Message 25017 in TECH.WW
From: G8MNY
Date: Thu, 02 Jul 26 08:40:00 Z
Newsgroups: TECH.WW
Subject: ICOM 735 PA instability
Message-ID: <63947_GB7CIP>
Path: N2NOV|KC2NJV|VE3CGR|PY2BIL|GB7CIP
R:260702/0853z @:N2NOV.#RICH.NY.USA.NOAM $:63947_GB7CIP
R:260702/0852Z 8263@KC2NJV.#NASS.NY.USA.NOAM LinBPQ6.0.25
R:260702/0852Z 83577@VE3CGR.#SCON.ON.CAN.NOAM LinBPQ6.0.25
R:260702/0851Z 45904@PY2BIL.SP.BRA.SOAM LinBPQ6.0.25
R:260702/0840Z @:GB7CIP.#32.GBR.EURO #:63947 [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)
________________________________________________________
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.
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..
When Bias OK No BIAS
But this fault was confusing..
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
------------------ 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.
Class A Stage's Collector
THE CURE CLASS A STAGE CIRCUIT
I doubled the NFB base to
470R and all was then tame. 270R )||(__ Pull Class
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
← Return to Bulletin List