Colin,
I don't claim to be an expert on storage CRTs but I spent a surprisingly pleasant couple of weeks tracking down the several problems on a 7623A which not only had some real faults but also had the storage board wrongly wired up after removal by the previous owner. The performance of the 7623A was so good that when I spotted a 'spares or repair' 7633 at a bargain price I thought I would have another go. I did get it working but the storage tube was well used and its writing speed was not up to the (good) 7623A.
The methodology I used was to set up a repetitive store and erase cycle and follow the waveforms that the manual shows for each of the storage electrodes and each storage mode. The waveforms are complex and rely on some logic and 74123 monostables to set up the timing and then the logic switches different voltages into the op-amps that drive the relatively high voltages on the storage electrodes. Between the two scopes I must have replaced four of the 74123s and one or two 74xx logic ICs. The other failure common to both boards was (one or more of) the set of four transistors that control the 600V line (Q1728 ..Q1743) and resistors around these had drifted significantly in value. The 7633 also had internal tracking of the PCB traces (four layer board) in the vicinity of the high voltage area which I finally managed to fix by drilling and filing until I had cleared the leakage and adding jumpers for the tracks that had been removed in the process.
Having a digital storage scope to follow the erase and store cycle made life a lot easier than trying to remember what had just flashed on the screen and wouldn't return for a few seconds. A wild guess based on your symptoms would be that one of the 74xx logic ICs has failed but finding the culprit may take time.
Regards,
Roger
