Yes - I was thinking it was digital noise coming from the readout character generation circuits.
I tried the switch in the GATED position and that does indeed eliminate [almost] all of the gaps in the trace. Oddly, there are still two or three gaps that stay in the same position along the trace. Furthermore, adjusting the HOLDOFF on the time-base moves the gaps along the trace in "steps". I'm not sure if that's normal but perhaps it is.
I like the readout in this mode. Yes, it gets odd with slow sweeps but when in FREE RUN, the readout characters bounce/wave under certain waveforms and input frequencies. I think someone (maybe that was you?) that told me there are past posts regarding how to fix that but I don't think I ever found that (or wasn't able to get it adjusted properly). In any case, in GATED, that's eliminated (albeit it becomes a bit of an issue having the time-base selected by the GATE SELECTOR switch).
While looking through the manual, I found that removing Q3416 causes the system to display full rows in the upper and lower rows allowing proper centering, etc., of the readout characters. Fun stuff.
Thanks again,
Barry - N4BUQ
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----- Original Message -----
From: "Harvey White" <madyn@...>
To: "tekscopes" <[email protected]>
Sent: Thursday, December 30, 2021 9:59:43 AM
Subject: Re: [TekScopes] 7704A - Noise on sine-wave traces
There's likely no relationship between the chopping frequency and the
sweep speed.? I'd go for random.
You're thinking that this behaves like digital, perhaps?
Harvey
On 12/30/2021 10:46 AM, n4buq wrote:
Hi Harvey,
I'll see if that resolves the issue. The way that the blanks move along the
trace, it seemed a more random effect than I'd expect when writing to the same
locations each time but maybe not.
Thanks!
Barry - N4BUQ
----- Original Message -----
From: "Harvey White" <madyn@...>
To: "tekscopes" <[email protected]>
Sent: Thursday, December 30, 2021 9:42:50 AM
Subject: Re: [TekScopes] 7704A - Noise on sine-wave traces
Do not do anything.
There is one beam.? It has to be "borrowed" to write the readouts.
There are two ways:
1) Borrow the beam when it's needed.? This chops tiny holes in the
trace.? it's going to be obvious at some sweep speeds, but will work on
slow sweeps.
2) refresh the readouts on the sweep retrace.? This eliminates the
"noise" but for slow sweep speeds, you will get a flickering readout.
There's generally a switch on the readout board to control this behavior.
Harvey
On 12/30/2021 10:14 AM, n4buq wrote:
When displaying a sine-wave where several cycles/division are present, when the
READOUT is switched on, there is noticeable "noise" (for lack of a better
definition) in the trace. It appears as tiny blank spots in the trace that
move rapidly along the trace. The noise is not in sync with the triggering and
appears at many points along the trace. Switching the readout off stops the
effect.
The power supply has fresh filter caps as well as the horizontal board and the
HV board so I don't think this is PS ripple and more likely digital noise from
the READOUT circuitry but that's just a guess.
I've looked for places in the READOUT board and/or interface board where there
might be bypass filtering components that might have been designed to eliminate
this but I don't see anything. I do note that switching on the READOUT
connects it to the +15V rail and possibly that's the source of the noise.
I've mostly observed this with sine-waves but it might be present with other
waveforms. As a side note, with mostly square-waves, with the GRATICULE
ILLUMINATION turned up, I can see a much smaller presentation of similar noise
in the trace. This is much less noticeable but could be related.
Any ideas what I can look for as a cause for this?
Thanks,
Barry - N4BUQ
