Tek 485 Trim Pot is gone??? - Page 1
Forum section: electronics / repair

author: Zenwizard
author_rank: Regular Contributor
timestamp: July 05, 2018, 02:22:44 am
content: The attached picture is from a Tektronix 485 Scope I am in the middle of the doing a full cal on the scope. There should be a trim pot R863 according to the service manual however on mine there is just a fixed resister. I am curious if this is a bodge or if this was a design change from Tek. The pot should sit in between the 3 dog bone resisters at least so says the service manual.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 05, 2018, 02:53:40 am
content: The solder looks original.You should be able to look up official Tek changes based on serial number.

author: Zenwizard
author_rank: Regular Contributor
timestamp: July 05, 2018, 08:44:06 pm
content: I have been able to confirm that this is a bodge resister. This will be replaced with a POT.

author: High_vacuum_house
author_rank: Newbie
timestamp: July 05, 2018, 09:53:53 pm
content: If on calibrating this stage in the oscilloscope there is no need to adjust its resistance I would leave it as stands as it does look original factory fitted part. It could be that in earlier models the pot caused drift or other instability issues. The solder is clean and untouched. What does this part of the circuitry do.Christopher Capener

author: Relayer
author_rank: Contributor
timestamp: July 05, 2018, 10:19:48 pm
content: Hello Zenwizard,Wow, a 46 year old scope.I don't blame you for wanting to re-calibrate it. There's definitely got tobe component and age drift with it.Just looking at some of the components reminds me of the days when Ifirst got interested in electronics, I delved into an old B/W telly encounteringplenty of electric shocks, but all the components in the unit looked just likethe ones in your cro.A 500Ω 25 turn pot would do well in there to give you some niceaccuracy.Regards,Relayer

author: lowimpedance
author_rank: Super Contributor
timestamp: July 06, 2018, 12:24:59 am
content: Quote from: Zenwizard on July 05, 2018, 02:22:44 amAre you sure those are resistors and not capacitors !.

author: Zenwizard
author_rank: Regular Contributor
timestamp: July 07, 2018, 01:42:41 pm
content: Yea they are caps. I have not worked on much gear that has precision capacitors in it. For being 46 years old the scope has very little time on it. I was able to reference a service manual for this from a late serial number scope. All service manuals call for a 500 Ohm pot here. The control is A sweep display linearity adjustment. At the beginning of the sweep the ramp is not smooth there is a bit of ringing. This pot helps tune out the ringing. Especially at the beginning of the sweep.

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2018, 09:09:04 pm
content: Interesting that a Carbon Composition resistor has been fitted there, it's surrounded by carbon films. On the one hand it has poor tolerance, large TC very high long term drift, on the other hand, it is low inductance. I'm not sure if that has any relevance in that location. It does still look original though.

author: SeanB
author_rank: Super Contributor
timestamp: July 08, 2018, 04:42:53 pm
content: Could be a "select on factory final test" resistor, which was soldered into the board during production after they determined that, for that particular batch of boards, this was the optimum resistor value. Saves time on the adjustment process if you already have the particular section pre a ligned already, and less time in production as well, especially as they probably saw all the finished boards had about the same small range of adjustments per batch, so removed the adjustment and made it fixed.