TheRadioBoard

Golf,
thanks for the tips - can see it now! The set itself is a vast improvement on all previous MK484 designs and once again, hats off to Dan for his efforts in drawing it all together. I can get 1638 Goanna and the rest (to varying degrees) with very little interference form other strong locals.

After about 8pm Queensland time, I can get another four or five DX stations. I got one just last night up above 1638, playing country music but couldn't hear the callsign - there are no english speaking locals that high up on the band here in Brisbane, so it was a genuine call. Am persevering with that one until I find the culprit...

In the meantime - here's the 'boxed' version - not quite finished yet, but on the way. Coloured pot knobs are: yellow - DC feed, red - Volume (when amp is added on) - blue - RF Gain. Switch and earphone jack will be added soon.



Austin Hellier

Nice! Are you using a transistor amp for the audio or direct out of the MK484 to the earphone?

Joe,
it's a little rough around the edges at the mo but I'm going to tidy it all up over the weekend. The set is unamped in these pics but after I've done it (poured rain onto my balcony last night - didn't get a chance) I'll post a couple more. The set works quite OK now and I'm looking forward to some good DX from it too - selectivity on locals is the best its ever been for an MK484 style receiver.

As for the change in casing, I had it all mounted on a wooden baseboard with a smoked perspex fron panel, but the current front panel had most of the right hardware, with only one pot value having to be changed and a ground plane intact. All I needed to do was swap over a half dozen components and add a little jumper wire and there it is!

Austin Hellier

Good on you Austin.

Glad the sets are working out for you and Joe.

73, Dan

Dan, Joe and others,
thanks for the comments. Can anyone tell me where I can find a decent downloadable dial scale (a legit one - I don't want to 'borrow' someone elses' work unless they've put it up for grabs).

Would like a nice dial scale when I add the switch and phone jack if I can get one in time - thanx!!

Austin Hellier

There's a bunch of scans of radio dials here; they are apparently free to use.

You can also make one up using a simple CAD program, or a graphics program. It's tedious, but the results from a laser printer are great.

Just an update, I've eliminated the 60Hz notch filter I brewed up as I cannot hear any of the choppiness anymore. I basically had too much audio gain after the MK484, so I was always keeping Vb below 0.6V, which really exaggerated the problem.

Austin, I'm curious to hear how your MK484 radio is pulling in the DX compared to your crystal sets.

I am going in too many different directions since returning to work after the new year to do much tinkering lately.

Joe

Joe,
sorry I've not been back to the board lately, it's just that the Brisbane floods got in the way... As for the MK484 receiver, I've not gotten back to that yet with the improvements I wanted to make, but did in fact create a makeshift one (pics to follow soon) using that iniquitous cardboard tube and pvc covered wire inductor - tut, tut, tut!

And it's not too bad either. I needed a quick fix for me to listen to the impending rise of the flood waters near my town, so I've dubbed this one the "Pre-Flood Radio". It kept me informed for some four days before I was forced to flee to the evac centre for another five days. And when I got back, it was still going - still pumping out news from 612 ABC about the flood...

Austin Hellier

Joe,
pic as promissed of the 'Pre Flood Radio'. Looks pretty rough, but is sporting the said coil, and the latest "scavenge" - a dual ganged 415pF tuner cap, with several small 'puffences' for the FM band.

Works a treat - picks up all 13 locals (best Rx at night) but a good strong 11 locals during daylight hours - MK484 driven but no BJT audio amp as yet.

Austin Hellier

double post - sorry

Excellent information, thanks for sharing all the research.

I made my first ZN414 radio about thirty years ago, and I made amplified and barefoot versions. A few months ago, I bought a couple of the new version TA7642 and this week I got around to building one into a circuit. I live about 16 miles from a 50KW station and about three miles from two low power 2kW AM stations. My radio has a smallish ferrite rod antenna, of about 5cm long, so it won't pick up as much signal as one twice as long.

