[TenTec] OmniVI problem2

Harald Rester harryrester at gmx.de
Wed May 10 13:42:44 EDT 2017

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

More information about the TenTec mailing list