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yaesu fl2100Z open resistors and more cooling

Discussion in 'Amateur Radio Amplifiers' started by VE3UB, Apr 26, 2012.

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  1. VE3UB

    VE3UB Ham Member QRZ Page

    Hi Guys , here is what i have fl2100z amp new tubes powered 120vac .
    Every thing good , power the amp 2.3 KV reading tubes lite up fans running .
    Kenwood hooked to the amp via relay , start to tune with radio no out put .
    Power off the amp discharge all high voltage and start checking for for signs of any thing burnt .
    Pulled the back plate off to see the bottom of tube sockets everything looked ok so i got the wiring diag.out and started to check resistors at the tube socket ,found 2 33 ohm 3 w on each tube open on the bias of the tube .
    So i am not sure why both or for that matter even one as there were no signs of burnt they look as new .

    Has anyone had this kind of problem or know what would cause thes to open .

    As well when i do get it running i would like to do a better job of cooling the tubes from the top of the amp suggestions .

    Thanks Steve VE3smy
  2. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Steve, the only way I can see those resistors opening up is if there was a momentary application of high voltage that went through one tube to the other. If you haven't checked the 572B tubes for shorts now would be a good time to do that. Some tubes just arc over because they are gassy. It might be a good idea to purchase new 572Bs. Replace the 33 ohm 1W resistors, put in the new tubes and see if that makes any difference. Get the tubes from some place that tests them first. RF Parts and Ameritron are two examples that test their tubes and they have a warranty to replace them. The 572B is produced in China and there are continuing problems with their QC. They are reported to be a bit better now.
  3. VE3UB

    VE3UB Ham Member QRZ Page

    Thanks Gary , so how do i check these tubes for shorts ?.

  4. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Yes, that would be a good idea. This will only work on tubes that have a hard short. This is where the internal parts of the tube are in physical contact with another part. A tube that flashes over may not happen because of a shorted element. Sometimes they have gas in them and that gas will break down when high voltage is applied. To check this you would need to hi-pot the tubes. This is where you have a variable high voltage source and a meter that reads the leakage current. You turn the HV up until you're at the point where you start to read the leakage current. This is the maximum voltage the tube will withstand.
    If it's below your operating voltage then the tube is bad. If it's at the HV level you operate at the tube is still bad. You need a little head room. Some says it's twice the voltage that the tube will be operated at. There are others that say it's even higher.
    Most amateurs do not have a hi-pot unit. They usually just replace the tubes when a problem occurs.
    Last edited: Apr 28, 2012
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Many tubes only show a short when the filament is activated. I have several 3-500 tubes that show G to F short only when it has voltage on the filament. if you put an ohm meter on it cold it shows good. Once you put it in the amp and slowly bring up the line voltage you can see the grid meter strt going negative. I don't have a high pot tester, but I do have a variac. Using the variac I can usually spot a bad tube befroe it does any damage. Also a variac can help you spot many other amp problems before they case any damage. A small 5 amp variac for $10.00 at a hamfest will pay for itself many times over. The blown out 33 ohm resistors probably was from a gas arc in a tube.
    Last edited: Apr 28, 2012
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