Tektronix 2235 Oscilloscope Restoration

In this video, I restore a Tektronix 2235 analogue oscilloscope.

Below is all the details of the obscure parts I used:

Schematics and documents:

Digikey Parts List:

TekWiki link:

Details on parts substitutions:

- 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.
- The large silver 450V 75uF cap was 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)

[nortonfan]

I'm really pleased you posted this video. I, too, have this model scope in need of attention; the scope does not show a trace on the screen. What I discovered was, the push-on terminals to the crt were quite loose and not making a proper connection to the pins on the crt. I located the failed wire terminals in the Allied Electronics catalog, fortunately;' and, to my complete amazement, they were quite inexpensive. I plan to make the upgrades you have so clearly shown in this video. Thanks for that!....and keep up the good work! Cheers from California, USA,

[jrmiller]

Thank You for posting the parts that you replaced on your scope. I picked up a model 2235 that was on sale at a going out of business sale. Another buyer turned on the scope and it smoke. They unplugged it and the smoke continued for about 10 minutes. It poured out alot of smoke. They placed the scope outside and at the end I purchased it for $10.00. I took it home and opened it and it did not show any burnt components and it turned on but does not fully operate correctly. I ordered all the parts you replaced and will attempt to restore it.

[fullwaverecked]

NFM, I watched this a while ago and now I'm about to get one. Thanks for all you do making awesome vids. Totemo ii desu yo!

[robhampshire]

WOW is this helpful! I turned on my 2235 last night and was met with nothing but a dead scope - no power LED, no sounds, no smoke, no nothing. I'm sure it's a power supply issue, the thing was sketchy when I bought it in "beaten like a dead horse" shape several years ago - but I got it CHEAP! I pulled it apart and am about to make your digi-key parts list order - had to make two subs as a couple caps are backordered, no worries there. But the area of the PCB around the fan output area of mine is TOTALLY different than both yours and the service manual so I'll have to sort that one out eventually. To NFM and others - I may be back with questions if that's cool! I'm not exactly sure where some of the caps in the digi-key list go - most are obvious but some are not.

[garyfiber]

I have the same model scope, You have listed some of the more obscure parts, but do you have available and can list all the Caps and items you purchased from DigiKey for this project?. I would like to do the same project to my 2235. The Link at the video bottom helps, but does not include and entire listing of the parts used.

[SoddingaboutSi]

That multi turn pot is standard. I have one of these in the loft. My Vertical attenuator know has faded too.

[grumpy_ken]

great repair/update tips. I just drug home a 2236 (same scope + frequency counter) today that doesn't power on

[DishNetworkDealerNEO]

I just bought a 2235 last night. I am going to be revamping the power supply as this vertical wavers a bit from power supply ripple. The Digikey list has three obsolete items but most items are still available.

[oldwhitelodge]

I have just acquired a virtually unused 2235 Option 1 AN/USM-488 DoD version of this oscilloscope. I've successfully changed all the electrolytics in the PSU and added a fan as the pass transistor does get rather warm. Interesting, the PSU layout is different in parts to the standard 2235 and the location of the empty pads for the fan is in a completely different position but still fed from pin 20 of the transformer. I'd add a photo if I could.
Now, that is where all my troubles began!
The scope went bang three times during burn-in runs – yes four times!
#1: a big flash that took out P9070, Q908 and R906. I replaced the ‘594 and all seemed well.
#2: Pre-regulator cycling – at least this showed that protection was working) but the inverter wasn’t. I eventually tracked this down to a shorted capacitor, C944. This was a tiny one but had shorted Q947 and Q946 bases to ground. Replaced and all sprang to life.
#3 Again pre-regulator cycling (smile). There was a dead short across C940. After taking half the components out of the inverter I eventually discovered that CR907 had gone short and started a burn-in cycle yet again.
#4 The 2235 stopped working after 30 minutes so I concluded it was a heat problem. Again CR907 gone short. Replaced and things came to life. I found CR907 to be exceedingly hot so I made a small heatsink for it out of aluminium and this kept the temperature to less than 40 degrees.
All seems OK so far but we will see I’m sure.
I am in the UK with a mains supply of 240V so that may help explain the overheating.
That pre-regulator is a bit of a pig.

