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G8MNY > TECH 27.11.24 16:00l 64 Lines 2818 Bytes #999 (0) @ WW
BID : 20669_GB7CIP
Read: GUEST
Subj: FT290/690/790 mk1 PLL fault
Path: IW8PGT<LU4ECL<I0OJJ<GB7CIP
Sent: 241127/1029Z @:GB7CIP.#32.GBR.EURO #:20669 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Nov 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
________________________________________
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³ (_) ³====³ ³ 4.5000 ³ "" "" \./ ³
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³ ./"~"\. |__| |__| /~~\ ³
³ /~\ /~\ | | |__| |__| | |³
³ \./ \./ `\._./' |__| [__] \../ ³
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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.
FAULTING SAGA
Tuning into 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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