Re: PG506 repair question.


Andy Warner
 

The first round of candidates for Q745/755 in the PG506 high amplitude
output section arrived today.
I have uploaded screenshots using the BD179G parts.

https://groups.io/g/TekScopes/photo/265034/3246294 is 10us period,
amplitude max, terminated into 1MOhm.
https://groups.io/g/TekScopes/photo/265034/3246295 is the same, but
terminated into 50Ohm.

The good news is that the full amplitude output is now restored, I think
Ozan's current-based analysis was right on the money here, and without the
ability to dump the current with both transistors, the full output voltage
swing was not possible.

Note the rise and fall times, which preclude the 1us period being any use
at all - the amplitude of the output signal is limited by the rise and fall
times.
This is clearly shown in https://groups.io/g/TekScopes/photo/265034/3246296

I think I clearly need to look for faster parts than the BD179G. That has
an F(t) of 3MHz, while the venerable old D40E7 has F(t) of 230MHz.
I think I have one more chance at reworking the Q745/755 pads before I
start lifting traces.

As an aside, the aberration on the falling edge at low frequencies (which I
now accept as a fact of life) goes away when terminated in 50Ohm.

On Tue, Jun 15, 2021 at 11:44 AM Andy Warner <andyw@pobox.com> wrote:

I have another off list confirmation that the falling edge of the high
amplitude waveform, at least on first gen PG506's, routinely has an
aberration as shown in: https://groups.io/g/TekScopes/photo/265034/3242704
The exact period & amplitude it kicks in at varies between units, but for
the next person that trips over this: it seems like it is just part of the
design, and the rising edge is the one that matters for this particular
unit's role in life.

On Mon, Jun 14, 2021 at 9:14 PM Andy Warner <andyw@pobox.com> wrote:

Thanks for the additional verification, and advice on diodes.

Given the data points from the group, I plan to ignore the falling edge
artifact, and focus on getting the amplitude correct once I have
replaced D745/755 with modern parts.

On Mon, Jun 14, 2021 at 8:07 PM Tom Norman <jtnorman@fire2wire.com>
wrote:

HI Andy-

I wanted to follow through on my earlier response. My PG is serial
B035037, so similar to Dan's I think. It also shows the same negative
going edge distortion you show in your posted picture at 100 Hz, and
pretty
well cleans up around 250 Hz. Output voltage into 1 Meg is 64V P-P, at
maximum amplitude setting, across the frequency range of the
instrument.

I too saw the reference to the rising edge being the "reference" edge,
so
after repairing the PS issue, I just accepted that they (Tek designers)
didn't care too much about what was happening to the negative going
edge
at
longer periods. It's good to know that at least two of you are seeing
a
similar thing. My ignorance is keeping me from understanding why the
designers would have chosen to regulate the entire switching supply
rather
than just locally regulating a fixed -72 rail.

And on the -72 supply diode replacement, I ended up just trying some
1N914
diodes, which seem to have worked well. Also used those to replace
CR80
and CR125, which had taken an absolute beating as a result of the
strange
switching transients that resulted from the "backflow" due to the slow
rectifier recovery times. 1N4148 seems like it would work too.

Tom






--
Andy





--
Andy





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Andy

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