New facts: The power amplifier seems ok so far, at least I have all
voltages of the driver and final bias. Basically, it was really my fault
as I shortened one base of one transistor for a measurement.
Now I tested the PA under following circumstances:
RF 0
Driver Bias 1mA
Final Bias 300mA
Pushing PTT results in an output of 2W with a current of 2A, but the
current is increasing rapidly - after 10 seconds the current is 3A with
abt 3Watts output.
Second crazy thing: SWR is still indefinite (into dummy or antenna).
Opinions are welcome!
73, Harry, DH1NBE
Am 06.05.2017 um 23:26 schrieb Harald Rester:
Hi guys
Three weeks ago my Omni went off the air and I tried to find the
problem but had no luck so far, although I got lots of help from Gerd,
DF9TS, (tnx es Servus Gerd: I dont know what to do anymore...)
Maybe some of you have an idea what is going on and can give me an
advice what to do next (Yes, I searched the list...many times...)
So lets go (be aware - longer story :-)):
1. Suddenly I had RX-Problems: RX-Volume decreased, but it went away
after transmitting
2. I become aware of the fact, that my SWR got worse during TX
3. Finally, maybe after 10 minutes my Elecraft W2 showed indefinit
SWR, power output abt 200 Watts(!?): Shutdown from power supply / Omni
4. First, I could decrease power output to abt 30Watts and SWR was ok
again
5. After some attempts even at RF out=0 everything shuts down and I
had the same readings as mentioned above
-> Suspected bad feed line / antenna, but Spiderbeam is fine (Hi
Rick!), same problem with dummy load and with different cables
6. After some measuring I found out, that the Power Amplifier needs
(propably) to much current, over 20A during TX even with RF out=0 (I
took a auto battery, because of the shutdowns from the PS)
-> Thought as well of a failed T/R relais, but even without that board
(signal out directly from the PA board into dummy), the failure
occured in the same way
7.I checked the bias, which was ok, and was able to (re)regulate the
50mA for driver bias and the 500mA for the final bias
8. I plugged in the cable from the low driver board and increased the
current: 50mA driver was ok, but already 20mA more (of the final bias)
increased the current from the PA from 50mA of the driver to over 20A
9 In order to check the regulations of the bias I desoldered the
collectors from Q3 and Q4 (the MRF 458) to get rid of the high currents
10. I checked the regulating voltages around U1B and Q6 two times
(using R16 for the final bias), and suddenly the regulation failed
(12V on Pin 7 even without Q6, short between base and collector of Q6
), the regulation of the driver was instead still working
11. I replaced the IC U1A/U1B = LM358 and the Q6 = MJE520, measured
again the voltages, and regulation was working again, BUT
12. Now the regulation of the driver bias had exactly the same failure
(12V at Pin1, short between base and collector of Q5)
13. Yesterday (finally) I replaced again the IC (wasn´t sure if it
works correctly after the new failure) and the failed Q5, but nothing
changed, except for the fact, that Q5 got destroyed again (short)
-> What I checked as well:
* No shorts of Q1,Q2, Q3, Q4,
* Diodes ok (D2,D3)
* regulating voltages from R15 and R19 ok (between 0,5 and 0,9V)
So what the heck is going on? Am I already in a kind of vicious circle
trying to repair self-created failures, which probably have nothing to
do with the main problem of my beloved Omni?
Any suggestions welcome, before my wife gets angry again because of
more endless hours in the shack and a frustrated husband!
Tnx for reading es 73,
Harry, DH1NBE
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