Sunday, November 7, 2021

Tektronix 7D14 Fix-Up Part I

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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