Monday, December 28, 2015

RADIO SHACK 120-586 Pocket Radio -- a 'Sleeper' MW Long Distance Radio

A 'sleeper' product is usually a product that is an excellent one, and one that you wouldn't think of as an excellent product just by looking at it.

The old Radio Shack Pocket Radio, 1978, on Left -- and the Radio Shack Pocket Radio, circa 2012, Model 120-586, on right. Both are simple radios with two IC's, one to run the radio circuits, the other to push audio through the speaker. You can tell that Radio Shack's newer Pocket Radio has retro styling from this picture.

When it comes to radios, there are many 'sleepers'. The Sangean PR-D5 I mentioned in two of my posts earlier on this blog is one of these 'sleeper' radios. It was marketed as a table radio -- it looks like something a soccer mom would have in her kitchen to listen to soft pop music. And it looks like a mini-boombox -- which it actually is. But the PR-D5 has excellent performance, especially for those MW listeners who want to hear stations from far away. In fact, it's one of the best new long distance AM radios you can buy.

The Sony SRF-59 is another 'sleeper': it looks like a cheap headset radio you would give to a little kid so they could tune in the local pop FM station.

Until a few years ago MW radio aficionados thought it was just a cheap headset until a couple long distance listeners ("DXers") discovered by accident that it pulled in stations like crazy and worked great with an external loop.

The Radio Shack 120-586 AM-FM Pocket Radio is another such 'sleeper' product. I first heard of it on the internet. Some guys were saying it had amazing AM broadcast band reception. It was available at the local Radio Shack for around $14 -- that's $4-5 cheaper than a Sony SRF-59. So I got a Pocket Radio about six months ago.

What the guys on the internet said is very true: for a small pocket radio, which resembles the traditional transistor pocket radios of the 1960's and 1970's, it has amazing AM band and very good FM band performance.

It is what it says it is: a Pocket Radio. A small grey and black radio with a small speaker, and a stereo headphone jack wired for mono. You can place it in a small room and hear sports, talk, or music adequately. But like the 60's and 70's transistors it resembles, it won't fill a room with full sound. But you will still be able to hear talk or a football game OK.

The 120-586 Pocket Radio uses two AA batteries, and it seems to do well on them. I am keeping track of how long they last. If what I read on the internet is any indication, battery usage may be quite low. The fact the radio has no microprocessor and firmware operated power buttons, LCD readout, clock, etc. mean that the only time the batteries are being drained are when you actually are listening to the radio. I'm still on the two AA's I put in the radio when I got it.

Because of the low battery drain and great performance, the 120-586 would probably be a good emergency preparedness radio. It's small enough to shove in a back pack, a car's glove box; keep in your camper, or tool box, or even place in a shirt or coat pocket -- and it pulls in a large number of AM and FM stations. And on top of that, although you can use headphones, the speaker makes it a good choice for emergency preparedness.

I have used my Pocket Radio to listen to local sports stations while working out in my storage room, and I have used it on headphones while reading or falling asleep.

I take my Pocket Radio to work with me -- it slips inside my briefcase easily. It's unobtrusive enough and small enough to either use with the speaker or headphones. From work I can listen to either local AM news and sports stations, or local FMs -- or listen to California stations when working at night, as well as stations from other areas of the NW US.

The Pocket Radio pulls in a lot of distant stations from my workplace -- for example I can get KXTG 750 Portland, OR (about 150 miles / 200 km S); KEX 1190 Portland, OR; KGA 1510 Spokane, WA (200 mi E); KFBK 1530 (Sacramento, California -- about 800 miles S) and KGO 810 San Francisco (800 miles away) quite easily with listenable signals at my workplace in Seattle.

I use the Pocket Radio around the house or in the yard sometimes when I am doing chores.

I sometimes DX with it for fun.

