Anyone using the K7DYY rigs?

[N2DTS]

I came across this transmitter a while ago, and it seems interesting.
Anyone use one of these rigs on a regular basis?
Do they hold up if you don't short something to ground?

I never hear of them on the air, never worked one....
I would have thought they would have sold like hotcakes, 300 watts carrier with good fidelity in a small box.


Brett

[WA4JK]

They are a small package fixed freq. to five on one band.
There seems to be both good results and not so good. Search in archives and other boards for additional info.

[N2DTS]

Been searching, nothing much found in the last year or so.


Brett

[ve6pg]

..brett...i'm using one here. it is on 160, has 6 fixed freqs., i've had it about 2 yrs, and have no complaints at all...310 watts carrier output...larry w8er has one on 75m...great little box, and as i mentioned...no problems with it at all...it would be nice if it has a vfo, but that might be coming soon...you cant go wrong with it...

..tim..

..sk..

[Jim, W5JO]

Brett, Bob/W1PE bought one of them back some time ago.  It lasted just a couple of weeks and went belly up.  They replaced it with another and it did the same in a short time.  At that point Bob was tired of the trouble and requested a refund which the company did.

[WA4JK]

I had the same experiance within two weeks and a local hear sent his back  four times before bruce took it back. Bruce's thought to me is there is some reactance intermittent change large enough to short the output but not large enough for the swr protection to catch a very high current draw thus pulling the FETs beyond their rating and boom there gone. I did extensive megering on my coax connections and was clean and green to no avail. Another idea is if you get to close to 100% neg modulation it will do the same to the FETs. I liked the package but was disappointed in the results.
I must say this Bruce is a stand-up guy and will work with you where ever he can.
Wished I could talk him out of a Super Sr.

[flintstone mop]

I have the JR. Class D for all of the 80M band (no tuning, no channels). Just shoot a crystal or VFO into it and I get 180W out. Reasonable audio. Not real great for extreme high end audio. 5KC is pushing it for audio, but good enough for Amateur use.

Mine went tits up, caused by me, by soldering onto the PTT line which is one side of the A.C. circuit. I guess isolation transformers were too expensive to include, but a known, and warned about shortcoming. Cost me $90 to have Bruce repair.

Fred

[ve6pg]

..again, no problem with mine...frequently get great reports, audio, strongest sig on the band, etc...but...let me add, that i have not opened it up, and dinked-around in it...i'm guessing...most of them that had problems were opened-up by their owners...'nuff said?...

..sk..

[WA4JK]

I wished I could figure out how to determine a true cause to the failure. I had a tuner inline after the Sr. so it would have supplied a constaint load which would not have reacted to that ultra-quick supposed intermittent imbalence which never appeared on the scope or metering. It truly is a very nice option espically for a RV'er. I just could not see rebuilding it every two weeks.

Now the supposed Super Sr. design which will not become a production item is very desirable.

 Here's the scoop in his own words.

I sent an email to K7DYY asking him if the rumor was true or false. Here is the reply:

Hi Ted:

The rumor may have gotten ahead of reality.

I am indeed working on a Super Senior. It is adapted from
my commercial design. It uses two finals similar to the senior
through a combiner to provide a conservative 1500 watts PEP
with each module running at less than 200 watts carrier. This is the
same derating I use in the broadcast transmitters and has proven
to be very rugged and stable.

The transmitter is in a 2U rackmount package, about 3" high, 19" wide
and 10" deep. It operates on 160 and 80 meters and has a built in
synthesizer for 5 Khz steps from 3600 to 4000 KHz and 1800 to 2000KHz.

An LCD display with a modulation monitor is built in. It shows frequency,
carrier power, peak power, reflected power and has a bargraph representing upward
modulation. It flashes an overmodulation warning if the downward modulation
exceeds 100%.

Unfortunately, if I were to offer this as a product, the price would likely be
a bit high for most hams, probably on the order of $1,400. At the moment
this is more a personal indulgence than a business and I was planning to
do a one time build of three or four transmitters. When the design is complete,
I plan to post the details on my website so those who are interested can
adapt the circuitry to their own projects.

