Re: Another scope 7854

Joe Laffey

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

On Thu, 4 May 2017 21:14:41 -0500, you wrote:

Trigger off "Mode" (in between Ch1 and Ch2) also has the glitches
"Mode" means that the trigger source depends on the vertical mode
Yes. Thanks for clarifying.

I will keep digging around... any insights appreciated.
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

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 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.

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?

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.

Joe Laffey
The Stable
Visual Effects

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