Quick and dirty and you might get lucky. Swap out the tubes in stage 4 and stage 5 and see if the issue moves to stage 5.
If it does fix it my diagnostic fee is $85. If it doesn't fix then tuff shit and get troubleshooting.
You greedy old man !!!
I understand you have to pay for your new A/C unit, but still !
I swapped the tubes but no joy. So troubleshooting it is.
I got somewhere, fixed that stage, now moving on to the next kaput stage.
So.... here goes.
Now we understand how the dividing effect is produced, I could make more sense of what was going on.
This defective 50us stage was showing either 40us intervals, or a blurry / unstable picture... which I calmed down using the single shot feature of the scope, in digital mode.
This revealed that the blurry image was not due to an unstable signal as such, but rather to the fact that the scope could not present me with a stable picture because... the signal was not periodic / recurrent.
That is... the interval between two markers /spikes would be 40us then 50us, randomly, no "pattern".
But now, we know taht 40us is not just any number, it's a 4x10us, the preceding stage output.
So I started to wonder if all that was wrong was that the monostable RC time constant was not adjusted properly, was too close to 40us and therefore every now and then, would divide by 4 rather than 5.
So I checked R and C. C is a tiny 47pF thing. Pulled one leg so I can measure it reliably. Used my WaveTek 27XT DMM as it's more of a little LCR meter than a mundane DMM. It can measure pF range caps with precision and reliability. Turned it on... without me even doing anything, it was already registering 47pF, bang on ! But I had not connected the probe to the cap yet !
It's just the parasitic capacitance of the probes. You can null it out but I have a brain capable of sustracting 47 from whatever the meter would measure. So... I got 100+ pF when I meausre the cap. Minus 476pF, so the cap is about 50+ pF. Close enough, so soldered it back onto the ceramic strip.
Next check ' R '. Made of the trim pot in series with a fixed 2Meg resistor. the latter measured spot on but the trim pot did not. 1.65Meg for 2Meg nominal. According to the parts list, it must be 20% tolerance, so technically it should be good but.. well it's close to the edge.
So at this point, seeing as modifying the time constant / RC value might get me somewhere, and seeing as the the fixed R anc fixed C were good, and only the pot was on the low side... I though OK maybe the problem is indeed the RC value and maybe I just need to tweak the pot. So, I did. I am glad. It fixed the problem(S).
First, I had to make myself a big isolated flat head screwdriver as I don't have any and didn't fancy shorting things out in that kind of instrument ..... so I shove a piece of heat shrink tubing around my screw driver, leaving only 1mm of exposed metal, just enough to be able to stick the tip of the blade inside the slot in the pot shaft. That worked a treat.
I moved the pot from end to end to see what it did. doing this, I can make that stage divide by 3, 4 or 5. It could then see that my unstable signal was indeed due to the divder hesitating between 4 and 5. Once you put it firmly onto the ' 5 ' side of things, the markers become perfectly stable and set on 50us as they should !
Also. I noticed that the "ripple"/undulation effect I described earlier, where the amplitude of the markers was "waving".... well I can reproduce this, and make it go away, at will. It happens when the divider is transitioning between 23/4 or 4/5 pulses. It's just trying to make its mind up. Just add some more trim to make it lean firmly to were you want it to go, here ' 5 ', and the waving effect disappears, all markers now are the same amplitude/height, perfect !
So, I was able to make the this stage just perfect, cool. However, I had to crank the trim pot all the way clockwise. Markers look good but I don't have much headroom/safety margin. I am tempted to increase the value of the fixed resistor a bit, to give me a bit more margin. But for now it looks OK, crossing fingers.
So that's all excellent news ! Not only is it fixed, but cost me nothing, and learned how to cure the instability and "ripple" effects.
So, now that this 4th / 50us stage is working, let's see if that improved the following stages !
It sure did : next one, 5th, 100us, which previously showed 40us and was unstable, now works just fine !
So, next one still, 6th, 500us : ahem... doesn't work. It does produces markers, strong ones, good amplitude, and stable but... time interval is 100us not 500. Also, the trim pot has ZERO effect what so ever. As if it was disconnected or something.
So it's not deviding anything.
Next stage, 7th, 1ms works fine, but of course it produces time intervals 5 times shorter than expected since that's what it's fed with... so not its fault !
OK, so now let's see if we can fix this 500us stage then.... stay tuned