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Old 08-06-2018, 12:29 AM   #1
redwire
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Default Recapping Tektronix 2235 PSU

I'm recapping the power supply in a Tektronix 2235 late-1986. The scope seems to work but just to ensure no troubles. It's lean for electrolytic capacitors, they seem to be the minimum value that would work.

The primary capacitor is a Sprague can-electrolytic 75uF 450V that tests OK at 90uF and 0.17 ESR for Tek P/N 290-0978-00 (C906). But 31 years old.

I initially selected UCC 82uF 450V but I see Tek upsided the cap for the 2235A model to 270uF ?!

Another problem is modern capacitors are all -20% verses the older -10% spec.
So my new 82uF cap ends up smaller than the original, measuring 72uF... which annoys me.

I can fit a 10mm snap-in which I think I will do, but what value? The PSU is rated 40W, 10R NTC so the on/off switch


The Sangamo 840uF 12V (290-0945-00) did not age well. They have a hard epoxy seal which is cracked on most of them. One took a leak on the PCB and had high ESR. I put in 1,000uF 25V UCC KYB's.

I replaced the Rifa PME271 X, Y caps notorious for burning up. Damn things are still for sale at Digikey.
Attached Images
File Type: jpg IMG_0698_enh.JPG (124.1 KB, 50 views)
File Type: jpg IMG_0700_enh.JPG (83.7 KB, 45 views)
File Type: jpg IMG_0681_enh.JPG (266.8 KB, 59 views)

Last edited by redwire; 08-06-2018 at 12:30 AM..
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Old 08-06-2018, 03:26 AM   #2
Per Hansson
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Default Re: Recapping Tektronix 2235 PSU

The 2235A has a 75w power rating, that's why they upped the cap size on it...
The original 2235 cap has a tolerance of -10% to +50% so I would not fret at all over any differences, it's not like it's a critical cap.
75F does not sound like a small value for a 40w PSU to me, but if you want you can go up to 112F and still stay within the original design guide specifications following that 50% tolerance...

That said your 82F cap that reads 72F is almost spot on to the correct rating of 75F so not sure why you are unhappy about it?
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Old 08-06-2018, 05:19 AM   #3
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Default Re: Recapping Tektronix 2235 PSU

pay less atention to the uF and more to the ripple current on big smoothing caps.
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Old 08-06-2018, 02:28 PM   #4
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Default Re: Recapping Tektronix 2235 PSU

Part of my confusion is I am also recapping a 2235A, as well as the 2235.
I'm not sure why the 2235A has a larger power supply spec 75W vs. 40W and upsized some capacitors so much, like 75uF->270uF primary C906 and 840uF->4,700uF +5.2V rail C970.

I didn't want to replace the electrolytic capacitors with less than the Tektronix originals.

I'm finding new United Chemi-con parts are low bogeys, at least -10% to -13% of rated value, where the 30 year old parts, Spragues were +17% and the Nichicon are bang on.

It means oversize by over 20% just to make up for cheapness, lol.

+42VDC pre-regulator has 1,000uF 50V +50/-10% UCC SME so I put in a 1,200uF 63V UCC KYB part.
The original measures 980uF and the new 1,200uF measures 1,070uF or -11%.

The 2235 primary cap, one rule of thumb is 2-3uF/W output so 80-120uF is reasonable.
The 2235A primary cap is Nippon Chemi-con 270uF 450V 4-pin snap-in. I am looking to see what will fit as 4-pin are obsolete here.

When I finish I will post the capacitor sizes of 2235, 2235A originals and the replacements I used.
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Old 08-10-2018, 02:42 PM   #5
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Default Re: Recapping Tektronix 2235 PSU

I can't find a replacement for the 2235A primary cap C906 270uF 450V 4-lead snap-in. Tektronix p/n 290-1206-00. Four pins on 22.5mm circle, big spacing and about 35mm dia and 35mm height.

I see only 2-pin 10mm snap-ins out there.
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Old 08-11-2018, 04:32 PM   #6
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Default Re: Recapping Tektronix 2235 PSU

What about a slightly larger cap?
P.S: look at the datasheet, not the picture!

http://uk.farnell.com/kemet/alc10c39...-in/dp/2456748
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Old 08-12-2018, 05:13 PM   #7
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Default Re: Recapping Tektronix 2235 PSU

Thanks for the possibility.
At 35mm height and 35mm dia. C906 is very close to the input-filter cover, in 2235A see pics.
There is a Kemet 270uF 450V part 35x35mm ALC10(C/E)271DC450 but no stock could I find.
Tektronix P/N 290-1206-00 is also used in 22xx series and TAS455, TAS465, TAS475.

