modified 12-08-08


If you are blowing fuses in your FT-102 in transmit there are a couple of things you might want to know. Below is my email answer to a fellow ham who had the above problem.

Hello Mario,
    Yes, that problem is the second most common problem with the FT-102 and is caused by the 6146 going into thermal runaway.
    The problem is as you described - an increase of the resting cathode current which starts out at 75 ma and gradually and then quickly increases until the fuse blows within a 10 minute time span. This will happen in SSB, even if you do no talking and shut the mic. gain down. There is only one cure and that is to get a new set of tubes as the ones you currently have in your radio have had the screen and control grids permanently deformed by excessive heat.
    I mention cathode current as your meter does not indicate plate current. The measurement circuit is inserted in that part of the circuit between the cathode of the 6146s and ground so that high voltage insulation is not required for the measurement circuitry. If you look at the bottom of the power amp board (solder side) you will notice a white component (before it is thermally damaged and charred) which looks like an inductor because it is coiled. This is just a coiled resistance wire with a white fabric insulation over the wire. This is the shunt resistor connecting the cathodes to ground. It is about 0.6  ohms and the meter circuit works from the voltage developed across this resistor when current flows in transmit.
    The problem of the runaway cathode current is almost always caused by improper tune up procedures. Most hams know the "10 second rule" where they do not key down for more than ten seconds but many forget that they must stay keyed up for ten seconds because the tubes are only  rated for 50% duty cycle. In fact I recommend that you key up for 15 seconds for every 10 seconds on time when tuning up. The extra heat which is generated by doing the procedure wrong deforms and warps the grid assemblies and once this happens it is a permanent abnormality. Most hams only know the fuse blows as they keep their meters in the power position and you will not see any abnormality when looking at the power output since the abnormal extra current does not generate any additional power to the antenna.
    Most hams especially if they are older get angry with the radio and so they keep putting in larger and larger fuses or slow blow fuses. This results in several problem such as the set catching fire, blowing the transformer, shorting the choke supplying the 6146s (should measure more that 4.4 ohms) and cooking the shunt resistor. I have seen these damages over and over. This is the main reason that the 102s wind up in the radio graveyard. I now have ten parts radios which were purchased from people who did not want to go to the extra expense of replacing the transformer and didn't want to pay to ship a broken and smoke smelling radio back. There are many other radios that I would not make an offer to buy as I now have plenty of these parts radios.
I recommentd the following test:

     Place the set into transmit SSB and turn the mic gain to off with the meter switch in the IP position so that you are looking at the cathode current. Your static cathode current should read about 75 ma.. Keep the set in transmit for the next ten minutes and make sure the current does not show an increase of 25 ma over the starting value after a ten minute time span. The best tubes will show no increase on your current meter  while fair tubes will show no more than a 25 ma gain. Anything that exceeds a 25 ma. increase, will blow fuses - if not at the time then in the near future. 

    So please get a certified set of new tubes from a reputable wholesaler (not on ebay) so that you can be sure the tubes you purchase are new and not someone else's pulls because they noted the same problem. Most people when replacing tubes put the pulls in the new boxes and over the years forget (sometimes intentionally) that they are not good.
    In addition please check that the resistance between the top and bottom connections of your choke (please make sure that the high voltage has been discharged) is more than 4.4 ohms as when the choke is subjected to excessive currents and sections of the choke short out. That results in decreased output on the higher bands like 10 meters.
      If your shunt resistor's insulation (different from the choke) is charred leave it alone as there is no with problem with this. It does not need to be replaced.
     And lastly make sure that you do not use more than a 5 amp quick blow fuse.
    73 de NC4L           Mal
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