Monday, January 13, 2014

Tektronix 2445B Capacitor Replacement

Due to the fact that my Tektronix 2246 is currently out of commission (although parts are on the way as I did find a parts unit 2246 scope) I have focused my attention to my other favorite Tektronix scope, the 2445B. I primarily use my 2246 as it was the first 'real' scope I purchased about 10 years ago and is a rock solid analog scope. A few years back I came across a 2445B for a good price and have used it mostly for portable use as it is slightly smaller than my 2246. My 2445B also has Tektronix calibration stickers intact from 2002 indicating at least that no one has been inside mucking around with it.

My specific 2445B is the 150Mhz version and is in excellent condition although there have always been a few issues. The on screen display would jump around and not have a clear focus regardless of the focus adjustment and other internal astig calibration controls. When changing volts or time functions there would also be erratic display behavior. Knowing that the scope was over 20 years old and has unknown operational hours, I decided to start with a full capacitor replacement of the power supply. Electrolytic capacitors do go bad over time from several factors, so replacing them is a common step in all old electronic equipment.

The first step is to take it apart and inspect it. I will say the 2445B is one of the better scope designs I have worked on from Tektronix. The design is very modular with the power supply being very easy to access. Once removed, the power supply pulled apart into two boards shown here:

Tektronix 2445B Power Supply
There are quite a few caps between the two boards, luckily most are radial caps and not axial. As I began inspecting the caps themselves I was surprised to see that some were rated for only 85 Degrees C instead of the higher 105 Degrees C rating. I also noticed that some were definitely showing some 'bulging' which is usually indicative of high heat and ultimate failure, although none were leaking. Some other things I noticed were that all of the high voltage film capacitors looked stressed. They had slightly bulged as well and had crack marks on their casings. A few resistors also looked bad including two 15 Ohm ones within the surge protection circuitry in the line level input stage. One had actually blow open and had a physical crack in it.

Additionally looking at the A5 Control pcb I noticed that there were four surface mount electrolytic capacitors on it. I learned how leaky these caps can be after a complete Tek TDS-420 restoration which needed a total of about 75 of them replaced. Looking closely at the board I could already see they were leaking and causing corrosion to traces, so they would be replaced as well.

Tek 2445B A5 Control PCB Caps

Ultimately I decided to replace every capacitor on the power supply boards and the four smd caps on the control pcb. I would leave the caps on the A1 main pcb alone for now as they are primarily low voltage and would be considerably more work to replace. If there was still issues post-recap I would then consider replacing them as well.

For electrolytic capacitors I always go with Panasonic 105 Degrees C units. I will use Nichicon 105 C unit if a Panasonic value is not available. The film capacitors were replaced with Kemet units which were actually also the originally spec'd components. I upped the blown 15 ohm resistors to 3W units. For those interested in re-capping their own Tek 2445B (or variants) here is my Digikey parts list:

Quantity Part Number Description
6 P13476-ND CAP ALUM 100UF 25V 20% RADIAL
2 P13125-ND CAP ALUM 47UF 25V 20% RADIAL
4 P13123-ND CAP ALUM 330UF 25V 20% RADIAL
2 P13131-ND CAP ALUM 220UF 50V 20% RADIAL
1 P13465-ND CAP ALUM 4.7UF 50V 20% RADIAL
1 P5874-ND CAP ALUM 3.3UF 450V 20% RADIAL
4 P10769-ND CAP ALUM 10UF 100V 20% RADIAL
4 493-10252-1-ND CAP ALUM 1UF 50V 20% RADIAL
2 P15W-3BK-ND RES 15 OHM 3W 5% AXIAL
1 989-1206-1-ND RES 270K OHM 3W 1% AXIAL
1 399-7494-ND CAP FILM 10000PF 250VAC RADIAL
2 399-5410-ND CAP FILM 2200PF 250VAC RADIAL
2 PCE3777CT-ND CAP ALUM 33UF 10V 20% SMD
2 PCE3833CT-ND CAP ALUM 10UF 35V 20% SMD
2 493-1421-ND CAP ALUM 330UF 250V 20% RADIAL

Most of the Tektronix 24NN family (2445, 2455, 2465, 2467, etc) has the exact same power supply, so the component list above should apply to all. Here are the rebuilt power supplies once all components arrived:

Tek 2445B Capacitor Replacement
Once completed there were about 45 components in total that needed to be replaced. While the large Sprague 290uF caps were probably were fine I replaced them for good measure anyway.

Tek 2445B Capacitors

All completed and reassembled my 2445B has a beautiful display and extremely sharp digital readouts. I don't see any need to replace the A1 pcb caps at this time but will still consider it for the future. With a total component cost of $31 and about two hours of time I would recommend this rebuild for any original 24NN scope.

Rebuilt Tektronix 2445B


  1. Hi, shouldn't be P10729-ND instead of P4639-ND? Thanks for the list!

  2. Thanks for the list. I used to it replace the power supply caps on my 2465BDM. These differences may be a result of my particular unit:

    I had a 1 uF, 50V radial left over.

    There was a 4.7 uF, 35V radial hiding under the 220 uF capacitor holder. I replaced it with a 4.7 uF, 50V I had in my parts bin.

    There was a 10 uF, 160V radial adjacent to J304. I replaced it with 10 uF, 250V, 493-13127-1-ND.

    I put a heatsink on the infamous U800. I trimmed down the length of HS273-ND with a dremel cutoff wheel and applied it with thermal tape BER347-ND.

    The fan was replaced with 12 VDC, 80x20mm, 603-1048-ND.

  3. You seem to hve left out C1132 - 10uF 160V. It's on the 87V unregulated line, and the schematic notes that the voltage on that line is 98.7V, so if you're using a 100V, you're sailing close to the wind!
    I switched my 2465 on for the first time in a year or two, and after about 3 minutes, it made megasmoke and I had to take it outside so it wouldn't stink the house out. It turned out to be one of the oblong, high voltage caps in the HV power section that had blown apart.

    I have been finding it very difficult to source allof these caps. Nobody has all of them, and some, like the high capacity, high voltage ones (eg 250uF 200V - I see you used 330uF 250V) are on a 16-week lead time.

    1. Hello Mij. OP is using the partnumbers from Digi-key. For the Netherlands you could try Mouser Electronics. I have a problem with my 2445B and found everything over there.

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  5. Is there an updated parts list available?

  6. also need to replace 4.7uf@35 Tant and 270k resistor on A3 and A2 boards respectively. I have been working on 2465b and 2467bs have several of my own and most I buy and refurb