Re: Another scope 7854

 

On Fri, 5 May 2017 01:24:00 -0500 (CDT), you wrote:

yOn Fri, 5 May 2017, David @DWH [TekScopes] wrote:

The 7000 series uses a separate internal trigger path which originates
inside the vertical plug-ins and the dual trace plug-ins have their
own internal trigger channel switch which is shown on schematic 4.
This is what allows the horizontal slots to use any of the 4 channels
as a trigger source no matter what is displayed.

My guess is that there is a problem at the trigger channel switch
having to do with channel 2. Maybe the levels going into pins 4 and
14 are wrong or one of the parts connected to pins 8 and 9 is open or
shorted.

I doubt the channel switch is bad but U270 and U470 can be swapped to
find out.
I will try this. I did re-seat the two ICs.

The fact that the glitch waveform appears like it is being triggered by
the opposite slope I wondered if it didn't have to do with the inverting
part of the circuit.... The bias network around R428 perhaps (between
Q426 and Q526)? Note I have Option 6 (the DC offset).
I have seen this before where the waveform edges were distorted by a
lot of ringing. This might be produced if one of the active stages is
saturating or clipping.

I am curious why enabling DC Offset coupling seems to fix the issue (I am
not 100% sure it fixes it completely; I may have seen the glitch once with
it on). All that DC Offset switch does is add DC bias to the base of Q520.

Also, with the invert switch turned ON (ch 2 inverted) the glitches occur
MUCH less frequently, like every 2-5 seconds vs. 1-3 times per second.
In both cases, the operating point of the active stages changes. The
position control does the same thing; does it have any effect on the
glitches?

I did see that the base of Q426 was about 4.6V DC, rather than the 3.2V on
the shcematic. (in non-invert, as described in the manual). If Q426 and
Q526 were near the edge of cutoff perhaps it would cause this issue?
There are two version of the 7A18. The schematics show that the older
design has +3.2 volts at the base of Q426 and the newer design has
+4.7 volts at the base of Q426.

The same signal is applied to the signal channel switch and the
trigger channel switch so if it was distorted at that point, then I
would expect it to show up on the screen.

With the traces centered, I would compare the DC voltages between the
two channel switches with different channels selected to see if one
obviously has the wrong operating point.

I would also apply a square wave to channel 2 and measure the waveform
at the output of the trigger channel switch and at the collectors of
Q480 and Q580 to see if it is distorted. The signal levels at those
points are high enough to allow this.

FWIW the voltage rails in the plugin are spot on.

I did finally realize that most transistors are indeed in sockets as you
or another list member said. At first I thought those sockets where just
the old plastic spacers that hold transistors off the board. I was wrong.
Anyway, re-seating made no difference.
Those are Berg Miniserts. If you are interested and cannot find the
datasheet online, then I can make it available.

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