Friday, June 18, 2021

Lunar Link LA-62 Amp - Fail - Repair

I've been using a Lunar Link LA-62 now for about 3+ years on 6m (50 Mhz).  It's worked Almost flawlessly.  There have been a few minor repairs needed in that time.  A single $0.99 cent diode in the power supply.  Several #85 lamps for the meters went last year spent about $5.00 to get 20, and it only needed 3.

Yesterday I started to notice that from time to time my output power would be only equal to my input power.  So no amplification.  33w in, was about the same out.  I tried retuning, and no change really.  The one tell-tale sign was that this would happen sporadically.  I'd key up and get no amplification on the output, or I'd key up and get full power out.  All the indicators on the amp looked normal, including Plate, and Grid for the most part.  I just wanted getting full power out reliably.

The LA-62 uses external TX/RX relays.  And this one is equipped with the "VR-2" (vacuum relay - 2) which is meant for QSK.  

So to debug what was happening...I removed the VR-2 assembly.

I opened it up by unscrewing 6 screws and looked inside.  To my utter amazement...all that exists inside is two GigaVac GH-1 vacuum relays, and a micro coax with ground strapping, and some jumpers.  There is absolutely NOTHING special about the VR-2, nothing fancy or exotic.  It's just two relays configured for TX/RX normally. (if your familiar with such things, and thankfully I've created them like this myself on another amp of mine)

I connected the power leads of the VR-2 to a 26.5vdc supply I have, and tested for RF through it.  I found that I could get continuity from XCVR IN/OUT PORT to AMP IN.  But I couldn't get continuity from AMP OUT to ANTENNA PORT like 95% of the time.  Of about 50 attempts I saw continuity from AMP OUT to ANTENNA about 4-5 times.  The rest of the time the really was failing to function.

Thankfully, the grid of the tubes acts like a dummy-load in the case where the RF OUT has nowhere to go.  Which to me...just seems like Black Magic.  But thankfully this saves things from getting ugly!

But anyway...I continued the tare down and removed the failing GH-1 relay, and then I removed it's coil pack and inspected that, it tested OK for resistance about 346 ohm instead of it's spec of 335 Ohm.  Close enough I think.  I put it back together and testing continuity from COM (common) to NO (normally out) and found that I could feel the relay energizing, but it was failing to make contact from COM <> NO.  This was the issue, and why I wasn't getting RF out.  

These relays are spec'd at about 2 million relay cycles, which in normal FT8 operation for me would be about 50k to 100k cycles per year.  So that's a LONG life time when you break it down like that.  I can't speak to why the hardware armature has failed in this vacuum relay.  But it I ordered one from Max Gain Systems, Inc. overnight FedEx today and I hope it'll be here before tomorrow or Sunday. (it's Friday now) This is the season for 6m, and every single day matters BIG TIME.  We're just getting into the peak.

So I'm very confident that once the new GH-1 arrives it'll only take about 25 minutes to get thing back together and running again.  But for now I'm relegated to 170w max.  Which does get me some flags in EU, but it's a struggle when the band is marginal.

I'll post any updates to this status as they occur :-)  I hope to report good things sooner than later!

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