[TenTec] Omni 546 - C RF Power amplifier

[Marinus Loewensteijn]

> Or the smaller capacitor may have let the ALC work fast enough to
> minimize the spike. Since the ALC generally has a fast attack slow decay
> time constant, the first key closure is the one to have the spike, even
> with QSK the ALC gain should hang about constant and show no spikes on
> subsequent key closures.


Jerry

Thanks for the input.

The 33 uF is not in the ALC loop - the low level driver board takes the output from the TX mixer. From the manual:  (quote) " The first two stages, Q1 and Q3 are ALC controlled 
by transistors Q2 and Q4. The thrid stage operates at a fixed gain. Bias for the second and third stages is provided whenever the "T" voltage is applied to the "T" terminal
, with diodes D1 and D2 determining the value of the bias voltage. The first stage bias is similarly derived from diodes D3 and D4, but the turn-on is delayed a small fraction of a 
second. Voltage applied to these diodes from terminal tD is delayed from the normal "T" volatge by circuits on the control board assembly. The delay is a necessary part of the 
sequential keying circuit that provides clickless CW."

The delay circuit is nothing more than a 470 Ohm resistor coming from the Voltage supply going to the two diodes with parallel to them the 4.7 uF  (but 33 uF implemented) capacitor
and then feed the base of the transistor through a resistor.

I see it just as during manufacturing they implemented late changes which were not documented in the manual.

I see the damage as being done slowly (cumulative) over time by just excessive drive to the final power transistors since the intermediate stages have enough grunt.

I might be mistaken however I have put a higher value cap back in place of the 4.7 uF.

73, Marinus, ZL2ML

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