Saturday, May 9, 2015

Tektronix 2246 Repair


Last year I had talked about how I accidentally destroyed my Tektronix 2246 oscilloscope by removing the cover. There was a small dent in the metal bottom panel that while removing it had snagged a heatsink on a Motorola 151-0846-00 labeled TO-39 transistor, specifically Q702 on the A10 main board. This ended up ripping the pins out of the transistor can leaving me with a unusable scope. Unfortunately this was not a common part and trying to locate one is next to impossible. I had attempted to use a 2N3866A with no success, the scope was definitely not happy with that transistor in it, an original replacement part would be necessary.

I did ultimately find a donor board. Late last year I found a complete used replacement board on eBay for around $40 which contained both of the 151-0846-00 labeled transistors. This option was definitely cheaper than a whole parts scope and I would hate to ruin another 2246 which may be repairable just for this one transistor. Once I replaced this transistor in my 2246 it was good as new.



I want to focus the attention now to a second Tek 2246 scope that I own with its own set of troubles. I purchased another 2246 a few years ago for cheap, I think it was only around $100. This scope had a lot more use than my original one, while it was in good cosmetic shape it had some issues mostly related to old capacitors. The display was not stable, the character generated osd and cursors would jump around and upon probing the supply rails you could see there was some noise present. A re-cap would be necessary and possibly some additional caps on the A-10 main pcb if necessary.

The 2246 main power supply is pretty basic, much more simple to work on than the Tektronix 2445B power supply which I have also done. One interesting observation upon accessing it is that all of the caps look to be large axial electrolytics:

Trktronix 2446 Power Supply

The bottom of the board told a different story, and once removing a capacitor and testing it was indeed a standard radial cap with an interesting third lead out the top that wasn't connected to anything. Maybe it was designed for additional stability? I know this scope was a popular portable model so my guess it was just some additional ruggedness built into the design. Interesting regardless, at least I can replace them with standard radial caps which are much easier to source.


Replacement caps were all Panasonic 105 degree C. units which are always my first choice, then using Nichicon capacitors in cases where the Panasonic's were not available. The final rebuilt version looked like this:

Tek 2446 Power Supply Rebuild
Quite a difference in size. For good measure I also replaced all of the high voltage film capacitors on the board, many looked stressed as they did in the 2445B. I left the large primary switching capacitor alone, they are rarely every a source of trouble. Everything else looked okay.

This is where things started to get interesting. After putting everything together and powering the scope back up, I had strange display issues. The entire display was shifted left. I started looking around to see if I had missed a cable or possibly had a connector loose during the reassembly but didn't see anything obvious. This scope was working just fine before the power supply rebuild so this issue was definitely something that I caused. At this time I went to turn the scope over on its side while powered up that the display snapped back into alignment. After some more poking around I realized that if I put pressure on the chassis, twisting it just lightly I could get the display back in alignment. So it must be a bad ground, loose connector, bad solder joint, or some other mechanical failure where putting pressure on the chassis would complete whatever broken connection was occurring. It will just be a matter of tracing down where the issue is at. More to come in part II.

7 comments:

  1. Hi Brad Boegler,

    Trying to recap 2247A PSU having same design as this 2246. But need to find matching parts either Panasonic/CHEMI-CON/Nichicon CAPs.

    Really appreciated if you could share your part numbers list used in this rebuild?

    ReplyDelete
  2. Hi Darren,

    I believe this was my order for this recap. It may not include everything as I did have some of the caps on hand, but it should cover most of it:

    Quantity Part Number Description
    2 493-4807-1-ND CAP ALUM 39UF 20% 200V RADIAL
    1 P11280-ND CAP ALUM 1000UF 20% 63V RADIAL
    10 P14397-ND CAP ALUM 1000UF 20% 16V RADIAL
    5 P14419-ND CAP ALUM 470UF 20% 25V RADIAL
    4 P10768-ND CAP ALUM 4.7UF 20% 100V RADIAL
    2 P15288CT-ND CAP ALUM 100UF 20% 63V RADIAL
    3 P4619-ND CAP FILM 1500PF 20% 250VAC RAD
    10 S5.1HCT-ND RES 5.1 OHM 1/2W 5% CF MINI
    10 P5.1BBCT-ND RES 5.1 OHM 1/2W 5% AXIAL

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  3. Hi Brad
    I bought a brand new 2246 scope that the US Navy had on the shelf untouched for the past 28 years (now almost 29). Good old eBay. It seems to power up fine and go thru it's logic sequence but the characters on the CRT shimmy and the H line has many small noise spikes. I hit it with a sine wave and it tried to display amplitude and responded to frequency but the display looks chopped (non-continuous) and it won't lock on sync. The voltages are also interesting. Using the Service Manual I checked all of the voltages. They are all "within specified range" EXCEPT for the reference +7.5V which is out of recommended range (6.58V). Adjusting this immediately moves every other voltage out of it's specified range.
    Any ideas? Are you in Canada? Me too.

    ReplyDelete
  4. Hi Brad,

    Thanks for the part number list.


    Hi A2DD2A,
    Recap PSU likely solve those problems

    ReplyDelete
  5. Hi Brad,

    Due to shipping cost, I decided to order from local vendor but they mostly sell Chemi-Con, Nichocon and some Panasonic.

    End up I choose Chemi-Con first follow by Nichicon or Panasonic.

    ==>some increased value based on SVC Manual which could go up 100% or some even 4000%
    My Part list:-
    SYMBOL Replacement PartNum
    C2203 ELXZ500ELL222MM35S
    C2221 ELXZ160ELL152MJ30S
    C2222 ELXZ160ELL152MJ30S
    C2223 ELXZ160ELL152MJ30S
    C2224 ELXZ160ELL152MJ30S
    C2225 ELXZ160ELL152MJ30S
    C2226 ELXZ160ELL152MJ30S
    C2227 ELXZ160ELL152MJ30S
    C2228 ELXZ160ELL152MJ30S
    C2208 EKMG161ELL4R7MF11D
    C2210 EKMG161ELL4R7MF11D
    C2238 EKMG161ELL4R7MF11D
    C2232 EKMG161ELL470MK20S
    C2233 EKMG161ELL470MK20S
    C2229 25ZL680MEFC10X20
    C2230 25ZL680MEFC10X20
    C2234 25ZL680MEFC10X20
    C2236 25ZL680MEFC10X20
    C2204 UEP2A101MHD
    C2248 UEP2A101MHD
    C2213 100pF, 250VAC, ceramic cap
    C2214 100pF, 250VAC, ceramic cap
    C2217 100pF, 250VAC, ceramic cap
    R2272 CPF210R000BHE14
    R2273 CPF210R000BHE14

    ReplyDelete
  6. A bit of an old post - but do you still happen to have that parts board and could spare the other 0846? I have a scope that either had that issue previously or I caused it taking it apart. Either way i need a new one to continue troubleshooting. Thanks!

    ReplyDelete
  7. Dears, I have 2 of 2246 that I would like to recap. My first question is: how to discharge the crt before take out the power supply?
    Second: do you used a isolation transformer to test it?

    ReplyDelete