Tek 2465 vertical calibration? Hardware problem? User error? - Page 1
Forum section: electronics / testgear

author: i386
author_rank: Contributor
timestamp: November 27, 2024, 01:49:35 am
content: I have a 2465 (not A or B) that I bought years ago and never really used or knew how to use it. Recently I dragged it out of storage and it blew one of the capacitors. I replaced all of the Rifa caps and one carbon comp resistor that was clearly damaged in the blast. The scope starts up with no errors and I’ve spent a great deal of time in the operators and service manual as well as watching hours of videos. I’m now pretty familiar with the scope’s controls. I also recently scored a BK precision function generator.What I’m noticing is when measuring voltages on Channel 2 is they read exactly 5x higher on Channel 2 than on Channels 1, 3, or 5.I have a 50ohm BNC cable connecting my signal generator to my scope inputs. Starting on channel one I set the frequency to 1Khz and the amplitude so that the scope measures it at .1V. Channel 2 will measure that same signal as .5V. I have my V/Div set the same on all 4 channels. I’m triggering on whichever channel the cable is plugged into. All 4 channels measure the frequency correctly as 1Khz.It seems odd to me that it’s off exactly by a factor of 5. I’m not sure if the vertical calibration is off, there’s a hardware problem, user error or what. Everything else is functioning normally as far as I can tell. I’ve read through the CAL02 procedure several times but haven’t mess with that yet.I’m using this for tube amp work. I’m mainly interested observing clipping in my sine wave signal and measuring voltages at various points in the signal path. I will rarely have a need for channel 2 anyway. Still I have the itch to fix things when they don’t work.

author: i386
author_rank: Contributor
timestamp: November 27, 2024, 04:09:00 am
content: Update I found a post on the tekscopes group where a guy had a similar problem. His was more intermittent and changed when he fiddled with the V/Div knob but he had the same 5x voltage problem. Turns out he was able to fix it by cleaning the contacts in the attenuator. I think I’m in the right neighborhood. I should have time to work on it this weekend.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2024, 04:48:43 am
content: You should try it with a few different amplitude signals and some different V/div settings to see if it is consistent or changes. It likely will change and it probably is an attenuator issue. The attenuators on those are pretty delicate and fiddly, so tread carefully. If it becomes hopeless, let me know as I have a few parts mules packed away somewhere.

author: i386
author_rank: Contributor
timestamp: November 27, 2024, 05:32:31 am
content: Quote from: bdunham7 on November 27, 2024, 04:48:43 amThanks, I checked something else. At the appropriate coupling settings I’m able to measure 1M Ohm and 50 Ohms through the attenuator on Channel 1. On Channel 2 where I should be seeing 1M Ohm, I have an open circuit and where it should read 50 Ohms I’m getting fluctuating readings (and none of them are 50 Ohms.)The service manual recommends replacing the attenuator if it’s bad so no cleaning instructions there. However, I read on the tekscopes group that they are “quite serviceable”. I’ve seen some pics of it disassembled and yes it looks pretty delicate in there. I’ll try to find an article or video of someone cleaning it so I’m not just going in blind. This old scope is in great shape. I’d love to see it working 100% again.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2024, 05:48:42 am
content: That does look pretty mint! IDK if you've already heard this warning, but you should never run that scope with the cover off unless you have a fan blowing on the underside circuit board (you'll typically have the scope on its side or propped up) and if you end up having to disassemble or repair the cooling fan you should read the instructions at least 3X.

author: i386
author_rank: Contributor
timestamp: November 27, 2024, 06:01:54 am
content: Yea, I’ve read that about keeping it cool.I had to replace the Rifa caps recently because one blew. That’s what started this adventure. I stopped and read here about the fan when mine didn’t want to come off easily. I’m glad I paused because what it looks like and how it actually comes off are two different things. Good news, mine came off without breaking. Also there is no vibration. It runs whisper quiet.

author: i386
author_rank: Contributor
timestamp: November 27, 2024, 02:45:36 pm
content: I found some more info on this topic in this video:https://youtu.be/DX6ipBe3WGQ?si=nv3SOoZg5caQJYvWIf nothing else it shows some things to not do. “Scraping” the contacts would be one example. Shimming the contacts with folded paper would be another. Maybe IPA would be the preferred cleaner for the contacts and not Deoxit.Several people in the comments recommended gently bending the the “leaf springs” to increase the force between the pins and the contacts. Sounds like a really delicate procedure.I’m going to keep reading and researching until I have time to put my hands on it. I believe this is 100% fixable one way or another. Will continue to share my findings.

author: tggzzz
author_rank: Super Contributor
timestamp: November 27, 2024, 05:59:20 pm
content: Quote from: i386 on November 27, 2024, 02:45:36 pmLooked at about 2mins of the video..."think I heard somewhere the [main PSU] electrolytic caps are three legged..." Nope: he should have bothered to look at the service manual. And that's the 4x5 series.He scrapes the gold finger contacts with a tweezer, and uses DeOxit D5. Shouldn't scrape, should use IPA (and if DeOxit then ought to use the DeOxit specified for gold contacts).

author: i386
author_rank: Contributor
timestamp: November 28, 2024, 02:03:15 am
content: I ordered some Deoxit gold (G100L). It will be here on Saturday sometime and I should have time to work on it. My initial plan is to just examine and clean the contacts.From the pics I’ve seen posted I can identify the contact springs and their mating contacts on the circuit board within the attenuator housing.There are also the pins that extend through the unit. Are these conductive or just serve as a guide? I don’t quite understand how that part works and if they should be part of the cleaning procedure.I found some other posts here with the same problem and also a few over at the tekscopes group. It seems this problem is most common on channel two. I wonder if it’s because many people don’t use channel 2 that often and thus the contacts don’t get exercised as often.

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2024, 09:12:38 am
content: Not sure what you mean by "the pins". Are you thinking of the relay coil connections?Have a look at the schematic in the service manual, available at all the usual places.The fundamental switch contact mechanism looks like that used in the Tek 4x5 series - where the contacts were controlled by cams. Tek recommended cleaning those by putting IPA on shiny paper, inserting that paper between the open contacts, closing the contacts, and pulling the paper out while making sure the delicate gold "fingers" weren't bent.

author: i386
author_rank: Contributor
timestamp: November 30, 2024, 04:38:47 am
content: Quote from: tggzzz on November 28, 2024, 09:12:38 amYessir, I understand how it works now.It was a delicate operation getting this out. The contacts are visibly dirty. I’ll be working on cleaning those tomorrow.

author: i386
author_rank: Contributor
timestamp: November 30, 2024, 07:45:20 pm
content: We have a working channel 2 nowBeforeAfterThis has gotten me checking marketplace and Craigslist for old scopes to fix.

author: tggzzz
author_rank: Super Contributor
timestamp: November 30, 2024, 09:04:52 pm
content: Quote from: i386 on November 30, 2024, 07:45:20 pmWell done, and welcome to the rathole. Satisfying isn't itRead the TEA threadAnd be careful of the EHT components; they can bite.