Ebay 45W linear amplifier - Heat sink & Bracket

Some pics of the heat sink and bracket that I'll be using during testing.  The heat sink came from a junk PC, and the bracket is bent from 26 gauge copper sheet.

Heat sink and bracket, drilled and tapped.

Bracket mounted to heat sink.

Test fit.

Of course, something always goes sideways when working on any project.  In this case, I planned to use an existing hole in the heat sink (already drilled and tapped) for mounting the 2SC1971 driver, but neglected to measure it first.  As luck would have it, it's too large, so I'll have to fill it with JB Weld, redrill and tap.  That's a project for another day.  

Chinese "EBAY" 45 Watt linear amplifier "kit"

Looking to add a few dB to the QRP output of my HB rigs, I stumbled across a number of these Chinese Irf530 amplifiers on Ebay at very reasonable prices - generally $15, including shipping.  Not being one to pass up a potential bargain, I popped for one and it arrived today.  Above shows the contents of the package emptied into one of the XYL's best cake tins.  She'll never miss it.   Why a cake pan when I have a nice, white, melamine surface?  Because these parts are small and have an annoying tendency to pop free from the tweezers or forceps at the worst possible time, and a cake tin works perfectly to constrain those parts that attempt to go rogue.

Because the parts are so small and my eyes are not a sharp as they once were, I made use of the camera in my tablet to help identify the various resistors:

While it may be difficult to see here, I was able to clearly read the lettering on the part during assembly.

It's worth noting that there are a couple of different versions of this kit in circulation (v303 and v306), and the documentation provided (for download) from the vendors suffers from poor translation.  An improved english manual is available for v303, but it proved of limited value because, while the kits are very similar, there are some significant differences in both the design and parts used.  Best to suffer aong with the Chinglish instructions - which is really nothing more than a parts list (in .xls format), pictures of the top and bottom of the board, and instructions (again in Chinglish) for winding the three transformers and RF-choke.  These were adequate - I was able to understand what they were saying, but I'm at leat an intermediate-level builder and have wound countless ferrite transformers and chokes.  Still, it's hardly rocket science.

So, in constructing the board, I follow a practice of installing the small SMD parts first - it's easier to do so now than after adding larger parts that take the board from the 2nd to 3rd dimension. With this one, I started with the .01 capacitors, then the .1 and then the couple of 100 pF caps.  The large electrolytic and mica can wait.

After installing the caps, I moved on into the realm of resistance, soldering all the SMD resistors to the board.

It should be noted that, while the capacitor values are unmarked, when you inventory the parts, it'll be apparent which are which based on the quatities included with the kit.  The resistors ARE marked - not long ago, I could read the markings with the naked eye, but over the past 10 years, this has become challenging and I've had to resort to magnification.

Once the SMD stuff is installed, you can move on to the larger parts.  Easiest was to first install the two pots, and, while you're at it, set the center terminal of each to the ground side.  This way, the '530s will be turned-off when you fire it up, reducing the chances of accidentally frapping the silicon.

After this, I installed the two molded chokes and the 2 large 22 Ohm resistors, leaving enough wiggle room in the lead to solder in the '530s when the time comes.

I wound the transformers next.  The instruction sheets (downloaded) caution that the edges of the cores are sharp and should be relieved to prevent nicking the wire's insulation.  They mention scraping the edges with needlenose pliers, but I find it easier to chamfer the edges uing various sizes of phillips screwdrivers.

There's nothing tricky about winding these - a single winding is nothing more than passing the wire through both holes in the binocular cores, so do what it says and you'll be fine.  T3 has a tapped primary - put two one-turn windings through the core, twist the opposite ends of each winding togeter to form the center tap.  Not a big deal.

I still need to prepare my heat sink, so I left the board at this point before knocking-off for the night:

It took only a couple of hours to get to this point, and I've probably got about that much left in prepping the heat-sink, wiring up the power supply, low-pass filter and T/R relay.  With any luck, I'll be able to bang that out tomorrow and get on with some testing!  Wish me luck.

73 - Steve

"Broadbanded" 40 meter vertical antenna

Can you spot the antenna?  Of course YOU can, but it's proven relatively invisible to the typical non-radio cuckoo person thanks to a can of camouflage paint (Rust-Oleum "Deep Forest") from Lowe's.

There's nothing remarkable about the antenna itself, it's just 40' worth of aluminum sections mounted to a treated 4x4.  The sections telescope into one another, allowing the length of the antenna to be adjusted to bring it into resonance.  Very simple, but, no vertical will perform worth beans without a decent ground system.  Again, mine is nothing special: just a bunch of radials, between 30 and 100' long, buried just under the sod.  It's been several years since I installed this antenna and I don't remember exactly how many, but it's somewhere around 20...  I had a bunch of partial spools of wire and just kept burying them until I ran out :-)

Here's the cool part: On the lower frequencies, it's tricky to make a single antenna present a decent match across the entire band without some kind of tuner. A long time ago, someone showed me a simple way to "stretch" the bandwidth of an antenna by adding an electrical 1/4 wave section of 75 Ohm line to the base. So, at the feedpoint of this antenna is an electrical 1/4 wave section of RG-11, which connects to an electrical 1/2 wave section of RG-8 that feeds a bandpass filter located in a weatherproof box at the base of my tower, where it joins the section of RG-8 that runs through the underground conduits (about 150') and into the shack.

