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Reviews Categories | Ham Radio education & exam prep materials | MFJ-554 Code Practice Oscillator Help


Reviews Summary for MFJ-554 Code Practice Oscillator
MFJ-554 Code Practice Oscillator Reviews: 2 Average rating: 4.0/5 MSRP: $99.95
Description: A code practice oscillator ('CPO') oriented towards a classroom environment (has a good audio amplfier and internal speaker)
Product is in production.
More info: http://www.mfjenterprises.com/Product.php?productid=MFJ-554
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WB5AGF Rating: 4/5 Jun 5, 2018 23:27 Send this review to a friend
Needs ESD Protection On Key Input (and perhaps ext speaker)  Time owned: 0 to 3 months
(This is a follow-on to my original review of the MFJ-554 Code Practice Oscillator.)

I've recently become aware that the MFJ-554 Code Practice Oscillator ('CPO') has a design flaw that may result in the unit failing.

The key input circuit goes directly to the gate of a 2N7002 field effect transistor (FET). There is no electro-static discharge (ESD) protection applied, just a pull-up to Vcc (+8 VDC) through a 10k resistor.

Under certain conditions (typically low humidity) plugging in a Morse key to the CPO, or touching the metallic parts of a key that is already plugged into the CPO, could result in sufficient static shock to the gate of the 2N7002 to damage-or-destroy it.

I'm at present considering if it makes sense to include dedicated ESD protection circuitry on the key input or if a change in the method the design implements control of the rise-and-fall times of the audio tones (at present accomplished by varying the way in which the DC control Voltage is applied to the gain input of the TDA7052A audio amplifier) could be used to provide protection.

While thinking about the need for ESD protection on the Morse key input circuitry I also started wondering if the external speaker audio output circuitry might allow ESD damage to the TDA7052A audio amplifier. At this time my concerns on this have not progressed any further (just 'thinking about').
 
WB5AGF Rating: 4/5 Mar 30, 2018 22:37 Send this review to a friend
A Good Code Practice Oscillator (very pure tone)  Time owned: 0 to 3 months
(The reason I gave a rating of '4' is that I think the MFJ-554 is too expensive but, from a purely functional standpoint, it works very well.)

(explanation - why I bought one)
I 'collect' code practice oscillators ('CPO's) and have been curious about the MFJ-554 for quite some time ... finally my curiousity got the better of me and I purchased one.

The MFJ-554 works quite well ... it generates a very clean (i.e. sinusoidal) audio tone (NOT the raspy square-wave sound generated by an inexpensive 555 IC-based CPO) and it is equipped with a good audio amplifier and internal speaker.

(this part is inserted so that no one is confused by what is meant by a code practice oscillator)
The MFJ-554 is a code practice oscillator - NOT an electronic keyer circuit. The input jack on the front 'expects' to see an alternatingly open-and-closed circuit as comes from a 'straight' Morse key or a semi-automatic key (e.g. from a Vibroplex or similar). When there is no closed-circuit on the input jack the MFJ-554 is silent (i.e. no audio tone comes from the speaker). When the external key has its knob pressed (i.e. the circuit is closed) then the MFJ-554 generates an audio tone that is a pure sine wave (to the human ear it sounds very 'gentle'). As the operator alternately presses-and-releases the Morse key the MFJ-554 generates alternating audio tones that follow the pressing of the key.

MFJ is not shipping a printed manual with the unit ... instead there is a note directing the owner to down-load an electronic copy of the manual from the MFJ Web Site.

The MFJ-554 is in a sturdy, wrap-around, steel case. (Take out one sheet-metal screw on each side of the case and the 'clam-shell' design comes apart ... with the one caution being that the speaker is bolted to the top of the case and the electronics are bolted to the bottom of the case. When the case is being taken apart some caution needs to be observed as the two wires going from the circuit board to the speaker are not very long and not connectorized.)

The speaker is mounted so that it points out the top of the case through a series of slots. Working together - the audio amplifier and speaker do a good job projecting audio tones into the room.

The MFJ-554 is designed to be powered by an external low Voltage DC power supply with a 2.1 mm coaxial-jack on the rear of the case. MFJ specs the unit to operate from +9 to +12 VDC (center pin of the jack is positive). I've been running mine from a +12 Volt supply and there has been no evidence of the internal regulator (sticking my finger on it with the case open) overheating.

The only peculiarity I ran across during the check-out of the unit was when I hooked-up an external speaker (I have an old Skytec acoustically-resonant speaker specifically designed for listening to Morse signals). At first there was no sound coming from the speaker (I checked it on another CPO to confirm that it was functioning). MFJ has chosen not to provide a schematic (in the on-line manual) and I prefer having one when I begin trouble-shooting a circuit. I contacted MFJ's tech support and the guy I spoke with immediatly said that he would e-mail me a copy of the schematic ... I had it within just a few minutes. The MFJ-554 uses a TDA7052A IC for the audio amplifier (I searched on-line and got the apps guide) and noticed that it is of the bridge-tied load ('BTL') type - where there are two amplifiers on the output ... one connecting to one speaker lead and the other amplifier connected to the other speaker lead in a push-pull configuration. When I saw that something 'clicked' in my head and I took a close look at the external speaker connector (a 3.5 mm 'stereo jack' on the rear of the case). Whoever designed the MFJ-554 (in 1997) must have been thinking about the audio output being 'balanced' (from the push-pull architecture of the audio amplifier) as they wired-up the external speaker jack as would be done in a balanced audio application. Instead of the external speaker connector being wired for the typical stereo headset connector (with the tip and ring of the jack being for the left and right audio channels and the sleeve of the jack being the 'common') the designer of the MFJ-554 wired the external speaker jack with the audio amplifier outputs on the tip-and-ring connectors (and no connection to the sleeve connector) of the external speaker jack. That explained why my Skytec speaker didn't originally work when connected to the MFJ-554 ... it is wired as though it is 'one-side' of a typical stereo connector whereas the external speaker jack on the MFJ-554 is wired for an external speaker to be on the tip and sleeve connectors. Once I threw a test cable together, to adapt between the two wiring configurations, my Skytec Speaker 'sang' just fine (I was surprised at how good the MFJ-554's internal speaker sounded when compared to the Skytec).

All-in-all the MFJ-554 is a quite nice code practice oscillator with its price being its only limitation (in my opinion).
 


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