Steve,
Re-check the voltage on Q6. The emitter should be 0.6 volts *lower*
than the base - which should put it at the manual's voltage of 0.4
volts. The power setting should not matter for that measurement.
Check the emitter resistor on U6 - it should be 1.5 ohms. If this is a
K2 below SN3000 without the A to B upgrades, R50 may be a 1/4 watt
resistor, and it was replaced with a 1/2 watt resistor. If R50 has been
overheated, it is conceivable that its resistance is lower than the 1.5
ohms.
Another thing to check is the TX buffer stage U9. First measure the DC
voltage at pin 3 - if Q20 is working properly, it will be at 12 volts
(or power supply voltage) during receive, but drop to 1/2 that value
during transmit (or TUNE). If it does not change, replace Q20 (2N7000
which is static sensitive).
Also check the resistance of R93 (820 ohms) and R94 (82 ohms) as well as
the value of C150 (330pF). Note that R94 and C150 are in series which
makes the gain of U9 greater as the frequency is increased.
73,
Don W3FPR
On 11/9/2014 10:56 AM, Steve Kavanagh wrote:
Don:
Many thanks for the tips! RFC3 looks to be right. T2 was sitting right on the board but I was able to pry the side next to T3 up so that it is certainly clear of all the leads around it and there was no change. So I guess I will have to wait a day or two until I get home and have a go with an RF probe.
I did do a few DC measurements and noticed that the emitter voltage of the driver, Q6, is about 0.25 V rather than the 0.4 V specified in the manual (at 0.1 W power setting...the manual doesn't clearly specify the power setting for the DC checks). I wondered if the beta of Q6 might be a bit weak...but it doesn't quite feel like what would drop the power by 10 dB. The bias is about right...around 1.05V.
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