G8MNY > TECHNI 09.10.21 08:30l 67 Lines 2890 Bytes #999 (0) @ WW
BID : 49023_GB7CIP
Subj: IC 735 Battery Backup Mod
Sent: 211009/0624Z @:GB7CIP.#32.GBR.EURO #:49023 [Caterham Surrey GBR]
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
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.
I found the 12 memories can occasionally be lost, as the old memory battery was
Looking inside I found the battery was not completely flat, & was a soldered in
type & difficult to change without removing the whole large PCB.
+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.
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
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
8/ Reassemble & test memory stays OK for a few more years if the rig is used
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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