Transistor keeps dying. Need suggestions (schema attached ) SOLVED

[mr ed]

I am restoring a little used tektronix tiny digital 222 scope. The z axis CRT drive transistors keep dying.
Not right away. It seems to take a dozen turn on/off cycles. ( I am adjusting battery charger).

The z axis data stream is 10v with 0.5us rise and fall. First failure , it half worked and I changed both mmbt5501  to restore a working display.

Again, after a few more power cycles, it's dead again.  Pen down 100%.

These transistors are rated 160 v cb ce. Supply is good 120 vdc.  The scope is 95% SMD, all tiny and compact but spotless being sealed.

The z axis circuit drives a 2kv rated SMD cap modulating the electron beam. There is -1600 vdc on the other side of that SMD cap, I measured it too.

Original design had no clamping diodes (bad idea Tek guys) . My early version 222 had no clamping diodes so I neatly added through hole diodes.  This is still killing transistors.

Any ideas?  Data sheets for tr and diodes here...

https://drive.google.com/drive/folders/1urmiwkh-uDJXATBpUcBoPrcSbcubWjtp?usp=sharing

[RoGeorge]

Breakdown reverse voltage of any BE junction is very small, of only 5...10V or so.  CR408 should protect Q405 against reversed Vbe voltage.  Is that diode OK?

[BillyO]

Also, is C414 in good shape?  Any leakage could be very detrimental.

[mr ed]

Bad solder connection to R416, 1/2 Meg open,  should be a copper trace. .
Cap 21M ohms in circuit. Working now by accident but needs solder reworking. There is No room in there to work.