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G8MNY  > TECH     14.06.18 07:30l 65 Lines 2819 Bytes #5 (0) @ WW
BID : 25163_GB7CIP
Subj: FT290/690/790 mk1 PLL fault
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 180614/0526Z @:GB7CIP.#32.GBR.EURO #:25163 [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)
 ________________________________________
│  _  ┌────┐ ┌────────┐              /~\ │
│ (_) │====│ │ 4.5000 │   ""   ""    \./ │
│     └────┘ └────────┘  |__| |__|       │
│              ./"~"\.   |__| |__|  /~~\ │
│  /~\   /~\   |     |   |__| |__| |    |│ 
│  \./   \./   `\._./'   |__| [__]  \../ │
└────────────────────────────────────────┘
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 in to a beacon & testing in SSB, it look like the 1uF ( │▒▒│ ) 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
┌──────────────────┬─────┐
│  [¨]       D  [¨]│[¨]  │                            +5V
│              D  c│(+)  │inter-          ┌───┐    ┌───┴──┐ 
├───────┐   *:│~│: │(+) D│polation        │VCO├─┤>─┤PLL IC├─Xtal
│   o(-)││▒▒│:│ │: │    █│                └─┬─┘    └─┬─┬┬┬┘
│       │  D :│ │:▄│    ▀│X 10.7          DC└─┬<├┬───┘ DATA
│  0    │D   :│_│:█│ [¨] │  MHz               └CR┘
└───────┴──────────┴─────┘
    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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