G8MNY  > TECH     17.05.21 09:00z 67 Lines 2890 Bytes #172 (0) @ WW
BID : 34284_GB7CIP
Read: GUEST
Subj: IC 735 Battery Backup Mod
Path: ED1ZAC<ED1ZAC<IZ3LSV<I3XTY<I0OJJ<GB7CIP
Sent: 210517/0755Z @:GB7CIP.#32.GBR.EURO #:34284 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                            (Updated Jan 20)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
 ________________________________________________________
³         ŚÄÄÄÄÄÄÄÄæ ŚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄæ  ==  ==  == ³
³ ßßßß == ³  \     ³ ³ Rx 14.273.5 vfo A 12³    _    _   ³
³ /~\     ĄÄÄÄÄÄÄÄÄŁ ĄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŁ   (_)  (_)  ³
³ \_/                          ____  IC-735              ³
³      ßßß ~~~~~~""""""~~~   /'    `\    ßßß   /"~"\  [] ³
³  0   ÜÜÜ                  |(~)     |   ÜÜÜ  |     |    ³
³           Ž Ž Ž Ž Ž Ž    ³  ~       ³        \._./  [] ³
³ /~~\ ßßß  Ž Ž Ž Ž Ž Ž     |        |   ßßß  ==  ==  == ³
³ \__/ ÜÜÜ  # # # # # #  []  \.____./    ÜÜÜ  [___] [___]³
ĄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŁ
I have 2 of these old rigs, a heavily used one for /P use, club events, &
training courses etc. & a clean one in the shack.

SYMPTOMS
I found the 12 memories can occasionally be lost, as the old memory battery was
going flat.

SIMPLE CURE
Looking inside I found the battery was not completely flat, & was a soldered in
type & difficult to change without removing the whole large PCB.

         D4
+5V>ÄÄ´>ĆÄÄÄæ             So I looked at the circuit
            ³             & found a very small trickle
    Ś-10K-æ ³             current, bridging around the
    |     | ³             battery steering diode D3,
 +ŚÄĮÄ´>ĆÄĮÄĮÄÄ>RAM       would bring back the battery
 _³_   D3       MEMORY    to a good voltage, & give
  Ü                       many more years of life.
 _³_BT1


FIXING IT
Access is fairly straight forward, but a few screws!
 1/ Remove power lead, remove top & bottom covers, including the 4 anti rattle
    screws around the LS.
 2/ Unscrew the 4 screws for PA unit, remove external Phono Rx patch lead,
    release the LS lead, RF drive lead from underside PCB & 2 other connectors
    to the upper PCB. then rotate the PA unit.
 3/ Look at the top PCB & locate the Disk battery BT1 & nearby diodes D3 & D4.
 4/ Measure the voltage on BT1 against earth. You should see 3.7V on the +lead,
    if you do, a poor battery is not the fault!
 5/ Check that D3 has the same battery voltage one side & 0.3V less on the Ram
    side.
 6/ To apply the charging modification, tin both ends of D3 & carefully solder
    in a small 10K across it. The 10K limits charging current to just a few uA.
 7/ Now put everything on its side, & make power safe, power up the rig & check
    D3 volts. You should see 4.5V on the Ram side & an increasing battery
    voltage.
 8/ Reassemble & test memory stays OK for a few more years if the rig is used
    occasionally!


Also see tech buls "IC 735 No RF Output Fault", "IC 735 No PA Bias Fault",
"PA instability in ICOM IC735" & "Autocaller with old Maplin kit".


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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