TS530SP blown fuse after transmitting

Discussion in '"Boat Anchor" & Classic Equipment' started by NJ3T, Nov 5, 2019.

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  1. NJ3T

    NJ3T Premium Subscriber QRZ Page

    I have a Kenwood TS530SP and a MC50 microphone I had bought from a friend, loaned it to another friend who used it for over a year until he got his own HF rig, and then it was stored in my hamshack/computer room/mancave closet for another year. I got it out the other day to talk to some local hams on 10 meters, (within 70 miles from my QTH). I was thrilled with the receiver and the NB that actually worked! I got great signal and transmit audio reports. I noticed some 1 kc drifting in the receiver that I "chased" to correct during my QSO's. I not only could hear the drift but could also see the frequency change on the digital display. I then noticed the rig seemed to get very hot over the PA section of the case. The fan was working. I was going to sign off to investigate and right after transmitting the AC 6 amp fuse blew. At the same time I heard a buzzing in my headphones. It took me several minuets to find a 120v 6 amp fuse. After replacing it I was back on the air and again received great reports. I watched as the temperature increased once again over the PA part of the case, verified the fan was running, and again the fuse blew right after transmitting. Yes, I again heard a buzzing sound as the fuse blew. In fear of damaging the rig further I have put it back in storage until I can find out what may be wrong.

    I'm thinking the 6146 finals need replaced or maybe something is wrong in HV supply? Anyone ever experience this problem?

    I did a little research on replacing the finals. I believe they are 6146B finals, 12by7a driver. I read I should replace them all. I also read that I should "re-solder" the tube sockets? I have never neutralized tubes before but it sounds like all I need is a RF voltmeter (or a vtvm with RF probe???) Any advice would be appreciated.

    I hate the idea of giving up on this ole rig. If I can get the transmitter fixed and then find out why the receiver drifts and fix that, I'll be a happy camper.
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    1. Bad finals or driver should have no effect on the PLL or frequency display during receive.
    2. No, the driver and finals do not have to be replaced as a set.

    Symptoms don't ring any bells. First suspect would be the PS caps. I would start with visual, and sniff check inspection of internals.

    p.s. I have a well traveled TS-530. It is what a modern radio should look like, especially big control knobs!! :)
    K9ASE likes this.
  3. W5RKL

    W5RKL Ham Member QRZ Page

    What is the final amplifier IP meter indicating when the final amp is hot?
    Perform the following test:

    1. Turn the MIC gain fully CCW
    2. Set meter switch to IP
    3. Set mode switch to either USB or LSB, it doesn't matter which one.
    4. Press PTT button and note the IP meter indication

    The meter should point to 60ma. Is the IP meter indication stable or does it
    fluctuate either side of 60ma?

    I agree you do not have to replace the driver tube AND both final tubes as
    a set.

    Mike W5RKL
  4. G3YRO

    G3YRO Ham Member QRZ Page

    With any rig with Valve (tube) PAs you should ALWAYS monitor the PA Current (= IP on this rig)

    Mike is suggesting you look at the Standing Current, as if it is suddenly climbing very high, this would cause the heat from the PA AND the fuse blowing !

    You need to look at the meter in Transmit on SSB, but without talking (best turn the mic gain to zero)

    The PA Current should be around 60 mA . . . if it starts climbing high then there is a problem with the bias circuitry (could be as simple as a dirty Bias Set pre-set pot).

    To explain - the PA Valves need a certain negative voltage to set the Standing Current at a low value . . . if that voltage diasppears for any reason, the Valves will try to take a LOT of current, and this is all dissipated power !

    And a lack of bias will also cause this problem on Receive - just that you won't see it on the Meter unless you are in Transmit mode.

    If you see the meter climbing towards full scale, immediately turn the mains switch off (otherwise you may destroy the PA valves, as well as damage the power supply)

    Roger G3YRO
  5. N8YX

    N8YX Ham Member QRZ Page

    I'd look at the Bias pot, as was mentioned above. An intermittent one will cause the PA tube idling current to do exactly what's being described here. The fact that the rig sat in storage for a while reinforces this suspicion.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, sounds like the bias supply may have failed and the tubes just get hotter and hotter until they finally blow the fuse -- if not the tubes!