It pulls in a lot of stations on AM, and does very well on FM -- the only drawback being that FM is in mono, even through headphones. Even so, the AM and FM sound through headphones is pleasant and full. Although it's a mono radio, the headphone jack is a stereo jack, so any set of stereo headphones will work -- you'll just hear everything in mono. But at the same time, you won't need a mono adapter to plug your headphones or ear-buds into the radio's headphone jack.

It has decent, full sound through headphones, and OK but tinny sound through the speaker.

The first time I turned on the Pocket Radio at night, I was pleasantly surprised. It was pulling in stations that radios with much larger antennas were pulling in.

The radio does so well because it has a SiLabs digital IF chip inside.

Since the 1980's, most radios -- whether a headset, small portable, car radio, or boombox -- have had "I.F. chips" inside them ("I.F." stands for "intermediate frequency").

The radio's "IF chip" is the chip that is the heart of the radio. An IF chip is just a bunch of transistors crammed into a small chip half the size of your average USB flip drive. It grabs the radio signals from the FM whip antenna or AM loopstick antenna, and then amplifies, filters, and re-amplifies the radio waves, converting them into audio sound, which is then usually sent to another chip that powers the speakers or headphones (some IF chips have their own audio amp included). All the other components you see crammed around the chip (when you have the radio opened up) are just there to make the IF chip do its job. 

A photograph showing the radio PCB board inside a Sanyo boombox. In the upper left you can see the main chip, the IF chip, possibly a UPC1018 in this model of Sanyo. There are three "IF cans" near the chip, which are tuned coils that the chip uses to filter and amplify the AM/MW and FM signals. Directly below the IF chip are several, similar, tuned coils for FM. In the upper right of the picture is another chip that takes the FM signal from the IF chip and converts it to FM stereo (sometimes these are called "multiplexer" chips). The audio amplifier chips for this boombox are on another board on the other side of the boombox -- to the far right, off the picture.

There may be an audio amp chip to drive the speakers. There may be another chip to help you get stereo FM. And another chip to boost FM signals coming from the antenna. And there may be another chip that runs the clock, or makes the buttons on the front of the radio work. But the "IF chip" is what takes the radio signals and makes them usable for the speakers or headphones.

Until recently, all "IF chips" were analog -- just tens or hundreds of transistors, all amplifying, filtering, mixing and refining the frequencies for the radio. Some of them could be rather complex (the chip in the Sony SRF-59 would be one example), but they still operated as analog devices.

But that changed with the introduction of digital, software operated IF chips during the 2000's.

Digital radio IF chips are like hybrids -- they have a software program inside them that does much of the amplification, and all of the radio frequency filtering and processing (it's called DSP, Digital Signal Processing). Because a lot of the work is done by the software program (which doesn't need tuned coils, ceramic filters, trimmers, and other components), the number of outside parts needed is minimal.

The Pocket Radio with the front off, showing the parts. Compared to many other radios, there are very few parts, which is one reason the radio is $14-$15 or so. The top thumbwheel runs a variable resistor that tunes the radio. The bottom thumbwheel is the on-off and volume control.

SiLabs radio chips are probably the best known brand of these digital radio chips, at least among radio DX'ers. The typical SiLabs radio chip has an analog low noise amp (to maximise the signal coming off of the FM or AM antenna) -- and then a section in the chip converts the analog radio waves into digital signals, which are then amplified, tuned, filtered and processed by the software inside the chip. After that processing, the chip converts the digital signals back into analog audio signals, which are then sent to an amplifier chip that drives the speaker or headphones.
A pic of the Pocket Radio's PCB showing the location of the two chips that make it work.
SiLabs packs a lot of processing into their IF chips. On the MW band, where you have a lot of interference from nearby stations, such interference is surprisingly minimal. On most MW radios, if you tune to a weak station that is right next to a strong station you'll hear "splatter" -- a ker-chush ker-chush sound that is splashing over from the strong station on the next channel. With the SiLabs chip, you hear very little of this.

