In starting out on this, I compiled some notes on the 7D14 and its misadventures. I believe
the problem is in the "front end" where the signal is processed
from either the "A" channel BNC or the "B" channel internal routing
from a vertical amp. The 7D14 service manual bears reading -- particularly the circuit operation description. Here are some preliminary testing observations:
(1) The symptoms are that the counter will display the frequency
accurately for a while -- about 20 minutes to one hour -- and the
display will wilt to zero.
(2) For a while, I was able to input the signal (60 MHz - same as
input to the 7A26) to the "A" channel BNC but and thought I had
"isolated" the problem. But not so much anymore.
(3) I chose to start at R222 as it is the input to the input
amplifier stage -- Q130A/Q130B, the curious dual N-channel FET --
to determine if where the signal was getting dumped. Starting up
the cold 'scope, I saw the signal there at approx. .091 Vpp and
even after the 7D14 stopped displaying the frequency, i.e.
displayed zeroes, the signal was still there albeit somewhat
diminished at .041 pp.
(4) The subsequent point immediately after -- Q132/Q232 base,
Q141/Q241 base, and Q136/Q236 base -- all showed no or erratic
signal.
The voltage measurements below are the signal measurements from the scope -- though they
are little more than the numbers behind #4 above. I appended the voltage measurement pictures.
One more thing I gotta tell you is that I am no fan of the Rigol DS 1102E scope I recently got. I am absolutely unable to operate the thing! The trace is fuzzy and erratic but it's probably me but it still makes me no fan of digital scopes -- so far!
SCOPE TEST MEASUREMENTS
MEASURE 7D14 7D14
POINT RUNNING GOOD CRAPPED OUT
R222 .092 Vpp .042 Vpp
Q132 base .048 Vpp NONE
Q232 base .048 Vpp .027 Vpp
Q141 base .310 Vpp NONE
Q241 base .260 Vpp NONE
Q136 collector .300 Vpp NONE
Q241 collector .230 Vpp NONE
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