IC 735 No PA Bias Fault
From
G8MNY@4:902/6 to
WW on Wed Dec 11 05:58:02 2024
To : TECH@WW
By G8MNY (Update Jan 20)
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I now have 3 of these old triple convertion HF PLL rigs, a heavily used one for /P use, club events, & training courses etc. a spare, and a cleaner one in the shack.
SYMPTOMS
I have repaired 2 of these old 1985+ 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 the cast 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, or temporarily disconnect as needed.
3/ Remove all the PA & Filter screening plate self tap screws, & remove.
4/ Connect a Dummy load, PSU & Mic.
5/ On the PA board locate the screwed down Tab NPN transistor with white past
and wire diode on top, nearest the middle and front of the PCB. On Tx it
should be +12V on collector (Tab), 1.3V base (from PA Bias Pot), and +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 & de-solder 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 will do OK.
7/ Put in a 2 tone test signal on Mic (acoustic), and adjust PA Bias pot for
perfect crossover on a scope. Check quiescent PSU current is OK.
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","IC 735 Lamps Mod","IC 735 No RF Output Fault", "Autocaller with old Maplin kit" & "CW/SSB Rig Frequency".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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