Al Holt <grovekid2@...> wrote:
I believe the bias is set by R17 & R18 show in the detail from the
schematic below:
T2 is the trifilar wound inductor. I think one interesting test would be
to see what happens to the voltage at the R17/R18 junction when the
problem is there and when it's not.
You're right. On 40m there's 3.694 V there, on 20m (BPF 1) 0.348 V.
If it is then my guess is something up stream is getting shorted to
ground. Maybe reflowing T2's connections?
Would T2 be affected with the band selection? It shouldn't...
I think you might want to consider the if T/R switch Q6 is working properly.
I checked, it seems fine. There's 8 mV on the drain during receive (on all
bands) and 12 V during transmit.
I'm familiar with Rev 3, 3a and 4 boards. If you have an earlier version
some of this might not apply.
It's 3a.
Good luck with your troubleshooting!
Thank you!
I might have found it. Maybe. Probably.
It seems that I have two unrelated problems (but if there was some spike
from the antenna, they might have the same source).
I cut the track between T2 and IC3 (pin 7) and measured both ends.
T2 didn't vary much, but IC3 pin 7:
- 20m (BPF 3): 46 mV
- 30m (BPF 2): 42 mV
- 40m (BPF 1): 3.841 V
- 60m (BPF 1): 3.841 V
- 80m (BPF 0): 1.569 V
I desoldered IC3, shorted pins 3 with 7 and 9 with 13, making a mono-band
receiver... and it works. At least on the audio sweep. I can't test it
with the antenna right now, because the antenna is in my other QTH.
But!
Even if audio sweep for 20m looks okay now, there are still strange
voltages on IC7 and IC9.
- IC7A in+ (pin 3): 0.887 V
- IC7A in- (pin 2): 0.890 V
- IC7A out (pin 1): 0.808 V
- IC7B in+ (pin 5): 0.886 V
- IC7B in- (pin 6): 0.888 V
- IC7B out (pin 7): 0.807 V
- IC9A in+ (pin 3): 2.376 V
- IC9A in- (pin 2): 2.497 V
- IC9A out (pin 1): 2.394 V
- IC9B in+ (pin 5): 2.396 V
- IC9B in- (pin 6): 2.516 V
- IC9B out (pin 7): 2.616 V
I suspected that IC7 might be bad, so I desoldered it and bias voltages
returned to around 2.5 V. I swapped it with IC9 just to be sure and...
now the new IC7 (old IC9) doesn't work (has 0.8 V), and new IC9 (old IC7)
sometimes works, and sometimes doesn't.
My suspicion is that a spike on the antenna killed IC3 and made IC7 and
IC9 unstable. I'll order new ones and see.
The other thing is that I want to avoid similar situation in the future...
I'm wondering what should I do. Maybe I should buy some protective device,
a surge arrester or something... or just a transil... or two transistors,
like T1 and T2 here:
I have two antennas right now, one is the open half-wave dipole (20m) and
another is a grounded end fed (it was just a random wire tied to a tree,
but works fine on 30m after adding a transformer), if that's relevant.
