Building The Retro 75 - Another Joins The Group

[W1UJR]

Fixed the photo links, photos and complete story at http://www.w1ujr.net/bruce's_bench_2011.htm

My sincere thanks to Bob W1KBW for his tips and suggestions, plan to implement some after the shake down cruise.

Been as busy as a beaver over the last few weeks with radio kits, and have more coming this week! As from Elecraft, I've also enjoyed kits from the good folks at Small Wonder Labs, www.smallwonderlabs.com, building a number of their kits over the years. Recently Small Wonder Labs came out with a simple, QRP power AM transceiver kit, called the "Retro 75". I had looked at the kit many times, but always had other projects on the bench, finally ordered it a few weeks back.

Saturday night I started the Retro 75 build, was up to 4:30AM, lost track of time and really was enjoyed the project. I finished up the details this afternoon, and I estimate that it took me about 12 hours of total time. One could do it in less time, but I wanted to enjoy the process, and the moment it ceased being fun, I stopped and took a break. I also spent some additional time to make sure the solder connections and lead dress looked just right. In fact, I initially ran the wiring for the control and jack leads above the chassis, but it looked a tad too cluttered for my taste. Wanting to really enjoy and be pleased with the project, and perhaps having too much time on my hands, I redid the wiring, and ran it under the PC board. The extra efforts were worth it, as the rig played the first time out. I did find one wiring error during my pre-power up testing, I had reversed the T/R and Power leads, easy to do as they are next to each other on the board, and I was working under the board with a mirror image rather than on top. During the alignment stage, the set was hearing shortwave stations with just my finger as an antenna, and even the two variable indictors, T1 and T2 did not require much tweaking, both were just about spot on.

Some notes about my build, as I mentioned earlier and per the suggestion of Dave K1SWL, I ran my control and jack leads under the PC board, rather that on top, it makes for a cleaner looking build, used heat shrink tubing to keep the lead dress neat. The 3A/B/C capacitor selection confused me at first, it was not until I read later in the manual that I fully understood that those different values were used to set the turning range.

Since the instructions are rather free form, I chose to build using component types, rather than board areas, this seemed to speed things up as I could sort the values before install. So the caps went first, then the resistors, inductors, etc. The system worked pretty well, and I used the component listing as check sheet, marking off each part on install. I'd ideally like to see some sort of plug in header for the wiring, so the board could be removed for service, inspection, but this is by no means a necessity, also a plug in xtal holder would be nice, so could sway out xtals, guess I can implement pretty easily, and both options would only add to the very reasonable cost for the kit.

I bought the optional enclosure, glad I did, works well with the rig, With that said, I would really love to have the anodized aluminum case like the ones offered with the Small Wonder Lab DSW series back in the late 1990s, but that is something which I am working on. Power levels, for AM carrier, varied between 3-4 watts without modulation, right on spec, as you can see by the Elecraft power meter photo. RX side seems to work well, nice and sensitive, pleased about that, looks like a real bargain for the money. In summary, the Retro 75 is one very fine kit, a great weekend project, quite well designed, well priced, looks like Dave has hit another homerun with this one!

[K1JJ]

Bruce,

Nice looking rig and circuit board!  I'm thinking of picking one up myself.  I have a few basic questions after reading the site info...


Price is right. So it's $69 for the board kit and $40 for the enclosure. Except for more crystals and a 13.8v PS, that's all we need, right?

I see it includes a built in mic, maybe an electret condensor, I presume.  I've read the basic specs on the website but don't see anything about TX freq response or audio cleanliness.  I'd be curious what you see if you sweep it from 0hz to 8kc. What is the maximum % of modulation, is it flat throughout this range and does the waveform of the scope look good?

How about the receiver?  What is the 75M RX tuning range or is it a RX VXO based on the inserted Xtal? Is it a regen or something else?  It says 4kc crystal filter... is this +- 2kc bandwidth for RX? Do you have a sig gen to test sensitivity?

Maybe these questions are already answered in a review somewhere.


What a cool little rig for the price.

T

[Steve - K4HX]

It's a PW rig. Who cares if it has good audio response or if it's clean?  

It uses a mod transformer. The low end is purposely limited. It's too bad the designer didn't copy the approach many CB radios use - essentially series mod with no tranny. Anyway, it's still a cool rig.

Discussion on frequency response here.

http://amfone.net/Amforum/index.php?topic=24493.0

User group here.

http://groups.yahoo.com/group/SWLRetro75/

[W1UJR]

Tom and Steve - I have not swept the audio on the unit, not a bad idea, but suspect Dave's got it tailored to the freqs which are for ideal for intelligibility rather than HiFi. I was really impressed with Larry's military rig during the net, that kind of piercing audio is ideal with low power - but you know that with your 11 milliwatt signal.

I hope to have more time this week to do some bench measurements, Bob RKW has some good suggestions on the set, perhaps he'll post them here. Bob didn't think the RX was too hot on this set, mine seems to be fine, picking up stations with just my finger on one of the resistors, think it may make a nice little monitoring set.

Lots can be done to dress this up and add options, Dave kept it pretty basic and functional, sure to meet the price point.

