[ICOM] IC-775 - low output power in higher bands

[paul at w8aef.com]

A friend of mine asked me to take a look at his IC-775DSP.  It put out 100 
watts on the lower bands but the output power gradually dropped to 10 watts 
as you went up to 10m.

My usual approach to signal tracing, the goesouta is more bigger than the 
goesinta, failed me this time.

I found two stages in the radio that had negative gain.  That is the 
goesouta was not a big as the goesinta.

Q10 on the IF UNIT had 35 mvpp on it's gate but only 25 mvpp on it drain, 
and Q19 on the RF UNIT had 480 mvpp on G2 but only 300 mvpp on it's drain, 
but replacing both devices did not cure the problem.

After much head scratching I obtained a 2nd 775 so I could try the besideya 
method.  (compare a working unit with a non-working unit).

Turns out Q32 on the RF UNIT had 135 mvpp on the base and 1.16 vpp on the 
collector.  The working radio had 115 mvpp on the base and 5.0 vpp on the 
collector.  Replaced Q32 and the radio works fine.

During my signal tracing I made made level observations that may be of 
interest to others.

Cable to PA UNIT unplugged at RF UNIT J8 to force maximum gain in IF UNIT 
and RF UNIT.  Mode RTTY.  Observed with Tektronix 465 oscilloscope.

IF UNIT         14.1 MHz

Q10      G      35 mvpp
         D      25 mvpp

Q33      B      55 mvpp
         E      55 mvpp

IC19     pin 6 135 mvpp

D36      C     240 mvpp
         A     240 mvpp

D48      A      65 mvpp
         C      55 mvpp

Q78      B      60 mvpp
         C      80 mvpp

Q45      G2    110 mvpp
         D     6.7 Vpp      28.1 MHz

D72      A1    560 mvpp     550 mvpp
         C     490 mvpp     430 mvpp


RF UNIT

C272           165 mvpp     175 mvpp

D9       C     220 mvpp     220 mvpp
         A2    220 mvpp     220 mvpp

Q16      G2                  45 mvpp
         D                  900 mvpp

Q19      G2    310 mvpp     480 mvpp
         D     300 mvpp     300 mvpp

D26      C     250 mvpp     250 mvpp
         A2    250 mvpp     250 mvpp

D27      A2    180 mvpp     140 mvpp
         C     180 mvpp     140 mvpp

D37      C     145 mvpp     140 mvpp
         A2    140 mvpp     115 mvpp

Q32      B     140 mvpp     115 mvpp
         C     6.5 Vpp      5.0 Vpp

R148           1.3 Vpp      1.2 Vpp

The normal disclaimer: your smileage may vary.  :)

de Paul, W8AEF