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G8MNY > TECH 30.11.09 09:36l 65 Lines 2803 Bytes #999 (0) @ WW
BID : 43756_GB7CIP
Read: GUEST
Subj: FT290/690/790 mk1 PLL fault
Path: IZ3LSV<IK2XDE<ON4HU<IW0QNL<F4BWT<VE2PKT<ON0BEL<GB7CIP
Sent: 091129/2337Z @:GB7CIP.#32.GBR.EU #:43756 [Caterham] $:43756_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Nov 09)
(8 Bit ASCII Graphics use code page 437 or 850)
The fault symptoms on one of my FT290 are impulsive noise on FM that kills Rx
packets & it is not so noticeble on SSB until the fault is realy bad. Turning
the rig off & on fixes it for a while (usually days). I have a simular fault
before years ago, but I don't know what was done to fix it.
________________________________________
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FAULTING SAGA
Tuning in to a beacon & testing in SSB, it look like the 1uF ( ³±±³ ) C34 in
the PLL filter was breaking down, warming or cooling that area changes the
fault (OK cold & hot, bad at room temp!), but changing the capacitor
(connections burried under soldered in PCB underside screen cover) has no
effect. :-(
PLL Box
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄ¿
³ [ù] D [ù]³[ù] ³ +5V
³ D c³(+) ³inter- ÚÄÄÄ¿ ÚÄÄÄÁÄÄ¿
ÃÄÄÄÄÄÄÄ¿ *:³~³: ³(+) D³polation ³VCOÃÄ´>Ä´PLL ICÃÄXtal
³ o(-)³³±±³:³ ³: ³ Û³ ÀÄÂÄÙ ÀÄÂÄÂÂÂÙ
³ ³ D :³ ³:ܳ ß³X 10.7 DCÀÄÂ<ÃÂÄÄÄÙ DATA
³ 0 ³D :³_³:Û³ [ù] ³ MHz ÀCRÙ
ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÙ
VCO X Fo
5.76MHz
Further investigating found the PLL IC MN6145(uPD21819C) itself seemed to be
the culprit as heating/cooling caused the fault to change. re-soldering up all
18 pins had no effect.
Connecting a 10:1 scope probe to local Xtal Pin 2 & 3, or the buffered osc
drive Pin 11 had no effect, so it was not RF levels that were low.
But putting the probe on the IC Pin 1 (+5V), I found caused the PLL noise to
increase, as this is decoupled with 2 caps I was supprised. So I concluded the
10uF (c) C55 may be low in value, & as the it was difficult to remove I put in
a small 2u2 on the track side on pin 1 directly. This solved the probe noise
effect & made the hot & cold effect on the IC less apparent but still there!
I then tried an additional voltage drop diode (5.6V is fed via a diode post (*)
to this IC for 5V) in series with the IC supply diode giving it only 4.28V,
this worked OK & I can't reproduce the fault thermally now.
Result, my ageing IC does not like the supply volts & becomes unstable at a
specific temperature range. Changing the +5V to 4.2V has fixed this.
See my Tech bul on "CW/SSB Rig Frequency" & also "FT290/690/790 DC Power mod"
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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