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G8MNY > TECH 21.01.26 19:45l 74 Lines 3084 Bytes #9 (0) @ WW
BID : 50074_GB7CIP
Subj: PTT line for Drake L-4/4B PA
Path: JH4XSY<N3HYM<N2MH4<VE3KPG<GB7CIP
Sent: 260121/1043Z @:GB7CIP.#32.GBR.EURO #:50074 [Caterham Surrey GBR]
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)
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿﾚﾄﾄﾄﾄﾄﾄﾄｿ
ｳﾚﾄﾄﾄｿ _ _ ｳｳ ｳ
ｳﾀﾄﾄﾄﾙ (_) (_) ｳｳ ｳ
ｳﾚﾄﾄﾄｿ _ ｳｳ ｳ
ｳﾀﾄﾄﾄﾙ (_) � ｳ ｳ
ｳ o [|]ｳｳ ｳ
ﾀﾄﾂﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾂﾄﾙﾀﾂﾂﾄﾄﾄﾂﾂﾙ
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
())))ﾄﾄｴ>ﾃﾄﾄﾂﾄﾄﾄﾂﾄ/oﾄﾄﾄｴrelayﾃﾄﾄﾄﾄﾂﾄﾄｴ>ﾃﾄﾄo o)ﾄﾄ> \ Tx reed
===== +ｳ ｳ ~~~~~ ｳ PTT ｳ ｳ relay
=== ｳ 10n ｳ ｳ ｳ contact
100uｳ 2K2 === GND ｳ ｳ
0Vﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄo ﾀﾄﾄﾄﾄﾁﾄﾄﾄ
This old large valve HF PA uses an inductive 12V PTT relay that is runs on
unregulated 24V.
MY CURE
Originally not wanting to modify either peices of kit, I chose to find a simple
fool proof solution in the inter connecting lead.
To stop the back emf and in-rush current to the IC735's reed relay SEND contact
I added a 35V 4u7 in series with a 100R 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
())))ﾄﾄｴ>ﾃﾄﾄﾂﾄﾄﾄﾂﾄ/oﾄﾄﾄｴrelayﾃﾄﾄﾄﾄﾂﾄﾄｴ>ﾃﾄﾄo ﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄ(o o)ﾄﾄ \ Tx reed
===== +ｳ ｳ ~~~~~ ｳ PTT 100R ｳ ｳ ｳ relay
=== ｳ 10n ｳ ｳ+ ｳ ｳ ｳ contact
100uｳ 2K2 === GND === 4u7 ｳ ｳ ｳ
0Vﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄo ﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾁﾄﾄﾄ
L E A D
The added operate and release delay is not detectable (a few mS?), and 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 and 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
())))ﾄﾄｴ>ﾃﾄﾄﾂﾄﾄﾄﾂﾄ/oﾄﾂﾄｴrelayﾃﾄﾂﾄﾄﾂﾄﾄｴ>ﾃﾄﾄo ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ(o o)ﾄﾄ \ Tx reed
===== +ｳ ｳ ｳ ~~~~~ ｳ ｳ PTT ｳ ｳ ｳ relay
=== ｳ ﾀﾄﾄﾄｴ<ﾃﾄﾄﾄﾙ ｳ ｳ ｳ ｳ contact
100uｳ 2K2 1N4001 10n=== GND ｳ ｳ ｳ
0Vﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄo ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾁﾄﾄﾄ
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", "Drake PA 3-500
valve repair" & "Using 2 HF PAs".
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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