Heathkit IB-1103 frequency counter only counts squarewaves?

[KI4YAN]

I'm having a hell of a time using my ahem, NEW frequency counter. Works fine, twitchy sometimes, but it only counts when you give it a squarewave to listen to. For the life of my, I can't get it to count anything but a good sharp square.

Any ideas on where to start? I have the manual and a very large copy of the schematic, but finding parts may be difficult...

I have a simple 6AG7 crystal osc running great, nice clean sine, no harmonics unless you tune the plate tank to them, but the Heath still won't count it. i've tried capacitive coupling, a wire hangin out the front of the counter, a schmitt trigger to try and square the waveform up for the counter, but nothing has worked.

[KI4YAN]

well, after consulting the screwdriver and the manual, i found a spot where the counter input board had been on fire...

seems like that could be a reason for it to act funny...

EDIT: after repairing the fire damaged section of the board as best as can be done, it appears i've done no damage, but no good either. Still only counts squares...

[w3jn]

Look at the input section.  There are probably a few transistors that amplify and square up the incoming signal.  TTL doesn't like sine waves so there's a fair amount of pre-conditioning that oughta be done.   99 times outta 100, freq counter problems are in the front end where Hammy Hambone connects the counter input directly to the plate supply of his leenyar or something.

Also see if there's a 7413 Schmitt trigger on the input chain somewhere, if bad, it could cause trouble like this.

Git at 'er with a scope and trace the signal thru, while feeding a sine wave into it.  You should see where the problem is pretty quickly.

[KI4YAN]

Well, after putting a ringing squarewave to it, I have traced out what I *think* has happened.

As long as the edges are clean and there is little to no ringing, it counts fine. As soon as the ringing takes up more than 50% or so of the squarewave tops and bottoms, it stops counting.

The Schmitt trigger, not in this case a 7413, but instead a 741 configured as such, is not working.

I'll swap the 741 and see if that fixes it, if not, I'll just have to trace it down. Gonna be a pain since I don't have that special heath extend-o-card...

[WA1GFZ]

a 741 is as slow as a mule. Get yourself an op amp with some speed. It may be configured as a comparator. Someone must have blown the front end and replaced the op amp with the wrong part. There must be 100 different parts that will drop right in with the same pin out.

[KI4YAN]

Nope, heath had it there for a reason. It's not my problem part!

Turns out that someone had replaced ALL of the old ECL logic chips with newer ones. What I missed during my first, rather sleepy investigation, was that one of the pins was broken off the flip-flop that precedes the Schmitt trigger. Isn't this what the Derb calls a "Hammy Hambone" moment?

Lo and behold, I had one of the esoteric 95H28 chips in my junkbox...from god knows where.

So, I just plugged it in. Seems to work fine now.