TEK 7704A with dual sweep funky??

I have two 7B53As and one 7B92 sweep plugins.
In a 7704A, they all have similar problems.
There's no pretrigger leadtime, can't see the leading
edge of the waveform.
The trace unblanks about 4 nS after the edge. If I crank
up the intensity enough to see that 4nS, the rest blooms badly.
Twiddling the trigger level control moves moves the edges all
around during the first 150 nS or so. The sweep isn't even
monotonic in that first 150nS. The sweep actually reverses
in 16nS intervals creating a bright spot and severe distortion
if you move an edge through the region. Acts like there's some
serious ringing on the first part of the sweep ramp.
There's nothing in the cal procedure about compensating this
region.

I've got three 7704As that all exhibit this problem.

A 7B80/7B85 combination in any of the mainframes works flawlessly.
I've got 20 nS of pretrigger visibility. No nonlinearity of the
sweep. No blanking issues.

So, is there a failure mode in the dual sweep plugins that can
be corrected? It's hard to believe they could ship 'em this way.
Fixes? Mods? Suggestions?

Thanks, mike

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