6 Meter Amp Testing Problems

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Richard Solomon

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May 29, 2014, 6:21:41 PM5/29/14
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I replaced the two blown FET's (high power spike took them out) and adjusted
the bias current on all 8 for 500 ma. each.

I added a 6 dB attenuator on the input lead just as a precaution.

Initial test results show very low efficiency, on the order of 20%. However the Power Output
does seem to increase somewhat linearly with increase in Power Input.

For example (now remember to subtract 6 dB from the drive power).

40 watts of drive gives 300 watts out
50 watts of drive gives 400 watts out
60 watts of drive gives 500 watts out.

Efficiency does increase slightly with increased power output levels.

I am thinking that the power spike did more damage than just take out two of the FET's.
One of them actually had a cracked ceramic top piece. (verified before removal).

Is it possible that some of the combiner caps went bad ? In circuit they are hard to test.

Any thoughts ??

Thanks, Dick, W1KSZ

Bob, K1IW

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May 29, 2014, 8:01:00 PM5/29/14
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Was it working better before the repairs?

You should be getting at least 20dB of gain.  Captain Obvious says you are not.
The gain varies from 50 - 54 MHz.  Perhaps this article would be helpful:



73,
Bob

Richard Solomon

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May 29, 2014, 9:41:31 PM5/29/14
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It came as a "turn-key" amp that is advertised and sold by one known to us, so I am operating under
the assumption it was a working unit prior to the high power spike disaster.

I have read that article as well as numerous others but I see nothing there about troubleshooting. So I
am thinking that spike did more damage than just pop a couple of FET's. There must have been
some awfully high RF voltage flying around for a few milliseconds !!

The chap who sold me this amp reported a loud bang and a large flash.

So. I'll play around a bit more but given the circuitry testing caps in place does not seem possible.

73, Dick, W1KSZ

Dave Olean

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May 30, 2014, 9:30:20 AM5/30/14
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Dick,
It is possible to test the combiners. It is also possible to test each of the four individual amplifiers. First you must lift the far end (away from the FET) of the small approx. 11” semi rigid coax center conductor. (This is a 50 ohm point)  In its’ place tack a small 50 ohm resistor for each amp on the combiner side, not the coax center conductor.. Then check the VSWR looking into the output connector of the amplifier. Any blown components will show up as poor VSWR. Use low power or you will pop the small resistors This is best done with a return loss bridge and a swept response.  The same lifted center conductor can be used to hang a wattmeter and measure the output of each stage. Remove the fuses of the stages you are not testing. Input VSWR will be wrong with the fuses removed, but you should see equal output among the four amp stages. A bad one will show up quickly. Use a sliding attenuator to adjust the rf drive, not a switched attenuator. Also keep RF drive low. You want to make sure all the amps are working equally. Efficiency will go up as power output is increased. You should see around 40-45% at moderate power levels, say 200 watts per stage.  Also make sure that the combiners have been modified from the original circuit if you are operating at 50.0 MHz.
 
Dave K1WHS
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Richard Solomon

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May 30, 2014, 6:14:25 PM5/30/14
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Your last sentence got me thinking ... all along I expected this Amp to be a "turn-key" Amp, so to speak.
What if it wasn't ... so I re-tested, this time at 54 MHz.

Results showed improvement but nothing near expected.

With 40 watts of drive (less 6 dB = 10 watts), my efficiency at 50 MHz was ~22%, at 54 MHz it was 27%.
That's using the panel meters to calculate power input, granted not accurate, but for a comparison they
should be good enough.

The board looks like a Lo-Lo board, ie: no strap across the striplines, but beyond that I cannot tell.

What if I retested by pulling all but 2 of the fuses, measured the output and repeated that for the other
3 FET's. Could I isolate the alleged bad combiner that way ? After that my only recourse it seems is
to replace ATC caps until I get the bad one(s).

Anybody got a reasonable source for these that doesn't require giving up a first born ??

