Tektronix 2225 Focus Issue - Page 1
Forum section: electronics / repair

author: thevamper
author_rank: Newbie
timestamp: April 15, 2021, 10:38:25 pm
content: Hello everybody,I just purchased a Tektronix 2225 oscilloscope. It seems to be having an issue with the focus. I can get it to focus but there is an extremely small range where it stays in focus. I have linked a video below that better shows the issue. I have checked all of the high power resistors in the focus circuit and they are all reading nominal values. I am thinking it could be the focus potentiometer but those are hard to source. I was also thinking it could be the transistors in the focus circuit. Anybody have any ideas?Thanks,Tony

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2021, 11:21:13 pm
content: Check those again, especially the one that is parallel across the focus potentiometer. You need to check it twice, once with the focus pot turned all the way clockwise, then again when it all the way counterclockwise. You should get a bit less than 510K one way and something very low the other. Does your small window of in-focus happen to be near one end of the range? Also, make sure you check the small 180K resistor in that chain as well and if one is bad, replace them all with Vishay HV metal film resistors, HVR25/HVR37 series.

author: CaptDon
author_rank: Super Contributor
timestamp: April 16, 2021, 01:58:31 pm
content: it looks like you have WAY too much intensity from the picture. It would be hard to maintain focus at that brightness level. How does the focus act with the beam at lower brightness? There may be other problems like the beam blanking or beam limiter / CRT bias being somewhat off. As the other posters mentioned, this series of Tek scopes was plagued with CRT focus and brilliance resistor problems as they aged. I know of two 2213's that got trashed because of the factory screwup that put too much voltage on the C.R.T. filament. The cause of the early C.R.T. death wasn't known by the company I worked for that owned the scopes and they chose not to repair them and decided Tek quality was no longer worth the insane purchase price or factory repair price. Tek in the 60's and 70's were bulletproof. Then they started 'costing out' and their reliability went to crap. Because they had so many in house built hybrid modules and I.C.'s and made them unavailable for do-it-yourself repairs, as well as obsoleting all parts within a short timeframe other players in the oscilloscope game were able to gain traction. Tek really got laughed at when they were selling portable battery scopes (212 etc) with 500khz bandwidths and the 5110 / D10 series with 2mhz bandwidth for multiple thousands of dollars. I still own a lot of Tek stuff but can't afford and won't buy any of their newer stuff. I have Tek scopes here at work with probes costing over $3000 each. They get boogered up easily and cost over $2000 to repair if they can even be repaired.

author: David Hess
author_rank: Super Contributor
timestamp: April 17, 2021, 01:43:40 am
content: The focus resistor chain is known to be unreliable in the early 22xx models but the 2225 is a late model which should have that resolved. Still, I would start be checking the focus resistor chain because that is relatively easy to replace unlike the potentiometer itself.

author: djsb
author_rank: Super Contributor
timestamp: April 17, 2021, 10:20:19 am
content: I had the same problem on my 2225 a few years ago. I think I replaced one resister in the focus chain and bought a potentiometer from sphere research in Canada.https://www.sphere.bc.ca/test/index.htmlDavid.

author: jdragoset
author_rank: Frequent Contributor
timestamp: April 18, 2021, 10:21:20 pm
content: Mine, purchased from a Pensacola Fl area, seller had several with the same kind of focus issues.When I purchased, figuring no problem, find the pot, CITEC 2 M ohm pot had several resistance path cracks.If it turns out to be the pot itself, I've got several pix of replacement ideas that did work in my case.

author: jdragoset
author_rank: Frequent Contributor
timestamp: April 20, 2021, 01:42:41 am
content: This might be a good replacement but will need the shaft sanded down and flat added:https://www.ebay.com/itm/1-2-Meg-Piher-Linear-Panel-Pot-Plastic-Construction/372862571979This was not available two years ago when I used:https://www.ebay.com/itm/One-Tektronix-Focus-Potentiometer-311-2282-00-2-5-meg-from-SC504/154073879417Used a 3/8" x 32 die to thread the Nylon OD with nut, flat and lock washer each side of HV enclosure.Just tested and holds focus and intensity low to high sweep rate very well after two years.

author: thevamper
author_rank: Newbie
timestamp: April 28, 2021, 07:53:38 am
content: Quote from: bdunham7 on April 15, 2021, 11:21:13 pmI just checked with the pot in both positions you mentioned and all of them read 1M just like they say in the service manual. The smaller one says 200K in the service manual and reads 200K in both pot positions. David Hess mentioned that the resistor issue was fixed in 2225.Quote from: CaptDon on April 16, 2021, 01:58:31 pmMy camera makes it look like I am using more intensity then I actually am, however I turned down the intensity and had a little more success finding the focus, but the same problem persists. The focus issue is in the middle of the range. This makes me think it is the pot because you can eventually get to the focus in the middle, it is just hard to find because there is such a small area where it actually focuses.I will probably move forward with replacing the pot since djsb and jdragoset seem to have success with this. I couldn't find the pot on sphere research in Canada. I just emailed them to see if they might have it. I found this replacement on ebay:https://www.ebay.com/itm/124679162494?hash=item1d07750a7e:g:VyoAAOSwQ3Zgdbpvhowever it is not the exact part and needs to be modified as described in the eBay description.jdragoset if you can send those pictures of your replacement ideas that would be great.Sorry for the late reply everybody I've been putting off this project for awhile. Thanks for all of the great suggestions!

author: jdragoset
author_rank: Frequent Contributor
timestamp: April 28, 2021, 11:30:39 am
content: When I did this there were no all-plastic replacements available.This was a Clarostat 2 Meg from an earlier Tek scope.Today I would just use the all-plastic 2.2 Meg pot on Ebay.

author: bdunham7
author_rank: Super Contributor
timestamp: April 28, 2021, 03:17:17 pm
content: Quote from: thevamper on April 28, 2021, 07:53:38 amI'm sorry that wasn't clear--I was only saying to check the resistor that is in parallel to the potentiometer in the two positions--the others won't change. Actually, looking at the schematic, that resistor R892, won't appear to change either because of the later model revisions connecting the focus grid differently. However, R892 should read quite a bit less than 1M (about 688k) unless one side of the potentiometer R893 is open. If it reads 1M, it looks like a new pot is in order--as you've already concluded.It looks like the pot, 311-2357-00 may be exclusive to the 2225 and the pot appears to be a Tek-exclusive model. So your eBay inexact replacement may be the way to go.

author: thevamper
author_rank: Newbie
timestamp: April 28, 2021, 07:09:42 pm
content: Quote from: bdunham7 on April 28, 2021, 03:17:17 pmThanks for the clarification. Just checked R892 again and it reads 1M no matter what. So it looks like I will move forward with replacing the pot.Quote from: jdragoset on April 28, 2021, 11:30:39 amThanks for the pictures. I'll order the ebay plastic pot and let you know how the repair goes

author: thevamper
author_rank: Newbie
timestamp: July 18, 2021, 04:35:15 am
content: Thanks for all the help everyone. I replaced the pot and the scope focuses fine now. I also 3D printed an adapter to fit the new pot to the old control rod since the new pot is different physically. I have attached a pic of the 3D printed adapter. It just converts the knurled knob on the new pot to the D-type interface type that the old pot used. I also attached a pic of the new pot installed to the rod with the adapter.

author: jdragoset
author_rank: Frequent Contributor
timestamp: July 18, 2021, 10:45:07 pm
content: Great