Re: Help with 7A18 problem

Albert Otten


Your suggestion should work I think but only helps when the 7A18 Input Amp itself is out of order or can't be trusted. In that case it would even be simpler to test the 7A18 stand-alone, not in a mainframe. With small signal levels, say less than 0.5 V, there will be no leakage yet via conducting semiconductors (like CR210) and no harm can be done to the FET Q210A. The only load is formed by the probe tip.


Very good that you solved the problem and explained what the problem was. With my suggested V/div check I overlooked that the cam activated contacts are not the only possible cause of troubles. I could have known better since I have had similar problems with 7A19 or 7A29. I have a dozen of 7A18/7A26 but never felt he need to look inside a switch compartment. Now I opened a 7A18N which seems easier, missing the long strip PCB over one compartment side. But in the 7A18N the compartment is completely closed. Hence it wasn't immediately clear to me that there was just one PCB inside the compartment and that this pcb has switch contacts at both sides of the PCB, in this case "left" and "right", not "top" and "bottom".


On Sun, Mar 3, 2019 at 07:53 PM, Colin Herbert wrote:

Something that occurred to me today, after what Albert pointed out about the
5mV/DIV setting being possibly compromised by dirty switch contacts, was that
it is possible to see if the attenuator assembly has any problems by putting
an appropriate signal into the input and a scope probe on to the output, which
is the junction of R130, C210 and R210 or the equivalent components on the
other channel. This of course requires either an 7000-series extender or some
other way to access the 7A18 plug-in with a probe, plus another
fully-functioning scope. Turning the VOLTS/DIV switch will allow examination
of the signal-path, to some extent. This could be achieved without an
extender, with one scope of the 7000-series having two vertical amplifiers,
by removing both the side-panel of the DUT amplifier and the left side-panel
of the mainframe. If the DUT amplifier plug-in is in the left bay, then I
think it would be possible to connect the scope probe from the working
vertical amplifier plug-in to the DUT one? You wouldn't need to be accepting
the signal from the DUT amplifier, though this may be of use. Is there any
sense in my suggestion, or have I got it completely wrong?


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