[ANTALIFE]

So glad I found your video! By the way do you think doing all this restoration requires recalibration?

[Mezmorize]

Glad to see a new video from you, love your repair videos of test gear :-)

[grahamcarter]

NFM, many thanks for the informative video and extra info! I have a 2235 which I've had from brand new (sometime in the early 80s) and although it's never had a fault I'm sure the RIFA caps and others are living on borrowed time. Like you I'm in AUS and I'd like to order from Digikey but your link to a Digikey cart of parts appears to go the USA Digikey website, not the AUS one. Or at least all the prices are in USD and it looks like the US website. So did you buy from Digikey in the US or down under here in AUS?

[fichambawelby]

Excellent restoration, indeed, and excellent and well explained procedures, as well!. BTW...could you post or recommend some web sites to buy one of this antique marvels?. I am in Madrid, Spain, and I am looking to buy a CRT scope of almost 200 Mhz BW. TIA to everybody.

[mikemoran]

I just bought one of these and it works great, but i want to preserve it. Would you happen to have a list of all the caps you replaced (new ones)?

[jorgeandrade]

I really enjoyed the video, I have a question and a suggestion: How did you order the components? Do you manually select each item or do they have some sort of BOM? Instead of simply showing the before/after, it would be more beneficial for your viewers, especially beginners, if you actually replaced a component or two on camera.

[barryballenger]

thanks so much for this video, i have watched it several times and constantly stop it and review your advice. I have got two 2235s that I am working on. Had to replace a CRT in one as it was damaged in shipment. Can I as future questions as I go through your process of restoring and upgrading?

[tallB]

Great info, thanks so much! Made good use of your documentation and the Digi-Key list too. I'm only replacing a few things to start. The can cap was previously replaced so I'll do RIFA's and six 1k electrolytics. Anyway, dumb question: how does the black plastic PS cover on the bottom come off? It has 1 bolt, then I thought it would pull out like the top. Need to get the RIFA's out, I see the lower one looks cracked. (The scope works well, but hasn't been powered up for a while - no worries)

[stephentreadwell]

Thanks for this video. I was just given a 2235 by a friend who has been retired for many years. It works (amazingly as it has sat unused in an unheated metal building for years), but needs a really thorough cleaning and adjustment. I'm going to recap it and install the mods. I have a couple of questions: the ECO says to replace R949, a 0.51 ohm resistor with a 0.75 ohm resistor. My scope has a 1 ohm resistor in that location (my manual also lists it as 1 ohm). I noticed in your video that you also have a 1 ohm resistor there and didn't change it when you did the mods. Looks like it works OK, so maybe I'll leave it alone, especially as the ECO say not to change the resistor as the "2235 instruments already have the correct resistor". But I wonder if the "correct" resistor is 1 ohm or 0.75 ohm? My second question is regarding the two transistors Q946, 947. The original parts list refers to these as a matched pair. Did you replace these with TIP41C, and if so, did you attempt to match them and how - also, which parameters need to be matched: Vbe, Vce(sat), hfe?

[gregj.gotham]

My problem with my 2235 is in the sweap circuit all I have is two stationary dots with no movement across the screen. Looks like someone already did power supply caps except the Rifa caps. Darn I just got it from eBay to late to send back.

[tomb]

Very nice video for I am just starting into electronics and I bought a 2235 that sat for 10 years and went to turn it on and no power light, I should have sent it back but I would like to fix it. it does make that woosh sound but stops and does it over again every second, not really firing up the whole way I guess. I can tell by some of the solder joints in the high voltage area that some parts where replaced.
I think I will start with that large 120uf cap in the power supply, it is original.
Then go the the 3 2200pf you said to replace if I can find them.
Big project for a beginner.