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Hey everyone,

short story:
After one of the PA transistors died, I replaced all of them. Now I only get 3,5W power on 80m but less than 0,5W on the other bands.
Is this because of insufficient (fake) replacement transistors or could something else be damaged?

long version:

I built the QDX and after some wire pushing on the coils I got 5W@9V?at all bands and was super happy. I am measuring DC on the detector diode of a QRP dummy load.
After a some FT8 operation using an antenna, the output power dropped to 2.5W on all bands. Then I accidently shorted one of the transistors during some measurements and the output power went to a few mW.

I removed the BS170s from the circuit and measured them using a diode tester and one was definitely broken.

Then I bought replacments for the BS170 transistors at three different online shops (not really trustworthy ones).

However during testing after replacing the transistors, the QDX behaved identically with each of the different BS170 sets.
There was about 3,5W output power at 80m but only less than 0,5 W on the other bands.

I supect that the transistors are fake because their performance decreased with higher frequency. But I am not sure. Is there a way to validate this?
I used a LCR-T4 transistor tester to measure the transistors. This showed differences in capicatance. The original BS170s from the kit have 50pf, one of the sets I bought has 60pf and the others 30pf. Does this give any indication if the parts are correct?

Or could it be possible that someting else in the QDX is broken? Maybe the IC that drives the PA (74ACT08)? However, I measured 2.5V at the BS170 gates so I guess it still works. Unfortunately I do not have an?oscilloscope at the moment.
Or is maybe the PIN diode switching circuit for the LPFs not working anymore?

Reception is still fine!

I would be really grateful for help on finding the issue!

regards
Chris

Hey,

just an update for others who are experiencing the same issue. I replaced IC5 and that fixed the issue. Now I get 4W on 80m and about 3.3W on 20m. I guess that might be because of the replaced transistors. Maybe I'll try some others. However at least the QDX is working again now :)