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G8MNY  > TECHNI   10.10.21 10:00l 52 Lines 2467 Bytes #999 (0) @ WW
BID : 49100_GB7CIP
Read: GUEST
Subj: IC 735 No PA Bias Fault
Path: PI8DRE<PI8CDR<DB0RES<ON0AR<GB7CIP
Sent: 211010/0749Z @:GB7CIP.#32.GBR.EURO #:49100 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                               (Update 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

SYMPTOMS
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.

FIXING IT
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
    reach!
 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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