Electronics > Metrology

IFR FM/AM 1100S Radio Test Set AN/GRM114A scope inverter repair

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Paul29E:
Hello, I need help fixing my test set, any calibration specialists still around? I am trouble shooting a scope inverter module on a IFR FM/AM 1100S radio test set. I was monitoring over the air signals on the combination oscilloscope and spectrum analyzer when a burning smell started and the scope waveform distorted. After unplugging the test set, I removed the cover and discovered a blown tantalum capacitor C11 150uf 15V that filters the 12vdc and a burned resistor R2 68ohm 1/4 watt that is on the emitter of Q1 transistor MPS-A13 which is a start up circuit. After replacing both parts it happened again after turning on the scope, I immediately turned off the scope and unplugged the set. After removing the blown capacitor I monitored the 12vdc feed for the inverter module (4000 scope inverter), 12vdc feed tested ok. I took Q1,C10 and C2 out of circuit and they tested ok. What do you think I should look for next?
Thanks, Paul

Grandchuck:
C1 and CR1.

Paul29E:
I will check C1 and Cr1 when I take it back apart. I did a visual check and they weren't burned up. C11 blew up twice and R2 burned with it. I need to check Q2 and Q3 also. Maybe it’s a common failure. Thanks for the help.

Paul29E:
Replaced blown C11 150uf 15V that filters the 12vdc and also a burned resistor R2 68ohm 1/4 watt. Found C2 capacitor reversed and R1 was the wrong value. I replaced tantalum C10 and C11 with 25vdc 150uf electrolytic caps and repaired burned circuit board. Q1, Q2, Q3, CR1, C1, C2, R1, R2, R3 and R19 all replaced with correct values on schematic. Oscilloscope and Spectrum Analyzer work good now. Thanks for the help.

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