Preventive maintenance on 30+ year-old Tek scope - Page 1
Forum section: electronics / testgear

author: blisterfingers
author_rank: Newbie
timestamp: June 24, 2018, 09:55:51 pm
content: Hi all - long time lurker, first time poster.I've just picked up a second-hand Tektronix 2235 100MHz scope to help me with troubleshooting guitar amp builds (multimeter alone isn't cutting it anymore). It has way more functionality than I'll ever need for this purpose but I got it for a good price.It's in good working order as far as I can tell. I've run through most of the familiarisation procedures in the operators manual and it displays a good clear trace when hooked up to a cheapo function generator.Given the age of the scope, the next step is to test the key internal voltages and check the power supply electrolytics with an ESR meter. Looking for advice on:Should I check all the electrolytics, or at least a sampling of them?What other components should be checked?Is it worth doing any preventative replacements other than out of spec caps?I mostly subscribe to the "if it ain't broke don't fix it" philosophy, but unsure about the best approach with a device of this age.I have the service manual and schematics, I know how to safely discharge capacitors and work on energised devices. Not planning on messing around with the CRT at this stage if I can avoid it.I've found a few articles and threads on resurrecting dead devices but nothing to date on care and feeding of old scopes.Any pointers would be greatly appreciated.

author: bd139
author_rank: Super Contributor
timestamp: June 24, 2018, 10:14:57 pm
content: I’d leave it until something goes wrong. The only weak points on the 22xx are the inverter assembly and the intensity pot really plus any axial Philips capacitors inside. The inverter and intensity pot are virtually impossible to get hold of.Honestly if you really like the scope, see if you can get your hands on another working unit as a backup if one goes past your eyes. If this one blows up you can try and fix it and if that fails use it as a parts mule for the other one.

author: rstofer
author_rank: Super Contributor
timestamp: June 24, 2018, 10:51:28 pm
content: Scopes are really 'about' kinds of gear. If the trace moves 'about' the right amount as a function of input voltage, good enough. Is the time base anywhere near right? If so, leave it alone.Until it breaks, nothing good will come from opening the case. I bought a well used Tek 485 about 14 years ago and I haven't opened it yet. It is surprisingly accurate and for my purposes, that is good enough. I have no idea when it was built but probably around 1980 (produced from '72 - '86) so it's probably 38 years old, give or take.If I want to know what is inside, there are plenty of YouTube videos. Let them mess with their scopes, I'll leave mine alone.I like the idea posted above of watching for a good deal on a duplicate.

author: nctnico
author_rank: Super Contributor
timestamp: June 24, 2018, 11:48:00 pm
content: Quote from: bd139 on June 24, 2018, 10:14:57 pmI totally agree! If it ain't broken don't ruin it!

author: tggzzz
author_rank: Super Contributor
timestamp: June 24, 2018, 11:59:04 pm
content: 30 year old equipment tends to have RIFA mains filter capacitors. Pro-actively replace them before they detonate, spread carbon everywhere, and remove some of the PCB's prepreg.https://www.google.com/search?q=bad+rifa+capacitor&tbm=isch

author: chris_leyson
author_rank: Super Contributor
timestamp: June 25, 2018, 12:57:48 am
content: QuoteI would replace the electrolytics for sure especially in the power supply where the ripple currents are higher. I'm re-capping a Chase HFR2000 that's about 35 years old, one of the caps in the power supply went short circuit.

author: blisterfingers
author_rank: Newbie
timestamp: June 26, 2018, 02:45:42 am
content: Thanks everyone for the advice.I think my plan is to be as non-invasive as possible but replace the RIFA protection caps, and possibly the other power supply electrolytics if their ESR is too high.Will post some pictures as I go along.Cheers

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2018, 02:51:24 am
content: Dig out the Service manual for it online and have a good study of its workings and especially the PSU where these seem to be the most problematic.Not that you need this but it's good to have when you need it:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf