Timing of Ref Power measurement

Dave - WB5NHL

I too am having problems with the Power Foldback. My Flex 6500 is driving an ALS-600 linear amplifier. I have the TX Delay set for 20 ms (per ALS-600 to allow for the amplifier relay).

I have done quite abit of checks using Flexmeter to monitor the Fwd Pwr and Ref Pwr.  When operating with amplifier I have noticed some strange behavior of the Ref Pwr reported by SmartSDR.  Upon keying the Flex with the amplifier on using Tune, the Ref Pwr immediately jumps to a very high value then over a couple of seconds it sets down to a stable value.  At low Tune powers this behavior doesn't cause any Foldback. But when approaching 40W drive the Flex does a foldback.

The question seems to be; what is the relationship between Ref Pwr measurement and TX Delay?  Or is there some other explanation for the strange Ref Pwr behavior.   BTW, changing the TX Delay does not change the results.

I like the idea of power foldback and would like to be able operate with it in SmartSDR.

George KF2T

Hi Dave,

Try manually shorting the amplifier's key line (putting it in transmit), then keying the Flex. Same problem? Doubt it will fix it, but it would certainly rule out the relay.

There should be no relationship between TX Delay and power measurements. They are (supposed to be!) independent.

Foldback begins at 25w reflected power, I believe (not at an SWR level). So, if the amplifier input is not matching your rig, this _could_ explain what you're seeing. How much, and what type, cable is between the Flex and the amp? It is possible any input circuit mismatch is being exacerbated by the reactance of the jumper. It's the same effect when you read different SWR with different meter locations on a feedline. 

If you can minimize the length of the jumper, or perhaps put a line isolator in place between the amp and rig. Confirm your single-point ground is effective, too. Sometimes over time the heavy braid can work its way loose.

Good luck!

Dave - WB5NHL

thanks for the ideas.  Answer to some of your questions. I  have about 4 ft of RG-8x mini between the rig and the amplifier. The ALS-600 is a Pi network fixed tuned input solid state amplifier. I can try an SWR meter in the line but the typical cross needle meter is damped and unlikely to show the rapid response that Flexmeter records.

I have tried different jumpers, different lengths without any change.

A Radioworks T-4 Isolator in the line between Flex and amplifier has no effect.

My single point ground utilizes 1/4 wave counterpoises cut for each band since I am in a second floor shack.

BTW, when the SWR settles on the Flexmater it is no higher than 1.1:1    All these measurements have been using Cantenna dummy load to eliminate any antenna mismatch effects.

 

 

Bill W2PKY

Hello Dave- 20 ms sounds like a short delay for the type of antenna relay in the ALS-600. A Jennings Vacuum Relay, for example, is rated @ 16 ms; And that's a fast relay. Perhaps you might try a longer TxDelay just for grins.

Dave - WB5NHL

Tried 40ms no change.  BTW from ALS-600 manual

"Note:  The ALS-600 T/R relay enable time is typically 15 milliseconds.  Many transceivers permit setting a time delay between their relay-control output, and the actual full-power RF output.  If your transceiver has this capability, set this delay to 20 milliseconds to ensure you don’t transmit before the amplifier relay fully energizes (which can result in the loss of part of the first Morse character)."

Dave - WB5NHL

George, I tried your suggestion regarding keying the amplifier manually by shorting the TX relay.  Ref Pwr is solid every time I key the Flex on every band.  It is absolutely related to the TX Relay Output timing and Ref Pwr measurement by SmartSDR!

Eric-KE5DTO

Could you try a much larger value (400?) and let us know if you continue to see the fold back issue with the TX switching plugged in normally (i.e. not shorted)?  This may help us to understand whether this is a timing issue related to when we switch that line and when we allow RF output.

Dave - WB5NHL

Eric,  thanks for your quick interest.  I tried TX delays from 400 to 700 ms and there was no change in the way Flexmeter reported Ref Pwr ie. peaked then settled down.

An example of the foldback, on 40M I read a peak 35W FWD Pwr and 27W (multiple tries at Flexmeter set for peak)   The SWR ultimately settled at 1.02:1

George KF2T

Interesting results, Dave. Thanks for the report. I'm slightly suspecting some kind of RF ingress, but you do seem to have done everything right! Are you running an ALC line? If so, disconnect it. Is the problem on all bands? I agree with Bill that the specified delay is probably too short, but you seem to have no better luck with the longer delays. On a lark, can you use TX2 or TX3 to key the amp?