I did some math and diagonally the main/side-pins are 11.25mm apart so a 10mm snap-in could fit, if I jumper one not-connected side pin. So putting the cap between (+) PCB pad and a side-pin, with jumper to (-) pad.
But it looks like ~22mm dia part is max. and would still hit the ferrite bead or input-filter cover.
Attached Images
File Type: jpg Tektronix 2235A PSU pic.jpg (113.7 KB, 26 views)
File Type: jpg Tektronix 2235A C906 pic.JPG (200.0 KB, 31 views)
File Type: jpg Tektronix 2235A C906 bottom pic.jpg (116.7 KB, 24 views)
File Type: jpg qservice Tektronix 290-1206-00.jpg (6.1 KB, 21 views)
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Old 08-15-2018, 02:07 PM   #8
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Default Re: Recapping Tektronix 2235 PSU

220F 450v: ALC10A221DC450
330F 400v: ALC10A331DC400

Both are in stock at Mouser...
But if you want that perfect match maybe take a long shot and e-mail Kemet to ask for samples?
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Old 08-18-2018, 11:20 PM   #9
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Default Re: Recapping Tektronix 2235 PSU

I decided to make an adapter PCB, $2 ea for fab.
It converts a 4-pin snap-in footprint (leads on 22.5mm circle) to a two pin LS=10mm snap-in cap. The PCB is 28mm OD but 22mm or 25mm OD cap will be fine for 270uF 450V. Using machined pins to standoff the PCB from the 2235A main board.
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Old 08-19-2018, 01:56 AM   #10
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Default Re: Recapping Tektronix 2235 PSU

Very nice, be sure to post some pictures when it arrives.
Maybe also order a few extras incase someone else runs into this and wants one?
You could be the local dealer
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Old 11-24-2018, 01:11 PM   #11
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Default Re: Recapping Tektronix 2235 PSU

For the 2235A primary capacitor C906 270uF 450VDC, I found an adapter PCB is not necessary.
Because a 10mm snap-in cap can be made to work if you straighten the pins and add a jumper, putting the new cap across diagonal at the old cap's extra 4-pins.
I went with Nichicon LGN2W271MELA50.
Attached Images
File Type: jpg 270uF caps Tek2235a.jpg (76.3 KB, 23 views)
File Type: jpg Tek2235a adapter C970 270uF cap pcb1.jpg (118.2 KB, 23 views)
File Type: jpg Tek2235a C970 270uF pcb.jpg (173.5 KB, 26 views)
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Old 11-24-2018, 01:14 PM   #12
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Default Re: Recapping Tektronix 2235 PSU

Here's my spreadsheet with power supply capacitors for both the Tektronix 2235 and 2235A scopes, listing size and part numbers.
Just to save time measuring things, it's no fun with calipers.
Also to compare the two models, as the 2235A upsized some caps. Sort by column A model #.
Attached Files
File Type: xlsx Tektronix 2235_2235A recap list Ver1.xlsx (16.3 KB, 18 views)
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Old 03-07-2019, 08:16 AM   #13
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Default Re: Recapping Tektronix 2235 PSU

Thanks, just used this list to recap the SMPS in my 2213A. It appears to share all of the same caps in the list. All installed.

I have 2 questions:
  1. about your choice for 2235 C956;

    C956 Original: 270μF 10+100% 40V Mallory brand (pic attached)

    List C956: 47μF 20% 63V Nichicon UPW series

    The capacitance is quite a bit lower in the list's suggestion. I'm just curious why it is ok to use this lower capacitance for this particular replacement?

  2. For C906, the 4 leaded one. 3 of the leads are - and one lead is +. When using the replacement 2 leaded cap; I can just have the + lead in the + hole and the - lead in any one of the 3 - holes on the pcb, right? Added pic of how I have it connected.
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File Type: jpg 20190307_094843.jpg (1.67 MB, 16 views)
File Type: jpg 20190307_101125.jpg (2.89 MB, 20 views)
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Old 03-09-2019, 12:54 AM   #14
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Default Re: Recapping Tektronix 2235 PSU

Look again in the list, C956 is 330F 50v for the new cap on the 2235A scope.
As for the bulk capacitor just use your multimeter and check that you have zero ohm between the three negative posts.
If you don't look above at post #11 where redwire shows where he put a link.
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Old 03-14-2019, 09:29 PM   #15
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Default Re: Recapping Tektronix 2235 PSU

For C956:
2235A has a pi-filter with C956 10uF and Tek added C957 270uF, for the +30V rail. So there are two capacitors.
2235 has no pi-filter or C957, so only C956 270uF.

To replace C906, put the new cap (+) lead in hole 3 and the (-) lead in hole 4. Since hole 1 is pcb (-) add a jumper from hole 1 to hole 4 on the PCB.
Remember it is high voltage (mains) so the jumper should be insulated, no flux mess and not be close to other traces or short to the metal cover when you slide it back on.
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Old 04-02-2019, 03:59 AM   #16
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Default Re: Recapping Tektronix 2235 PSU

Thanks fellas, just pointing out that in the xml sheet that c956 for the 2235 has a typo, but the 2335a has the right one. Probably obvious to most but wanted to mention it. My fault for missing that, still greatly appreciated the .xml, well made.

For C906 on the 2335 (non A): Based on Hansson's suggestion, It appears that the 3 negative pads on the bottom side are all connected to the same trace and share continuity after testing.

Unless I'm overlooking anything else, I assume only the 2335A version needs the jumper.

Thanks again for your time!

Last edited by guy232; 04-02-2019 at 04:12 AM..
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