    You can operate the TS-530 with the cover removed and look through the PA compartment shielding to see the 6146Bs. They should not be generating much heat at all in "standby," and really don't get all that hot when transmitting on SSB, either -- the fan does a good job of keeping that area only "warm" and not excessively so.

    The added heat generated by PAs almost melting in their sockets, which it sound like what's happening, can cause more VFO drift. The rig is analog with an analog VFO and of course heat makes it drift -- and way more heat than it was designed for will make it drift more.
  7. W5RKL

    W5RKL Ham Member QRZ Page

    The bias pot can be a source of this problem as can the "-C" bias supply.
    However, I would recommend the OP check the stability of the final amp
    idling current in USB or LSB transmit with the MIC gain set fully CCW
    with the meter switch set to IP.

    The OP needs to power up the 530 and let it sit in receive for 30 minutes
    to an hour WITHOUT transmitting. Monitor the heat on the radio,
    especially above the final amplifier. If the radio does not get very hot
    like it does when in transmit when the fuse blows, then the -C bias
    supply has not failed.

    Perform the simple test I mentioned, meter switch to IP, Mode switch
    to USB or LSB, MIC Gain set fully CCW, press the PTT but. What does
    the IP meter show? It should show approximately 60ma. If so is the
    60ma stable? If not what does the IP meter show? Does the IP meter
    indication begin to slowly rise upward above 60ma?

    The fan runs 100% of the time when the 530 is powered ON. The fan
    does not have a heat-sensing component to turn the fan ON when the
    final tubes are hot or turning the fan OFF when the final tube cool
    down and/or in standby.

    Mike W5RKL
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    How much power is your radio putting out ?
  9. NJ3T

    NJ3T Premium Subscriber QRZ Page

    Hello all!,

    Thanks for the replies. As soon as I get some time I will get the rig back out and check for heat while in receive mode only, without transmitting for the purpose of checking the "-C bias supply" I will watch it close and if there is any excessive heat I will shut it down, let it cool. If the -C is ok, I will then check the IP current as described for 60 ma. I will also do a visual as mentioned to see if there is anything that is obvious. I will do the visual without the rig plugged into an AC outlet. One question, what should I use to "clean" a dirty bias pot? Can it be cleaned like any other pot? I have DE-OX-IT contact cleaner.

    Sorry but I have very little experience with tubes and HV supplies. I replaced the one 6146a in a Globe Scout Deluxe transmitter 40 years ago as a Novice. If I had to do the same today I'd be right here asking questions. I appreciate the replies.

    Don, I was on 10 meter USB and the CN901HP3 showed about 100 watts peak. At average setting in CW mode just after initial tuneup showed 120 watts. I did not see the meter just before the fuse blew. Both times I was watching the frequency display. I have not powered the rig since. I hope my 530's finals were not damaged.

    I will post my findings as soon as I can get the rig out again and test it. If I see anything during the visual check that looks suspicious, I'll see if I can get a decent picture to post of the problem area.

    Thanks again
    KA0HCP likes this.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    I WOULD NOT recommend doing that !

    The PAs have HT even on Receive . . . so if the Bias supply has failed, they could be drawing LOTS of current, even on Receive ! (which will certainly damage the PA Valves, and may destroy the Mains Transformer!)

    But you won't see that massive current draw UNTIL you go to Transmit, as the Meter only reads PA Current when you go to Transmit. (and if it starts to climb rapidly as the PAs warm up, immediately turn the mains off !)

    So even before the PA heaters have warmed up, I would recommend that you keep clicking to transmit to see what the IP is reading.

    As others have suggested, even a Dirty Bias Pot could cause this . . . so I would be squirting it with Switch Cleaner/Lubricant, and turning the pot back and forth BEFORE you turn the rig back on again.

    Roger G3YRO
    G4COE likes this.

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