Tektronix 2235 upgrades? - Page 1
Forum section: electronics / repair

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 24, 2018, 12:37:18 pm
content: I'm currently working on restoring a Tek 2235 scope.I have successfully replaced all the electrolytic caps in the unit, and am also installing a fan for the PSU.I've seen some info regarding upgrades to the power supply (http://hakanh.com/dl/docs/kitinstructions/050-2242-03.pdf) which apply to my serial number, but there isn't much detail on what the 'diode-resistor network' (Part number 119-3511-00) actually is..Does anyone have some info on this, and also any other upgrades or service bulletins etc that I can perform to this scope while it's on the healing bench?Oh, and if anyone has a source on those super rare vol/div knobs, let me know!

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 24, 2018, 06:32:30 pm
content: I found this in the 2230 service manual:

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2018, 10:52:17 pm
content: The 2235 and 2236 were the first oscilloscopes available in that series starting in 1984. The 2213 and 2215 do not count because they are significantly different.The last oscilloscopes in that series were the 2232 (early), 2235A, and 2236A from 1990, 2221A from 1991, and 2232 (late) from 1992. There may not have been any differences in the power supply between any of these so find a 2232 or 2235A service manual for an idea of what changed.Incidentally, and this does not have to do with the power supply, the only difference I have noted between the early 2232 and late 2232 is the high voltage film resistors used in the focus circuits. This might be explained if Vishay bought Mepco/Centralab and I doubt this difference is significant. The 2235 which was older used Allen Bradley carbon compositions resistors which we now know had poor reliability and this might explain why Tektronix changed them.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 25, 2018, 04:28:39 am
content: Awesome, thanks!Those diagrams will help me nail it down. Cross referencing the rest of the parts shouldn't be tooo hard now.I saw mention of those carbon resistors in another thread, I might stick some metal film ones in. I found some Vishay HVR37 series ones that are good to 3500VDC/2500VRMS

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2018, 01:39:42 pm
content: Quote from: TERRA Operative on September 25, 2018, 04:28:39 amI am probably the one who mentioned them in that other thread. The Vishay HVR37 series is essentially what Tektronix used although I am fuzzy on the differences between the HVR37 and VR37.If your 2235 has the original carbon composition resistors in the focus chain, then I would replace them.

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 01, 2018, 12:31:32 pm
content: Alrighty, I got the thing back together after all the repairs and upgrades and it still works!I guess I should tweak it some more, we can't have it working first shot...Here's a rundown of what I ended up doing:Full capacitor replacement. Swapped like for like, except.Two tantalums on the attenuator board were upgraded to 50V.PSU output caps were upgraded to 1000uF.450V 75uF cap upgraded to 120uF.Replaced Rifa mains caps with new x/y class caps.Installed 60mm fan, swapped the 20ohm resistor to 10ohm to increase fan speed slightly.Replaced R888 - R892, R894 with new Metal film Vishay HVR series 1/2W 510Kohm.Performed Service update 050-2242-03 with the following details:119-3511-00 Diode-Resistor network built from an 1N4148 and 10ohm 1/4w resistor in series, cathode of diode away from resistor.151-0565-00 substituted with S4010LS2TP (original part in my scope was a C106B2).151-0852-00 substituted with TIP41C (original part in my scope was 476-02).151-1245-00 substituted with IRFBC40LCPBF (original part in my scope was an IRF710 by International Rectifier).307-0456-00 substituted with V250LA20AP (original part in my scope was V250LA15A)The scope fired up first try, no problems at all.I hope that all helps someone else in their repairs/upgrades.