Re: 475 questions


ciclista41@...
 

Hi Chuck,

No, ever since the experience with the supposedly bad tantalums that were awful according to my GMI-236 "in circuit" ESR tester, but fine out of circuit (I returned that item), I have not depended on in-circuit measurements unless I've "lifted a leg" first. The measurements I included in my recent post were out of circuit.

Bruce

On Mon, Jun 22, 2020 at 03:18 AM, Chuck Harris wrote:


It sounds to me like you are using one of those transistor/capacitor/ESR/
SCR/diode/inductor/... testers on in-circuit components.

You shouldn't do that!

These cheap Chinese/Ebay testers are definitely not suitable for in-circuit
testing. Hook a storage scope onto the terminals and look at what they are
doing to the part. They are applying significant voltages, typically ramps
of +/- 5V to the part to see how it behaves, and voltages like that will
activate most of the semiconductor junctions in the scope, and certainly
will be confused by parallel combinations of parts.

The only testers that are truly suitable to use in circuit have to use less
than a diode/transistor junction voltage in their tests, and of course, they
can't test diodes and transistors that way...

ESR meters, like the Dick Smith variety, apply a couple of tens of millivolts
signal to the capacitor at 50KHz to do their testing.

-Chuck Harris

ciclista41 via groups.io wrote:
Hi folks,

Here's where I'm up to. All six large capacitors have been replaced, along
with the two tantalums that I pulled, thinking they were bad based on
in-circuit tests which turned out to be wrong--they were good. Made sure I
had correct polarity.

The large capacitors were good except for C1412 and C1414. C1414 was the
worst. It reads 0L on my Fluke 87-V for capacitance (same as shorting the
probes together). My "transistor tester" shows it as 12.59 nF with 14% vloss.
The C1412 still shows 399 μF (rated 360 μF), but the "tester" shows 347 nF,
6.5 Ω ESR, and 3.8% vloss.

Okay, so in replacing all these, I used Alex Cuoghi's PCB's. I wanted to
make sure I hadn't created any shorts due to poor soldering (my soldering
looks pretty good to me, but just being careful), so after soldering the four
pins into the PCB's, I checked for continuity among the ground pins to the via
where the negative of the cap goes, and from the positive pin to the positive
via. All good in all cases. Then I soldered the caps in place, followed by a
re-checking of appropriate continuities. Finally, I soldered the PCB pins to
the A9 board, making sure each was in its appropriate location and rechecking
for continuity and no shorting. Then I noticed that I had not replaced the
CR1412 rectifier before installing the C1412, so had to pull that again,
install the CR1412, then reinstall the C1412 cap. Again checked for
appropriate continuity and lack of shorts.

Time to plug the scope in again. I had a 200W bulb in series with the plug
as a current limiter. Switched on the scope, and immediately switched it off
again, as the bulb lit up brightly and there were sparks at the C1412 where
soldered to the board. Sounded like frying bacon, but this is not supposed to
be a MIG welder! Figured I had somehow created a short with my soldering, so
pulled the C1412 again. No sign of any burning, so sign of solder out of
place, everything still looked and measured as expected. Even pulled the cap
from the PCB with no sign of anything wrong. Shrugged my shoulders and put it
back in the board. Tried again with the same result. Pulled again (I know,
poor board, but it seems to be handling it all gracefully). Again, so sign of
my having screwed anything up. Let it sit for a day, hoping I'd think of some
reason it could be my fault that there was a short. Full disclosure, I may
have dropped a screw through the hole in the middle of the bottom of the
board, but I don't think so. Turned the scope every which way while shaking
it and there was no rattle and nothing fell out. The only other thing I could
think of was that I may have cooked the CR1412 with soldering it, but I doubt
that, and it did measure as good before I put it back in.

So, I'm guessing that there is a short somewhere in one of the boards, and
the fact that the C1414, which is in series with the C1412 that gets so upset
when I switch on the scope, was protected from that effect by the fact that
the old C1414 measures in the MΩ range, and maybe the short was what killed
that capacitor in the first place.

Any ideas?

I don't yet have a Variac, but one should arrive in the mail tomorrow or the
next day. Alternatively, I have a DC power supply that is fixed current or
fixed voltage with up to 10A and 30V. Not sure where to feed sub 5V into the
circuit to try to find the fault. Also I tried pulling all of the power
connections to other boards and turning it on, but still got a sizzle at the
C1412.

Bruce





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