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QRO-3KDX hum on carrier

Discussion in 'Amateur Radio Amplifiers' started by W9BHI, Jan 9, 2019.

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  1. W9BHI

    W9BHI Ham Member QRZ Page

    I have a QRO-3KDX that is showing a bearly audible hum on the carrier on my REA mod monitor.
    A few people said that they can just bearly hear it, like a 120hz hum.
    It's not obtrusive but it's there.
    I went so far as to replace the electrolytic caps in the HV supply and while I was in there, I replaced the bleeder resistors also.
    No change, hum is still there.
    I have had many amps in this operating position and never seen this before.
    I have disconnected my audio rack from the radio that is driving it so the hum is not coming from the radio.
    I checked for loose grounds on all of the equipment.
    The amp is grounded also.
    Besides the hum on the carrier, the amp is working perfectly.
    Any thoughts?
    Don W9BHI
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Does this amp have a fan speed switch?
  3. W9BHI

    W9BHI Ham Member QRZ Page

    I was hoping you would jump in.
    I tried changing the blower speed but it doesn't seem effect the amplitude or frequency of the hum.
    I actually removed the plate transformer from the chassis and set it beside the amp as far away as the cables would allow and that made no difference.
    I need to put a switch on the blower leads so it can run until the filament timer runs out, then shut the fan off momentarily and key up the amp to see if the blower motor is influencing the filament transformer that is in close proximity.
    Don W9BHI
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I wouldn't rule out the transceiver.

    I've had 60 Hz hum induced into transceivers or transmitters by simply having a fluorescent lamp on the same desk as the rig, with zero cables connected to the radio except for the coax antenna connection.

    It can also be induced by a ground loop if all the equipment is not bonded to a single-point ground system. One way to help determine that is to disconnect all ground connections to "everything" and see if it makes a difference.
    WA7PRC likes this.
  5. WB5WPA

    WB5WPA Ham Member QRZ Page

    What do on-air reports give - do they hear any hum?
  6. W1QJ

    W1QJ Ham Member QRZ Page

    I don't know for sure what your problem could be buy I can tell you what I know and perhaps what I tell you may help you. First off. I do know that N4ATS Bill had a similar problem and as I recall he said it had something to do with the fan blower unit. You may want to contact Bill and ask him. Secondly, I found a problem with a QRO 2500 which has the same basic circuitry for control. I looked at the schematic for your unit and the fan speed control circuit is indeed the same. The fan speed control circuit is a flawed design. It really needs to be rewired and in my case, rewiring it stopped the problem with the amp I was fixing. If you closely study the circuit used to switch fan speeds you will notice the flaw if you know anything about switching AC loads. If you don't see the flaw I will tell you what it is and how to fix it. In my case rewiring the circuit fix the problem. The problem was due to the RFI spewed by the switch every time it was switched. Assuming you switch has been damaged over the years it just may be spewing the RFI that is on your signal. I don't know but certain my condition was weird. Lastly, take a close look at the bias circuit. Unlike almost every other 8877 amplifier out there this amp is unique in that the filament transformer has a center tap and the bias is applied there. Just about any other 8877 amp you will see the bias is fed indirectly to the cathode through a choke. Compare the bias to that of the Ameritron AL-1500 or a Dentron DTR-2000L. There has been debates about tying the cathode to the filament in these tubes. I know that when I do conversions on Ameritron amps, I aways make sure that I place the bias on the cathode/filament connection and not the other that goes just to the filament. It would be easy enough to experiment with biasing the the tube on the opposite side of the filament choke (See Ameritron AL-1500) or you can try bias through a choke and disconnect the connection between the filament and cathode and bias just the cathode through a choke. Not saying this will fix the problem, but it makes me wonder why this amp is unique in that regard. I do know that my problem with the timer circuit being attributed to the fand switching was certainly a mystery until I figured out the problem.
  7. W9BHI