A brief description of how I understand it to work is: At resonance, the inductance (L) and capacitance (C) of the antenna and feed system are equal.  As you tune in frequency away from resonance, the L and C of the antenna and feed system change in opposite directions, countering each other; As the antenna "goes capacitive", the feed system "goes inductive".  Obviously, there are limits, otherwise the thing would look like 50 Ohms across the spectrum, but in this case, it's enough to provide a decent SWR across the band.

It's a simple trick that doesn't add appreciable cost or complexity to the system, give it a try!

Base of antenna - lag bolted to 4x4

View looking up from eye leve

NC-109: The final chapter.

Since the last post, I was able to "tweak" the Pullen Mixer circuit enough so that the radio works reasonably well, so I'll leave it in place as a tribute to W8NYI, the fellow who I believe perfomed the modification.

When I say "reasonably well", the radio now aligns properly - the dial tracks the oscillator and RF amp perfectly, but the RF gain seems down a bit, probably due to the difference in conversion gain between the two mixers.  Nevertheless, the sensitivity is totally adequate for shortwave.

So, the old girl is cleaned-up, tuned-up and, as shown at the top right of the above picture, happily among friends.

Oh, one interesting thing that I meant to talk about the other day - this set has seen a LOT of use: Check out the wear on the dial string "pulleys":

These pulleys don't turn (not meant to), the cord is meant to slide in the somewhat polished surface. In this case, it actually started to saw it's way through the copper!  Fortunately, an easy work-around was to simply restring the cords (one main, one bandspread) to use the unused half of each pulley, which works just fine.

So, another project added to the "done pile".  I've got a couple of ideas for what to take-on next, but first, I really need to clean the shop/shack... it's gotten completely out of hand :-)

73 - Steve N8NM

NC-109: Day 3 and archeology

Thanks to my favorite chassis cleaning product, aptly named "Super Clean", I was able to get the chassis looking respectable in less than an hour.

I saw someone recommend this stuff, available at AutoZone, on one of the old-radio forums, and it works better for removing old nicotine residue than anything else I've tried - literally, spray it on and wipe it off.  Awesome stuff!

Here's what it looks like after a quick spritz and wipe.  Amazing!

The finished product:  Came out pretty nice, little corrosion toward the rear, but much better than before.

Under the chassis, I cleaned-up the wiring around the modified mixer stage (more on this later), replaced the few tubular paper caps and the mica wafer inside of Z2 (dreaded silver migration disease).  

Above is a shot of the underside of the mixer after all cleaning-up all of the flying leads.  Unfortunately, that all may have to go!  When I started alignment, I found that, no matter what, I can't get the dial to track properly across any of the bands, and this seems to be due to interaction between the RF and LO inputs of the mixer - there's not enough isolation, and the alignment of the RF side is affecting the LO frequency - probably why the AM band was inop.  So, unless I can correct this, I may remove the Pullen circuit and put everything back to it's original spec.  

Now, for the archeology part:  I forgot to take a picture while I had the front panel off, but penciled onto the chassis behind the panel is the callsign of, presumably, one of the previous owners: W8NYI.  I looked through back editions of the Callbook (scans are at archive.org) and found that W8NYI was Richard Smith, who lived on Prairie Street in Detroit - which was surprising, because I live in the Detroit area and purchased the rig in Eastern Ohio... So, the rig has come back home!

Unfortunately, the last Callbook in which W8NYI appears is 1975, so it's quite possible that he became a silent key. Since the 6DJ8 tube used in the mixer dates the modification to the late 60s/early 70s, I envison OM Richard toiling over it in the shack of his QTH in the old house (now gone) on Prairie Street.  This is why I'm reluctant to remove the modification - It's part of the rig's history and W8NYI's legacy; I sure hope I'm able to get it to work!

NC-109 Day 2

A couple hours into the NC-109 and it's starting to show.  Sometimes, it's surprising how much difference just a little cleaning-up can make - this thing is filthy!  Fortunately, the ancient nicotine and goo isn't putting up too much of a fight.

Electronically, I haven't run into any major problems, but did find a surprise - more on that later.

Initially, the "A" (AM Broadcast) band was dead, and in troubleshooting, I fell victim to the habit of "overthinking" the problem.  Initially, I suspected a defective component - dirty switch contact, open oscillator coil, or something along those lines, so I spent quite a bit of time checking components.  When I measured the L and C in the oscillator circuit, I discovered that the inductor was out of range and the problem was simply alignment.  I didn't expect that to be an issue because the rest of the bands were dead-nuts on, but the "A" band coil had just been screwed down a few turns too many.  Easy fix!

