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Reviews For: BG5MVE Code Practice Oscillator (being sold on eBay)

Category: Ham Radio Education & Exam Prep Materials

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Review Summary For : BG5MVE Code Practice Oscillator (being sold on eBay)
Reviews: 1MSRP: 19.00 - 23.00 (varies slightly)
Description:
A pre-assembled code practice oscillator mounted in a small aluminum box (decent quality - extruded). Includes a built-in speaker (very small) and USB power cable.
Product is in production
More Info: http://
# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
0015
WB5AGF Rating: 2016-05-31
improved audio tone Time Owned: 0 to 3 months.
This past weekend I pulled out my Tektronix 465 Oscilloscope, heated up my almost-as-old Oryx soldering iron (made in the UK in the 1970s), got some paper and 0.9 mm Pentel mechanical pencil (held together with tape) .... and got-down-to-the-business of :

a.) determining more closely the schematic of the BG5MVE Code Practice Oscillator

and

b.) figuring out how to lower the distortion in the audio so that it sounds correct to my ears ( no square waves for me ! )


After several hours I am pleased to report reasonable success for both 'a' and 'b'.


One section of the 4558D IC is being run as a twin-T oscillator .... that then goes to a transistor that serves as the keyed 'gate' (controlled by the external Morse key) and from there into the second amp in the 4558D. The CPO's volume control works by controlling output of the second amp in the 4558D IC that is fed back to its inverting input (the more output that gets fed to the inverting input serves to reduce the output).

The audio amp in the BM5MVE CPO is a LM386 audio amplifier IC and even with its gain at minimum (20) there is WAY TOO MUCH signal available from the 4558D so there is an attenuating network on the input of the LM386 but it isn't enough (except that, as you will learn, having that excess gain turned-out to be useful).

The audio signal in the 4558D twin-T oscillator section looks to be fairly clean ... i.e. on-the-scope it look very close to sinusoidal. From what I could see there is some distortion visible on the audio once it's gated by the external transistor and is run into the second section of the 4558D (serving as a buffer. That same (slight) amount of distortion appears to be what is getting sent into the input of the LM386 audio amp.

What I did to provide a cleaner (i.e. more nearly sinusoidal) audio output from the BG5MVE :

1.) I added a capacitor to the negative feedback circuit that controls the output of the second amp in the 4558D. (none of what I did has any 'elegance' ... it was all based on a.) what parts were readily available and b.) did they work ?) The capacitor jumpers around the potentiometer serving as the volume control and my thought was that a) when the volume is at minimum (i.e. there is maximum negative feedback to the second stage amp in the 4558D) then nothing is heard (so it doesn't matter) but b.) when the volume is approaching maximum (and so any distortion will be more noticeable) then the potentioneter (serving as the volume control) is set to a high value (for reduced negative feedback) and so the added capacitor serves to feedback more of the higher-order harmonics (of the audio signal) than is fed back of the fundamental (the newly added capacitor is connected across the volume control potentiometer and so the higher order audio harmonics are more readily conveyed around the potentiometer than is the fundamental) and so this serves to reduce the output of the harmonics (relative to the fundamental). The oscilloscope appeared to show that this was indeed being accomplished.

2.) I put together a two-stage RC low-pass filter network on the input to the LM386 audio amp (I started out by just adding a series resistor to see if I had the correct 'range' of the resistance to significantly reduce the output of the LM386 and once I was satisfied then I began rummaging through my containers of capacitors. The 'logic' (once again) was to reduce the harmonics of the fundamental audio tone (and thus more close approach a pure sinusiodal signal). (This was where having excess audio from the 4558D (for inputting to the LM386) was useful .... it allowed me to add significant additional series resistance (in addition to the existing 'flat' attenuating resistance that was already present), necessary for the RC filter function, while still having adequate audio output from the LM386.)

When I finished I compared the output of the BG5MVE CPO (listening to it through my old 'Skytec' acoustically resonant CW speaker) to the audio available from an on-line test signal source.

At the conclusion of my weekend work I had the CPO putting out what sounded to be a suitably clean sinusoidal audio signal. (at that point I turned-off the Tektronix 'scope, unplugged the Oryx soldering iron to let it cool down, and began putting away all the stuff that had accumulated around my desk during the 'work session')

- Paul, WB5AGF



----------------------
Earlier 4-star review posted by WB5AGF on 2016-04-05

I 'collect' Morse straight keys (it just sort-of happened) and, as a side interest, I also watch for interesting code practice oscillators (CPOs).

( When I was a kid virtually every large electronics retailer in the U.S. sold a two-transistor CPO circuit board for about a dollar. It was the buyer's responsibility to provide a speaker, appropriate DC power, all the external wire and connectors, and to mount the unit in whatever was available (mine went into a cigar box donated by an uncle). )

As I write this there is currently (spring 2016) being sold on eBay (U.S.) a code practice oscillator for around 20 dollars (U.S.). The device is made in China and I decided to take-a-chance (spend 20 dollars) and see what I got-for-my-money. (Previously I've been unwilling to risk buying from the several inexpensive electronic boards being offered on eBay from China but I've heard good things about them from people who've bought them.)

The BG5MVE Code Practice Oscillator is being sold by several (as I write this there are seven sellers on eBay-US) vendors with addresses in China and Hong Kong (I believe all offer free shipping). They all appear to use the same photographs and the same 'ad copy' (i.e. written description). The written description is pretty rough to read but for something as simple as a CPO I felt reasonably certain what I'd be getting for my money.

With free shipping across the Pacific Ocean included in the price I accepted-the-fact up-front (before buying) that the package would ship as cheaply as possible (i.e. it would probably take awhile to arrive). Mine took three and one-half weeks to arrive (which was OK I thought).

The BG5MVE comes pre-assembled in a decent little extruded aluminum box (a circuit board is used for the 'face-plate' of the box). It has a tiny little speaker mounted in the circuit board face-plate and it sounds OK for its size (a connector on the face-plate allows for connecting an external speaker). To keep the price of the CPO down the device uses a USB cable to extract 5 Volts from the buyer's computer (or other device with a USB interface).

The BG5MVE code practice oscillator circuit works as follows (I couldn't find a schematic on-line for the design and had to do a rough analysis from a close inspection of the circuit board which uses all through-the-hole parts (i.e. no SMT devices).) : The audio is generated by an oscillator using a JRC 4558D IC (as best I can tell the keying is applied to the IC's second amplifier which is acting as a buffer). The audio is then fed to a LM386 M-82 audio amp (operating at its lowest gain of 20) and then on to the speaker.

Once I was convinced the CPO was working OK I began to work on upgrading it :

- Stand Alone Power; I purchased a USB power 'wall-wart' that allows me to power the device
directly from an AC outlet.

- A Better Speaker; from a hamfest flea-market purchase several years ago I have one of the Skytec acoustic resonant CW speakers (originally sold back in the 1970s). It REALLY helps.

- A cleaner, and more properly shaped, CW Tone; The BG5MVE CPO does have some audio leak-through .... i.e. when the CW key isn't being pressed there is a slight amount of tone still getting through to the speaker (on the tiny internal speaker it is virtually inaudible). I am presently working on an improved audio muting circuit and will probably implement one using a FET similar to the 2N7000 (Fry's Electronics carries the NTE490 FET and its characteristics appear suitable) that will both reduce the audio leak-through as well as provide a shaped rise-and-fall characteristic to the signal's leading and trailing edges.

Overall, for roughly 20 dollars (U.S.), I'd rate the BG5MVE as being OK.