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Author Topic: Mirage B3016 Bias Adjustment  (Read 692 times)
M0LMK
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Posts: 4




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« on: May 08, 2019, 10:49:28 AM »

I've been working on a faulty Mirage B3016 VHF amplifier and have managed to bring it back from the dead! I had to replace the 100K pot that I believe controls the bias because it had fallen apart so now I guess I need to reset the bias.

I lifted the L3 inductor and put the meter between L3 and the pot but I can't see any current. Is this the correct way to bias it? I was expecting to see around 150mA, is that correct?
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AD0AR
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« Reply #1 on: May 10, 2019, 07:31:13 AM »

My schematic says the pot is 100 ohms, not 100k ohms.  Also the inductor is labelled L5 on my schematic.  It feeds the +12v to the collectors of the main power transistors.  I've gotten good results measuring about 150mA of collector current as you have.
I lifted the 12v+ side of the L5 and inserted a current meter then set the current to about 150mA, or about 75 mA each transistor.  If you are using FM, no biggie getting the bias perfect, just as long as you are above 100ma and under 200ma.  If too high, too much heat can be generated and bias can have thermal runaway, too low and you will have higher IMD byproducts/ non linear operation which is highly undesired. 
The schematic I am using had been downloaded from the internet here: http://bama.edebris.com/manuals/mirage/b3016
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M0LMK
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« Reply #2 on: May 10, 2019, 08:14:13 AM »

Thanks AD0AR! My schematic is a little blurry so the 100K that i've put in is probably the issue!

I'll swap it out for a 100 Ohm trimmer and post the results.

Some great info regarding the bias setting which confirms I was attempting to do it right.

Matt
M0LMK
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M0LMK
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« Reply #3 on: May 13, 2019, 12:13:17 PM »

100 Ohm trimmer in and still showing no current through L5.

I've read mention of these amps that have been modified for 'fixed' or 'no' bias. Could this have been modified for this type of operation?

FM operation is working fine but I would like to get the bias sorted for SSB!
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SM5GLC
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« Reply #4 on: May 13, 2019, 02:43:43 PM »

I had a look at the schematic, and bias voltage comes from a point labeled "6" and goes through the input coax, and is the picked up via a RFC 470nH and then a 50 ohm resistor, and blocked with a 750 pF cap not to hit the base directly. The 100 ohm pot or 100 kohm should not matter, if you have too high resistance the bias would be high. Measure the voltage at the stripline input just before the blocking cap to verify you do have bias voltage. This would read zero if the amp is in FM mode.

When trying to fix the bias issue, either have a power supply that is current limited to a safe value, or lift part in the circuit not to have the transistors to open up fully, as this will kill them swift and quickly. I would lift the 50 ohm resistor, preventing any bias to reach the base. Once you do have bias voltage, start with pot set to zero ohm, slowly increase the value.
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M0LMK
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« Reply #5 on: May 14, 2019, 03:05:07 AM »

Thanks for your reply. I am still very lost!

I think I'm in the correct area but I don't seem to be getting any results. I see zero volts across the stripline input just before the blocking cap. I have tested in both FM and SSB mode.

I'm sure i've tested at the correct point, see the attached image.

Point A = 0V
Point B = 0V
Point C = 13.8V

Is L5 where I think it is?



Any help is much apreciated!

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SM5GLC
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« Reply #6 on: May 14, 2019, 08:27:38 AM »

If you look at the schematic that is on the bama site..
L5 seems be on the correct spot. I would back trace from the bias diode, and verify that it is in contact with the 50 ohm resistor, and then further back up to point "6". This is where the bias voltage should be switched in, but beware it may be a less than 12 V at this point, schematic does not show it all.
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