Looking for Dentron Clipperton QRO Amp repair estimate

Discussion in 'Amateur Radio Amplifiers' started by KC8TMV, Dec 31, 2016.

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  1. KC8TMV

    KC8TMV Ham Member QRZ Page

    I would love to find some one within driving distance (75-100 miles) of greater Cincinnati, but willing to ship if necessary and with a good reputation. Really can't afford a new amp and would like to have this one repaired.

    Signal is "horribly" distorted when using the amp. Kind of happened out of no-where. Worked great for about 5 years. Wish I had the talent to diagnose, but sadly I don't.

    Any suggestions would be greatly appreciated!
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    The first thing that comes to mind is bad filter caps in the power supply. That amp is old and long over due for new caps if they have never been changed. Of course there could be other issues.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd agree, but "horrible distortion" is often created by the rig itself due to RF from the amp/antenna getting back into the rig -- can can definitely occur "suddenly" if anything at all changed in the station or the antenna.

    First thing I'd do is use a second receiver to listen to my signal in headphones and see what it sounds like, then check all connections, grounds (especially coaxial cable ground connections and microphone ground connections), unplug everything from the rig that isn't required for it to transmit, etc. Bad filter caps in the amp will usually lead to two obvious symptoms: The HV (DC) dropping more on key-down or voice peaks than it used to, and "hum" (120 Hz) that can be heard on the modulated signal using headphones (a scope will show this also, if you look at the edges of the modulation envelope).

    If all it needs is new filter caps, Mouser and others stock suitable replacements that aren't very expensive. When replacing a string of filter caps, I replace their bleeder/equalizer resistors at the same time -- often, when caps start going bad it's because the bleeder resistors drifted in value and some caps are stressed more than others -- which leads to a string reaction of them all going bad. But "age" alone, combined with elevated temperatures, does make them wear out over time and 30 years is a long time.
     
    WB1E likes this.
  4. KC8TMV

    KC8TMV Ham Member QRZ Page

    Well, here is how I tested: On air report that my audio sounds "like crap". I did as you had suggested and listened on 2nd rig. Yeah, I sounded like crap! Distortion sounded to me like total over modulation or audio being sent through an guitar distortion box. Turned off amp, test again, sounded fine. Kind of guessing that is leading me to the amp.

    I was really hoping to see if anyone that knew what they were doing would be willing (with pay of course) to do a good "going over" it and letting me know what they suggest for this or any other issue or suggested service.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd want more information than that observation (although good job on the observation!).

    An amp can't add much distortion to a signal, at least not much you could actually hear. Amps produce high levels of intermodulation distortion (IMD) at times, but you can't actually "hear" that, it just clutters up the band by making signals wider than they should be -- but "on frequency," even an amp with terrible IMD sounds fine.

    The only thing offhand I can think of that will create serious on-frequency distortion is terrible non-linearity, such as the tubes being cutoff by bias and then driven by RF -- like a Class C amp used on AM or SSB.

    With your Dentron, when you "key" the amplifier but do NOT modulate to drive it, what is the idling current indicated on the Plate Current meter?
     
  6. W1QJ

    W1QJ Ham Member QRZ Page

    I'd bet that you are getting RF into your radio somehow. Until you ruled out that issue I would not send the amp out for any repairs, and I do that sort of thing.
     
    KD2ACO likes this.
  7. KE0ZU

    KE0ZU Ham Member QRZ Page

    Maybe too much drive?

    Changed exciter rig?

    Mike
     
  8. KC8TMV

    KC8TMV Ham Member QRZ Page

    OK, fired up the amp today to try to get better information for WB2WIK. Low drive 20-25 watts and on the first key up to tune, I heard a new sound from the amp. Since I am sure you are all amused my my technical descriptions, it sounded like a "gurgling" sound coming from the physical amp itself. Double check the tuning knobs to assure I had not knocked them off their last position and they were right where they were last try. So I gave it another (very quick) go again and heard the same thing. Turned off the amp and heard another great sound that sounded like air being let out of a tire! (and a not so great smell and a tiny bit of smoke) I am guessing this was a capacitor letting go some where.
    As I said before I am out of my league on fixing this (smart enough to be honest), and really don't want to fight a fire in the shack.
    Looking for suggestions on anyone that would be willing to take a look at it. This might be cost prohibitive, but again, looking for those of you with more experience than me for advice.
    Thanks to all chiming in and especially appreciate not being belittled for my lack of technical (fix it) knowledge. I am probably one of those guys that need a auto-tune, key and forget it amps!
     
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    It sounds like a cap let go. People are good at different things and knowing your limitations shows maturity and smarts! Lou W1QJ is fantastic with these things. He can chime in on the best way to get it to him.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I suspect you may have to pack it up well and ship it for repairs; but it's a pretty good amp and should be worth repairing.

    If you do decide to "ship it" to Lou or anyone else who does this all the time, I'd recommend removing the four tubes and putting them aside prior to shipping the amp. Or if you want to be sure the tubes are good, ship them also, but ship them separately and well-packed, not installed in the amp.

    When UPS or FedEx or whoever picks up heavy cartons it's pretty common for them to drop them and then you lose tubes.
     

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