G8MNY > TECHNI 10.10.21 10:00l 52 Lines 2467 Bytes #999 (0) @ WW
BID : 49100_GB7CIP
Subj: IC 735 No PA Bias Fault
Sent: 211010/0749Z @:GB7CIP.#32.GBR.EURO #:49100 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Update Jan 20)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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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 repaired 2 of these old rigs with this fault, both with very bad
unreadable AF reports. Scoping the RF O/P when on a dummy load (sniffer near
a dummy load) hard gaps can be seen in the RF low level O/P.
PA Access is fairly straight forward, but lots of screws!
1/ Remove power lead, remove top cover including the 4 LS anti rattle screws.
2/ Unscrew the 4 screws for PA unit, then with PA unit upside down & flat, the
main unit can be balanced on its side on top, all leads should just about
3/ Remove all the PA & Filter screening plate screws, & remove.
4/ Connect a Dummy load, PSU & Mic.
5/ On the PA board locate the screwed down Tab NPN transistor with white past
& wire diode on top, nearest the middle & front of the PCB. On Tx it should
+12V collector (Tab), 1.3V base (from PA Bias Pot), & +0.68V on emitter
(bias linked to the 2 RF PA NPNs bases). Be V_E_R_Y careful not to short
+12V collector to either base or emitter on testing!
6/ If bias has failed (<0.68V), power off, unscrew the NPN & desolder it from
the 3 top side PCB pads. I found the fault was intermittent, as testing the
NPN looked OK, but it would not work. Replacing it with any Tab NPN > 1A
12V & gain B > 50 should do.
7/ Put in a 2 tone test signal on Mic (acustic), & adjust PA Bias pot for
perfect crossover on a scope.
8/ Disconnect leads & carefully reassemble, then retest.
Also see tech bul "AF 2 Tone Test Osc Design"" IC 735 Battery Backup Mod", "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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