The 7854 readout is part of the display board. The inputs use the
same Tektronix custom ICs (analog multiplexor and analog to digital
converter) as the other 7000 series readouts but the output is
generated by the display board raster line generator as it is
processed by the microprocessor leading to the interesting display
antics during acquisition when interrupts are being continuously
processed.
Since the problem only affects specific character positions including
at least most if not all of row 4 on the character map, I suspect that
the readout problem occurs from the row decoder (U1220 155-0014-01) to
the binary encoder (U1230 74LS147). Possibly one bit in the data bus
driver (U830 81LS97) is bad.
Just reseating the chips could be enough to fix it if those icky TI
sockets were used, U1220 could be swapped with U1520 or replaced with
a decoder from another 7000 series readout board, or U1230 or U830 may
need replacing. A replacement 74LS147 should be easy to find for
U1230. U830 could be swapped with U930 and if bad, replaced with a
81LS97 or apparently the 74LS467 equivalent.
I do not see anything that would link this problem to the intensity
issue.
On Tue, 10 Jul 2012 08:54:11 +0100, "David C. Partridge"
<david.partridge@...> wrote:
The readout in the 7854 is radically different - it is part of one of the logic boards.
Dave
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Rob
Sent: 10 July 2012 02:49
To: TekScopes@...
Subject: RE: [TekScopes] 7854: Excessive stored trace brightness.
P.S. All the numbers above are for a 7904. I assume they are the same in a 7854.
David Hess