73,

Bruce K7DYY

Ok $680X2 = $1360 for 160m and one for 80m 6 freqs. in each and that big red light on transmitt or the above for $1400.. duuuugh I'll take the Super please.

[WA4JK]

Nope never dinked in mine keyed and got N5 BOOOOMMMM that was it.

[KF1Z]

It would be an easy task to add a vfo, AND bandswitching to a "DYY SR."
And have a 160, 80 and maybe40 meter transmitter, that covered ALL of each band.

Gee, all you need to do is switch low-pass filters for each band...
And stick a vfo into it. ( I'd have to look again, but I think it has to run at 2x the freq of operation. MAYBE 4x. but I think 2X)

A slightly larger enclosure too...  maybe 150% the size it is now.

[N2DTS]

Well, it seems like a bit of a bust then...

I guess it does not take much to blow a fet!

Brett

[ve6pg]

...freqs are 4X...mine are all 7400kc, fer 160...i've used 2 different tuners with my loop fer 160. open wire feeders to tuner, then rig...it never has given me trouble at all...i was concerned about hi-swr, with tune up, but the little thing has never given me a whisper of trouble...the only issue was, on audio peaks, one of my tuners was zorching, due to dust on the variable caps...the tx was fine though...

..sk..

[WA4JK]

I don't think it's a bust, just work with bruce to try to understand what you need to look at and how to monitor the modulation. Like said some have never had one problem, while others could not keep it's fet under it. It's a neat little package.

[N2DTS]

So its the modulation that blows the thing up?
What about if you ran it at 275 watts out instead of 300?
I see there is some sort of modulation limiter, is that just for negitive modulation peaks or both?

The fets are rated at 90 watts, 400 volts and 8.8 amps, which seems robust, but maybe the 400 volts is on the low side?

Are there fets with higher ratings?

I don't have much of a junkbox for this sort of stuff,  so it would be easier to get something and maybe modify it than build something from scratch.

Not that I need another low power rig, but its interesting stuff to play with.

Brett

[WA1GFZ]

I wouldn't put more than 70 volts positive peak on a 400 volt FET. My 160 meter rig uses 500 volt FETs at 85 volts peak positive Vdd. I've blown around 3 FETs since 1996 due to my own stupidity.

[N2DTS]

Why is that?
That is under 1/4 voltage!
Does the voltage increase so much with modulation?

Brett

[W7SOE]

I have a Sr. and it works great.  I cannot speak to reliability as I built mine myself and so, if it fails, I would tend to blame myself. 

Rich

[steve_qix]

Why is that?
That is under 1/4 voltage!
Does the voltage increase so much with modulation?

Brett

Yes it really does - and the RF peaks are even higher.  Personally, I design everything with 20x headroom - carrier DC to RF device breakdown voltage.  So, a modulated RF amplifier using 40VDC at carrier should be constructed using at least 800 volt devices.

I'm working on 9 years with my main solid state transmitter - never have head a failure, and I've tuned up lots times into  nothing, and I've also had the antenna fail a couple of times when I was on the air.  But, the reliability is mainly due to the high headroom in the design.  That aspect cannot be stressed too much !

[N2DTS]

So could you plug 800 volt fets into the K7DYY rig and have it be reliable?

Brett

[W2VW]

I've never lost a FET in any of my transmitters

[steve_qix]

So could you plug 800 volt fets into the K7DYY rig and have it be reliable?

Brett

Well, it's not that easy.  There are lots of other factors what would need to be considered - the driver, for instance - and other elements which may be capacitance dependent.

[DMOD]

So what devices are failing?

Is it the Q1 modulator FET or one of the RF switchers in the RF deck?

It does appear from the schematic he might be driving the gates in the RF deck with a sinewave.

I often wonder if a good isolation transformer wouldn't help this design??

Phil - AC0OB

[KX5JT]

I've never lost a FET in any of my transmitters

Have you ever HAD a FET in any of your transmitters??