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Waveform in the schematics look like with a 10x probe based on waveform picture at EXT TRIG IN port.

Manual shows at base of Q309 signal has 10x50mV=500mV peaks as far as I can tell. Scope shot "07 19 38" shows ~200mV peaks and it is not clear if tunnel diode action is happening (could you DC couple for future scope shots?). I think you mentioned sweep works with a sharp edge but not with sine wave. A sharp edge doesn't need tunnel diode action as much as sine wave.

The circuit biases VR304 very close to 10mA (trip point) with no signal. If VR304 is in a socket you can use an ammeter instead of it and measure DC current with no input. Perhaps one of the components shifted or left sides of R300 and R306 are at wrong DC potential and stealing current. It may be as simple as adjusting the blocks prior to R300/R306.

When it is operating, signal coming from trigger amp pushes the tunnel diode VR304 to its high voltage region, and creates a fast spike at bases of Q309 and Q317. Q317 collector produces a pulse, and triggers VR358 through CR319. VR358 waits at the edge of trigger (current through R357+R356) until this time, and Q328 is off. When anode of VR358 jumps to high voltage region (~ 0.5-0.6V? depends on the tunnel diode) sweep is running. Q360 collector signal disables Q526 and sweep starts. When sweep hits ~ 10V Q408/Q409 and the network of devices in hold-off circuit send signal back to CR320/VR322, Q328 turns off and puts VR358 back to low voltage region for next trigger.

If you don't see ~ 0.5-0.6V at anode of VR358 at sweep rate (~10ms in test condition) it could be because it didn't get triggered, or Q328 is pulling it low. If Q328 is in a socket you can remove it to test if something in Q328 path is the problem.

However, most likely issue is wrong DC operating point of VR304 since you already verified VR304 is good and with a large enough signal you can get sweeps.

Does single sweep work?

Ozan

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