Follow up to the Yeasu 857D repair.

[Michael Chisena]

Hi all,


First off, to the guys who sent me high quality scans of the print, many thanks. It was critical to this repair.
Had an issue with my 857D that might be of help to others.

The symptom was that the 6m FM rx went deaf.

When I got it on the bench all the 6M functions were weak.

So rather than .3uV for 12dB Sinad, I was getting about 8uV
for 12 dB Sinad.

 

Transmitter was good.

 

Did some poking.  First
test, divide and conquer. Figure out where the signal loss is coming from. When
I injected signal directly into the Main Board, the FM performance was fine, so
I had a signal loss inside the PA board.

 

Tested the Relays and parts for the 6M filter chain. 

No problem. Next I focused on the little transformer and
transistor amplifier.  


T3005 and Q3034. Eventually noticed that there was no useful
DC getting to the transistor.

No voltage, no gain.

 

Back tracked the “50mhz amp 13.8" line.

It went to a pin of Q3020. It’s a DC switch transistor
array.

When I commanded the radio to change bands that pin changes
from zero to VCC when 6M is selected. 

 

Seems the DC from Q3020 never gets to T3005. The trace is
open.

So something disconnected on the circuit board traces between
the two points.

Ran a test. 

Tack soldered a wire to the junction of T3005, C1369 and
C1371.

The other end of the wire went to a 100 ohm current limit resistor
then onto a VCC point.

With the wire + resistor was connected to VCC the RX worked as expected.

 

The easy fix is to run an external wire from one end to the
other.

I’m not inclined to pull the PA board again and chase down
this one trace.

 

Guess what, 6M RX sense is now ok.  

 

From time to time I see people asking about
similar problems.

This might be an answer. 

Since it was a circuit board problem and not a component
issue.

 

Photo link

http://www.flickr.com/photos/90108848@N00/sets/72157625671195766/with/5290572457/

 

Be well all.

 

Mike Chisena

KA2ZEV

Herndon VA



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