7B80 sweeping off the screen


My R7603 mainframe came with a non-functional 7B80 plug-in. Back then I bought a 7B53A and happily used the scope until few weeks ago when I came across the 7B80: time to fix it, or find a new home. I am asking for guidance because it doesn't seem too broken: it triggers, but no trace is drawn on the visible area of the screen. I have no other oscilloscope, only analog and digital multimeters. Frequency counter, signal generator and VNA won't be useful in this mission.

First of all, on the "+ sawtooth" output in the back of the mainframe, the working 7B53A produces a down sloping negative signal, sweeping between -4.5V (trace @left) and -5.5V (trace @right). Measured on the low-ish impedance analog voltmeter.

OTOH the 7B80 produces an up going positive "+ sawtooth" signal between +1V (theoretical left) and +10V (theoretical right). Its frequency follows time/div changes. Trigger is honoured (no sweep in single-shot for example).

Which reading is correct?

If I turn CRT intensity to the max I see the green halo coming from the right and it flashes off when the beam is swept back. In these conditions, pressing the "Position" button shows the cursor leaving a trace entering from the RHS of the CRT and a dot is positioned a couple of divisions right of the timing value compressed position. So the beam is trying to sweep somewhere behind the second vertical plug-in bay :)

I have run the 7B80 base without Q344 and Q346, therefore having the sweep signal (TP345) pass through R343 and off to "+ sawtooth" output. The result is a negative sloping signal between -2V and -3.5V on the "+ sawtooth" output, so much closer to 7B53A's values, but not enough to see a trace.

So far I have tested Q344 (hFE~=50), Q346 (hFE~=50), Q458, U416, all apparently good.
Not owning a plug-in extender I found a trick to measure voltages inside the 7B80 in-place, and +50V, +5V and -15V picked near Q344/Q346 stage measure OK, few 10's of mV low and no sign of AC.

Why on Earth do I get a reverse and amplified sweep?! If voltages were somehow reversed the whole plug-in would be totally dead. Or the 7B80 isn't meant for a 7603 mainframe?!

Simply put: help!

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