Tektronix TDS220 Repair: Missing Capacitor on Input  - Page 1
Forum section: electronics / repair

author: fotte
author_rank: Newbie
timestamp: March 18, 2025, 12:42:19 pm
content: Hi everyone.I recently got a Tektronix TDS220 oscilloscope. It was in a bad condition and I made it worse. Its an early revision of the pcb where the BNC-Connectors only have 2 Ground connections instead of 4. While fixing the broken connectors I managed to blow off two ceramic capacitors. These are located on channel 2 and trigger and have been marked: C155 and C191. The package measures to be 1206. Since the input structures for channel 1 and channel 2 are symmetrical I tried to measure this capacitor on channel 1 but I dont trust the result. My meters read 0,000nF - 0,001nF. Since I could not find the schematic anywhere I'm asking you: Does anyone know what caps (C and Rating) should be placed here? Maybe someone has an educated guess by looking at the schematic. The first parts for the three inputs are build the same way.Any help is apprechiated.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: March 18, 2025, 01:31:34 pm
content: If they are connected like you show, the voltage rating of them would need to be no less than the voltage rating of the input. 300V?? It should be marked on the front panel. They are likely pretty small as scope input R & C are probably something like 1M and 20pF (check specs) so these are either that input C or in parallel with the attenuator input C.

author: fotte
author_rank: Newbie
timestamp: March 18, 2025, 10:21:32 pm
content: Hey PwrElectronics, thank you for your input.You are right, the input specs are 1Meg||20pf. I traced out some more of the input. Maybe someone else has additional ideas.EDIT: Sorry everyone - I am tracing out channel 1, and the cap missing is on symmetrical channel 2... If you look at the schematic C105 is missing.

author: tom66
author_rank: Super Contributor
timestamp: March 18, 2025, 10:42:41 pm
content: Did you measure Ch1 cap in circuit or out of circuit?I expect sum of C104+C105 won't exceed 10pF since other parts of attenuator will contribute at least 10pF capacitance.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 19, 2025, 12:14:19 am
content: Tom is probably correct and the two caps would be about 20 pF each.Do you have something that can measure the input capacitance of the scope? If you do you could put a couple 20 pF caps in place and see if you need to change them a bit......You need something that will measure the capacitance with the resistors in parallel though. I used my GenRad 1650 on my scopes and it seems to do an excellent job,

author: fotte
author_rank: Newbie
timestamp: March 19, 2025, 08:43:27 am
content: @Tom66: I tried both, to measure in-circuit and after desoldering (with a hot air station). The problem is I only have access to an Extech EX520 and an Aneng 681. And their respective resolutions are 10p... Best thing about that is, that the Aneng displays 0 .. 1pF when measuring, even though it cannot really measure it@Wallace Gasiewicz: Unfortunately I dont have access to something like that.I will test it, with a 10pF 1206 500V NP0 and post the 'results'.

author: tautech
author_rank: Super Contributor
timestamp: March 19, 2025, 08:59:18 am
content: Quote from: fotte on March 18, 2025, 12:42:19 pmVery evident from your picture.The BNC inputs in these are fragile as they don't go through a metal bulkhead and aren't securely fastened with nuts.Being a 2 post BNC housing doesn't help either as the posts break or crack the solder as your one has.You can make them more robust by removing the solder mask from the ground plane and soldering a copper strap over the BNC to give it more strength.Also check if the SN# isn't part of the worldwide recall for TDS200 models.

author: fotte
author_rank: Newbie
timestamp: March 19, 2025, 08:28:42 pm
content: Quote from: tautech on March 19, 2025, 08:59:18 amVery evident from your picture.Got it from a heavy smoker and it was stored in 'not so dry conditions'. However I got it for free and I tested it before opening it - It works, so I am willing to put some effort into making it work (and clean) againI'm still a bit unsure if cleaning with 99% rubbing alcohol gets rid of all the tar..Edit: It was recalled and I have the stronger 'Earth-clip' to provide sufficient current carrying capacity..

author: fotte
author_rank: Newbie
timestamp: May 29, 2025, 01:36:11 pm
content: It took me a while to get back at this scope.I replaced the caps with 10pf / 300V and the input signal looks good. But there is another issue.The screen is readable but in the center there is a large round spot where the images starts to look inverted. I took a few pictures. Is this a dead LCD, a bad backlight, or is this some sort of smoke/tar deposit? It looks like it is behind the glass-layer and the polarization filters - so maybe the coating is dissolving? I don't have much experience with LCDs... Any suggestions are welcome!Furthermore the PCBs and especially the probes and the cables still smell heavily of old smoke. Can anyone recommend something to clean them? I used rubbing alcohol for the PCBs already and worked well - but they still smell and i dont want to flood the encoders, connectors and transformers.Again any help is welcome

author: tom66
author_rank: Super Contributor
timestamp: May 30, 2025, 10:13:15 pm
content: It looks like a bad LCD, but I remember these Tek scopes had some contrast adjustment in the menu, maybe worth looking around and seeing if that is set incorrectly. These scopes did use rather poor quality LCDs for the time period they were sold in, a bit like 90's laptops, so it wouldn't surprise me too much if it is just failed.