New 4-1000A modulated by 4-1000As AM Project - Fabio II Reborn

[K1JJ]

Well, I finally got around to running some THD and IMD tests on Fabio II.  Had it on 160M last night and got some good audio reports. However, the THD test is what tells the story.

Running a single tone THD test while loaded up to full power, I'm seeing about -35dB down for the 2nd harmonic peak and about -45dB for the 3rd peak.  Some of the higher order peaks are down -50 to -55db to the noise floor.   It's actually cleaner than my modified FT-1000D on AM.  The heavy negative audio feedback certainly helps the THD. I'm real pleased considering it's a plate modulated rig with a mod transformer, phase shift, etc..

I found by playing with heavier loading and different screen voltage, I can dial in better numbers. This tweaking must be tailored to the individual rig. I've also found what works for some rigs, doesn't work for others.

Anyway, here's a screen capture off my HPSDR.

I plan to get Fabio II on 75M AM tonight, so check it out if you are around.

T

[K1JJ]

Here's some pics of Fabio II.  I still need to tidy up the cables and screw the cabinets on, but it's almost finished.  (and tested)    The enclosures, when unscrewed, slip off in seconds for full, easy access.

The first pic shows the RF deck that swivels on a pedestal for easy servicing.


In pic 3 see the solid state (WA1GFZ design) MOSFET audio driver and the MRF-150 IPA RF driver.

T

[K1JJ]

First pic is the new pair of 4X1s in linear with its cabinet on.  Its name is Rico Suave II.  GG with regulated screen and grid voltages.  Very clean amp and fast band changing.

The small accessory boxes in the second pic are an RF unbalanced to unbalanced input tuner, a six step variable sequencer and a failsafe shutdown system. The shutdown monitors 4X1 plate, screen and grid currents of both the RF and mod decks. If a preset threshold is exceeded, it will break the PTT line.  Works FB so far.

T

[K1JJ]

It appears Fabio is still in the shakedown mode. It had about seven on-air crapouts tonight.  There were severe flashovers and three Teflon PL-259s coaxial connectors burned up.   These symptoms are clues to the main problem - HV arcing to the antenna 50 ohm line!

At first I thought the burned PL-259s were from testing the rig in high tap. But after finding soot marks I found two problems.

The first one was plate cap parasitic suppressor placement. Even though the suppressors were about 1.5" from a grounded bracket, they still arced to the bracket.  The suppressors were melted. I added a Plexiglas shield over the bracket.

Tried again and the flashover repeated, but in a different spot. I blew out another PL-259 in the antenna line and all the wire fuses in the HV supply opened up.  I found some more soot. This time the top of the RF plate choke was arcing over to C2, the loading cap. It was about 2" away, but not enuff for RF I guess.  This 4KV + 4KV (8KV)  voltage was going through my antenna line, thus burning up the PL-259s.  I even smoked an SO-239 on the Bird wattmeter.   I guess the HV pulse wasn't totally squelched by the RF choke across the 50 ohm line.  This is a strapping choke wound with #20 wire. It is still good.

I noticed the small 2.5 mH RF choke across the 50 ohm antenna line, used for static discharge, was open. More evidence that there was HV there.  Cheez.

Anyway, I added a second Plexiglas shield to the plate choke and hopefully tomorrow will be a better testing day....       This plate modulated HV stuff can be squirrelly until isolation and spacing gets worked out. I've been through this before. Eventually the rig runs FB without arcing.

T

[kb3ouk]

The new rig sounds good, I heard you last night after I got done talking up on 40 meters, you were troubleshooting the triple carrier problem you were having on 3875 when I heard it.

[K1JJ]

The new rig sounds good, I heard you last night after I got done talking up on 40 meters, you were troubleshooting the triple carrier problem you were having on 3875 when I heard it.

Thanks for the report, Shelby!


Yes, the FT-1000D driver has a strange birdie problem on 3875. It's in the design and puts out two additional carriers 2KHz above and below.  If we look at the spectrum picture in a few posts back, we see a peak on 3882 and 3878. They are down only -40dB on 3875 and can be heard as weak beat notes. But they are down -60dB when on 3880 as shown in the pic. Not sure if anything can be done with it since other FT-1000Ds do the same thing. Probably synthesizer design.  Might need a different driver when on 3875. I have an HPSDR that might work.

Hmmm.... I wonder if this is caused cuz I bypassed the AM TX filter? Seems like 2Khz is too close to make any difference though. Anyone else have a stock FT-1000D? Please put it on 3875 TX AM and listen on another receiver to see if there are weak carriers on 3873 and 3877...



It was a lively session last night with many coming and going...

T

[Opcom]

Would you mind posting the schematic of the modulator including the feedback? It would be beneficial to anyone concerned with a clean signal.

[W1RKW]

Tom,
Make sure that the HV blocking cap from the top of the plate choke to the tank is up to the task.  The 2.5mH choke shouldn't have let go due to HV. Don't want HV on the ant feedline. Could make for a deadly situation.

BTW, nice looking and sounding rig.

[W1AEX]

It looks reeeeal purdy Tom! I didn't have any rigs on last night but noticed the southeastern sky was flashing so I suspected you might be operating. Looking forward to hearing how it all sounds.