And the amount of signal a radio with one of these chips pulls from a small loopstick antenna is phenomenal. The Pocket Radio has a small AM loopstick -- it's 50mm long and 8mm wide, and works very well considering it's smaller than standard (most boomboxes and digital-readout SW portable radios have 100 to 120 mm AM loopsticks, for example).

But the Pocket Radio pulls a lot of signals from that small loopstick antenna.

On FM, it pulls in a lot of stations even with the whip antenna collapsed.

When you look inside the Pocket Radio, it has relatively few parts. There is just a small circuit board with surface mount resistors and capacitors, the tuner control and volume control, the antennas, and two chips. There are no coils, no 'cans', no trimmers, no bulky variable capacitors, no DC-DC converter chips, none of that.

In fact, because it's so simple, and the parts seem fairly tough, the radio seems well built. One would probably have to stomp on it, or run it over with a car to break it.

The radio is inexpensive because it has such few parts. The lack of stereo is due to a small mono audio chip. They could have added a stereo audio chip and made the radio stereo through the headphones, but they probably decided to go mono to decrease cost. They may have also figured no one is going to listen to FM stereo on a single-speaker pocket radio.

The Pocket Radio with the back of the radio off. You can see a total of six electrolytic capacitors, and few other components on this side of the PCB. The other side has 30 or so more -- but still a lot smaller part count than the 1978 model Pocket Radio pictured at the top of this blog post. The loopstick antenna is at the top - it's a 50 mm by 8 mm loopstick, which is fairly common among portable radios today. Although it's short, it's a decent working antenna. The Sony ICF-38, a high performance radio on the AM band, also has a 50 x 8 mm antenna.

Because the Pocket Radio has an analog (or mechanical) tuner control (a thumbwheel on the side of the radio -- no tuner buttons) there is no need for a microprocessor chip or PLL chip in the radio to tune it or control it. One less chip on the printed circuit board to pay for or worry about.

The Pocket Radio is very much a simple machine.

The Pocket Radio is great for carrying around the house and yard, or taking on a bike, or possibly jogging, as it puts out enough sound to hear a sports event, or even music (although music will sound better using headphones than through the speaker).

I take mine on my bicycle when there is a football game on, and I want to go riding. If I'm near a highway with a lot of road noise, I won't hear much, but if it's a quiet residential street, or a quiet section of a bike trail, I can hear football games O.K.

On a bicycle a radio like this is preferable to a headset-only radio, because it's not exactly smart to be wearing headphones or earbuds on residential streets, for safety reasons.

What the Pocket Radio has over a Walkman or headset is the speaker. Sometimes you don't want headphones or earbuds on, especially on bike trails where you need to be able to hear what's going on around you.

Last week I took my Pocket Radio with me to hear the Monday Night Football game. It was raining lightly out. A few drops got onto the pocket radio (which was in my coat's top pocket) but the raindrops didn't get inside the radio (of course). Although the radio is not waterproof, it's so simply built that a few raindrops aren't going to affect it much. Whereas with some digitally tuned radios you don't want any water getting near them.

On FM, it seems to be considerably better than the SRF-59. In my location it will pull in fringe FM stations clearly that the SRF-59 does not hear. For example, in my area, there is a Spanish language religious station on 93.7 that is fuzzy or MIA on many of my FM portables. The Pocket Radio brings it in well, even with its whip down. The SRF-59 -- which uses the headphone lead as the FM antenna -- won't bring the 93.7 station in at all.

In this case, it's too bad the Pocket Radio isn't stereo through headphones, as it works so well on FM.

As a long distance AM listener (or "DXer", as we often call ourselves), I have recently been using this pocket radio to DX the MW / AM band at night. The radio works quite well. I can hear the majority of signals on it that I hear on my other DX radios, although a few signals are sometimes missing -- especially very weak stations right next to a local station running at high power.