I built it according to the resistor values shown on the Rapid Electronic data sheet with a 1k resistor connecting the chip's output lead to the positive voltage supply (passing through the series connected 70 ohm total headphone load). I used the usual 100k resistor between the output pin and the far end of the tank circuit (opposite end to the chip input.

I could tune the band and hear stations but the volume was VERY poor. I checked through the circuit with a magnifying glass but there was nothing wrong. The voltage on the output pin was 1.0 volts with the specified value resistor at a no signal point on the scale. While I was messing about with the arrangements looking for the cause of the unacceptably low volume, I connected a 72k resistor in parallel with R1 (the standard 100k resistor). The volume sprang up about ten fold and the radio performed well right across the range.

I am well aware that this is an unorthodox thing to do, but it works and although I can get faint breakthrough from a local 50kw station on 695KHZ when tuned to a 2kw station on 603 khz if I null out the 603 khz one by rotating the radio so it is off the end of the rod, at other times, I don't get any breakthrough.

I am wondering if by chance I picked up a faulty TA7642 which is behaving in an uncharacteristic manner over the needed value of the feedback resistor R1?

I also improved the audio quality by turning the single BC548 output transistor into a Darlington Pair. The original single BC548 had its collector and emitter connected across the +ive and -ive supply rails with the series connected 70 ohm headphone connected between the battery positive and the positive rail to the output transistor collector and the TA7642. Now the Darlington Pair is connected in its place. This was a real doddle of a modification and made a good increase in volume and sound quality at the cost of taking full volume power consumption from 2.5ma unmodified, to 5.5 or 6ma on a very loud station. All I had to do was lift the emitter of the old output transistor and connect it to the base of another BC548, soldering the emitter of the new one to the ground plane and its collector to the positive rail alongside its pal's collector.

Three photos here:

https://picasaweb.google.com/Volpoon/TA ... 9773654354

These were taken before the Darlington Pair Mod was added.

EDIT - ERROR WARNING!!!

The diagram below has a mistake in it. The base of the first transistor does not go straight to the power rail at all. It has a 100k resistor to the power rail... Sorry about that.



This is the original circuit that I used. I found it elsewhere:

G0BZB - wow, that’s really a compact layout. Nice.

Thanks for posting your results when you change the 100k resistor!

I vaguely remember Bruce Kizerian (ElmerDude) posting a bit of information on Rap ‘n Tap about the effects of changing that resistance. Unfortunately I can’t find a reference for you. I think it was back around ‘04 - ‘06. You might search Rap ‘n Tap under “MK484”, or “Bruce Kizerian” or “ElmerDude”.

Fiddling with the 100k resistor is certainly something I’ll try when I get a chance.

73, Dan

Thanks Dan. I am just experimenting with the circuit, and I am pretty sure that my need to swap the 100k resister indicates some sort of fault condition. Messing about with this value is likely to mess up the AGC and possibly the selectivity, so it may be interesting, but is probably not a good idea in a well working circuit, unless you like 'fiddling' for the sake of it. Mine was an act of desperation, really and it paid off for me, but might not for others.

I was talking to a mate of mine, G3XXQ (Len) this morning and he told me he used a radio based on the ZN414 in his workshop years ago. Instead of using a ferrite rod antenna / coil, he used an old 455khz IF transformer with the internal capacitor removed. He operated this with a local AM transmitter with a few feet of wire as the antenna. This would make a VERY compact layout possible, and I have a scrap AM radio board with a very nice looking Medium Wave oscillator coil on it. This is very near perfect for tuning the band, and if indeed a couple of feet of wire will work, I will try coupling one coil of the oscillator coil pack (it contains two coils, a tapped one and a shorter untapped one) to the chip and the other to the headphone lead via a 1n capacitor. This will maybe then act as the antenna if I earth the lower end of it. I'll need to fiddle about with this to get it right, but it is worth a try if it means an ultra compact radio, which I need really to carry around all day. Of course, I could rely on a fixed capacitor in the tunning circuit and vary the inductance of the coil by using the tuning slug. I only listen to two stations, one on 603khz and one on 693khz. The first one is three miles away and is broadcast at 2kw of output power, and the second is 16 miles away and comes out at 50kw.

