SB-104A problems and solutions

[R. Schmitz]

Hello to the list!
Some weeks ago, i sent some Mails to the list.
Problems with my SB-104A were low output on 20 and 15 m.
Got several helpful messages, so thank You all,esp. to
Mike,w8krr
Mike,wg8vge
Kees,K5bcq
Brad Allen
Charles,ka8jvx
Dave,w8dlk,hope,I have nobody forgotten.
Heinz DH2FA, KM5VT borrowed me an extender board and now the rig is working 
fine.
What were the bugs?
First, i bought the rig without CW Filter,used almost as RX but TX should 
work OK.
It was nice, finding a CW Filter built in, after i got the rig.
Next:Fuses blew,when TX with Hi-Power.
I feared a problem in the final, so i needed a while to solve the Problem.
The fuse holders were confused, so prmary was fused with 20A and secondary 
with 4 A.So-do not believe in anything,You have not soldered yourself,hi.
Next, i located the output problem.
Made varible caps to board C C334,337 for 20m and C329,333 for 15m.
Result was 70watts 0n 15m and 60watts on 20m.
then i made variable alsoC361 for "0m.Helped a bit.
When testing TX,I found vy distorted audio when using Hi-Power,with 
Low-Power,modulation was fine.
Checking the Bias Voltage of the PA: only 0,1Volt ---Class C Operation.
Bug was in the Relay,so I replaced it with two small relays.
Now Power up to 190W on 80/40m a bit lower on 20/15m.
Looked like an ALC Problem,but ALC from Pin4,Board K and Pin 4 Board J were 
OK and properly at Pin 4 board C.Found the Emitter Voltage of Q303 was a bit 
higher 1,0V at TX,1,2V at RX as in the Voltage chart and only little Voltage 
difference at Pin5 IC301 between Mic/CW-Level open or close.
I found the problem in R323,replaced it and whoow:
100W-110W on all bands from edge to edge after abit readjusting the variable 
caps.Dont know , if it was necessery to make the caps variable-
but now,I dont change to the fixed caps.
Will make some Mods from the Heath Service Bulletins and  give him a extra 
7MHz Filter to the ReceiverFrontEnd to solve some IM problems in the 
future.But at this time the Rig seems to be very fine.
Best 73´s de Randolf,DL6KAI

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