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G8MNY > TECH 31.05.19 10:51l 49 Lines 2274 Bytes #999 (0) @ WW
BID : 6816_GB7CIP
Read: GUEST
Subj: IC 735 No PA Bias Fault
Path: IW8PGT<IZ3LSV<F1OYP<ON0AR<GB7CIP
Sent: 190531/0949Z @:GB7CIP.#32.GBR.EURO #:6816 [Caterham Surrey GBR] $:6816_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (New Apr 19)
(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
load) hard gaps can be seen in the RF low level O/P.
FIXING IT
PA Access is fairly straight forward, but a lot 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 connector, 1.3V base (from PA Bias Pot), & +0.68V on emmitter (bias
linked to PA & RF bases).
6/ If bias has failed, unscrew the NPN & desolder the 3 top side PCB pads.
I found the fault was intermittant, as testing the NPN looked OK, but it
would not work. Replace it with any 0.5A 12V Tab NPN HFE> 50.
7/ Put in a 2 tone test signal, & adjust PA Bias pot for perfect crossover.
8/ Disconenct 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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