Tektronix 475 - Still Stuck

Short Trace

So I am still waiting on parts to fix the B Sweep and to make progress with the B Trigger. Last time I was working on this odd problem where the start of the trace is lost when running at the three fastest sweep rates.

I'm pretty stuck. The problem is it sort-of works so there is no component that is clearly broken. Something in there is marginal but I'm not sure what.

Things I got wrong

I got a couple of things wrong when I was looking at this last time. First of all I was looking at the Z BNC on the back for output but this is in fact an INPUT <forehead smack>.

The second thing is that the signal coming out of J1333 is a current signal not a voltage signal. This is why the waveform looks really messy (depending on where you look). This is also why it looks quite different at Q688 if I disconnect the coax.

Theories

Here is the circuit diagram again as this will be useful here

Unfortunately I have a few theories but none of them seem to be leading anywhere. What it worse is that I tend to cycle between them as one theory becomes less likely! The theories I have examined (and none look good so far) are:

1. There is a delay introduced somewhere (bad connection, open circuit resistor, bad capacitor). Looking at the signal before Q1338, after Q1344 and even at the output makes me think this is not the case.
2. The gain on Q1338 is somehow too low so the output is not climbing fast enough. I haven't totally discounted this one and someone on the Tek forum suggested this also. They suggested I check C1330 in case this was broken as it could lower the gain of this amplifier but unfortunately it checks out. I also checked all the resistors, diodes and inductor in that area and they check out. I checked the voltage across R1342 is zero when beam finder is not pressed so the diode doesn't appear to be leaking. The voltage drops across the CR1343 and CR1341 make sense.
3. Q1354 and Q1358 are not turning on fast enough. They both have baker clamps across them to improve their turn-off performance (to prevent them saturating) but this won't help their turn-on time. I thought perhaps if the diodes were leaking this could slow down the turn on but both checked out fine. I even pulled CR1354 as it looked suspect but it checked out Ok when out of circuit.
4. The charge 'boost' isn't functioning. Ok I'm not sure I understand this circuit but essentially the capacitor C1358 gets discharged through Q1358 and Q1354 when the push-pull drives the output high. Q1362 will charge the capacitor up again when the output if the Z amplifier goes low. I presume the capacitor is there to provide a sharper rise curve which is why I looked at this. I can see the voltage on C1358 fluctuate but it seems to drop fast and then bounce back up again (getting charged via R1348). If I look at the voltage on the emitter of Q1362 it drops and then rises like a capacitor charge curve so I have to presume this is working. I even replaced C1366 as I couldn't test it (1pF) but it made no difference. Interestingly if I remove Q1362 from the circuit entirely it makes no difference to what is displayed on the screen! I don't understand the voltage trace on C1358 at all really - I would have thought that it would go low and stay low until recharged by Q1362.
5. The 6,2V Zener diode (VR1362) reads about 6.0V. Interestingly there is a small chunk of the glass missing and the soldering on one of its legs looked dodgy. I touched up the soldering but given it is a 5% tolerance component 6.0V is still within spec.

So overall I am pretty stumped! Help!