Bill W2PKY

Hi Dave, agree with George that it's probably some kind of feedback from the amp on the keying line between the amp and TX1, rather than a timing issue. An other thread involving a solid state amp showed problems with increasing power output. Perhaps adding some RF decoupling in the ALS-600? Hope this helps.

Dave - WB5NHL

George, I tried some other things this morning and I have to disagree with RF problem. First, I do not use an ALC line. Yes, the issue is on all bands. I'm using a dummy load to eliminate any antenna mismatch issues.

Second, TX2 and TX3 show the same behavior. Finally, I did another test manually keying the amplifier , then the Flex. This time instead of disconnecting the TX1 and shorting it (effectively shorting the relay input at the amplifier) I tried something different. I removed the TX1 line but connected the RCA connector shell to the Flex ground. I then keyed the amplifier by shorting the center pin to Flex ground terminal. This way my grounding and interconnection to the amplifier are in play. With manual keying, Flexmeter reported no erratic behavior of the Ref Pwr and there was no foldback at full drive power up to 50W.

Bill, not sure what you mean by "RF decoupling"? I have already tried a Radioworks T-4 Line isolator between the Flex and amplifier with no effect. 

Bill -VA3WTB

Dave, this is one strange thing, there are lots on here who have no problems at all with it. Just keep digging to find it. We do know the spec that sets it off, and so you have gone over the threshold.
I think the problem I see is, while understanding why they incorperated the fold back, it seems to be yet another thing that will bother many. There are just so many different setups that could trigger it. Flex is not as plug and play as we like, many things must be just right for things to work.

Bill W2PKY

Hi Dave- was thinking of bypassing the [keying line] RCA jack inside the ALS-600. RF could be on the keying line and feeding back into the FLEX. Since your experiments show that unplugging the keying line eliminates the problem, seems the keying line needs decoupling/bypassing on the Amp side. Line isolator was on the coax jumper, right?

Dave - WB5NHL

I tried a .1uF capacitor directly across the keying relay input at the amplifier.  No change in Ref Pwr or foldback.

Dave - WB5NHL

Bill, yes the line isolator in the coax line. Check Radioworks for an explanation. It is designed to eliminate ground loops in the shack.

Bill W2PKY

Hi Paul- Dave says if he keys the amp manually everything is okay. It's when he uses the TX1 keying line to the amp that the issue appears. Perhaps FRS has some suggestions. Do you have a help desk ticket Dave?

Dave - WB5NHL

Haven't yet, but Eric did comment so Flex has seen the discussion.  At this point I think Eric or Tim would convert to help desk ticket if necessary.

Tim - W4TME

I have data mined all of the comments here for the defect report so a HD ticket isn't necessary.

Dave - WB5NHL

I agree that a HD ticket isn't the answer i.e. there doesn't seem to be any in the Flex to repair. I guess I don't understand?  Is somebody at Flex going to look into the issue? Or are you saying I'm on my own trying to understand why Flex doesn't seem to respond correctly to a keyed the ALS-600 with resulting foldback?  My impression from from Eric's question and my answer was that there might be something to investigate.

K1UO Larry

Dave,  Sorry ...late to the party with this but...wrap your Key line a few turns through a choke core and try that...  Sounds like a problem I once had when I had the Elecraft amp with my first Flex rig (6300).    Choking the key line fixed the erratic behavior.

Good luck

Larry  K1UO

Tim - W4TME

Is somebody at Flex going to look into the issue?

We are actively looking into the issue.

Dave - WB5NHL

Larry, just tried it with no effect.  Note, the keying is NOT erratic. The Flex keys the amplifier perfectly well. Once keyed and within the nominal damping of cross needle SWR meters I get a nice stable signal output at low levels.  When the drive is increased to 40-50W I get the foldback with a nice stable key of the amplifier.

Dave - WB5NHL

Thank you Tim    I knew I was probably over reacting.  BTW, I suppose it is obvious I am willing to try any experimentation/ data gathering.  This including an experimental SmartSDR version.

George KF2T

Dave, what is your grounding configuration? I'm intrigued by your report about the "Flex ground" and the "interconnection." Are both cabinets tied to a single point with heavy braid?  Your experimental change shouldn't have made a difference. Another thing to maybe try, if you can get an isolation relay between the TX1 output and the amp PTT input, I wonder if that will fix you up.

Dave - WB5NHL

My comment about "Flex ground" was referring to grounding stud on the back of the Flex. The "interconnection" refers to keying ( RCA plugs at both ends) connecting TX1 and ALS-600 keying relay input.  Both cabinets are tied directly to single point station ground. This ground goes down to an 8' ft ground rod outside the house. However, since the shack and single point ground are on the second floor I use 1/4 wave counterpoises for each band attached to single point ground.

