The Alda 103 “A” Tranceiver CW mode


Figure 1: CW oscillator level control on the Alda 103 (see text).

I had been operating the Alda 103 CW/SSB triband ham radio transceiver, a gem from the seventies, for a couple weeks on the SSB end of forty meters. I’d received nothing but good reports, such as “very clear and clean” – and “an easy to listen to signal” since I’d resolved an issue with the microphone input impedance, which on the Alda is slighty different than *any* other amateur transceiver. This was done by putting a Triad audio impedance transformer between the Alda and the Shure 414A Hi-Z microphone.

The Alda’s microphone input circuit is low/medium impedance, and it requires a high output microphone. This is why the regular, standard, 600 ohm (Icom, Yaesu, Kenwood, etc) low impedance microphones do not work with it. The original microphone supplied with the Alda was the Mura DX-114. It was relabeled for the Alda, and it had an impedance in the low k-ohms range. I used the AC VTVM method, along with a known audio signal source, to compute the impedance of my old Mura microphone at about 3.75 kohms (1 KHz source). However; its output was near nil. It was a microphone known to lose its output with age. With the transformer and the 414A Shure mic, the Alda could really do the walk. The Shure 414A is not very easy to find, but is less of an unobtainium item than the Mura. I happened to have one because it works well with the Ten Tec Omni.

Having fun enough with the local short skip contacts, I managed to snag a couple DX stations along the way as well. Yet, on CW, there remained a slight issue to work out before I would put the radio to the test there.

I am witness to the fact that the Alda 103 can put out a very nice, clean, CW signal, with no chirping or clicking. This required a simple adjustment. So, the Alda 103 transceiver that I have in my possession indicated a little alley-oop (chirp) on the highest band. I remember that in the early eighties, when there were more Alda radios still in service, that they often did have a little chirp. I believe the reason for what I heard is simple – and is the result of a CW oscillator level that was set incorrectly.  In any case mine was set too high.

In figure one can be seen a close-up of the Alda’s PC-201 board, and in particular the CW level potentiometer.  The original position can be seen to be set at about nine-thirty.  This control needs to be set to a point where the oscillator just barely starts.  It’s at that point where the waveform will look good, and there will be no chirp.  On my unit, that level was between the 11 oclock and 11:30 positions.  The component number is P-202. It is labeled “sidetone” because it is also used for the CW monitor. This is nice because you can actually hear any “alley oop” in your signal if there is mis-adjustment, versus an independent sidetone that doesn’t derive from the actual signal.

The direction of travel is counter intuitive, unless you’ve looked at the schematic.  One must turn the pot clockwise to the point where the oscillator stops, then counter-clockwise until it barely starts.  Since the oscillator itself runs all the time, and a buffer is keyed, this setting results in a good waveform on CW.

board201-on-alda-cw-osc-level-1Figure 2: PC-201 board on the Alda 103

The PC-201 board is the “messiest” of the bunch.  The CW pot can be seen on the left side of the photo, behind the electrolytic cap.  It requires a board pull because it’s mounted flush on the PCB, unlike the other controls to the right of it, which are mounted at 90 degrees.


Figure 3: PC-201 board from the Alda (showing the edge connector)

Most of the pots on the Alda boards are easily accessible without removing the card/board.  Unfortunately, the PC-201 board must be pulled in order to adjust the oscillator level.  It has to be adjusted, put back into the slot, tested, pulled, adjusted again, re-inserted, pulled, etc.  It took about three rounds of this to get it tweaked. This procedure may explain why some operators never optimized their Alda 103 radios for CW.  Maybe the test procedure at the factory was amiss, but for my unit it could be a matter of the aging of the components, and therefore the slight shifting of component values.

The Alda has been a wonderful trip back to the days of Analog dials and simple discrete circuitry.  My particular unit can output 150 watts at 16 volts, but I prefer not to push the old girl that hard.  I’ve been running it nominally at about fifty watts to a dipole.  This morning I had a chat with a fellow in the Florida keys.  Too bad it’s not 80 degrees Fhrenheit here in central NC. Maybe in a month or two …  🙂

Addendum 11/13/2019:

Some amateurs have mentioned that in spite of careful adjustments of the CW level control, their  ALDA was still exhibiting some “pull” in the CW note.  I think I’ve discovered the reason for that problem.  I have always had the ALDA connected to a very well-tuned antenna and tuner setup.  Recently, I was playing with antennas and connected one that was not quite in perfect tune.  The CW note on the ALDA did seem to pull a little when I did that.  It was not real objectionable, but more than I would live with.  When switching back to the perfectly matched dummy load, the CW resumed a perfect tone.  This makes me think that the ALDA engineers always used a dummy load when they did their testing.  Anyway, there is some way that the loading of the output stage is affecting the CW oscillator.  Looking at the schematic, the reason is not immediately clear – although the ALDA is a very low parts count radio, so there’s less isolation between any part of it and any other part of it.  Until I discover the exact mechanism for the cause of this, it looks like I have to stick with perfectly tuned sky wires or SSB on the 103.   Stay tuned (is that a pun?)   – LOL.

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