Re: 465B Horizontal Trace Issues - Help requested


Tom Lee
 

Terrific hanging in there, Gary. There’s no problem leaving a fixed resistor in there until you get a replacement pot.

Now go celebrate!

—Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Oct 1, 2020, at 6:23 PM, gary@pincus4.com wrote:

Eureka!

I just executed the idea of bypassing the 1X pot (1k ohm) - first with a jumper I had handy that I use to discharge capacitors (a 5W 1K resistor in a wire between two minigrabber probes). That brought the resistance in line with what it should be reading through the pot. Then I replaced the resistor with my Heathkit IN-3117 Decade resistance box. Even set to zero ohms, when i turned on the scope the trace was stable and only slightly compressed. Changing the resistance on the box allowed me to set the trace to any level of compression including no compression at all. I have it exactly where it should be - right to the edges of the grid (390 ohms is just about perfect). When I unplug one of the leads from the decade box, the trace immediately collapses, plug it back in and trace is perfect. Shailendra you suggested that the problem may be the pot - good call.

I do believe this confirms the 1X pot is the culprit. Shailendra you suggested that the problem may be the pot - good call. Since i have a 390 ohm 1/2W resistor and not the the pot, I think i will put in the resistor and run it for a while to see if everything else is working properly. Then when I have collected all necessary parts (pots, tants, electrolytics), and increased my skill level, take it apart, clean it and preemptively replace the usual suspects. Since a pot is just a variable resistor I can't think of any reason not to leave a fixed resistor soldered in place for a month or two. Other than this being a "kluge" repair to a precision instrument any reason not to? But if anyone can think of an electronic reason, please let me know.

I feel like the lawnmower mechanic that just "fixed" the Ferrari with a hair pin.

I will put together a BOM for pots, tants, and lytics and run it by this group.

Thanks for all the help and encouragement.

Gary




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