Re: 7A26 Probe sense oddity


Hi Nigel,
The on-screen readout relies on very small currents to properly decode what letters, digits, symbols, and multiplication factors to use.

It is not unusual that the switch contacts that select these currents will build up oxides, dried lubricant, flux residue, or corrosive chemicals from the air over time. This can affect the currents enough to start showing random or erroneous information on the screen.

The usual solution is to cut blotter paper into narrow slivers about 2" by 0.1" and soak it in alcohol. Carefully slip the blotter paper under each of the contact fingers, turn the knob until the contact tries to close on the blotter paper and gently slide the blotter paper back and forth to dissolve the contaminants. Do this one by one for each contact finger.

Then using another bigger piece of blotter paper soaked in alcohol wipe down the rear connector of the plugin, especially the pins that the on-screen readout currents will be passed onto the mainframe through.

Dennis Tillman W7pF

-----Original Message-----
From: [] On Behalf Of NigelP
Sent: Monday, June 29, 2020 9:36 AM
Subject: [TekScopes] 7A26 Probe sense oddity

Had a strange problem earlier. I have a 7904A fitted with 2x 7A26 and I suddenly found the probe x10 sense wasn't working on just one of the CH1 PI inputs (the same probe was checked on all four PI inputs). Shorting the ring to ground produced the expected OSD IDENTIFY result but touching a 10K resistor there didn't toggle the OSD to x10 (I think 10K is the correct value for this).

I assumed an internal PI issue but randomly pressing the Identify button and switching both the CH2 polarity and the BW (for no particular technical reason.... they were just there at hand!) the problem just disappeared! A 10K resistor then tapped on the ring confirmed the functionality.

Just because it's now working again doesn't stop me from getting curious about what "fixed" it in case there's a next-time!

Anybody got some suggestion as to what might have been the issue? Obviously the probe sense function is based on the degree of current passing into (out of) the ring; it was working for identify, but not for x10.

Dennis Tillman W7pF
TekScopes Moderator

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