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Reviews Categories | Ham Radio education & exam prep materials | MFJ-557 Code Practice Oscillator Help

Reviews Summary for MFJ-557 Code Practice Oscillator
MFJ-557 Code Practice Oscillator Reviews: 9 Average rating: 2.7/5 MSRP: $29.95
Description: Battery powered code practice oscillator with attached straight key on metal base.
Product is in production.
More info: http://
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VE7MXL Rating: 2/5 Oct 17, 2017 12:15 Send this review to a friend
Needs a pitch change and a filter.   Time owned: 6 to 12 months
First, you need to change the pitch of the oscillator. Replace R2 with a 10K resistor to change the tone range to be 600 Hz to 1000 Hz. It ships with a 4.7K, IIRC, which makes the tone range way too high. Go with an even bigger resistor for a lower range.

Second, you need to filter it to avoid listening fatigue from the truly horrific distorted sawtooth output waveform. A single-stage W7ZOI filter with follower input and (possibly) follower output will be enough to get you a sine wave into headphones. This is a pain, but an audio CW filter is usable with a radio after you've learned CW so it's not a waste.

Another alternative is to replace the oscillator, but the best choice at several hundred Hz is a Wein bridge, which is notorious for settling in to the tuned frequency. Or, one could use the brain dead MFJ circuit and augment it with a single-stage W7ZOI between the 555 and the audio amp (LM386, I think).

Maybe you could replace the LM386 with a multi-stage peaked low-pass filter, which would eliminate the ability to adjust the output frequency in exchange for a much better tone, even mostly fixing the frequency variability by filtering out the early wrong-frequency rise time.

Anyway, you should make yourself comfortable with the idea that the purchase price gets you a project box and a key (maybe some components can be re-used). The oscillator itself had to have been designed by someone who hates hams. You will either fix it or abandon the device.
G0DJA Rating: 1/5 May 3, 2014 12:31 Send this review to a friend
Poor quality  Time owned: more than 12 months
In my opinion there are a lot of issues with this key.

I was given mine some years ago, and I'm glad that I did not buy it.

The audio is very raspy due to the audio oscillator being an NE555 'timing' chip which produces a very triangular 'saw' wave output. The adjustable tone goes from just about OK to *stop it NOW*.

The key itself is thin and the knob is a simple dished disk. For beginners I would suggest that a proper knob with a skirt would be better.

If I were to want to help someone by sending Morse to them I would use a better straight key and oscillator and I would not inflict this on them to start to learn how to send. It might put them off for life!
KG6QNK Rating: 3/5 Sep 2, 2010 17:20 Send this review to a friend
Nothing to get excited about  Time owned: more than 12 months
It's OK for starting out; however, I will have some suggestion for you. Uses 555 IC timer chip, thus output is not a pure sinewave note, rather it is a squarewave-somewhat distorted. Practice Key as supplied is not that bad; and in fact could be used as a somewhat expendable QRP Key. There is another set of mounting holes drilled, that will allow you to mount a Nye "Speed-X" Key directly to the Base, instead of the supplied practice key- works well. Workmanship on the board could be better, I had to resolder bad connection; enclosure and base are solidly built. My main problem with this unit is the Tone Quality- I now use MX-T-Tone CPO (Kit) from Morse Express- produces clean Tone Note. At this point I would not buy another MFJ CPO like this. If MFJ would replace the 555 timer chip, with the type of Oscillator Circuit (name escapes me) used in the MX-T-Tone a relatively simple matter- they would have a winning product and would be doing Ham Radio a service.
N1ERF Rating: 3/5 Apr 21, 2010 08:48 Send this review to a friend
Chirpy  Time owned: more than 12 months
Not bad for a beginners CPO.
An all in one package is good.

It's unsteady tone is because it
switches the power on and off to
the timer with the key. Kind of
a poor man's battery saver. Not
quite enough time for the oscillator
to stabilize.

It would require a power switch,
a steady running oscillator and a
slight rewiring to get the tone a
little more steady.

My son and I both use it for fun.
Hopefully he'll be burning the speed
trails in the future and I'll have to
follow him.

Highly recommended for beginners.

599 C
KC0GLA Rating: 2/5 Jan 10, 2009 18:08 Send this review to a friend
Good Base and Key but Useless Oscillator  Time owned: more than 12 months
I bought this to learn code 7 years ago and have not been happy with this as a package since I bought it. I installed a good battery but the oscillator keeps changing pitch with volume and with voltage or how well contact is made. I even tried a cheap wall power supply. For the price get something else.

I found out after taking a look inside of the oscillator portion that is uses a single 555 timer IC. A much better solution would to be to use a programed PIC micro controller as used in most keyers to generate a clean output or at least some other design.

The straight key and the base I am happy with. The base has plenty of weight for a straight key. The key was easy to adjust and had good contacts.
W7SPK Rating: 4/5 Nov 18, 2007 05:06 Send this review to a friend
You can play music with it too!  Time owned: more than 12 months
I bought this 8 years ago and really never used it until just recently. Like another reviewer said, open a book and practice sending the text. Kinda fun when you get bored of watching grass grow.

The audio on mine over the years as become pretty quiet. The speaker inside is glued to the top and the glue came loose. Might be part of the problem.

I also discovered that if you have a ear for music, this can become an interesting instrument! By rotating the tone knob while tapping the key you can play tunes. For my 2 year old girl, I've mastered "Twinkle Twinkle Little Star" and "Baby Beluga".

My daughter just loves this thing, carries it all through out the house all day making all kinds of noise. My wife hates it.

For what it is, not a bad buy. :-)

Sean Kelly W7SPK
KC9HTV Rating: 4/5 Jul 5, 2005 19:26 Send this review to a friend
Pretty good for what it is.  Time owned: 0 to 3 months
I've had this for a couple of months, and I'm satisfied with it for what it costs. Along with the MFJ-418 code tutor, I am getting good practice for the code test. It's great to be able to practice sending, and work on my technique, and it's portable enough to take along to work and practice at lunch. The only 2 caveats are that you'll learn right away that you'll definitely want a better key later on, and unless you use alkaline 9 volt batteries, it will drain them fairly quickly (around 3 or 4 days). Otherwise, not a bad little gizmo.
KI4AFO Rating: 4/5 Jun 19, 2003 14:45 Send this review to a friend
Good for it's purpose  Time owned: 3 to 6 months
I just recently passed my general ticket. I made quite a bit of use of this tool during the lead up to the exam. For what it is intended I found it a good piece of gear. Maybe I am a little diferent but I used it to learn each letter by sound. I was having a heck of a time changing tones/frequencies, and from other posyts there are others like myself. I could learn a character at one frequency but when I listened to it at a different freq it sounded different and I had a hard time getting them down. By using this tool and adjusting the tone I was able to learn the characters by different tones. It is also nice to practice sending once in a while as it breaks up the monotony of copying code all the time. Crack open a book and start sending the page.

For what it is intended it is a good piece of gear.
N4EKV Rating: 1/5 Oct 8, 2002 14:48 Send this review to a friend
Look elsewhere for a CPO  Time owned: 0 to 3 months
The sound this CPO generates has poor tonal qualities and is unsteady, starting at one frequency and then quickly dropping a tad as it 'settles in' to the idea of making noise.

I was going to use it to tune up my bugs, but the sound is too annoying. Then I though of saving it for my two-year old to use to learn morse code, but I don't want to turn her off to the otherwise sweet sound of fine CW!

Look elsewhere for a CPO.

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