Thought I'd give you a small update as to the proceedings of my PM2421 repair.
It was all downhill after that.
Although this referred to Nimrods, that sums it up nicely.
The first session didn't go well.
The second session didn't go well.
The third session went far worse than those.
I really didn't expect this to become such a task. It took me some time to realize that the beautiful manual was for an earlier model. The manual from ElektroTanya is for my model, but the scan quality is not as good. And both manuals have missing pages.
![Sad :(](https://www.eevblog.com/forum/Smileys/default/xsad.gif.pagespeed.ic.L3FGyzQrjB.png)
After lots of wasted time, I realized that the logic was stuck in the down-integration phase. Looked like the comparator never signaled the zero crossing to ending that. Pulled out the resistor at its input and applied -0.1 V / +0.1 V. Output didn't change. However, that comparator is a µA710 in a 14-pin DIP.
![Sad :(](https://www.eevblog.com/forum/Smileys/default/xsad.gif.pagespeed.ic.L3FGyzQrjB.png)
Unlike the majority, this one has push-pull outputs, so most of my collection is pretty useless. For testing's sake I build an adapter and went with a LT1018 (slower, not really dual supply, but as a quick test ...). After switching the thing on, I saw a light in the corner of my eye, which was also the corner of the power supply PCB. The display changed to 2 Nixies out, 2 Nixies gibberish, and the 5 volts were gone. Getting at the PS PCB is not too easy, but after pulling it out I found the light source: a long, thin glass tube with a fine wire in it, which the schematics showed as a 1
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
resistor.
That was on Friday, and I still don't have the slightest idea why this happened. I was nowhere near the 5 volts rail when it happened. I was just
watching, as I hoped it would spring to life. Even if I had made any short with that comparator, this
could not have resulted.
I'm planning on powering the thing with my lab supplies (except the HV for the Nixies) so I can limit the current easily and safely. After replacing the dead fuse resistor with an ordinary 1
![Ohms \$\Omega\$](https://www.eevblog.com/forum/Smileys/default/omega.gif.pagespeed.ce.G89vbH4KGC.gif)
resistor, the 5 volts from the PS are fine again (tested with an electronic load and DC lab supply instead of transformer).
Of course this might deserve a posting in the repair thread, but if I fail to make it work, I'd be too embarrassed.
![Embarrassed :-[](https://www.eevblog.com/forum/Smileys/default/xembarrassed.gif.pagespeed.ic.zcm3EmNu9T.png)
I hate to admit defeat.
Not sure if I find some time today. The weather's too nice to stay in the house all day.