Rob

[AB2EZ]

The new rig sounds good, I heard you last night after I got done talking up on 40 meters, you were troubleshooting the triple carrier problem you were having on 3875 when I heard it.

Yes, the FT-1000D driver has a strange birdie problem on 3875. It's in the design and puts out two additional carriers 2KHz above and below.  If we look at the spectrum picture in a few posts back, we see a peak on 3882 and 3878. They are down only -40dB on 3875 and can be heard as weak beat notes. But they are down -60dB when on 3880 as shown in the pic. Not sure if anything can be done with it since other FT-1000Ds do the same thing. Probably synthesizer design.  Might need a different driver when on 3875. I have an HPSDR that might work.

Hmmm.... I wonder if this is caused cuz I bypassed the AM TX filter? Seems like 2Khz is too close to make any difference though. Anyone else have a stock FT-1000D? Please put it on 3875 TX AM and listen on another receiver to see if there are weak carriers on 3873 and 3877...



It was a lively session last night with many coming and going...

T

Tom

My stock FT1000D also puts out a pair of sidebands +/-2kHz from the carrier in AM mode on 3875kHz. The sidebands are 40dB down from the carrier... independent of the carrier level (adjusted with the drive control). The sidebands are much lower at other nearby carrier frequencies, and their distance from the carrier also changes at other carrier frequencies. They are 60dB down and at about +/- 5 kHz when the carrier frequency is 3880kHz. They are 80dB down and at +/- 14kHz when the carrier frequency is 3885kHz. At 3870kHz, there appear to be some close in sidebands (approximately +/- 500Hz) about 60dB down from the carrier,

Nice rig!

Stu

[K1JJ]

Thanks for the info, Stu.  Looks like the 3875 carriers are normal for the 1000D.

Opcom, I didn't draw up a schematic yet, but will in the future.  The NFB is derived from a 5 meg (string of 180K 1 watt resistors) across one end of the mod transformer secondary to ground.  I tap off a 10K resistor that's in the string, at the bottom of the string into a capacitor for audio NFB. This low level (1 volt audio signal) gets fed back to the input stage of the MOSFET audio driver, thru a pot.

Bob, I think the coupling cap is OK. It's a big 15KV job and today after a few hours work I see no more signs of arcing in the rig. I'm sure my big RF choke across the 50 ohm ant line wud cause fuses to blow if so.

Rob, I might get on tonight, so maybe catch ya.

Early today I fired it up and found yet some more arc over problems.  The 160M and 75M tank coils were arcing to one another even though the terminals were spaced 2.5" apart. I added another Plexi shield. Now there is no arcing anywhere, even at full strap.  Finally.

The bad news is my Bird wattmeter SO-239 connector arced over again. It is weak and disfigured  from that HV arc last night. I cleaned the Teflon, ordered a new connector on eBay, and will have to run the rig in low to medium tap for now.

T

[W1RKW]

T,
Odd that the Bird took an HV hit. Is it HV or just very big RF? If it's HV and  there's no loading coil down the feedline or at the ant you'll have HV down the feedline especially if the 2.5mH choke is open. The fuse may not blow in that condition and if the cap is flaky. The cap being a 15 Kv cap doesn't mean it can't go south.  Don't want you to have an accident. I know you're diligent but please be careful OM.

[K1JJ]

T,
Odd that the Bird took an HV hit. Is it HV or just very big RF? ase be careful OM.

Bob,

The arc that caused the problem was from the 4X1 plate cap to C2 loading cap.  There is both DC HV and RF high voltage there.   Evidently the 50 ohm safety choke bypassed the DC and blew the breakers, but the RF voltage present there was not bypassed to ground by the choke.  Thus, instead of a normal 500 V of RF on the 50 ohm line, there was probably 4KV.    I'm glad that C2 didn't get damaged.

This was a momentary flashover that has been fixed thru isolation.  But still the remnants of this 4KV RF pulse may rear its head in the form of weakened parts in the 50 ohm line - like the SO-239s that once zorched badly with carbon tracks, cannot be made to handle full power again.   I'll just have to weed them out over time.

T

[KL7OF]

Tom... I have found that the so-239s and the pl259s that are used on the smaller hardlines   (5/8---7/8") are very well made and can be easily adapted to standard coaxes by someone that is handy with a soldering gun  I found some high quality chassis mount So-239s and silver soldered a pin into it replace a fried connector on my bird..working good at strap so far...Gud LUK...hope to hear you....Steve

[W1RKW]

Tom,
it sounds like you have a good handle on the situation. I know I don't have to tell you to be careful. You're pretty knowledgeable.  Just wanted to make sure.

[Opcom]

For noise in weird ducting:
If a perpendicular transition or flange in the airstream can't be avoided, some "DAP" caulk can be used to give a fillet or rounded profile to the joint or a corner if it is making noise. This worked well mating the intake snout of a 500CFM blower to a 2 LB coffee can. YMMV.

[K1JJ]

To maintain continuity -  the next related thread - please go to:

http://amfone.net/Amforum/index.php?topic=35856.0

There is discussion about testing and troubleshooting Fabio II to improve frequency response and cleanliness.

[Opcom]

And the IPA. No name for it yet...

Chico?  just a thought.