Drake L-75 Hum on Transmit problem

Discussion in 'Amateur Radio Amplifiers' started by K5JCJ, Jun 26, 2019.

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  1. K5JCJ

    K5JCJ Ham Member QRZ Page

    I had a local tech replace the caps and HV resistors and diodes in my Drake L-75 (single 3-500z) amp. A few days of operation later the tube filament died and I replaced it with one of the new RF Parts tubes. The amp is operating again but some folks I've had a QSO with have said that my signal sounds like it has hum or noise in it. HV, Grid and Plate current all look good and within normal parameters, and I'm not driving it with more than 50 watts.

    I don't have sufficient test equipment to really explore the problem easily on my own so could any of you speculate as to what kind of flaw would cause audible distortion in the transmitted signal?

    I'm including pictures because you might enjoy looking at amps as much as I do.

    John K5JCJ

    Attached Files:

  2. K7RJB

    K7RJB Ham Member QRZ Page

    Easy fix with a scope and an HV probe.
    I'd guess an issue with the power supply.
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    ~~~radio porn~~~
    gotta love it!

    Did anyone mention trouble after the work but before the first tube failed?
  4. WR2E

    WR2E Ham Member QRZ Page

    Some 'less than good' rework can be seen... I'm being kind.
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Yeah, just took a good look at hi-res pictures... it's really bad. Coffee break's over... back to work.
    Did the amp give you a reason to have the work done by acting badly or was it "just time" to recap it?
    I'm just curious if there was an indication that something needed to be done right away vs. preemptive action.

    I'm not pooh-poohing preemptive action. I believe that in many cases, 40 year old caps don't owe you much... but the work has to be done correctly or there will be trouble.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, tacking the capacitor leads onto cut off ones from the original caps isn't so slick.

    I wonder why the rectifiers would need to be replaced? Until they short out (!) they're normally good forever.

    If the filtering is a problem (now, after the work was done) you might be able to detect that by just carefully watching the HV indicated "key up" vs. "key down." With a stiff AC line, the HV indicated should only drop about 250v or so when going from "unkeyed" (standby) to "keyed and drawing 400mA Ip -- full output power." If you see it dropping substantially more than that, there could be a filtering problem that would be worth investigating.

    Of course "hum" might also be coming from your rig itself and just made "louder" by the amp, which wouldn't be the amplifier's fault.:p
  7. N6UH

    N6UH Ham Member QRZ Page

    Effective filter capacitance is maybe a bit light too. 8 * 100uf where the originals were 8 * 120uf
    K5RT and WQ4G like this.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Was the filament giving up was a coincidence? The timing is suspicious but that could be a red herring.

    There looks to be a lot of old connections that have been heat soaked for years and possibly brittle. I'm wondering if there's a cracked solder joint or two in there from rummaging around.
  9. K5JCJ

    K5JCJ Ham Member QRZ Page

    Some of that sloppy re-work was there when I bought the amp last year, but the re-capping really was tough on the existing wiring. I plan to recreate the PCBs using modern replacements in the near future. And I'm embarrassed to say that I didn't even notice that the tech I hired actually put in lower values of the capacitors than the originals. I had them replaced after reading that 40 year old capacitors would likely be unreliable, so I was hoping to be preemptive in getting it back to top condition. Looks like this time I'll have to do it myself and ensure that the correct capacitors are installed. I just hate seeing this great amp in less than perfect condition.

    The amp worked with the original Eimac tube for a few days after I got it back in the shack, but I assumed that old age killed it. The old tube filament wouldn't heat up anymore.
    I've eliminated the rig as the source of the hum. I have an Icom 7300 and Yaesu FT-450 and get reports of the hum regardless of which rig I'm using, and when using my other amps (ALS-600 & FL-2100B) I'm told the audio is clean.
    Is there any other capacitors or components besides the HV ones that could contribute to audible hum in the TX?

    I'm going to start taking it apart and embark on a proper restoration soon so all of your feedback is greatly appreciated.
    K2XT likes this.
  10. WR2E

    WR2E Ham Member QRZ Page

    It's really not THAT much different.

    The caps are in series, therefore 8 pcs of 100 microfarad = 12.5 total, and 8 pcs of 120 microfarad = 15 total.

    That's probably within the value tolerance of either one. I think typical electrolytics like that are something like -20 +50 percent... maybe less, but chances are that the small overall difference you might have is likely not really a big deal.

    If you do major work on the filter boards, you might want to look for the best quality 105°C (temperature rating) parts that you can find. You might even design for 'snap-in' capacitors with a re-layed out board to accommodate them.

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