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G8MNY > TECH 09.02.26 16:51l 120 Lines 6314 Bytes #9 (0) @ WW
BID : 51738_GB7CIP
Subj: ICOM 735 PA instability
Path: JH4XSY<N3HYM<GB7YEW<GB7CIP
Sent: 260209/0749Z @:GB7CIP.#32.GBR.EURO #:51738 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jun 25)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
________________________________________________________
ｳ ﾚﾄﾄﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ == == == ｳ
ｳ ﾟﾟﾟﾟ == ｳ \ ｳ ｳ Rx 14.273.5 vfo A 12ｳ _ _ ｳ
ｳ /~\ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ (_) (_) ｳ
ｳ \_/ ____ IC-735 ｳ
ｳ ﾟﾟﾟ ~~~~~~""""""~~~ /' `\ ﾟﾟﾟ /"~"\ [] ｳ
ｳ 0 ﾜﾜﾜ |(~) | ﾜﾜﾜ | | ｳ
ｳ ﾞ ﾞ ﾞ ﾞ ﾞ ﾞ ｳ ~ ｳ \._./ [] ｳ
ｳ /~~\ ﾟﾟﾟ ﾞ ﾞ ﾞ ﾞ ﾞ ﾞ | | ﾟﾟﾟ == == == ｳ
ｳ \__/ ﾜﾜﾜ # # # # # # [] \.____./ ﾜﾜﾜ [___] [___]ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
I have 3 of these rigs, an old one for /P use, a spare and a clean one in the
shack. I've been looking into a problem with my newer clean shack one. It has
developed a fault (may have always been there?), on long overs, on low power,
the Tx gain goes down & results in distorted Tx AF.
Using 2 tone testing (Mic processor off), I checked the 2 stage push pull bias
& it seemed to be thermally vicious cutting off the driver & PA when warm.
ﾚﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ
ｳTwo ｳ _ /ｳ ｳ RIG ｳ ｳDUMMYｳ ｳ ｳ
ｳToneﾃﾄﾄｴ_ｳ ｳ MicﾄﾄｴUNDERﾃﾄﾄﾄｴLOAD ﾃﾄﾄﾄｴSCOPEｳ
ｳOsc ｳ LS\ｳ ｳ TESTｳ ｳ& O/Pｳ ｳ ｳ
ﾀﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ
So I had previously turned up the PA bias to be OK on 50W 2 tone HOT, e.g. too
much standing current when cold! But this only delayed the problem, not fixed
it!
On a different IC735 I had seen PA Bias failure of the Bias emmitter follower &
that produced the typcal no RF output at low levels..
PEAK ｴ .ﾄﾄ. .ﾄﾄ. Round peak PEAK ｴ .ﾄﾄ. No .ﾄﾄ.
RF ｳ /::::\ /::::\ ｳ /::::\ Gain /::::\
ｳ ｳ::::::ｳ ｳ::::::ｳ Sharp Null ｳ ｳ::::::ｳXoverｳ::::::ｳ
ｳ ｳ::::::::ｳ ｳ::::::::ｳVｳ ｳｳ::::::::ｳ ｳ::::::::ｳ
ｴX::::::::::X::::::::::X ｴｴ::::::::ﾃﾄﾄﾄｴ::::::::ﾃ
ｳ ｳ::::::::ｳ ｳ::::::::ｳ ｳ ｳｳ::::::::ｳ ｳ::::::::ｳ
ｳ ｳ::::::ｳ ｳ::::::ｳ ｳ ｳ::::::ｳ ｳ::::::ｳ
ｳ \::::/ \::::/ ｳ \::::/ \::::/
ｳ `ﾄﾄ' `ﾄﾄ' ｳ `ﾄﾄ' `ﾄﾄ'
When Bias OK No BIAS
But this fault was confusing..
PEAK ｴ .ﾄﾄ. .ﾄﾄ. Round peak PEAK ｴ .ﾄ. Lower .ﾄ.