I'd ideally like to put it in one of the aluminum enclosures and have nice plug in jacks for the mic, internal PTT, additional xtal holders, etc.

I agree, a bargain for the price, lots of fun!

[W1RKW]

The audio response is not flat. It peaks between 1Kc and 2kc maybe 3Kc.  As Bruce said not hi-fi.  Stu/AB2EZ has given me a few suggestions to improve the low end.  I'll have to dig up those notes if anyone is interested.

The transmitter is limited by a limiter. It holds modulation down to about 90%.  This can be over ridden by removing the LED but the Retro will produce products very easily.  Stu even suggested a negative peak limiter to improve modulation index.  I tried it but could not see any improvement and didn't delve into why or tweaking.  Will do that at some other point in time.  

As Bruce mentioned, my RX section seems a bit hard of hearing. Few have this malady and most have hot receivers.  It maybe due to the SA612 but I ended up installing an RF preamp on the front. This improved sensitivity tremendously. It's now as sensitive as my FT102.   I'm working on an AGC to automatically tame it somewhat but using an RF attenuator on the front end seems more like the ticket especially if you like tweaking controls. I set the AF gain to a preset position then vary the volume using the 'RF gain' control.  Works quite nice.  I've posted an RF preamp on the yahoo Retro75 group in the photos section. Here: http://groups.yahoo.com/group/SWLRetro75/

RX tuning range will be determined by NPO caps supplied. I'm not sure what the tuning range is but I'm guessing more than 50KCs.

Audio bandwidth on the RX is respectable.  I'm using a hi-fi speaker and it's not bad. Audio power is more than sufficient too.  Speaker is driven by an LM386.

I also have a switching arrangement to accommodate a 25watt solid state linear.  

My retro75 has been modified to use more than 2 crystals.  I'm using a 6 position rotary switch to select 4 crapstals with a range starting at 3870 to 3885 in 5kc steps.

As far as setup and building:  Building the board with parts provided.  You'll need a few 'external' parts such as controls, jacks and switches and wire to connect them.  You'll need a jack if you want to be able to connect a mic or you can wire a mic directly to the PC board. The same for an antenna and power.  

I wasn't aware the kit came with a mic.  Initially, Dave suggested a couple of mics for optimum audio. Nice that he has a mic. Not sure of the type. I'm feeding audio at the present moment from my audio chain. I've tried a D104. It works but you have to be right on top of it to modulate near 100%.  A dynamic mic works but some can be pretty hot.

The enclosure is optional. I have mine on a chassis from something I had kicking around. PC boards on top and switch and other add-on underneath. Face plate is made of Plexi.

Overall it's a great little kit.  It has a lot of modding potential and one can turn it into a little big rig quite easily. All depends on how crazy you want to go.

You won't be sorry for spending $70.  Another Small Wonder

[K1JJ]

Thanks much for the summary, Bob.

I'll bet with the right attitude, one could make a series of mods and have quite a nice performing pissweaker.  Do Stu's fidelity and neg peak limiter, add a preamp, some more xtal ositions, dick around with the TX audio curve and you will have a cool, self contained rig.   

It's important with a PW rig to have a minimum of overhead infrastructure wasting power.   A poor example of this would be my SDR rig running 11mW with a 200w+ computer running to keep it alive. That might be about .00005% efficiency.....terrible - 

I would love to see a tiny rig like this set up to run on a pair of 9V batteries. Maybe one batt set per QRP net, but WTF.

T

[W1UJR]

Great summary Bob, and mine did not come with a mic either, just the 1/8 jack, wish that was at least 1/4" for the D-104.

I've been in communication with both Dave from SWL and Dean from American Morse about a series of aluminum cases for the Retro 75.
Dean made some beautiful CNC machined and blue anodized cases for the SWL DSW series CW rigs a few years back - more like decades now!

Not sure if Dean was the same fellow who made the later DSW cases, see the gray colored one below.

I'll post any good results here...

Orignal DSW Case



Later DSW Case

[Steve - K4HX]

I want one with a brass front panel.

[W1UJR]

I want one witha brass front panel.

Steampunker!

Bruce's New Retro 75 Case :-)

[KF1Z]

It's a PW rig. Who cares if it has good audio response or if it's clean?  

Because...

For reasons unknown, some guys are dumping it into a linear amp, for "more power".

Why? I don't know for sure, it's meant as a PW , QRP rig.

Why not leave it at that?

[W1UJR]

For reasons unknown, some guys are dumping it into a linear amp, for "more power".

Why? I don't know for sure, it's meant as a PW , QRP rig.

Why not leave it at that?

Agreed, might be cool just for the engineering exercise, but really, what is the point...

[WA1GFZ]

Tom Vu worried about efficiency when he uses 4 4-1000As to make 375 watts AM modulated
Softrock makes transceiver kits also.

[K1JJ]

Tom Vu worried about efficiency when he uses 4 4-1000As to make 375 watts AM modulated

Past tense, my boy - past tense.

I hear Mr Vu tore it down and later built a 24 piller that runs 99.99% efficient. He was schooled by some guy who liked brass panels, Steve.

T