I've had the "pleasure" of fixing way too many 5370A and 5370B front panels.
It's been a while so these musings may be clouded... also beware of the two
or three different front panel designs. Also there are "hybrid" 5370A's out
there with 5370B front ends.
There are 4 pots on the front panel: the 470K display update rate, a 10K or
200 ohm arming level (200 ohm on 5370B and later 5370A's), and two 5K trigger
level. All are linear taper, all have switches, all are small pots with
(1/4"? bushings). Finding pots with proper switches ain't easy.
I have never had to do the trigger level pots. They have metal shafts and
seldom break. Their switches are what you would expect (closed when the pot is
turned clockwise). In the far CCW direction the switch is open and selects the
0V trigger level.
The other two pots are an exceedingly crappy hp design. They have fragile
plastic shafts and really poor quality switches. The switches need to be
closed when the switch is counterclockwise. HP's pots have both normally open
and normally closed contacts. Frankly these pots are seldom used and you can
use a switchless pot and just hardwire those pads.
The display rate pots is particularly useless. You can just hardwire it to max.
The biggest bugaboo in buying a 5370 is the state of the HP custom input
amplifier chips. They can be damaged by overloading the input. Blow one and
you are screwed. Buy a 5370 with a blown input amp and you are screwed. The
5370A and 5370B used different chips. The 5345A counter is a source of input
amp chips.
It would be VERY nice if some clever person built drop in replacement "chips"
built of modern SMD parts on a DIP header. Those input amp chips are socketed.
They are little more than a comparator.
Quick check of the 5370A inputs is to set the unit to "SEParate" inputs and
connect the back panel 10 MHz ref signal to each of the inputs. The signal
level LED should flash. A steady LED means that input is toast (or the input
PCB slide switches are full of navel lint and need cleaning).
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