The value of R309 was a typo on my end. Per the schematic and “actual” resistor on the board, it is a 820Ω (Not 820kΩ). Before I replaced it, it measured very high over 1MΩ, so definitely went open. Hopefully that clears up that confusion. That fix restored the “Horizontal Position” Control, and allowed the trace being displayed to be moved left & right (along the x axis).
After some more testing, here are some additional findings:
– Setting the “H In/ Sync” to “Line Sweep” produces a full trace, and probing using an (Tektronix TDS460) at R303 which is the feed into the “grid” for the Horizontal Amp (V301A). This shows a square wave with approximately 5v pk-pk (@ 60Hz). This seems to make sense as the sync is set to “Line Sweep”. This setting ties the input of R303 direct to R118 from the power supply section and bypasses the sweep circuitry.
– The other “H In/ Sync” settings do not provide and signal to R303 (seeing less than .02v and no discernible waveform) . This is leading me to believe that the Horizontal Amplifier is working correctly, and that the other “H In / Sync” settings are not producing / passing the waveform that is being sent in.
– If I use a scope and look for the waveform as it comes into the Horizontal section (using a 1v pk-pk Sine wave @1kHz connected to a 1x probe on the scope input) I can see it at the + side of C401 which is where the input starts for this area. I then tried following it, to see where it is lost.
– Able to pick up the waveform further in at the base of Q402 and Collector of Q402, from here it goes to both Q401 & Q403.
– Unable to get the waveform at Base of Q403 and Collector of Q401, removed both of those transistors and I can see the waveform at those points without the transistors in circuit.
– Testing Q401 and Q403 out of circuit and they both test good. I socketed their spots on the PCB (to make any future swaps easier), and then as a precaution tried 2 new transistors as replacements, same issue. However this does not mean the transistors are they culprit, as with them in circuit it puts all the other components that support them and past them in circuit. So will need to dig deeper.
– Ultimately I feel the waveform coming in, is getting blocked from making it to the Horizontal amplifier input at R303.
I am chasing the wrong thing here ? Or does this theory hold water ?
@dropwire No that makes sense to me. If line works then it is for sure in the time base section. Stay on the track you are on and I think you should find it pretty quick. I threw out that resistor when you said you tested and verified them all, but that changes when you put time on it. Anything can change really fast, especially when you say something is good on the forum, then it turns bad!
Radios + Tubes + Scopes + Cars= Nothing better!
Eureka! Persistence for the win … 😀
The scope is fully functional now. There were a few culprits here:
– Sketchy trace on the Emitter of Q402
– While checking Q404 and Q405, and putting in sockets to make any future repairs / troubleshooting easier. I found the new replacement 2SC458 transistors I used, had a different pinout than the original “Hitachi” 2SC458. The Original “Hitachi” transistors were Base / Emitter / Collector, and the new NTE replacements are Base / Collector / Emitter. This prompted me to also check the two 2SC717 transistors (Q401 & Q403), which were also pinned different than their original “Hitachi” versions. I corrected the pinout connections on all the transistors (so much easier now, that I socketed all of them), and voila the Trace is fully functional.
Not sure how I missed double checking the pinout on the transistors, I guess I put too much faith in what the data-sheet showed, combined with just frustration at the amount of time troubleshooting. 🙄
I ran the scope through its paces, checking various waveforms, different sweep timing. Adjusted the intensity (CRT), focus (ASTIG), and Vertical input. Excited to get this all wrapped, just have to finish “powder coating” the case, and we can put this in its new home on the workbench. May try pairing it with my “Model-A (Jud Williams)” Transistor Curve Tracer, I think it would work well with this scope.
Thanks again @radtekman for the tips and insights, and just being there to listen to my testing thoughts. Now… What else can I work on … 🤔
@dropwire Great! Happy to help, sometimes that is all you need. I always check pinouts when doing something that technical. Mainly because I don’t remember pinouts well so it is always safer to verify, same with tubes. They make them in so many different ways old or new it is hard to keep them straight. Sounds like it should be ready to go now.
Anymore, I don’t have to find things to work on, there is always a line of projects. Haven’t done many personal projects lately being backed up on jobs. My to do list is huge though, next up will be cleaning up and organizing possibly rearranging the lab (basement). I will post it up when I feel it is presentable.
Radios + Tubes + Scopes + Cars= Nothing better!
Here are some pictures of the Scope with the freshly powder coated case.
Apologies, forgot to add the individual images:
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@dropwire Looks fresh and ready to use for years to come! Great work
Radios + Tubes + Scopes + Cars= Nothing better!