Re: TDS744A input failure.

 

On Mon, 24 Apr 2017 19:21:38 +0000, you wrote:

There are high impedance attenuators (3?) between the coupling
capacitor and the buffer circuits. If the measured input resistance
is 139 ohms at all vertical sensitivity settings, then the fault has
to be before the high impedance attenuators. If the input resistance
changes with different vertical sensitivity settings, then the fault
is after the high impedance attenuators.
The input resistance is constant - irrespective of the vert sens settings.
So I guess it's before the high impedance attenuators.
Yes, so I think it is before the high impedance attenuators.

The TDS520B service manual shows the 50 ohm termination before the
high impedance attenuators but I do not see how that could apply to
the 1 GHz versions.

Didn't you say that the baseline trace is stuck at ground instead of
having any significant offset? Maybe the 1 megohm shunt resistor
shorted somehow. If so that should be visible on the hybrid.
I don't think I said that - not sure I understand.
Is there a significant DC offset that changes or does not change with
vertical sensitivity.

Interestingly, I came across this in my searching :-

<http://www.eevblog.com/forum/testgear/tektronix-tds520b-attenuator-swap/>

He seems to have exactly the same issue - even to very similar resistances. But in his
case when he swapped attenuators the problem disappeared - which smells
of a bad connection to me.

Many thanks,
Dave
I am thinking that the relays are not switching and that the problem
is an open circuit between the relays and relay drivers so they are
stuck in an unusable state. At least that could be easy to fix; clean
the contacts between the module and printed circuit board.

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