Daiwa CN801-HP Power VSWR Meter suddenly reads too high

Discussion in 'Ham Radio Discussions' started by ZL2TVT, Dec 17, 2012.

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  1. ZL2TVT

    ZL2TVT Ham Member QRZ Page

    If your forward (or reverse) meter suddenly reads high for no apparent external reading it could be due to a faulty 200pf chip cap inside the sensor head. After substituting new 6.8pf caps and no improvement I decided to look further instead of returning the meter to USA. First of all disconnect the existing connection wires off the sensor head, remove the head from the case and remove the cover. With a known 10 Watts at 7Mhz into the head with a 50 Ohm termination the DC reading using a 50k Ohm/voltmeter of approx 0.5Volts should be evident at the forward power feed-through out. In my case it was 2.5V DC! I traced the cause to a (not visually obvious) fractured chip cap causing an open circuit and subsequent high DC reading at the adjacent feedthrough cap termination. Paralleling another 150pf disk ceramic cap across the existing chip cap and removal of the chip cap (which came apart at the end) and restoration to an approx normal DC level proved the fault. Replace the chip cap with either another 200pf cap or a 150pf + 47pf 50V disk ceramic and recheck the calibration. I used two 100pf 50V chip caps and restored to nearly correct reading without having to recalibrate. If you find the repair gives a slightly higher reading then add a little series resistance between the meter wiring and feedthrough. Note: The original 200pf chip caps are fragile, and even a bad knock could cause fracturing of the ends of the cap~ZL2TVT
     
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