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G8MNY  > TECH     12.05.19 10:15l 72 Lines 3028 Bytes #3 (0) @ WW
BID : 4157_GB7CIP
Read: GUEST
Subj: PTT line for Drake L-4B PA
Path: ED1ZAC<ED1ZAC<IW0QNL<VE2PKT<N3HYM<KA3BVJ<AB0AF<GB7YEW<GB7CIP
Sent: 190512/0805Z @:GB7CIP.#32.GBR.EURO #:4157 [Caterham Surrey GBR] $:4157_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

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

THE PROBLEM
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
                                                              contact
        100u  2k2            10n===     GND                    
    0V컴컴컴좔컴좔컴컴컴컴컴컴컴컴좔컴컴컴o                 읕컴컨컴

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

MY CURE
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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