Agreed it is primitive, but it is an existing circuit put there by
Tektronix, and yes, the component tolerances are critical on the 0.1uF
capacitor and the 31.9K resistor - if they are off, you get a divide by four
or divide by 6 (typically). Here though it looks like anything from 3 to 7
based on scoping the output, and trying to scope the 0.1uF cap results in
nothing much that I can interpret as it won't trigger stably.

In this case, based on what folks are saying, is seems plausible that the
local supply decoupling caps (which are after all quite old by now) may be
dying/dead. Alternatively the 1mS input pulses my not be as regular as I
think (even though they 'scope up very clean).

Dave

"petrus bitbyter" wrote in message
l...

"Asimov" schreef in
bericht ...
"David C. Partridge" bravely wrote to "All" (14 Nov 05 20:38:19)
--- on the heady topic of " Monostable divide by 5 not working."

DCP Reply-To: "David C. Partridge"
DCP Xref: core-easynews sci.electronics.repair:348583

DCP If I were starting from a "blank sheet", I probably would use a
DCP digital divider chip or several. BUT, this is an attempt to repair
to
DCP a venerable piece of test equipment (Tektronix 184 time mark
DCP generator), which I'd like to keep pretty close to original ...

DCP TIA
DCP Dave


Geez, you guys, this student just wants us to do his homework for him!
Actually, the circuit is simply a monostable which is not retriggerable.
Basically, it's a pulse stretcher. It starts by discharging the 0.1uF
cap through the base resistor of Q2. Because the base is driven hard
negative, a series diode has been added to protect Q2 from reverse
bias breakdown.

The trouble with the circuit is that the input pulses might not be
properly biasing Q1 when high or low and then the trigger point is
constantly changing. Also if the input is driven through a capacitor
then its time constant may be further randomizing the trigger point.

A*s*i*m*o*v

... When I was your age, we carved transistors out of wood.



Homework? It's the most primitive divide by five counter I ever saw. Can't
imagine a master to invent such a thing for a question. Actually, as this
group is about repair, I guess it's an existing circuit. This type of
circuit is very sensible. Noise and/or disturbances on input or power make
it fail easily. It really needs uniform inputpulses. As I see no
possibility for adjustment *and* about all parameters of all components
influence working, I guess the components need to have high accuracy. The
original transistors might as well be specially selected. Aging of
components, especially the transistors, might be caused the problems. As
an aside, you may need to check or replace the decoupling capacitors of
the power supply near the circuit.

petrus bitbyter