Beaten Tektronix485 - Page 1
Forum section: electronics / repair

author: tatel
author_rank: Frequent Contributor
timestamp: June 13, 2023, 01:39:48 pm
content: I got a beaten Tektronix 485. €35, I couldn't really resist. I would like to restore it, but if this isn't a viable option, I guess it will be a good source of spare parts for somebody. I'm taking this as a big oprtunity to learn and a long term project.Power, slope, calibrator frequency selection, vertical atenuator, chop and X-Y buttons/knobs are missing. Screen protector is broken. I can hear something loose inside while turning it. There is a coax jack with no cable attached on the lower board, near the atenuators. Some of the comb connectors on that board were not fully inserted.I measured resistance between ground and the test pins, before and after fully inserting the comb connectors. No change. I got:+50 V ---> 1.9 kOhm+15 V ---> 93 Ohm+ 5 V ---> 68 Ohm- 5 V ---> 207 Ohm- 15 ---> 139 OhmPictures attached

author: tatel
author_rank: Frequent Contributor
timestamp: June 13, 2023, 01:40:18 pm
content: Last picture

author: tatel
author_rank: Frequent Contributor
timestamp: June 15, 2023, 01:48:15 am
content: Well it does turn on. There are clearly bad contacts in there. After using a rod to turn power switch on, then turning the plug strip on, it powered two or three times, no problem, but after some movement to look at the back of CRT, it didn't powered. Some more movement, and it turns on again. Since I got the back cover out, I'm not leaving it on more than a couple minutes each time.So far CRT looks dead. Can't see any light on it. I was hoping to see some light in the back side, but still don't know what exactly to look for or even if it can be seen without getting something out. Hopefully CRT is good, because I measured voltages on test pins on A13 board. It turns out there's no 180V, but about 58, and sometimes just about 15 VRemaining voltages tested on A13 board look good: 119,97, 59.50, 49.97, 25.45, 15.193, 8.992, 5.629, -5.606, -9.017, -15.244In A10 board got 49.98, 15.165, 4.794, -4.990 and -15.223 V. I think these are also OKTime to get the schematics and begin to look for something gone bad along 180 V rail, I guess. Since I need some way to turn on/off oscilloscope's power switch, it looks like I'll have also to get out some board or perhaps the front. Same to look for the bad contact, probably.I don't know what to say about the screws. Quite a bunch are missing: back cover had just two screws. On A13 all screws to fasten the board to the chassis are missing. Also I think there should be an aluminum board over the back side of the CRT. The guy that got into A16 board cover before me ruined some heads, so I had to carve a slot with the Dremel to be able to use a flat screwdriver toget the screws undone. I tried to undo some screws on A11 board; found they are very tight and it's too easy to ruin the heads. Maybe that's "steel" but to me it looks like a very low grade one. I can swear I see the diagonal lines that would mean they are pozidrives; but a Phillips screwdriver worked better. Now I see I could easily damage some screws, too.So looked for some screws, 4-40 3/16, pan and flat head. Phillips. Manual says flat heads are 100 degree countersunk, but these seem to be available only from the US and, even if willing to pay about $15 for a buch of them, shipping makes this option a no-no-no-no. At least ~20 shipping is insane, I think. Some guys asks for $60 shipping.If ordering from Aliexpress, 100 pcs each, total cost would be €6, shipped. Flat heads have 90º countersunk, so probably some epoxy will be in order to get these well seated.If you have any thoughts, I would love to know.