Tek 180A update :OK quick update, making progress.
I just fixed the 10ms stage.
I swapped tubes and no joy. Well it was the easy thing to try first.
Checked the value of every resistor. A few are slightly off tolerance but nothing that would cause catastrophic problems like I am witnessing. So I let it pass.
So I fired it up and started to check waveforms.. yes, real troubleshooting at last, yeah !
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/xcool.gif.pagespeed.ic.cOxV6sPNRd.png)
It confirmed the symptoms I was seeing yesterday :
- Output to the front panel no signal.. so it's not a wiring or switch problem, there is ACTUALLY no signal, and we know it's not the tube at fault since I swapped them with that of a know working stage.
- Output wired internally to the next stage does work, but sends pulses/markers of the wrong interval, that of the previous stage, 5ms, instead of the 10ms this stage is supposed to produce.
Schematic shows waveform on one side of the timing cap. We are supposed to have pulses of the correct period, 10ms not 5, but I get 5 not 10, so it's wrong.
Amplitude of the pulses is also 3 times smaller than it should.
There is no waveform given for the other side of the cap, so I probed the next stage, working, instead. One side showed spot on what the schematic said, what I just described. So I was happy indeed that I could take this stage as a reference point... so, I probed the other side of the cap, and I got a cycling charging exponential as you might expect. Amplitude also about 100V+
So, back to the defective 10ms stage, I probed the other side of the cap and... no exponential. Instead, I get the exact same signal that I get on the other side of the cap !
Huuu........ if we get the same signal, 100%, on both side of whatever component.... that means it's.... basically..... A SHORT !!!!
![Scared :scared:](https://www.eevblog.com/forum/Smileys/default/scared.gif.pagespeed.ce.oZptHNcbsD.gif)
WHAT ?! How could my cap be short ! I tested it just fine at 3.2nF before soldering it in !
So, could it be that it's leaking, and so bad that it acts like a short under operation ?
The evidence is there anyhow, electrons don't lie !
So, as a quick sanity check, I removed my cobbled together film cap, and put the old Sprague cap in place.
I mean it reads 50% high but at least it was behaving like a cap, and the stage was working fine with it.... not much headroom when adjusting the trim pot, hence why I replaced that cap, but still, I could get it to work fine.
So, put it back in there, and presto, the stage is back in business and waveforms are what they should be !!
SO ! Tough luck for me cap wise !!!
The famous "never fails" Mica... that reads spot on with a DMM but is leaky as hell to the point of keeping the stage from operating.... and now a modern(ish, salvaged) film cap that also read fine with the DMM but short out in service !!! And that old Sprague that beats them all and DOES work !
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif.pagespeed.ce.5xOqKkq0Co.gif)
Murphy is out there and kicking for sure !!
![Flogging A Dead Horse :horse:](https://www.eevblog.com/forum/Smileys/default/DeadHorse.gif.pagespeed.ce.kBWvHKqCd1.gif)
So, now the next defective stage as I said last night, is the 1s stage, which exhibits the same symptoms.
so of course I checked my film cap but here it seems to work fine : I do get my charging exponential, with good amplitude, on its relevant pin.
So the source of the problem must be something else.
Stay tuned.