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In older Elekta machines the charging of the Pulse Forming Network is controlled by the clipper circuit.
At a trigger point a thyristor activates the de-Qing transformer. When this thyristor is constantly open there is no de-Qing. This results in a very high dose-rate. When the thyristor is short circuited the de-Qing is maximum and dose-rate is very low.
When replacing the thyristor, pay attention to the correct replacement. not every BLW92 is a BLW92-1600.
On the scope: Look for the clipper-pip. Test if it moves sideways when you vary the HT Charge rate a little bit.