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G8MNY  > TECH     12.05.19 10:15l 72 Lines 3028 Bytes #3 (0) @ WW
BID : 4157_GB7CIP
Subj: PTT line for Drake L-4B PA
Sent: 190512/0805Z @:GB7CIP.#32.GBR.EURO #:4157 [Caterham Surrey GBR] $:4157_GB

By G8MNY                                    (Updated Jul 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
넬컴커    _    _  납       
냅컴켸   (_)  (_) 납       
넬컴커    _       납       
냅컴켸   (_)    膨       
 o            [|]납       

If wired directly to a IC735 Tx send connector, the IC735 reed relay contacts
weld up with the linear relay's back emf.

            L I N E A R                                    I C  7 3 5
18V AC            S4    _____                              SEND PTT
())))컴>쳐컫컴컫/o컴캑relay쳐컴컫컴>쳐훟                o)컴> \ Tx reed
=====      ===         ~~~~~           PTT                     relay
        100u  2k2            10n===     GND                    
    0V컴컴컴좔컴좔컴컴컴컴컴컴컴컴좔컴컴컴o                 읕컴컨컴

This old large valve HF PA uses an inductive 12V PTT relay that is runs on
unregulated 18V.

Originally not wanting to modify either pieces of kit, I chose to find a simple
fool proof solution in the inter connecting lead.

To stop the back emf & in-rush current to the IC735's reed relay SEND contact,
I added a 35V 4u7 in series with a 100 across linear's PTT 2 pin plug to
terminate the back EMF.

             L I N E A R                                   I C  7 3 5
 18V            S4      _____                P L U G       SEND PTT
())))컴>쳐컫컴컫/o컴캑relay쳐컴컫컴>쳐훟   컴컫컴컴컴(o o)컴 \  Tx reed
=====      ===         ~~~~~           PTT    100            relay
                                              +             contact
        100u  2k2            10n===     GND    === 4u7        
    0V컴컴컴좔컴좔컴컴컴컴컴컴컴컴좔컴컴컴o   컴컨컴컴컴   읕컴컨컴
                                               L E A D

The added operate & release delay is not detectable (a few mS?), & there is no
longer shocking back emf to feel or weld up the rig contact, if U manually
earth the phono plug etc.

But later I overhalled the PA & modified the PTT relay with the usual diode
across it, this may slow the change to Rx a few milliseconds, but stops all
relay back emf. 

             L I N E A R                                  I C  7 3 5
 18V            S4      _____                P L U G      SEND PTT
())))컴>쳐컫컴컫/o컫캑relay쳐쩡컫컴>쳐훟   컴컴컴컴컴(o o)컴 \  Tx reed
=====      ===        ~~~~~          PTT                    relay
                   읕컴<쳐컴                               contact
        100u  2k2    1N4001  10n===     GND                   
     0V컴컴컨컴컨컴컴컴컴컴컴컴컴컨컴컴컴훟   컴컴컴컴컴   읕컴컨컴

See my Tech buls on "ALC line for Drake L-4B PA", "Using 2 HF PAs", "Power
Factor of Drake L-4B PA" & "Valve Heater delay Drake L-4B PA".

Why Don't U send an interesting bul?

73 de John, G8MNY @ GB7CIP

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