[TenTec] Omni D final wiring

[Shawn Upton]

Back to this project today.
I hooked up an ammeter and tried ot adjust the idle current on the driver.  Out of TX I measured 0.5mA (rather high leakage on the MRF476's?) and >200mA in TX.  Pulled the amplifier and put onto the bench as the trim was hard to adjust.  Manually adjusted to 6mA, reinstall.  0.5mA on RX and TX.  Hmm.  I tweaked the pot and it went right off scale again (this time blowing the fuse in my meter).
I pulled the amplifier completely apart this time.  The MRF476's pass diode check, with each diode at 600-700mV.  But on Hfe test, one is at a  gain of 72 and the other at 32.  That seems rather mismatched and makes me think the low gain one is bad, and possibly leaky (the aforementioned 500uA in RX mode).
So I think I'll see if I can get some NTE235's next.
Shawn kb1ckt 

    On Monday, September 3, 2018, 11:58:18 AM EDT, Shawn Upton via TenTec <tentec at contesting.com> wrote:  
 
 Ack, apologies, not sure what happened to my formatting.
Yeepers, haven't worked on this in a while...  Background: Omni D, not a later C.  Last I fired this up, no RF output but the ammeter on the power supply registered 20A.  So I pulled the main feed to the PA and put in an ammeter into the smaller red lead, as per the manual.  Current went first to >400mA (supposed to be 4 to 8 milliamps) before settling down to 10mA.  During this time smoke decided to roll out.  Termination of testing.
Pulled the PA and it looks like R8 smoked.  R8 is 3.3 ohm feeding Q4.  But Q3/Q4 had no power so not sure why it would smoke.  The white/gray wire for bias is after this 3.3ohm so I'm a bit surprised.

I checked driver and final transistors, without removing them; the final shows typical diode connections.  The driver shows base to collector ok, but the base to emitter is shunted by 32 ohm (the bias adjust pot apparently).  Have not pulled transistors to test further.

Any idea while R8 would smoke?

The white/gray wire bias, hmm, something wrong there: With no PA installed I measured 4.5V and rising with the radio in TX.  Ah, stupid me, with the PA removed the bias diode is missing.  Clipped in a 1N4004 and I get 0.64V for bias to the PA.

shawn kb1ckt

  
  
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