When you enter the diagnostics screen, the Tx voltage is turned off, so it can be tested and we can see it turn off and on.
As soon as you exit the diagnostics screen, it turns back on.
If you read Hans' description, they found that certain things work better in receive if they leave the tx voltage on.? But in receive, the TX signal into IC503 is off, so the BS170s never get turned on, never conduct current.? At least if IC503 isn't broken...? So either:
a) IC503 was broken first from an over-voltage on the 5V, with floating or shorted-high outputs that caused the BS170s to fail, or
b) as I think Ludwig suggests, the BS170s had suffered some damage, and finally broke, sending a 12V over-voltage into IC503 outputs, frying it.
And I think Ludwig's version is more likely, since I have no idea how you could get a severe over-voltage on 5V without also damaging the 5V smps protection diode - it would clamp the over-voltage to ~5.6V, except for maybe a fast-and-furious spike that was faster than the clamping speed of the diode.
Or maybe it was something else that we haven't thought of...
?
Stan KC7XE.
?
