Hello and welcome back. This is a quick video, actually consisting mostly of photos, documenting
an HP 8447F RF amplifier that I recently got in an auction lot. I had purchased the lot for the
HP 8013 pulser at the bottom of the pile. By the way, the HP 8013 is the little brother the fancy
8082A that I just repaired, and lacks both the 200 MHz performance (it goes only to 50 MHz)
and the controllable leading and trailing edge ramps. But it takes little space on the bench,
so I like to have one on hand. For the chuckle, the one in the auction came with a surprise:
on top of it was a large “No Good” sticker, which was of course artfully hidden in the display pile
by the instruments sitting on top. But to my surprise, it worked perfectly.
So, two sticker hiding RF amps came along for the ride: an HP 461A HF amplifier
(good to 150 MHz) and this much more serious HP 8447F RF dual wideband amplifier, good to 1.3 GHz.
I already own an HP 461A, which you had seen in action as an improvised sense amp
in the core memory explanation video. It’s a cute 1960’s instrument,
probably one of the early transistorized amps to that frequency, but not very impressive by modern
standards. This one also worked right off the bat, and only needed a readjustment for gain flatness.
So, on to our piece de resistance, the 8447F.
The preamp section worked, but I had to affix the ignominious sticker of non-workingness
to the bottom power amp channel, which gave me absolutely no output.
Which is a shame, because the amplifier module that makes it tick looks downright
gorgeous. It's a beautiful ceramic hybrid, it looks like a piece of electronic jewelry.
Being no RF guru, I had asked Shahriar over at the Signal Path
for a hand on how to repair it. But I got no answer. So I decided the simplest and
most cost effective "repair" was to replace the module with a modern low cost RF amp.
There is not much to the HP 8447F, but it’s very conveniently packaged for lab use. Just a
20V power supply at the back. All the magic is in the two small RF amplifier modules in the front.
I did not think much of such a repair, so I didn't film it. However it predictably degenerated into
the usual unexpected surprise, and there is no video of the 8447 innards on YouTube.
So I decided to make this short video for future reference, mostly from pictures.
There are Chinese RF amp boards aplenty on ePay, some for less than $10,
but they have no specs - you all know what that means. Instead, I opted for a $60 eval board for
the new TI Silicon-Germanium wideband amplifier, that actually comes with specs, what a concept!
40MHz to 4GHz, 18 dB gain, with 19.5 dBm output at -1dB compression. I reasoned that if I used
it only up to 1.3 GHz, I might be able to get instrument grade performance out of it.
And it did fit right in, it's the little red board at the bottom.
Except it's powered by 5V only (miracle of modern day RF electronics),
instead of the 20V of the original. So I added this miniature DC-DC converter. Another miracle
of modern electronics, 95% efficient and good for 500mA for a few bucks only.
I also made sure all the mods I made were 100% reversible,
using this little screw post in an already existing hole for example,
so I could get back to the original configuration if I ever repaired the original RF amp.
Confident in my oh-so-simple repair, I reassembled the whole instrument, connected it to the VNA,
turned it on and... NOTHING! Signal 40 to 50 dB down instead of being 20 dB up!
How could I have messed up such a simple repair? I checked the 5V, or if I had reversed a connection.
Nope, all looked good. What else could go wrong?
The indestructible, semi-rigid RF cables to the front panel? Nah, that can’t possibly be.
I was suddenly overtaken by electro-guilt. I quickly wired up the HP amp to VNA,
gave it the required 20V. And voila, 20 dB of picture-perfect HP gain, flat as a pancake to
1300MHz, and most of the way up to 2 gig while we were at it. It had been working all along!
So I extracted the two semi-rigid SMA to BNC coaxes. The input one looked OK.
But the output one did not even register: below 50 dB of loss, blank screen on the VNA.
A quick look in the bino at the BNC connector revealed the problem - there was no center
contact! Nothing, gone. Just high quality ceramic. Not a connecting connector anymore.
I opened up the good coax and the bad coax, and you can see how the center contact is
missing from the one to the left. But fortunately, the center coax conductor
is still there - the contact must just have de-soldered or broken off and fallen away.
And that's likely why the instrument was junked, and found its way to an auction site.
So I just stole the center contact from another semi rigid assembly I had laying around
and re-soldered it onto my HP coax. As good as new.
I quickly tested the repaired coax assembly on the VNA. Now that’s a lot better!
I first put the coaxes back with the TI amp still in place,
to see if it was better than the vintage HP. But it wasn’t really.
Only 18 dB of gain (as advertised), big dip of 10 dB around 1.3 GHz. Also,
it does not go down to 100 kHz either, but stopped at 40 MHz, which is as advertised
too. Although not very clean in this particular implementation, this modern SiGe
circuit annihilates the HP when it comes to power consumption, noise figure, and price of course.
Now, mind you, I'm comparing apples to oranges. This just a cheap eval board. I could also have
gotten a few $100 Mini-Circuits or Nagra amp modules, which are similar circuits but
mounted on properly designed ceramics with nice connectors and metal enclosures,
and that would have outperformed the HP in all respects.
In retrospect, if you ever have a dead 8447, this is the Mini-Circuits module you want to buy:
the ZFL-2000. On eBay, they are not much more expensive than the new TI eval board I got.
Anyhow, I undid all my mods, and the original HP amp went back in. It looks
again as Bill and Dave intended, and it performs as they intended too.
Great, I can now remove the sticker of infamy. One more HP instrument saved!
I’m sure we’ll put it to good use in one of the future episodes. See you then!