Drake AC3 / 4 Power transformer

Discussion in '"Boat Anchor" & Classic Equipment' started by G4COE, Dec 5, 2018.

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  1. G4COE

    G4COE Ham Member QRZ Page

    Any one know the primary resistance of the two primaries? I was asked to replace one that was wired for 115V and kept blowing the mains fuse and reads reads 1.85 Ohms with primary in series out of circuit.. the guy bought another one that was wired for 240V..... this devil reads 1.4 Ohms - even lower, I'm reluctant to even try it without knowing first, thanks. Dave.
  2. W9GB

    W9GB Ham Member QRZ Page

  3. N2EY

    N2EY Premium Subscriber QRZ Page

    Are you sure they are phased correctly?
  4. G4COE

    G4COE Ham Member QRZ Page

    Sorry, not asking for the schematic, I've got the schematic, I'm asking for the Primary resistance. Yes, they are 'phased' correctly, primary in series for 240v .... and in parallel for 120V.

    It's the primary winding resistance I'm after.

  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I just measured the winding resistance of my AC4 transformer with the windings wired in parallel for 120V operation. I get 0.6 ohms measured with a Fluke 87 V it's a two wire measurement and standard Fluke, not 4 wire Kelvin probes so take that into account.

    That's from a properly functioning Drake AC4.
  6. G4COE

    G4COE Ham Member QRZ Page

    K7TRFE, excellent, many thanks for the info and your time, that's what I was
    after..... primary must have pretty hefty thick wire, reason primary is so low resistance .

    I gotta admit I was expecting some 10 -15 Ohms or so - I was testing off-load (transformer removed) and a 1Amp AT fuse, a bit low I guess, I'll try a 3 amp AT fuse

    I guess since you'll have the primary in parallel for 120V, I'll take that about 1.2 per winding meaning, for UK 240V it'll need be around 2.4 Ohms for series windings, the reading I got was 1.4 & 1.8 Ohms.... seems 'dodgy'. Only one thing to do is pop a 3 Amp fuse in n' cross my fingers and toes, which what I should have done in the first place.... ain't my transformer though.

    Thank you again for your good help.

  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I don't think your 1.4 to 1.8 measurements sound all that bad. Remember I'm measuring this with a stock DVM and two wire leads, I don't expect accuracy down to a tenth of an ohm with a measurement like that.

    Sure bringing the transformer up on a variac or using a light bulb current limiter would allow a bit safer initial test than just fusing the transformer but based on accuracy of our compared measurements (or lack thereof) I don't see huge problems. Now if you see a low resistance path to any of the secondary windings or to the transformer shell, that would raise suspicions but the difference between 1.2 ohms expected (based on my parallel winding measurement) and the 1.4 to 1.8 ohms you measured seems reasonable.
    G4COE likes this.
  8. G4COE

    G4COE Ham Member QRZ Page

    Yep, good idea to get the variac out and a light bulb as ballast the meter I'm using is a Greenlee DM860A, akin to a Brymen BM869A I think it is.

    I've not done any test as yet regarding the other windings, certainly worth doing I must agree. It'll get tomorrow now and I'll try a bigger fuse, the inrush, wondering if the inrush is killing it seeing it's only a 1A AT, I'll try a 3 amp slow blow well and get back probably tomorrow.

    Much appreciated, many thanks. Dave
  9. G4COE

    G4COE Ham Member QRZ Page

    Transformer was OK, lobbed a 3Amp slow blow in series switched on.... no big bang as I expected all voltages appeared - how disappointing! I went straight for the jugular..... didn't bother with the lamp bulb and variac.

    So t'was the in-rush that blackened the 1A 'test fuse'.... I'll go ahead with the capacitor mod kit from Electronic Specialist in Iwoa.

    Thank you for your time and information that you provided, the low 'R' got the better of me.

    K7TRF likes this.

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