This idea if it works will allow a board size around an inch square plus a battery. I won't need a polyvaricon tuning cap either, though I might use a 300pf compression trimmer which Len says he has a spare of. It all depends on how the oscillator coil tunes and whether I can get away with a couple of feet of wire as an antenna. I will revert to the 100k value for R1 as well, I'm sure my other chip has some anomaly.

OK - I removed the ferrite rod and coil and substituted the Medium Wave Oscillator coil as I mentioned above. It has a lower inductance than the normal coil and required an additional 100pf capacitor to be soldered across the tuned circuit to bring in the bottom half of the band where my stations of interest are located. It worked and would operate with satisfactory volume with a whip antenna made of a 2 foot long length of wire. However, when connected to the tank circuit directly, moving about alters the total inductance of the tuned circuit and re-tunes the frequency. I removed it and used the secondary winding of the oscillator coil. I grounded one end and attached the 2 foot length of wire to the other end. Once again it worked, but this time with a little less signal. Attaching a six foot length of wire in a vertical configuration brought in a large amount of signal and provided good volume on the stations of interest, but because of the residual 100pf capacitor I had soldered in and the 8pf minimum of my polyvaricon capacitor, I could not get far up the band.

If you wanted this radio as a fixed one in the home or workshop, it would be fine with this arrangement, but I want mine as a radio in my pocket, and since my body was constantly interacting with the short whip antenna, the signal would appear and disappear as I moved.

I removed the experimental set up and reverted to the ferrite rod antenna and coil.


Now I spent a couple of hours experimenting with Dan McGillis' pots idea from the top of the thread. I had a 50k and a 4.7k board mounted pot available and placed these as he describes for his 10k and 500k pots. I could take the radio from the point of oscillation by increasing the voltage at POINT A (see his diagrams at the top of the thread) and found for my radio the best voltage at Point A was about 0.975v. I have no problems at any setting with overload so selectivity doesn't seem affected by varying the settings. This will be because of a combination of my own local situation and the fact that my ferrite rod is only about 4.5 cm long and probably can't provide enough signal to overload the chip anyway.

On the matter of the 100k feedback resistor; I had already reduced this to 37k to provide a good volume. I have removed the second pot in the feedback line and soldered in a 470 ohm resistor between the output and the input pins and have fantastic volume, great sensitivity and no overload issues. As I said in my first post, I may have an anomalous chip, but I certainly have a nice and very portable AM radio.

My latest meddlings have also reduced the overall power consumption. With my Darlington Pair output transistors, The power consumption was over 5ma on a strong station with an ear-full of volume. It is now 3.5ma since I changed the resistors.

One last thing - my Sony Headphones (Sony MDR-W24) have a built in 1k volume control. Without stripping the things down, it is hard to know exactly how the pot works, but sticking a multimeter across the plug it appears to simply add resistance to the headphone circuit between zero and 1K depending on how one sets the little wheel control. Using this pot on the headphone has the effect of reducing the voltage at POINT A and at all points downstream of there, since the phones are in series with the positive supply. When I apply the full 1k, the sound level goes down to become barely audible, but is at the kind of level you might listen to in bed, and the power consumption of the radio then falls to 0.35ma.


THE FINAL ARRANGEMENT

Very impressive! Would you consider swapping in a button battery for the cell, and a varactor + mini-pot for tuning? At 350-uA consumption (!), you also should be able to charge up a large-capacitance/low-voltage electrolytic (you know the type) from a solar cell, then run the radio all night.

BTW, my paternal ancestors were from Newcastle, and some relatives still live there, one of whom is a ham, but I've never met them.

73,

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