Monday, April 13, 2009

DC20B disappointment

I completed the build of the DC20B QRP transceiver yesterday and today, but I am pretty unimpressed with the whole experience. It's true that I am probably hard to please at the moment anyway due to being in a foul mood due to back pain. But even so I doubt if I would think the kit was really worth the money.

The DC20B got off to a bad start due to a part being missing, causing me to have to make a 50 mile round trip to Maplins to remedy the deficiency. After obtaining the missing part I got as far as the initial switch-on stage, but when I tried a key to see if the keyer was working I noticed the first problem which was that the sidetone was at ear-splitting volume. There is no control for setting the level. In fact, the DCxxB transceivers have no volume control at all, which for a direct conversion transceiver with no AGC is probably rather a disadvantage.

This morning I completed the setting up, and found that the transmit frequency, which is supposed to be the 20m QRP frequency, is in fact just above 14.061MHz. What's more, the receive offset, which is supposed to pull the crystal so that someone replying on your frequency can be heard with a tone of 600Hz, moves it more than 1KHz so the received tone is much too high. I managed to solve that by substituting a capacitor in series with the trimmer with a higher value. But I still don't know how to get the rig on to the correct frequency. It drifts a bit, too.

The receiver did peak up after I connected an antenna, but because it is receiving on a little-used frequency I can't tell what the sensitivity is like because I haven't heard anybody. The receiver is certainly a lot quieter than the sidetone!

Despite claims that the DC series transceivers have good immunity to broadcast band breakthrough, the bugbear of direct conversion receivers, I can certainly hear some. Well-known QRPer Richard, G3CWI, built the 30m version and considered it unusable for this reason.

Mounting the board and connectors into the nicely made custom case is a real fiddle, due to the fact that the space has been reduced to a minimum. If the case had been made just a couple of millimetres larger it would have made things a lot easier. I managed to gash my hand whilst trying to force the DC power connector into its hole in the case. There is something odd about this connector too, because the barrel type plugs on the cables that work with the K2 and MFJ Cub are stiff to insert and then make only an intermittent connection.

The keyer is a nice touch, even though I can't get the beacon to work (though the memory is too small to get a useful beacon message into it anyway.) The transmitter is giving me more than 2W out on a 13.8V supply, which is better than spec, and better than the MFJ Cub, though whether that is healthy for the PA transistor which has no heat sink is another matter.

All in all, this is one kit I wish I hadn't bothered to buy, and I suspect it will soon end up in the junk box.


ddv22 said...

Je réponds par le traducteur Anglais > Français Désolé
Mon Anglais n'est pas bon
Je viens de finir la construction du DC30B , et il y a beaucoup
de problémes ...
je dois refaire le T1 avec du fil plus solide , l'isolant est
est fragile sur les fils fournis avec le Kit , pour le secondaire , j'ai mis du fil plus gros
Les capas CT2 sont fragile , il faut les remplacer , et l'accord
en RX n'est pas bon, je pense que je vais refaire peut être T2
et les capas parallèles , l'accord n'est pas bon
Il faur rajouter peut être une self et une capa avant et après le Quarts et mettre un support pour changer rapidement le quarts !
Beaucoup de modifications , peut être un potetiometre pour règler
le gain sur l'entrée de T2 ou ailleurs
Je reviendrais sur le forum si je peux lire en Français
73s a tous
Jean Paul F6DDV

ddv22 said...

J'ai oublié
J'ai soudé une résistance de 100 Kohm a l'emplacement libre
appelé R221 pour le "side tone" trop fort

ddv22 said...

Suite , Condensateur de 47 pF enlevé sur self du RX et remplacement de C1 par un 35 pF ! accord sur la bande a mi-course
sur le quarts il faut enlever CT2 et le 47 pF et mettre un CV de 80
pF en facade avec un trou dans la boite , + une self de 15 mH en série , et peut être que le Kit est meilleur ...

ddv22 said...

Aujourd'hui après avoir fait toutes ces modifications je trouve ce petit transceiver très bon, j'ai fait beaucoup de QSO , je regrette que le kit soit arrêté , je cherche un DC20B
Adresse OK sur QRZ.COM