Well it should have been obvious why the HP neon/photo-resistor decoders were different, the two designs are needed because the earlier ones used 1,2,2,4 binary code and the option 002/003 & 5248M I have use 1,2,4,8 binary code. Something to be aware of if boards have been swapped & the readout is incorrect.
I've confirmed the repaired high A18 readout board from the 5248M works, using a separate power supply to provide around -13V to -14V to drive the binary inputs (temporary wires added).
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1121604)
Hopefully the parts from Sphere will fix the lower speed display board I borrowed the decoder plate from.
Just need to sort out the remaining fault(s) with the A21 function control board, it seems to be passing a signal to the display counters when the gate is inactive and any input (or the internal check signal) is very intermittent when the gate is active. The area in question has had a lot of repair work done in the past.
Back at work tomorrow, so won't get much time to carry on with this till the end of year shutdown.
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David