acom1000 4m

Posted on Feb 4, 2014

Whether the tuning aid can be trusted on 70 MHz is a bit of a complex matter, but wtih me it seems to provide a suitable loading factor, when used for tuning when the attenuator is activated. This attenuator is actually a frequency dependent network of a coil and two capacitors (L7, C17, C19), and not a resistive attenuator. No doubt this has been done

acom1000 4m
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to balance the attenuation against the frequency dependent impedance of the grid. The input SWR is fine during operation on 70 MHz, but around 1:3 when the "attenuator" is inserted. If this SWR value causes your transceiver to decrease the output, tuning with the attenuator might prove difficult. The tuning aid works by measuring (D1A, D1B) the peak RF voltage on the plate, There is no reason to assume, that this measurement should not be valid on 70 MHz. The readout of the tuning aid is triggered by the detection of input power (>5 W), and this is where the problems start because the frequency dependent compensations built into the input circuit attenuates the amount of 70 MHz signal reaching the input detector (D2A, D2B). Actually the bad SWR introduced by the attenuator aids the input detector. The lack of input power detection could also affect the electronic bias switching (EBS) of the amplifier, as a 4m drive signal is more likely to be within the EBS transition zone. You are therefore encouraged to disable EBS by short-circuiting R11 on the MAINS PCB, and to re-adjust the bias setting (RP2) to around 120 - 150 mA. This modification will decrease the amplifier gain a bit, . and increase the required drive power. I assume that the computational link between the plate RF peak voltage and the tuning aid readout is dependent on whether the attenuator has been switched in or not. The resulting loading value might...

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