73, Dick, W1KSZ
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Dave Calhoun

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May 30, 2014, 6:45:01 PM5/30/14
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Hi Dick and list,

   If you do a youtube search on "w2vw larcan" you can see a really bad video of a lo-lo module under test on my bench. Pause it to read metering. There are two power output meters, one B&W dummy load with built-in meter and one Agilent. Panel meters floating in free air as mentioned may not be completely accurate but it gives an idea of what to expect.
frequency of test is 53.999 mhz, module unmodified.

Sorry for the crummy quality but there was no room to swing a camera and I took something like 42 videos of modules under test.

73,
Dave W2VW

Richard Solomon

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May 30, 2014, 9:04:29 PM5/30/14
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Save me the pain ... what were the results ??

73, Dick, W1KSZ
Hi Dick and list,

Dave Olean

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May 31, 2014, 8:17:51 AM5/31/14
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I would not just pull the fuses. The combiner will not work properly unless all amp stages are operating.  (Unkeyed FETs will not have the correct impedance.)  It is a simple matter to lift the center conductor of the output stage semi rigid coax and tack solder it to a small cable going to a dummy load. Test each amplifier into a 50 ohm load.  If all the amps work, you have a combiner problem. That can be tested and fixed by looking backward into the amp. You could cut the combiner in half and install more 50 ohm loads but I have not tried that. I looked at the entire combiner when I was converting mine.
    You can easily tell if the combiner was modified by looking at the cap values in place. A modified combiner will still show about a 1 db improvement when going from 50 to 54 MHz. I think you are seeing that.  My amp when driven by my low power K3 at 7.3 watts produced 1200 watts at 50 MHz and almost 1400 watts at 54 MHz. Gain was 21+ dB at 50 and 22+ dB at 54 MHz.
 
Dave K1WHS

Richard Solomon

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Jun 19, 2014, 1:03:19 PM6/19/14
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I finally got everything I need to test the individual Amps. Test results are:

Total Power Out (7.5 Watts Drive) = 243 Watts

Amp #1 125 Watts
Amp #2   60 Watts
Amp #3 110 Watts
Amp #4  ZERO !!

Now Amps #3 and 4 had the blown FETs and were replaced with new FETs.
They biased up OK, set each Amp for 500 ma.

At this point, I am stumped.

73, Dick, W1KSZ
Hi Dick and list,


   If you do a youtube search on "w2vw larcan" you can see a really bad video of a lo-lo module under test on my bench. Pause it to read metering. There are two power output meters, one B&W dummy load with built-in meter and one Agilent. Panel meters floating in free air as mentioned may not be completely accurate but it gives an idea of what to expect.
frequency of test is 53.999 mhz, module unmodified.

Sorry for the crummy quality but there was no room to swing a camera and I took something like 42 videos of modules under test.

73,
Dave W2VW
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Peter Sumner

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Jun 19, 2014, 7:29:15 PM6/19/14
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Hi Richard, if your seeing such low powers out of the amp it would suggest the combiners are still too high in freq. I saw a similar thing with mine, max power for me was 300 w. Then did the changes outlined by Dave k1whs in his article and things improved greatly for me. When the output stages are too high in frequency a larcan amp is a dog of a thing, once modified they Sing a sweet tune.

Peter, vk5pj.

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Dave Olean

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Jun 19, 2014, 7:49:36 PM6/19/14
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Hello Richard
I am confused. It looks like you are getting 0 watts from amp #4 after replacing the FET?  This sounds strange. Even if you do not have a modified output combiner, I would expect that gain at 54 MHz would be up at 21 dB or so.  If you disable the amps and lift each amplifier out put connection at the end of the semi rigid hardline away from the FET to connect to a dummy load, and you saw those numbers, I would say that those stages had a definite problem. Amps 1 & 3 look close enough, but 60 watts and zero watts is bad. Did the 0 watt amp draw any current above what its’ idling current was when  you applied some drive? Possibly some shorted or open caps?  You might see evidence by using some magnification and looking closely at each chip cap.
    Please give us the exact procedure you used to make the tests.
 
