Digimax D612 Frequency Counter problem

[madd0c]

Hi, long time lurker on this forum. I made an account to get some help with this.

I bought a Digimax D612 frequency counter on ebay which is from the mid 1980s. It worked great for several weeks. Then I made the mistake of soldering with a metcal soldering iron on part of a circuit under test. Metcal uses rf heating and I think it must have fried something in the frequency counter.

The frequency counter has a direct input for 50Hz to 50Mhz and a prescaled input for 10Mhz to 1.2 Ghz. Both 20V P2P RMS inputs.

After my mistake, the frequency counter functions fine in the prescale input (10Mhz to 1.2Ghz) and functions fine ABOVE 3Mhz in Direct input mode. The interesting thing is it is not a blown capacitor or resistor, because the direct input functions fine above 3Mhz (yes I replaced them even though they tested good,no change). Anything below 3Mhz and the counter doesn't register it at all even with a 5v signal from a digital signal generator.

The schematic is included in the operators manual here:
https://elektrotanya.com/digimax_d-1200_frequency_counter_operators_manual.pdf/download.html

I have traced the signal with an oscilloscope thorough the direct input to the MPF102(Q7) and it is fine to that point. I have an NTE457 replacement for the MPF102 on the way as I think that must be the problem, but I can not figure out why it would only effect signals BELOW 3 Mhz.

Does anyone have any ideas why the only thing effected is the counts below 3Mhz when it works fine above that?

Any help would be appreciated. Thanks!

[edavid]

C9 could be bad or the wrong value, or the 3MHz signal may couple through the MPF102's gate-drain capacitance, even if it's not functioning as an amplifier.

[madd0c]

Thanks for taking a look. C9 tested good, but I replaced it anyway with a known good part. It is correct .02uf value.

As for coupling, that may be why the signal dissapears at the MPF102. Will know more when I get a replacement for that part in, but that makes sense. I suppose that would explain why it counts properly at the higher frequency. That is to say I can measure and display a 3.455 Mhz signal properly , but not a 455Khz signal.

Thanks for the input!

[madd0c]

If anyone is interested, replacing the MPF102 fixed the issue, so it must have been capacitively coupling above 3Mhz.
This is just an update for anyone who searches on how to repair a D612 with this issue.

Also anyone searching about Metcal soldering irons  and damage to RF/UHF JFETs

Thanks!