    W9BHI Ham Member QRZ Page

    I would like to see the correct design of the fan speed circuit.
    Take a look at the picture of the bottom of the rf deck that he shows on his site and you will see an obvious mistake.
    All the cathode pins (2,3,4,6,7) are strapped together along with filament pin 1 but the rf input is connected to pin 5 of the filament (the tube pin numbers on the schematic are useless).
    I removed the rf input from filament pin 5 and moved it up to pin 6 (cathode).
    Also, rf bypass caps (C57 and C58 on the schematic) were missing on the cold end of the filament choke.
    In the picture you can see the filament power comes in from the terminal strip and feeds the filament choke.
    The far end of the filament choke has long leads that feed the tube filament pins.
    I unsoldered the wires from the filament pins and from the terminal strip and reconnected the short wires from the end of the filament choke closest to the tube socket to the filament pins and the long wires from the far end of the filament choke to the terminal strip.
    I added the missing .01 caps to the cold end of the filament choke.
    All of there changes I made are common building practices.
    Even in the picture of the bottom of the rf deck you can see the bypass caps are missing.
    I bet that all of the 3k amps he made have the rf input to the tube miswired and are missing the bypass caps.
    The people that are building his products are just following drawings (that are incorrect).
    I worked in the electronics manufacturing industry for over 30 years and have seen my share of
    production mistakes.
    Today I will try disconnecting the fan momentarily to see if that makes a difference.
    FWIW, I have repaired three of the HF-2500's and have never seen this problem.
    Don W9BHI
    KA0HCP likes this.
  8. W1QJ

    W1QJ Ham Member QRZ Page

    What you have stated here are obvious blunders in production and you can add to that the improper fan speed circuit. Yes you wouldn't want to feed the lone filament pin with RF. It is obvious you would want to feed the cathode cluster of pins. Good thing you noticed that. How could they forget the bypass caps on the filament choke? Only common sense. He probably hired some high school kids to do some soldering after school. Ok here is the skinny on the fan circuit. If you look at the schematic on the "over all" page you will see the section on the upper left for the fan speed circuit. As you'll notice he uses a DPST switch for the fan speed switching. The center of the switch (common) is the 120v feed to the fan. In one switch position the feed is connected directly to the fan (fast speed) in the other position the feed is then connected to 2 resistors in series which slows the fan down (slow speed). The circuit works but here's the problem. As designed the circuit momentarily disconnects the feed to the fan as the switch goes to the next position and then it makes up again. That make/break causes the switch to spark every time because of back EMF, over time the switch contacts get burned and the connection sparks worse as time goes on. In the amp I had in for repair every time you switched the fan speed the timer circuit would reset and you would have to wait the timing period to transmit. At first I could not believe what I was seeing and for the life of me I could not see the connection between the timer circuit and the fan switch. I went crazy trying to see the connection and obviously there is none. When I studied the fan speed circuit CLOSELY I immediately saw the propensity for the switch to cause an arc when switched. The RFI spewed from the arc was getting into the 555 timer and resetting it. The fix was to rewire the fan speed circuit properly. Once I did that problem solved.

    The proper way to rewire that circuit is to NOT use a DPST and switch the feed between a resistance and a direct connection. The proper way is to make a direct connection of the resistors in series to the feed and simply put a SPST switch across the resistors and short them out for high speed. The load never gets disconnected from the feed and when the switch shorts out the resistors there is no back EMF to cause an arc. The switch does not get damaged and no spewing of RFI from the arc. I simply used the same switch as a SP and rewired the circuit.
    N8CBX, AA7QQ and KA0HCP like this.
  9. W9BHI

    W9BHI Ham Member QRZ Page

    I finally fixed the hum on the carrier!
    I verified which filament transformer lead was common to the cathode and removed the bias lead from the center tap of the transformer and connected it to the cathode side of the filament transformer.
    Now no cathode current flows through the secondary of the filament transformer.
    The carrier now has no detectable hum.
    This configuration is what Ray should be using on his current run of HF-3KDX amps.
    W1TRY, N8CBX and KD2ACO like this.
  10. W1QJ

    W1QJ Ham Member QRZ Page

    Ameritron knew this problem existed years ago, most likely because W8JI designed it. When I get Al-82 and Al-1200 amps and convert them to 8877's I always mark the transformer end of the filament feed to the tube that connects to the cathode and apply the bias there, never to the lead that feeds the lone filament pin.
    W1TRY likes this.

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