Compared to my Sony SRF-59 headset radio (well known amongst MW aficionados), the Pocket Radio sometimes has better sensitivity, and better selectivity, on AM.

The only drawback when comparing the Pocket Radio to other portables is that the Pocket Radio is trickier to tune, if you are trying to hear distant AM stations. If you're just tuning local stations or strong distant stations, you won't have any problem whatsoever.

When it comes to finding weak signals, the Sony SRF-59 beats the Pocket Radio hands down. This is because the Sony is much easier to tune, and it doesn't tend to 'latch onto' a strong signal and skip over a weaker one. As you tune across the MW band with an SRF-59, you hear every channel except maybe a weak or MIA channel next to a local 50 KW station. On the Pocket Radio, there are many more of these skipped channels, because of the way the SiLabs chip tunes the band, and it can be frustrating.

The 'problem' is that the Pocket Radio is a mechanically-tuned, digital-chip radio. The tuner knob actually is attached to a device that is similar to a volume control (a variable resistor), which 'tunes' the SiLabs chip. It basically tells the SiLabs chip to tune upwards incrementally, or tune downwards incrementally. As you tune across a strong station you can hear the increments as the chip is tuning across it. And the tuning on the Pocket Radio is very, very tight. Because of muting built into the firmware in the chip, some channels 'mute out' and the radio will skip over them until a signal gets strong enough to rise over the muting. Then the radio will lock onto the channel.

A graphic depiction of how the Pocket Radio can skip over a weak station next to a strong station. In this case, the signal of the weak station on 1330 has faded just underneath the radio's "soft mute" threshold. Because of this, the radio's DSP chip will probably lock onto the stronger signals on 1320 or 1340. Once the signal on 1330 pops up above the threshold, the radio will lock onto it much easier. Once it is locked onto a signal, it stays there, even when the signal drops below the "soft mute" level. But when a signal is very weak, and there are stronger stations next to it, it is very difficult to tune to it.

Tuning the Pocket Radio for distant stations often takes a bit of patience. If you want to hear a weak station that is near a strong station you have to tune very carefully, and wait for the radio to 'latch onto' the weak station. Sometimes (on AM/MW) you have to turn the radio to weaken the local station, and then the radio will 'latch onto' the weak station as soon as it fades up. After that, it will stay on the weaker station even after it fades down for a moment.

An example: I have a strong local station on 770 khz, KTTH, which runs 5 kilowatts at night. Right next to it is KKOH, 780 khz, a station in Reno, Nevada that covers most of the Western U.S. Using the Pocket Radio, the local station on 770 will cover KOH if KOH is weak. But if I turn my Pocket Radio to weaken KTTH 770, and tune a little to the right, KOH will 'pop' through. Then the radio sort of 'locks onto' KOH, and will stay there until I turn the radio off, or re-tune it to another frequency.

This 'popping up' of weak stations is a feature of the SiLab chip inside the Pocket Radio. The SiLabs chip in the Pocket Radio has a 'soft mute' that suppresses noise (something my Sangean PR-D5 does not have), but the chip also will suppress a very weak radio station once it's below a certain threshold -- the station will go from full audio and then sort of drop out for a few seconds, and then pop back up to full audio. Sometimes it's a pain to deal with -- most times it's not that bad. A lot of times you can still hear the station underneath the 'muting', it's just at reduced volume, and the tuner may skip over it when you tune the radio across it.

This skipping of weak adjacents doesn't always occur: I've heard KDWN Las Vegas on the Pocket Radio, and KDWN is on 720 khz, right next to local 50 KW master blaster KIRO 710. I've heard CFRN 1260, which is right next to a strong local station on 1250. Sometimes 1250 covers 1260 completely (especially since they went IBOC HD radio) -- on other nights, it doesn't. I regularly hear KGO 810 from San Francisco, 800+ miles away from me, right next to local 820 KGNW.

