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HP 8657A low-band output level problem

Question asked by vr2whf on Oct 12, 2010

I have a HP 8657A with problem in output level for low-band frequency.

My HP 8657 always experiences more than 30db low for the frequency < 130MHz which falls ino the low band range (0.1 to 129.9MHz) generated by heterodyning the 800MHz signal with the 800.1 to 929.9 MHz signal. The output level for the frequency >= 130MHz works perfectly well, so it takes me to looking at the Heterodyne Section of the A6 board.

According to the service manual service sheet 5,  mentioning that the signal bewteen 800.1 to 929.9 MHz enters to the Heterodyne section, and attenuated before mixing with the 800MHz signal, giving 0.1 to 129.9MHz.  After the mixing, the downstream has the output amplifier to restore the RF signal to the same level as the input.

However, the service sheet 5 brings me little info. I have checked that those 4 output amp transistor (Q301 to Q304) seems to be biased correctly,  so I have no clue on what really happens.  Any clue?

Thanks and Regards,