Author Topic: Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?  (Read 6461 times)

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Offline youbecha

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Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?
« on: January 23, 2014, 01:16:22 am »
I have a small RF amp, the Mirage BD-35.  It is designed to be used with a low power radio like a handheld portable.

When you transmit, the amp automatically selects the proper frequency range.

Setup is a dual band handheld connected to the Mirage, connected to a SWR meter, then a VHF/UHF antenna. 

The antenna is tested with normal SWR.
With amp turned off,  the handheld displays normal SWR (with UHF or VHF) through the same meter, at the same location (between amp and antenna)
With amp turned on, UHF SWR is normal.

Problem is when transmitting in VHF the output SWR goes to the limit...as if the antenna was disconnected.

I have a schematic, but no test points or voltages listed...and I am not sure about which circuit is the UHF part, and which is the VHF part.

I suspect a relay isn't working properly...

The amp has two amp sections, one for VHF, one for UHF...the amp automatically switches to the correct section with a series of relays.

I hooked up an oscilloscope to the output, it properly shows the selected frequency and shows the amplification with the amp on.

That is as much as I understand at this point.  Obviously I need to test the relays, but I am not sure how to proceed, should I remove the relays? just replace them all anyway?  I don't understand how it can properly show the output freq with amplification...yet still have a problem.

Thanks in advance.
 

Offline youbecha

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Re: Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?
« Reply #1 on: January 23, 2014, 04:30:03 am »
Sort of update...Didn't desolder anything yet...but

I took it apart to look for anything obvious, and I tested it outside the case on the bench...the SWRs changed...not perfect by any means, but at least not full scale.

In fact, now I am not sure if the UHF part was working because now it has a higher SWR than the VHF...before I opened the case and removed the board, VHF was full scale SWR.
hmmm
 

Offline youbecha

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Re: Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?
« Reply #2 on: January 23, 2014, 11:12:46 pm »
You would think that, but the antenna check with an antenna analyzer and two other radios...also when this amp is off (bypasses some of the circutry) .

But as soon as the amp is turned on then it transmits off the scale.

Since the antenna system is ~50ohm, I assume if the amp is a different level, there would be a lot of reflected energy.

I just don't know how to test that.

And since the reflected energy changed for the positive as soon as I took the amp out of its box...it sure seems like something is wrong in the amp.


BTW, I first realized I had a problem when I was testing the system (handheld radio + amp) on a different antenna and realized I wasn't getting out with the amp on in one of the bands but not the other.
« Last Edit: January 24, 2014, 03:44:39 am by youbecha »
 

Offline w2aew

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Re: Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?
« Reply #3 on: January 24, 2014, 05:35:29 am »
What type of SWR meter are you using?  Does it have a CAL or SET point to adjust the meter for full scale before switching to read SWR?  If so, are you resetting this properly after turning on the amp?
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Offline DL5TOR

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Re: Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?
« Reply #4 on: January 24, 2014, 10:03:23 pm »
this is a common problem, 

if you look at eham http://www.eham.net/reviews/detail/3281 there you will see that there are a lot with the same problem.

If you look under http://www.qrz.com/db/DL3GAZ at the lower part, there is also a "fix" for the problem

Quote
Mirage BD-35: Fixing the problem with VHF-LED coming on while transmitting UHF. If both VHF/UHF LEDs are on, bad VSWR might kill driving TRX and the amp may oscillate on its own. Original 100µF cap has too high ESR at cold temperatures. In consequence, UHF stray RF triggers the VHF relay driver transistor.


I hope this helps, i have the same amp but mine works with out problems

73 de DL5TOR
 

Offline youbecha

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Re: Mirage BD-35 2M/70CM RF amp high SWR, how to troubleshoot?
« Reply #5 on: January 27, 2014, 11:49:39 am »
Thanks for the replies. 

I have two different standard power/SWR meters (dual needle) that are either low power or high power display.  (and like I said, I also checked the antenna with my antenna analyzer)

The reason I started checking SWR out of the amp was...
- I could hit the repeater using the HT without the amp on both bands.  (same antenna HT through amp, but amp power switch off)
- With the amp ON - I found out that in one band I could still hit the repeater nicely with the HT through the amp, but in the other band I couldn't.

I have spent some hours pouring over the limited schematic and data sheets for the main component (one mosfet and one dual op amp).  (schematic shows different mosfet than what is installed...).  Since the amp produces proper power and SWR in one band.

I can test and measure, but I don't understand which circuitry in a transmitter can make it incompatible with a properly tuned antenna. 

I have to guess that the output resonance has changed (no longer likes the 50ohm unbalance line)...but I don't know how to test for that.

I am now starting the process of matching the circuit board to the schematic and labeling things (yea, nothing is labeled) to isolate ...
 


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