However, standby for the ultimate decision tomorrow.  I have a friend with a Flex 6500 and ALS-600. We just tested and he does not seem to have the problem. As a final test tomorrow I will try my Flex (limited edition) and ALS-600 at his shack.  He also has K4POZ keying buffer (http://k4poz.com/keying/index.html) that I can borrow if indeed the problem is related to my shack keying connection between the Flex and the ALS-600.

Mark NS9N

I have been using a Flex6500 and ALS600 for over a year without any issues. For reference, I also have a 2nd floor shack with one end of my OCF Dipole about 10 feet from my radio. I have removed all "single" point grounding and only use the coax and shielded PTT lines between the 6500 and ALS600. These lines are 3' long and the coax is Mil Spec 9914. All my AC power supplies are connected to a single AC buss **** which is tied to a dedicated 20 Amp circuit back to the main circuit breaker box. The Antenna system is grounded where the coax comes into the house about 75 feet from the shack. I am not using any PTT delay. This set up works perfect on all bands. Only one last thought, you do have the antenna tuner disabled in the Flex when you use the Amp?

K1UO Larry

Sorry...  missed the word SWR before the erratic behavior comment..  I would now be scratching my head..  or anything else that came to mind...  over this problem.

Good luck es 73

K1UO Larry

The isolation  idea is good...  might be a voltage differential/residual sitting on the key line.

Here is an interesting post and response from Ameritron...

http://lists.contesting.com/_amps/2004-01/msg00191.html

Carl K5HK

I had same issues with my Als-600 and Flex 6300 when I upgraded to v1.8 Flex software/Firmware.  I downgraded back to v1.6.21 and the problem magically went away.   I checked or changed all the things suggested with no effect using v1.8.  So gave up on that version due to whatever was causing power output problems to amp.

Dave - WB5NHL

This is why Eric asked me to increase the TX delay to 400 ms. I tried values from 400 to 700ms with no change in the behavior of Ref Pwr.

Greg N8GD

OK.  I have a Flex 6300 and an ALS-600.  I just upgraded to v1.8.3 and I am not noticing any of the problems Dave or Carl are reporting.  I was particularly interested in Carl's report since he has the exact same setup as I do (6300 w/ ALS-600) but he's having problems and I'm not.  I did a quick check on 80 and 40M and saw no foldback issues.  My amp keys to 500+ watts with 55 watts drive on 40M and a little under 500 watts with 90 watts drive on 80M (the ALS-500 requires significantly more drive on 80 due to an incorrect cap in the output filter circuit, as reported by W1AEX, http://www.w1aex.com/als600/ALS600.html#75m).  I see no foldback at all!  I have not tried any bands above 40M yet.

Here's a thought:  Dave or Carl, what is the age of each of your ALS-600 amps and what usage have they had (intermittent or hard/daily use).  I ask because these amps have a 35 ohm swamping resistor in their input circuitry to help provide the proper 50 ohm match to the driving transceiver.  These resistors are known to change in value over time, especially when driven hard (as with the 80M high drive power I have seen required).  Also, this resistor, which is easily seen on top of the input filter board, is a large non-inductive type with gold-colored conductive bands at either end, about 1" in diameter and 3-4" long.  It sits in tension style clips much as a large 60 Amp fuse holder.  It is these tension clips I am concerned with.  If, in addition to the aforementioned possible change in resistance value, the contact ends of the resistor have developed come kind of corrosion (possibly due to heat) or the tension clips are no longer making good contact, it might be possible that the RF being passed into this resistor is poor until the current being passed through the tension clips finally causes an improvement in conductivity through heating or the very current flow that is occurring.  This would certainly cause changes to the input SWR over a short period of time as seen by Dave.  My amp has been in service for about 5 months with very light use.  It is essentially brand new.  If either of your ALS-600 amps has significantly greater age/use, this resistor might be the reason for the power foldback.  Again, mine does NOT exhibit that problem.

If either of you are willing to open up the amp's RF deck, remove the resistor, check it for the proper 35 ohm resistance, and clean the ends with some very light emery cloth or the like and clean the inside of the tension clips, you might find that your results improve.  If the resistor is significantly different from its 35 ohm nominal value, get in touch with Ameritron to obtain a new one.

Please post back on any results from the above efforts.  I, and I am sure Flex/Eric/Tim, would really like to know.  This might clear up at least one instance of foldback problems, which, I believe, heretofore have been reported exclusively on tube-type amps.

Greg - N8GD