Dave K1WHS

Richard Solomon

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Jun 23, 2014, 1:31:13 PM6/23/14
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I removed all the caps from the Amp #4 combiner (the one with zero output), cleaned everything and measured the caps
to make sure they are OK. They were. I replaced the caps and using my test cable (small Teflon Coax tacked to the lifted
output lead of the Semi-Rigid Coax), I retested ... still ZERO output.

At this point I am out of ideas and patience.

If anyone wants to take a crack at this thing, I will part with the whole pile, for short money. Contact me off-line at my
address on qrz.com.

This consists of one amp and a server supply, neatly mounted in what looks like an HP rack mount. Meters for Voltage
and Current on the Front panel. Two circuit breakers for On-Off on the rear.

Also an unmolested additional Lo-Lo Larcan Amp.

Pictures available to interested parties, preference given (also consideration) to parties willing to pick it up and save
me the packing and shipping chore.

73, Dick, W1KSZ
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Hi Dick and list,

   If you do a youtube search on "w2vw larcan" you can see a really bad video of a lo-lo module under test on my bench. Pause it to read metering. There are two power output meters, one B&W dummy load with built-in meter and one Agilent. Panel meters floating in free air as mentioned may not be completely accurate but it gives an idea of what to expect.
frequency of test is 53.999 mhz, module unmodified.

Sorry for the crummy quality but there was no room to swing a camera and I took something like 42 videos of modules under test.

73,
Dave W2VW
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Peter Sumner

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Jun 24, 2014, 2:39:05 AM6/24/14
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Hi Richard,
 sounds like to have a stinker, It seems unusual to have zero output power if the FET's are drawing idle current.  That sounds like a lack of drive from the input splitter.  I can tell from the tone of your email you are jack of it as I had days where I was going to cut mine up with a jig saw.

Can you monitor the DC current as you apply drive to this stage and see if the current rises to this stage when drive is applied? That will tell us if the FETS are being driven or maybe they are not getting any RF from the input splitter (have a look at the miniature coax and its ferrite BALUN on the input).

If you can muster one last offensive, I would be taking a few minutes to do some DC checks on the input board, your problems might be there and not on the output board (where most of us have had problems).

Mastering a Larcan can be a mind bender if there is a deeper fault there from transport damage or a pre existing fault.  My 1st attempt at conversion (they were HI-LO's originally imported from the USA) is still sitting on the shelf with a number of dead FET's but amp number 2 is humming along on 50.1 once I understood the output combiner was my enemy :-)

Kind regards,
Peter, vk5pj



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Richard Solomon

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Jun 25, 2014, 12:40:19 PM6/25/14
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I pulled down the test set-up yesterday ... so unless someone ponies up to take it,
it will sit in the corner.

If I get feeling masochistic some day, I'll give it another go.

Thanks for the reply,

73, Dick, W1KSZ

Jeff DePolo

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Jun 25, 2014, 3:14:58 PM6/25/14
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I have the original boxes for those amps if it helps...

------------------------
Jeff DePolo
Broadcast Sciences LLC

Sent from mobile device;  please excuse brevity and misspellings.

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Warren Stone

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Aug 22, 2014, 7:21:26 PM8/22/14
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I never drive the lo/lo amplifiers with more than 10W at the input.  A stock lo/lo should produce roughly 850W output with that drive.
A few capacitors at the input splitter, output combiner, and each FET output will improve amplifier response.  It will produce 1kW easily with about 6W drive power.
Increasing drive to 40W or 60W is sure to cause damage to any FETs that are still working.  Never use higher than 7A fuses and keep total module current to 40A (10A per FET).

You may also have a problem with the bias board or the output sample ports.  The overall amplifier is quite simple, straightforward, and forgiving - particular for 50MHz operation.

Here's a technique to check the FETs.  You will have to unsolder the input and output tabs for testing.

I have full alignment info, schematics, etc available.  Email me vo...@eastlink.ca if I can be of any assistance.

Robin Midgett

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Mar 6, 2015, 10:36:04 PM3/6/15
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Hi Jeff,
I sent Dick a note about his Larcan collection. If you still have those boxes & he still has the amps, maybe we can work a deal.
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