[TenTec] OmniVI problem

Harald Rester harryrester at gmx.de
Sat May 6 17:26:39 EDT 2017

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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