Monostable divide by 5 not working.

Wow, been awhile. It sounds as if you need a 5 stage ring counter. (pausing remembering just how to do it)

Using 5 J K flip flops you hook all the Ts together and in a ring (thus the name) you tie the J to the Q and the K to the not Q of the previous flip flop.Output at any of the five phases can be tapped at any of the Q terminals for a 20% on time, or the not Q for 80% on time.

How far in the cowbwebs of my mind did I have to go ? The last time I thought about ring counters was when I was builing a bootleg cable decoder for the old SSAVI2 scrambling method.

There are also programmable dividers if you want a one chip solution. With 7 terminals per FF, you only can get two in a 16 pin DIP so this means three chips minimum. If you go with the ring counter you don't use the other two, C and cD I think, you should consult the datasheet for whatever chips you use to see whether you should ground them or leave them float.

To sum it up, digital division is the only way to get reliable operation. If you wind up with three dual flip flops you can also use the last one to get on times other than 20% if desired.

Good luck with it.

JURB

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

TIA Dave

You've probably checked the obvious but is the supply to the divider the right voltage, well decoupled and free of glitches/ripple?

Dave

"David C. Partridge" bravely wrote to "All" (14 Nov 05 20:38:19) --- on the heady topic of "Re: 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.

Actually, no, not a student.

D.

Thanks, will check that again, may well be relevant. All the other countdowns are working (at least all the way from .1uS to 1mS). It's the

"Asimov" schreef in bericht news:MSGID_1=3a167=2f133.0 snipped-for-privacy@fidonet.org...

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

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