I've even heard Radio Rebelde on 1180 khz, which is a station almost 3000 miles away. I could hear music from Rebelde underneath two other stations on 1180 (KOFI in Montana, and local KLAY in Tacoma, WA). The music was parallel to Rebelde's 5025 khz SW station. And this was without an external loop!

If you use an external loop with the Pocket Radio you don't have as much dropout to deal with.

Using a loop with the Pocket Radio can turn it into a decent AM DX machine. The only trick is that in many cases it is best to tune the Pocket Radio to your target frequency before you place the external loop near the radio. If you don't, you may have trouble with the radio wanting to 'latch onto' a stronger local station instead of the weak, distant channel you want to hear. So tune the Pocket Radio first, then place the loop next it.

In summary, I think the Pocket Radio is a great performer for the size, and it's DX-able, but it can be frustrating to tune in a weak channel near a strong adjacent. It wasn't designed to be a DX radio, it was designed for someone to have a small radio with a speaker for carrying around or using in emergencies. It wasn't designed for DXing, but it can be done. Just be prepared for its peculiarities if you want to DX with it on the AM radio band.

I suppose for a lot of radio listeners, these Pocket Radios don't have much appeal because they aren't stereo on FM, and aren't the kind of radio that fills a room with sound. But for someone who needs a small radio with a speaker to shove in their backpack or book pack, or for emergency preparedness, the Pocket Radio is a very good bargain.

And for a beginning MW (or FM) Long Distance listener, the Pocket Radio is well worth the price.

ADDENDUM, 5/11/2016:
One thing I've noticed recently is sometimes when the AA batteries are low/weak, the Pocket Radio can take a few seconds longer than usual to turn on. I'm not sure if it's just my own particular radio, or a characteristic of the Pocket Radios in general.

The SiLabs chips take about a second to initialise, and maybe the lower voltages from weak batteries cause them to take longer to kick in.

I still use my Pocket Radio periodically at work (it fits easy in my briefcase or bookpack), or when on my bike.
CC 5-11-2016

ADDENDUM, 5/20/2016:
I thought perhaps it was weaker batteries (they measured at 2.35-2.4 volts together -- the radio runs nominally at 3 volts), then I thought perhaps it was the battery terminals needed resoldering -- the radio was taking up to 10 seconds or so to initialise.

Then, I decided to look at the SiLabs Si4831 Datasheet/manual.

It says that "part initialisation may become unresponsive below 2.3 volts."

So, use fresh AA's with your Pocket Radio, or just wait until the chip initialises when they're a bit weak. The Pocket Radio likes fresh batteries, but it will work on weaker ones. It just takes longer for the chip to initialise and kick in.

And although it will work on heavy duty batteries, Alkalines seem to work best. But you do get a decent life out of them.
CC 5-20-2016

I have concluded that the initialisation lag problem in my Pocket Radio is NOT the batteries, it's something else. It is doing it again (the 10-15 second lag before switching on) with brand new batteries that put out 1.6V each. It still works, just has a long lag before turning on which never happened when the radio was new.

I'll probably buy another one in a few days and see if there's a difference. There may be a bad part or solder joint -- hard to fix because it's surface mount. The saga continues. :-)
CC 5-24-2016

I might have found the problem with my Pocket Radio: the battery connection was still a bit intermittent. I resoldered the points where the battery connects to the PCB, and also 'tweaked' the positive terminal towards the battery compartment a bit, to tighten the connection with the AA batteries.

It seems -- so far -- to have fixed the problem. The radio now fires up within about a second.

If this indeed was the problem, it would make sense. If the battery isn't 100% connected to the holder, the radio wouldn't receive all the voltage necessary to initialise the SiLabs chip.

I bought a second Pocket Radio for about $14 US. It works well, and the logo is slightly different on the front of it. Last night I heard 1660 Khz KBRE Merced California, a rock AM station, featured in a recent post on this blog.

1 comment:

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