HI all,?
I am trying to repair an early model FT817. It is fine on all bands/modes in RX.?It is fine on TX in all modes, except for 18Mhz (17m) and 21Mhz (15m).?
In the affected bands, measured in CW, the rig draws 3A+ from the bench psu, and RF filter relays buzz loudly. This can trigger a shutdown, or the rig stays on and buzzes terribly.?
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My analysis so far - testing in CW mode: the preamp/driver stage works well. This feeds into the PA finals board, which seems to amplify happily - but supplies too much current in the affected bands. If I separate the predrivers/PA final stage from the RF filtering stage at the balun (T3001 in schematics), and insert a dummy load to absorb the PA final output, the TX works normally (~2A PSU current draw) on ALL BANDS.?
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This suggests the issue is the balun itself or after the balun in the RF filtering stages.?
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According to KA7OEI's fantastic info pages, , 20m & 17m use the same RF filter FIL5, while 15m, 12m & 10m all use FIL6. Given 20m and 12m/10m work ok, this suggests the issue is not in the RF filters themselves, since they work happily 'half the time'.
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Through experimentation, I notice that reconnecting the balun (T3001) to the RF filter board via an inductor of ~0.3uH (originally discovered after using basic crocodile clip cables to reconnect the stages together), the TX over-current stops - and the rig operates as normal, except for the introduced inductance that rises with frequency, hence lowering the output power (and lowering the PSU current draw) with rising TX frequency. This 'fix' itself causes problems on 50+VHF/UHF I think.
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It suggests to me there is some low impedance path to ground around 20Mhz (affecting 18Mhz & 21Mhz), and have pulled the board out of the rig to take a look. But nothing seems immediately wrong.?
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Has anyone experienced similar TX over-current on specific bands, where other bands work fine ? Could the issues lie in the RF filters? How best to test balun T3001 to check if it is at fault?
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Any ideas or suggestions are greatly appreciated,?
73 de
Matt Nassau, M0NJX
