Re: SG 503 Display Problem

Mark Wendt (Contractor) <mark.wendt@...>

On 8/12/2012 9:08 AM, David wrote:
On Sun, 12 Aug 2012 07:50:21 -0400, "Mark Wendt (Contractor)"
<> wrote:

while poking and probing is some odd voltages on U440 - pin 1 showed
164mV, when it should be 4.0V, pin 5 showed 178mV when it should be .1V,
pin 6 showed 4.2V when it should be showing 4.0V and pin 9 showed 173mV
when it should be showing 4.2V. Would that have an effect on the
display when the selector is above 1 MHz?
These signal change during normal operation depending on the
frequency. The schematic is marked with voltages for one particular
displayed frequency.

Note that Tektronix had a habit of leaving unconnected pins on 74
series logic instead of tying them high which is why the J inputs on
U440 show no connection.
Those voltages were checked with the Freq set to the ref 0.5 MHz
position, amplitude multiplier set to X1, Freq variable set to midrange
and Output Amplitude set to 5.5V, as called for in the manual for the
voltage and waveform checks.
You mean 0.05 Mhz, right? I see the same voltages on mine at that
setting. The difference between .178 and .1 and 4.2 and 4.0 are
insignificant for digital logic if it is not switching and just depend
on normal variations.

Pin 9 might be significant except I measured 170mV which does not
match my schematic either.

Incidentally, the display problem I had on mine where it does not read
correctly at 100 MHz and above came back for a bit. It is
intermittent and pushing on the board or looking at it funny fixes it
so I swapped out a bunch of the TTL logic with LS and HCT parts.

Yes, .05 MHz. That's what happens when you try to read small lettering without your cheaters in front of your eyeballs. ;-)

That seems odd though since at those settings, pins 1 & 9 should be reading 4.0V and 4.2V respectively, according to the schematic. I figured the pins 5 & 6 would be close enough for government work, but included them to show what I read off those pins as a possible help in troubleshooting.

Hmmm, back to the DMM and scope for some more poking and probing.


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