Re: Repair help Larcan Lo-Lo 6m

[Dave Olean]

I suspect that some kind of input peak power problem may have occurred.  What mods were done to the Larcan before you used it on 50.2 MHz? Were the output combining networks re worked or are they stock? They have a high VSWR at 50.0 MHz. If you are using an Icom 746 as a driver, that is way too much power as well. It is best to run the driver into a high power pad at full output to avoid any chance of instantaneous peak problems. The Larcan will deliver big power with just 6 or 7 watts.

    I see MRF151Gs on EBay for a bit over $100. I have popped a few myself.  I think you need all the FETs running for good efficiency.

 

Dave K1WHS

 

From: tkr...@azci.net

Sent: Tuesday, June 04, 2013 1:54 PM

To: larcan-6m-...@googlegroups.com

Subject: Repair help Larcan Lo-Lo 6m

 

I am new to the group so forgive me if I am repeating questions previously posted.

 

It appears I have blown two of the modules in a newly acquired amp.  I was testing it out on JT65 at about 200 watts out when I heard a crack and saw flash of light.  Found two fuses blown.  Replaced fuses and now can get max of 60-80 watts output.  Driver is Icom 746 which I previously checked for spikes and could see none.  Earlier in the evening I used the amp on SSB and drove it to 700 watts on CW with the same driver radio.  As near as I can see SWR is okay with the antenna, very little reverse power as measured with bird meter and 250 watt slug.

  1.  Does anyone have spare MRF 151Gs  for sale?  ( Yes, I have scanned over the info on MMRA page and understand RF Parts has them).

 

  2.  Is it possible to un-fuse these two modules and run with what are hopefully two good modules?  Not necessarily looking for 1 Kw out at this time, just more than the 200 watt brick I have.

 

  Thanks for any helpful feedback.  73 Tom NQ7R  Casa Grande,AZ

 

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[Kevin Hobbs]

Hi Dave

 

The Larcan in question was mine … sold to Tom. No mods were done as my efficiency was excellent stock, and as such didn’t seem to warrant any optimization (I did consider adding some C to output combiners only per your article). It ran JT fine here for many many hours at 1KW (drive was 7W from K3/10). I also suspect an input power spike occurred as those Icoms are known to do that!

 

73 Kevin

[Dave Olean]

Kevin,

My K3/10 put out 7.3 watts and drove that amp to well over 1000 watts out. I think I saw almost 1400 watts at 54 MHz, and something like 1150 at 50.0 MHz. That rig worked well with the LARCAN. I cringe at thinking about a 100 watt rig for a driver! All sorts of bad things can happen.

 

Dave

[tkr...@azci.net]

I used the amp on CW and SSB without problem unless it is cumulative.  I know of the problem with the 706, and checked my 746 with Spectrum analizer and could see not spike.  I kept the output turned all the way down, but that whole issue is now moot since the damage is done.   I have a K3/100 don't know if they have the same problem or not.  For that matter I have a FT817 QRP radio but never tried it on 6m.   

What I need is help repairing, finding someone who wants one for parts,  or finding someone with a boat.   Dave I will try to contact you directly if that is ok, but I will reply here in case others may need assistance.

 1.  What would be the best way to determine which FETs are bad or do I have to replace them all?

 

2.  Could I remove the bad modules and run it with just two for 500w?

 

3.  Could I just remove the bad FETs and defuse the modules?

 

4.  The PS generates a huge RFI broad-band signal on 6m, is that normal?

 

NQ7R

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[Lindsley]

Hi

I can get you a pair of the original transistors.

You can contact me

Regards

Lindsley

[tkr...@azci.net]

Hi Lindsley,

  Thank you for the response.  It looks like I will need three altogether.   Can you get or should I get an odd number?  Do you want to quote a price here or privately?  Not sure if you can get a full email address from what is above but mine is on QRZ.com.

73 Tom NQ7R

[tkr...@azci.net]

It looks like I need three mosfets all together.  Here is what I did for testing:  Put on dummy load for the amplifier.  Used my K3 at 0.5 watts to drive the amp and used my RCA RF probe to measure the voltage going into and out of the mosfets.  Only one showed significant output on both halves (module 3 from back).  Module 1 showed only half of that mosfet with the same gain.  Module 2 is completely shot and module 4 shows maybe 1/3 the output of module 3.  

  Can these be matched by adjusting the blue bias pots?  If so I don't know how to do that.

I really hate to but do I need to replace all four mosfets?  Thanks again for assistance.  Tom

[Kevin Hobbs]

Tom

 

If it is that bad, I recommend that you put out a want for a complete VHF Lo-Lo Amplifier (these have been available for approx. $250 at times and hopefully someone will part with one) … use the original as a model and do the same mechanical mods and replace the whole thing in the Chassis. I think that is the safest / most cost effective and leaves you with the bad amp for spare parts in case of future problems.

 

I also suggest finding a 100W 10dB Attenuator if you plan on using that amp with ANY 100W driver radio in the future … Initial Spikes are common and hard to capture! The Attenuator is the safest way!!!

 

Good luck with amp …

 

Kevin

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[stv...@gmail.com]

Tom, if you have not already tried, you may confirm which fets are blown by removing all fuses, adding a dummy load to the output and terminating the input also (no RF testing needed for this).

Then add one fuse at a time and monitor the quiescent current of the Larcan.  Each half of each fet should show some idling current when the bias is turned on.

Cheers,
Steve Bell, KJ7OG, Tucson