Capacitors in rectifier board ft102. 22uf instead of 33uf Or 47uf ok?

Discussion in 'General Technical Questions and Answers' started by KG7HVR, Aug 2, 2020 at 11:58 AM.

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  1. KG7HVR

    KG7HVR Ham Member QRZ Page

    recapping the rectifier B board
    I have several 22uf 400v caps. higher voltage than original ones.
    I suppose it might work, but maybe not.
    Can some explain why
  2. KG7HVR

    KG7HVR Ham Member QRZ Page

    So this board has a 33uf @250v and thats the one i dont have. I have only 22 and 47uf at 400v

    Schematic and pic of board Screenshot_20200802-034127_Drive.jpg 20200802_040903.jpg
    Last edited: Aug 2, 2020 at 12:35 PM
  3. KL7SG

    KL7SG XML Subscriber QRZ Page

    Without doing the analysis, the small capacitors would result in increased ripple on the supply voltages. The larger capacitor values may have an impact on load regulation, it could be better regulation or it could be worse. The voltages in either case could fall out of the design specifications. These are not regulated supplies.

    My question is why not just get the correct values?
    KB0MNM likes this.
  4. KM3F

    KM3F Ham Member QRZ Page

    Explain why....what?
    The 33 is the first filter off the 120ac supply.
    It's voltage rating is the peak voltage value it will see plus some tolerance after input rectification.
    Right after that is a 56k 1 watt, so not much current is being drawn to power a bias regulator.
    What you don't want is to have any Ripple at the top end of VR 01.
    Use a Scope to check this if in doubt.
    There is more filtering off the variable arm of VR 01 to be sure.
    I would think a 47 is overdoing it a bit but is your choice.
    KG7HVR likes this.
  5. N1OOQ

    N1OOQ Ham Member QRZ Page

    Looks like it's supposed to filter an ac voltage to make dc. Probably won't damage anything to use a different value. You could check the two bias outputs... Looks like bias for your finals? You'd have to make sure there's no hum on your transmitted signal.
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    If I were going thru the trouble I'd opt for using the stated values. Even if it meant slowing down a few days to get an order in from Mouser.
    KG7HVR likes this.
  7. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Aluminum electrolytic capacitors are not precision parts. They typically have tolerances in the neighborhood of -20%/+80%. If the value were critical, the designer would not have specified electrolytics. It's generally safe to substitute a somewhat higher value filter cap, and chances are reasonable that the substituted part will even fall within the sloppy tolerance of the originally specified part.
    W9WQA, KG7HVR, KL7SG and 1 other person like this.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    With a choice between 22uf and 47uf to replace a 33uf cap, pick 47uf. A bit more capacitance will mean less ripple in the supply output.

    W9WQA and AG6QR like this.
  9. KG7HVR

    KG7HVR Ham Member QRZ Page

    My meter lights were flickering. I replaced everything except that 33uf and so far no more flicker.
    Ill order a 33uf.
    Most of my new stuff tests pretty close. The ones removed were all over. The 47 read about 70 and the 22 was 33.
    Seems they drift up in value. Wonder why.
    Lots of good information here. Ill hold off and get the right one here.
    Thanks for everyone who commented.
  10. KA0GKT

    KA0GKT Ham Member QRZ Page

    One thing many amateurs forget is the Operating Temperature rating for the electrolytic capacitor. When replacing lytics in equipment used by the broadcast stations I maintain, I use +150 degree C capacitors whenever possible, but back when I did land mobile work on the Great Planes, I also would make sure that I used capacitors with a -40 or -55 degrees C. The electrolyte can freeze at low temperatures, and having something fail due to low temperatures when lives may be at stake es no bueno.

    The difference in price between 85 C caps and 105 or 150 C caps isn't all that much when you take into account the time needed to replace the caps or in my case, the down time for the commercial gear. If you are planning to use your rig in sub zero conditions (Think Mobile in Northern States or Canada), consider tow temperature lytics. Remember, -40 C is the same as -40 F. Where I grew up in Nebraska, -40 F temperatures were not that unusual in the winter and +105 F not unheard of in the summer.
    Last edited: Aug 2, 2020 at 9:22 PM
    KG7HVR and KB0MNM like this.

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