming the difference to design is the
in circuit influence.

So I considered that as these circuits are identical and if we believe that
potentially IC 3102E is bad on ch1 then my logic would say if I can remove the
chips successfully then swap IC 3102E from ch 2 into ch1 and see if problem goes
with chip? Any danger of damaging the chip other than physical damage of course.

Thanks

Kevin

On 16 May 2017 at 17:02 "David @DWH [TekScopes]"
<TekScopes@...> wrote:

On 16 May 2017 14:03:52 +0000, you wrote:

>Hi David
>
> Been a little busy so took a while to get back to this. So today lunch
> time I started to measure some voltages with my DMM as suggested,
> results as follows, something a little amiss (I think?)
>
> U555B Output pin 6 ch1 = 0.139V (0.2V) (0.06V error)
> U555B Output pin 6 ch2 = 1.476V (2.5V) (1.02V error)
>
> U310 Output pin 1 ch1 = -3.26V (-3.4V) (0.14V error)
> U310 Output pin 1 ch2 = -2.443V (-2.1V) (0.34V error)

You can check the values of R314, R315, R317, R318, and R319 around
U310 but I think the answer is going to be that U310 is bad.

The signal into pin 1/11 is suppose to be low when channel 1 is
selected. When that happens, the output of U555B is disconnected
(pins 1/11 of U310 should not draw any current) and should reach all
the way to ground which is what we are seeing and U310 should be
working but it is not.

When the signal into pin 1/11 is high, channel 2 is selected but pin
1/11 draws more current from U555B. In this case, it is drawing too
much current indicating that either U310 is damaged or R314 and/or
R315 are open.

The fact that U555B is not reaching its full output is irrelevant
because under that condition, U310 should not be forwarding the
trigger signal anyway.

> I also looked at the feed from the A3 Board Connection as follows:-
>
> Pin 18 (6.2V) = 6.5V (0.3V error), this also shows (0V) for XY but can't
> see how to check this on front panel.
> Pin 32 (0.0V) = 0V
> Pin 33 (5.0V) = 4.996V
> Pin 34 (5.0V) = 4.996V
>
> Some of the values are slightly higher / lower than they should be, is
> this the issue?

No, nothing is wrong there.

I wonder if there are any good sources for CA3102 transistor arrays. I
do not see any so if you cannot find one, blast me an email with your
mailing address and I should be able to mail one to you. I will have
to pull it from a junked board but will test it somewhat.

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