RF ｳ /::::\ /::::\ ｳ /:::\ Gain /:::\
ｳ ｳ::::::ｳ ｳ::::::ｳ Sharp Null ｳ ｳ:::::ｳXoverｳ:::::ｳ
ｳ ｳ::::::::ｳ ｳ::::::::ｳVｳ ｳ _-:::::::-_ _-:::::::-_
ｴX::::::::::X::::::::::X ｴ�:::::::::::�:::::::::::�
ｳ ｳ::::::::ｳ ｳ::::::::ｳ ｳ ｳ ~-:::::::-~ ~-:::::::-~
ｳ ｳ::::::ｳ ｳ::::::ｳ ｳ ｳ:::::ｳ ｳ:::::ｳ
ｳ \::::/ \::::/ ｳ \:::/ \:::/
ｳ `ﾄﾄ' `ﾄﾄ' ｳ `ﾄ' `ﾄ'
When COLD OK When HOT
So I have had another look at the problem, with a 100MHz scope, and I looked at
the class A stage in the PA unit. I saw it was "taking off" @ VHF when the
distortion occurred!
100mW 2W 15W >100W
ﾚﾄﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ
ｳ RIG'sｳ ｳCLASSｳ ｳCLASS B ｳ ｳCLASS B ｳ ｳSELECTEDｳ ｳ SWR ｳ ｳ ﾃﾄ>Ant
ｳ SSB ﾃﾄﾄﾄﾄﾄﾄｴ A ﾆﾍｵPUSHPULLﾆﾍｵPUSHPULLﾃﾄｴ BAND ﾃﾄｴBRIDGEﾃﾄｴRELAYｳ
ｳEXCITERｳ coax ｳSTAGEｳ ｳ DRIVER ｳ ｳ OUTPUT ｳ ｳFILTERS ｳ ｳ ｳ ｳ ﾃﾄ>Rx
ﾀﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ
------------------ PA COMPARTMENT ----------------------
The parasitic oscillation is above HF so it does not get far in the rig. The
fault is nothing to do with the class B driver bias as I thought, except that
affects the load seen through the driver transformer from the push pull class B
stage.
ｱｱｱ
ｱｱｱ The instability of the class A stage
.ﾄ. ｱｱｱｱｱ .ﾄ. at VHF is affected by class B driver
ｳ /:::\ ｱｱｱｱｱ /:::\ stage's bias setting!
ｳ ｳ:::::ｳｱｱｱｱｱｳ:::::ｳ
ｳ _-:::::::-_ｱ_-:::::::-_ What seems to be happening when hot,
ｴ�:::::::::::�:::::::::::� is that stage takes off until the HF
ｳ ~-:::::::-~ｱ~-:::::::-~ drive causes load to be seen on the
ｳ ｳ:::::ｳｱｱｱｱｱｳ:::::ｳ collector then it stops or reduces.
ｳ \:::/ ｱVHFｱ \:::/
ｳ `ﾄ' ｱｱｱｱｱ `ﾄ' The net effect is the HF gain is
ｱｱｱ reduced at the low level crossover
ｱｱｱ part of the 2 tone signal.
Class A Stage's Collector
THE CURE CLASS A STAGE CIRCUIT
I doubled the NFB base to
collector by paralleling +12Vﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾂﾄﾄﾄﾄﾄ
another 470R across the ｳ ｳ ﾚﾄﾄﾄﾄﾄ> To Push
470R and all was then tame. 270R )||(__ Pull Class
ｳ )||( ｳ B Driver
Q1 transistor seemed to ﾚﾄｴﾃﾄ470Rﾄﾂﾁﾄﾄﾙ ﾀﾄ)ﾄﾄﾄ> Stage
have too much VHF gain in ｳ ｳ ｳ
this stage when hot and ｳ ｳ/ ｳ
caused the oscillation! >ﾄﾄｴﾃﾄﾂﾁﾄﾄﾄﾂﾄﾄﾄｴQ1 ｳ
ｳ 220R ｳ\e 47uﾃﾄﾄﾄﾂﾄﾄﾄｿ Bias
Component tests around 330R ｳ ﾃﾄﾄﾄｿ === 68R _ｳ_Diode
this stage found ｳ ｳ 2n2=== 4R7 ｳ ｳ \_/
nothing wrong! ﾄﾂﾄﾄﾄﾄ)ﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄ)ﾄﾄﾄﾄﾁﾄﾄ
ﾀﾄｴﾃﾄｴ ｳ
150R 100R<ﾙDriver
+7VTx ﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ Bias
See my TECH Bul "AF 2 Tone Test Osc Design", "QRO 1kW HF Metered Dummy Load",
"IC 735 No PA Bias Fault","IC 735 No RF Output", "IC 735 Battery Backup Mod",
"IC 735 Lamps Mod" & "Autocaller with old Maplin kit"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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