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Q90 fried
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I've a stock uBitx (3rd build I think) and have made a few contacts on 75 Mtr with a dipole in the trees.? Also using an antenna tuner to get a close to 1:1 match.? Often let it on for extended times receiving ft8 and listing all rec'd stations.? Went to operate it the other day setting up for a demo and it wouldn't transmit.? Receive was working but possibly weaker than I remembered.
Scoped the signal path back to Q90.? Q90 was not amplifying and ohm'd a short base to emitter.? Replaced Q90 and it now will transmit again.? I suspect any strong signal or static pulse on the antenna during receive will damage Q90.? Another ham 2 houses away and DX Engineering near by.? I'm considering a reverse connected diode across b-e of Q90 as an experiment to see if it eliminates failures. By the way, into a dummy load it outputs 10 W on 80; 8 W on 40; 5 W on 20; and 1 W on 15 and 10 M.? My replacement Q90 may be limiting at higher freqs.? More scoping to do. Doug WA8UWV |
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Gordon Gibby
¿ªÔÆÌåÓý"Q90 was not amplifying and ohm'd a short base to emitter.? Replaced Q90 and it now will transmit again.? I suspect any strong signal or static pulse on the antenna during receive will damage Q90.? Another ham 2 houses away and DX Engineering near by.? I'm considering a reverse connected diode across b-e of Q90 as an experiment to see if it eliminates failures."
I've speculated before that the failure mode is a REVERSE voltage on the base-emitter, as the series capacitor charges up due to rectification by the base-emitter junction.? ?This document:???? ?on pp18-19 seems to suggest that indeed, overloaded reverse voltage bipolar junctions fail SHORTED, while forward current would be expected to OPEN them.? ?The reverse diode would elminate the charging of the series capacitor and protect the junction against reverse voltages, and someone tested this for ill effects and found none.? ?
So I put the reverse diode on any that I have.? ?No failures yet, knock on wood.? ?
Gordon
? From: [email protected] <[email protected]> on behalf of dougmansor@... <dougmansor@...>
Sent: Wednesday, September 19, 2018 10:09 PM To: [email protected] Subject: Re: [BITX20] Q90 fried ?
I've a stock uBitx (3rd build I think) and have made a few contacts on 75 Mtr with a dipole in the trees.? Also using an antenna tuner to get a close to 1:1 match.? Often let it on for extended times receiving ft8 and listing all rec'd stations.? Went
to operate it the other day setting up for a demo and it wouldn't transmit.? Receive was working but possibly weaker than I remembered.
Scoped the signal path back to Q90.? Q90 was not amplifying and ohm'd a short base to emitter.? Replaced Q90 and it now will transmit again.? I suspect any strong signal or static pulse on the antenna during receive will damage Q90.? Another ham 2 houses away and DX Engineering near by.? I'm considering a reverse connected diode across b-e of Q90 as an experiment to see if it eliminates failures. By the way, into a dummy load it outputs 10 W on 80; 8 W on 40; 5 W on 20; and 1 W on 15 and 10 M.? My replacement Q90 may be limiting at higher freqs.? More scoping to do. Doug WA8UWV |
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Hi,
During a calibration routine on my Nextion display (ver 1.09) I suddenly experienced no transmit and no receive.? (I may have hit the ptt without antenna connected)? But Q90 woulnt cause receiver to qkuit would it? There is no change when I unplug the antenna from the board and put my finger on the antenna terminal. Any Ideas of a likely suspect that would cause both to die?? Audio is working.. Andy, KM4TRT |