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Here’s my story, Tek 485, a ham fest purchase, it was dirty and nonfunctional. It's a pretty common issue with ham fair test equipment. After cleanup of the 485, I found it had a shorted Tant in the -15v rail, a quick fix, and it was all operational. Then a few hours later, another Tant shorted, this time it was C1748 through a small choke. The resistance was the hint, it wasn’t 0v to ground but about 0.26 ohms to ground, so I knew it wasn’t a direct to ground, this helped locate C1748.

Even though now all rails are up where they should be (I won’t list them all here but they all came up fine from +-5v to +180v). However, now there’s an issue with the Z axis. I’ll try to describe it, the trace is Z axis modulated, as in a dashed line ~!~ Strangely, the frequency of dashes appears to change a bit in accord with the intensity control. If I turn the trace intensity down the trace disappears completely. Here’s the kicker: if I touch the case (Collector) of Q1772 or Q1752 as they're common, the modulation disappears and the trace looks fine and intensity has full control. There might be other issues but the trace modulation is the first I’ve ever seen and needs to be put to bed first. How do you suppose changing the brightness changes the apparent frequency of the spacing of dashes on the trace?

I’ll post a short clip, let me know if it’s best here or in the files section.

Check power supply's for voltage and ripple first.

Then follow Z axis schematic and troubleshooting

Jon

Status update on the 485; as I mentioned in the earlier post, ALL the voltage rails were very close. I’ll list them this time as I was asked if I checked them. Here they are; +- 15v, +-5v, +-5.5v, +50v, +9v, +25v, +120v, +180v, and + 59.4v. There are others but these are the main rails. They all checked within allowable tolerance. While I was checking voltages, I checked for ripple. I used a Fluke 289 in ACmV range for reference and the ripple on all rails was also very close. I backed up my tests with a scope. I have to admit, this was after quite a few caps had already been swapped out.

After R&R a number of Electrolytics in the PS, another Tantalum reared its ugly head, C1748 was shorted. As I mentioned earlier, it had more resistance to ground than most failed Tantalum capacitors I’ve run across. I usually see somewhere around 0.03 to 0.06 ohms, however, C1748 was different, it was 0.28 ohms. I thought it might be through Silicon, but actually, it was via L1748, a small choke. After pulling the bad cap and replacing it, that’s when things went South.

Here’s what I found, after seeing all sorts of odd behavior on the screen I found C1743, a tiny 1.0pF cap had a weak/broken leg. For the life of me, I could not find a 1pF cap in my stash, I remembered some old-timer in the Radio/TV repair business back in the 60s needed a tiny value cap, not having one also, he fashioned one out of a short piece of insulated wire. I remember him calling it a “Tickler” cap, whatever it was for escapes me now. However, I remembered what he did so I twisted a couple of pieces of small wire tightly together, maybe 4 inches. I trimmed the two legs and slowly trimmed the wire till I saw 1.5pF on my analyzer. I think it was about ? inch of 22 gauge insulated wire, I soldered it in place of C1743 and everything began working again, with no issues. I put the real deal in place sometime later.

So, there it is… whether or not I broke the lead while replacing C1748, or it was weak/broke after 40+ years of service (I’ve owned this machine since the early 80s), who knows? The important thing is, it’s working again. I took several thermal images of all sides for future reference and filed them away in the manual. While in the manual, I noticed our store stamp, in made me smile, I ID stamped all our manuals, whether it was for bench equipment, or HiFi equipment, everything got stamped. I smiled remembering the day I unboxed the 485, along with a 475 and a couple of 466DM units. That was an exciting day in the shop… many years ago. <smile> At any rate, I just wanted to update the thread for the unlikely chance anyone was following it.

That's called a 'gimmick' -- a very useful thing indeed!

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070

I think the term for the cap using enameled wire is called a "gimmick".

Sam Reaves
ARS W3OHM
Owner / Moderator of:
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On Sat, 18 Jan 2025 at 04:29, Michael - AF7U via groups.io
<michael.rosanbalm@...> wrote:

...I found C1743, a tiny 1.0pF cap had a weak/broken leg...

At any rate, I just wanted to update the thread for the unlikely chance

anyone was following it.

Thanks; some of us are following it.

Interesting to find a different type of capacitor fault, one that might
conceivable be diagnosed by holding your finger in exactly the right spot :)

I currently have
- one 485 that is definitely a parts mule,
- one that was working until I came back from a coffee break (and I haven't
looked at yet)
- one that almost works and I ought to sell after getting the second to
work.

The most frequent fix I've had to do is C911 (!) and other 15V tants on the
"hidden" 13V rail.

The most interesting fix was to repair a fault inside U660 which drives the
vertical deflection plates. The internal resistor had gone open circuit,
but since both ends were exposed to pins, I was able to add an external SMD
resistor. Successful, except that the risetime was extended from 1ns to
1.25ns.