Re: Help for QMX in Germany
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Harry, one more thing. If IC407 should become damaged, that could also create a deaf condition. That stereo ADC chip is the most vulnerable to overvoltage of all the chips on the 5V distribution. A failure there is often accompanied by heating of the chip. I note all that here for completeness, though I think your problem is elsewhere.
JZ
Harry,
I have seen the following cause a "deaf on all bands" condition:
In order of declining likelihood:
a) a bad connection at either L401 or T401. This is absolutely the first thing to check. Wire enamel can be difficult to properly remove.
b) a failed 3253 mux chip. There was a rash of this early on in Rev 2. It was corrected in Rev 3 with the addition of a diode. I have seen only one failure of a corrected board, and it was due to an open circuit created at L503, voiding the T/R switch function. Checking for the correct bias voltage of 2.5V at C406 is usually a good test for the mux.
c) a short at the BNC connector. This is not so easy to determine, as by design an ohmmeter will show a DC short at that point while all is OK for RF. Only close inspection can help.
c) a failed T/R switch transistor. Rare, but it has happened. With power off, an ohmmeter check of resistance from each terminal to every other terminal should yield very high resistances for a good transistor.
d) a cracked capacitor C526. This will generally be accompanied by zero output power.
GL JZ
May I have to ask the seller, if it was an assembled unit, I do not know.
But how about the general deaf RX on all bands?
Winding L401 a thought? Mine have the older winding.
73 de Harry, DK2GZ
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