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Input Resistor in MFJ Auto-Tuners

Discussion in 'Radio Circuits, Repair & Performance' started by KC9SWV, Jul 19, 2017.

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  1. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    They are not switched between series or parallel, the resistance is either in series with the load or it is bypassed.

    With the resistance in series, then no matter how low of a load the antenna (or antenna + tuner) presents, the minimum will be no lower than 25 ohms. This would not be true if the resistor was parallel with the load.

    I would expect this is done so thst the SWR circuit will always give a voltage that the A/D converter and processor can deal with.

    The tuning algorithm happens so fast that I expect lower watt ratings are OK.

    73 and good luck with your project :)

    Mark
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    That is the way that I see it too.

    They are in series with the output, Or shorted across to take them out of circuit.

    The impedance detector has somewhere to start, And shorts the resistors out ASAP, To avoid smoke.
     
  3. WR2E

    WR2E Ham Member QRZ Page

    .. never mind .. dupe ..
     
  4. WD8ED

    WD8ED Ham Member QRZ Page

    I have an MFJ-998RT that I smoked a while back. Seems it would tune loads at lower power levels that weren't so friendly to the tuner when 1 KW was applied.

    Earlier this week I fixed it. This same 15 watt, 25 ohm 1% R121 resistor was GONE! Vaporized! Just too metal leads sticking up out of the board. There was a hole also burnt into the side of Relay 1 where R121 comes close to it. The relay that bypasses this resistor. It was also toast. A new resistor from Mouser and a relay from MFJ and still no tune. Passed all of the relay and internal self tests. But showed no indication of power or VSWR. Found C169, 33 pf cap bad and both D43 and D44 shorted. Replaced all three and did full alignment by the manual and it's now functioning flawlessly again!

    I almost gave this thing away or threw it out. They really aren't that difficult to repair. There's really not much to them.

    What exactly R121 does? I'm not totally sure but all you have to do is take the cover off and monitor when and what condition K1 is bypassing R121. But it looks as if it is providing some type of current limiting during potentially damaging tuning states. I know it didn't save the VSWR bridge but it couldn't have where it is located. But it did appear to save the rest of the tuner from my activities.

    Thanks,

    Ed
    WD4ED
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Any reason that you would do that ? Was you smoke testing ?

    It sounds like your smoke test worked. :)
     
    Last edited: Jul 22, 2017
  6. WD8ED

    WD8ED Ham Member QRZ Page

    Wasn't on purpose. It tuned and showed good VSWR. Kicked on the amp and things didn't go as expected. The antenna I was trying to use was way too short and the impedance was very low. MFJ-998 owners manual actually says not to do that. But I did anyway! I'm sure I'm not the first ham to do that! :)

    Thanks,

    Ed
     
  7. AI4WZ

    AI4WZ Ham Member QRZ Page

    I have had the same sort of problem with my 998. K1 was stuck not letting R121 limit the power while turning. R121 was not smoked due to never being in circuit while tuning. Replaced K1 and changed R121 to a 20 watt part ( up from a 15 W) and made a heatsink for it mounted to the rear panel. Still need to check C169 but D43 & D44 test good as do all of the parts related. Problem remaining is the forward power readout does seem low, as my IC751a meter reads 100w out but the 998 meter showes 14w w/SWR of 1.5. Have talked to contacts from Idiho to Spain to FL to OTN from NC. The battle goes on....
    Walt AI4WZ
     
  8. WD8ED

    WD8ED Ham Member QRZ Page

    That could be s handful of things.

    Try putting power into the tuner backwards. Yes it safe. In fact it’s how you calibrate the VSWR. See if the power reads similar to the forward indication. If the power isn’t similar you may still have a bad diode or a problem with the windings in the small transformer.

    Also, put a dummy load and a power meter on the output. Does power flow thru properly with a good load?

    The diagnostics are actually pretty good. It’s helped me several times to find bad relays. If the relay doesn’t sound exactly like all of the others, it’s probably bad.

    Hope that helps! It’s a start.

    Ed
     
  9. AI4WZ

    AI4WZ Ham Member QRZ Page

    All is well now.... relays replaced, load resistor increased in wattage and on a heat sink and tune on an FM carrier - not SSB. All is well, 73
     
  10. WD8ED

    WD8ED Ham Member QRZ Page

    Very good!

    Ed
     

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