In the process of troubleshooting that problem, though, is where I found the surprise: A previous owner had extensively modified the mixer - the original 6BE6 had been replaced by a 6DJ8.  That number didn't ring a bell, so I looked it up and found it to be a dual-triode.  A-HA! I quickly drew the circuit as it was installed, and confirmed the circuit as being the "Keats Pullen" mixer that I've used in some of my homebrew projects:

Since I know that this circuit can work quite well - and it seems to be in this rig - I think I'm just going to clean up the workmanship.  As you can see below, it's a little haywired:

Another modification that had been done - and I un-did - was installation of a 50uF cathode bypass on the audio PA.  According the the schematic in the manual, the cathode is unbypassed, and it was pretty obvious that the capacitor wasn't installed by the factory.  I imagine that the intent was to juice up the gain of the stage, and that it did, but it significantly increased the residual hum, especially when using headphones.  The stage has plenty of gain and sounds great without it, so: Bye bye.

Otherwise, I cleaned the switches and controls, and cleaned/lubricated the bearings in the tuning caps, which were quite stiff.  I may have to give some attention to the dial cords as well; the springs are stretched (probably from cranking the stiff caps) and the bandspread slips.  Fortunately, the stringing scheme is pretty simple, so this isn't as big a deal as it could be.

That's all for tonight!

Something Different - National NC-109

 I need another old SW receiver like I need another hole in my head, but I felt sorry for this old NC-109 when it came up at an auction I attended last summer and nobody wanted to bid on it.  I don't remember what I ended up giving for it, but it wasn't much.

Other than being dirty, it appears to be all there, so let's see what we have:

 Typical dusty, dirty chassis - looks like a layer of nicotine residue, but no rust or mouse damage, and all the tubes are there.  We're off to a good start!

This is interesting.  At the top of the chassis, it looks like someone hacked-in a 40uF electrolytic and 500 ohm wirewound, probably because the multi-section filter cap wasn't doing it's job. Sigh. Kids.  

That was an easy fix, though, and what's cool is that there aren't many tubular paper caps to deal with - National used ceramics for bypassing, and those don't usually fail.

Normally, I like to remove and restuff chassis-mounted electrolytics, but I really don't feel like this rig is worth the effort.  I might change my mind tomorrow, but for now, I've disconnected the can and installed new caps on a terminal strip.

Now for the fun part!  After changing the filters and inspecting for any other obvious problems - time to power it up!  I don't have any pics of this process (not much to see, really) - I use a variac connected to an isolation transformer, and bring it up slowly, watching for any signs of trouble.  This old National fired right up - no magic smoke was released!  Yippee!

At this point, I gave it a quick evaluation, starting a "punch list" of things still needing attention:

1. Band 1 (AM BC) is dead.
2. Strange AVC "pumping" and background noise - mixer?
3. Dirty controls/switches.
4. Main tuning and Bandspread caps stiff - need clean/lube.
5. Rudimentary cosmetics / degrunge.
6. Screw terminals for antenna disconnected/rerouted to added SO-239 - need to be restored (why do people do this?)

Nothing looks too bad.  On the good side: 

• It receives: Envelope and product detectors are both working.
• ANL works.
• BFO works.
• Crystal filter/selectivity switch works.
• Dial calibration is reasonably close.
• Only a handful of tubular paper caps and one LV electrolytic (AF cathode bypass) to replace.
• Nothings smoking!

So, barring anything unforeseen, should be able to whack this one out in a few nights.  Wish me luck!!!

73 - Steve

It's shiny...

The new audio board and isolation transformers (shown above) have been installed and working exactly as I'd intended; to quote Hannibal from the A-Team: I love it when a plan comes together!

Operationally, there are two input channels for transmit audio: The microphone and CW sidetone on one, and the soundcard interface on the other.  This separates the audio paths, allowing the level of each to be optimized without affecting the other. Also, as I mentioned in my last post, it mutes the microphone audio during digital transmissions, preventing any unintended voice transmissions while operating the digital modes.

I have had intermittent problems with hum on receive and/or transmit due to ground loops.  Mounting a bunch of modules to a chunk of wood is nice for experimenting, but it's not necessarily the ideal platform when it comes to ensuring no differences in ground potential exist between the circuits.  I'd tried a number of "easy fixes", none of which ended up being particularly easy or fixing the problem, so I finally said "screw it", pulled everything off the board, laid down a sheet of copper, and remounted everything.  This seems to have all but eliminated the problem, and has the added benefit of being shiny - at least for now.

So, for all intents, I'm calling this one "done".  Over the past few weeks, I've made dozens of QSOs on CW, SSB and JT65/JT9, where, just a few minutes ago I was able to "cross the pond" for the first time, working PB8DX in The Netherlands.  Even after all these years, it still amazes me to think that a bunch of parts on a piece of wood, connected to some wire hanging from a tree branch, can be used to talk to someone 4000 miles away.  Wild!

73! Steve N8NM