I finished assembly and started the calibration and checkout. All my
preliminary resistance checks were nominal. When I put the unit in tune
mode, the power indicated 3/10 watt and remained there while I tried to
adjust L1. After a few seconds I saw a faint puff of smoke and HI CUR
appeared on the display so I immediately shut the unit off. I touched
the driver transistor, Q6 (2SC2166), and it was extremely hot. I
remeasured the resistances on Q6 and found that Q6 base to ground, which
had read 118 ohms now reads 68 ohms. R50 (emitter to ground)which should
have read 1.5 ohms now reads 1.8 ohms.
I removed the bottom covers and saw where the smoke came from.....R50 was
burned but not open circuit. Resistance checks on Q6 (emitter to base,
collector to base and emitter to collector) look nominal as far as I can
determine in circuit. I rechecked T1 and T2 construction and
installation visually and with an ohmmeter and all looks okay. Resistance
checks on Q7 and Q8 are still okay. (As far as I could determine at the
time of the problem they did not overheat, at least not enough to warm
the bottom heatsink/cover.) Any ideas where to start looking? My
thoughts are to check for RF waveform at Q5 collector and Q6 base with a
quick push of TEST and then release. Am I heading down the right road?
I'll wait until I hear before proceeding. I'll also order a new R50 but
will wait to see if something else has fried.
Thanks,
Pat Haynes-K4BEH
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