[Collins] 32S-3 drive issue

[Dr. Gerald N. Johnson]

On Fri, 2008-04-04 at 20:38 -0400, Ken Plesser wrote:
> Gentlemen:
> 
>  
> 
> A problem with a 75S-3B that someone might be able to help with.

True the receiver can affect transmitting when connected for transceive.
> 
>  
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> When tuning up on 20 meters, full clockwise rotation of the MIC GAIN is
>  necessary in order to get anywhere near full-scale deflection of GRID 
> CURRENT in the TUNE position of the FUNCTION switch.  This is not a 
> problem on any other band.

You don't need FULL SCALE on the grid meter. You don't want any grid
current to keep the PA linear. Grid current should only tickle as the
ALL takes care of dropping the gain.
> 
>  
> 
> Similarly, full clockwise rotation of the MIC GAIN is necessary to 
> achieve the proper level of plate current and power output on voice 
> peaks.  In fact, full output (and appropriate ALC indications) on 
> voice peaks is only possible with a high-impedance crystal mike. 
> I can't get there with a lower-output ceramic mike.

And that is a bit normal, since the high impedance microphone has more
volts of output and this radio was made for the high impedance
microphone.
> 
>  
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> I have replaced tubes V1 through V5 with no apparent change.

Every stage, and so every tube affects the gain of the radio. Not just
the audio stages. Since its only one band, it might be a tuned circuit.
Is it the same in all 20 meter bandswitch positions? If its a problem in
only one band segment, its likely a crystal and with the cables
connected for transceive, its the receiver crystal since the crystals in
the transmitter aren't used for the HF conversion.

As for tuned circuits, its been common for transmitter output to fall
due to dirty bandswitch contact. Sometimes that's cured temporarily by
vigorously switching through the weak band. Cured more permanently by
applying a quarter drop of DeoxIT with a dropper bottle to each switch
contact. Its not a good idea to spray contact cleaner and to soak the
wafer insulation with juice that doesn't enhance its insulation quality.
Yes, its tedious to apply by the partial drop, to not spray but
replacing bandswitch wafers is a whole heap more difficult.

The other cause of a tuned circuit resonance comes from the trimmer
capacitors going open. That quarter drop of DeoxIT at their bottom
center followed by exercise of the trimmer can remove oxide there and
return the circuit to operation. These trimmers have a history of
sticking and not allowing adjustment without disassembly and sometimes
that's destructive. There were details on trimmer service in the most
recent CRA magazine.

To get to trimmers underneath and to bandswitch contacts, its necessary
to remove the bandswitch shaft and the shield cans. Be sure to clean the
shield can ground contacts with DeoxIT before reassembly.

Its important to realign the tuned circuits around any IF or RF tube
that is replaced. The techniques are in the operator's manual.
> 
>  
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> Any thoughts?
> 
> Ken, W3ESH

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer