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Ten-Tec Argonaut 505 #278

Way back in 1987 or so I was studying for the Radio Amateur exam. I put a wanted ad up on the board at Hamrads in Buitenchracht Street, looking for a Ten-Tec Argonaut 509, mainly because my elmer Jacques ZS1PL had one and I figured it was a pretty cool rig.

Got a call from Robin Kenward, he had an Argonaut 505, which I ended up buying* -- including the Model 250 power supply and the Model 405 linear amplifier. I only recently acquired the MFJ-16010 ST antenna tuner, it's off eBay.

My excuse is that I was young -- when you're young you want to hot-rod your car, your radios, your toys, your guns... as you get older you realise that stock is very often more than adequate. So some of these mods are good, some of them should have been filed under "leave well enough alone".

The frequency counter is based on a design from 73 Magazine and uses the 74C926 four-digit counter / LED driver chip. This is a fairly basic design, and doesn't make provision for USB / LSB / CW offset, so it's off by ... a bit. These days of course one would use a PIC. I kept all the bits (somewhere...) and I might revert to the stock configuration sometime. I'd count this as a "bad mod".

* For R450, which was more then (3 Feb 1988) than it is now. I also got a CTE CT1800 (a copy of the Icom IC-2E) handheld from him, which I used for many years.

Little known fact: The 505 uses a PJ-068 microphone plug. The shaft is 0.206", not the standard 1/4". Many Argonauts have been modified to use the more common plug.

Good mods

  • The previous owner (or Ten-Tec, but it's not in the schematic) added a 6k8/470p low-pass filter to the microphone input and a 10n cap over the key input.
  • I replaced the phono antenna connectors with BNC on both the 505 and the 405.
  • I added a header to the speaker so that I can unplug it when working on the radio. Such convenience. Many win.
  • I added a reversed diode in parallel with C44 (the main capacitor at the input from the external power). In theory this will provide some protection against reversed power.
  • The volume control pot failed and I replaced it with a switched unit. Moved the on-off function from the function switch -- to me the volume control is a more natural place to turn the rig off.
  • I added two diodes back-to-back on the receiver side of the antenna relay. That was before I fitted the protected-gate BF981, but it can't hurt.
Speaker fitted with 0.1 header

"Meh" mods

RF MOSFET mod: BF981

I replaced the 40823 RF transistor with a BF981. Lower noise, you know? The BF891 has Idss = 4-25mA, as opposed to the 5-35mA for the 40823. I measured Id for my specific transistor and replaced R5 (270 ohm) with an 820 ohm resistor to drop Id from 5.4mA to about 2.1mA. In hindsight this was kind of stupid, the BF981 has its optimal noise figure at Id=10mA. This is however moot. Even at its worst, the BF981 noise figure is adequate for HF, even without the masses of QRM we have these days with plasma TVs and Compact Fluorescent Lights.

For the mixer I measured the voltage over R4, which was 2.6V for Id = 2.6mA. With a BF981 fitted, a 560 ohm resistor gave the same drain current.

Revisiting the RF MOSFET

Argonaut 509 front end Firstly, let's inspect the original schematic. Vg1 = 1.1V because of the R1/R2 resistive divider. Rs is 270 ohm. We know that Vgs(off) = -4V max and Idss = 5 to 35mA (manufacturing tolerances) for the 40823. See Siliconix AN102 "Biasing for Device Variations", specifically Figure 6. You will see that the load line presented by the 270 ohm resistor crosses the Y axis at Id = 4mA. This means that the transistor will be correctly biased for any Idss a specific 40823 might have (basically, choose Vg1/Rs < Idss(min)).

The BF981 has Vgs(off) of less than -2.5V (i.e. between 0 and -2.5V) and Idss = 4 - 25mA (curve from datasheet). In practice, the 270 ohm Rs will work just fine.

Av (gain) = gfs Rd. gfs is the Transconductance, a.k.a. Transfer Admittance yfs. For the 40823 gfs = 12mS and for the BF981 yfs = 14mS so the gain is comparable.

2015-08-22: Went back to 270 ohm, and now using a BF964N I got from eBay (I bought 50 x BF981s which seem to be duds). Id = 5.2mA measured.

BF981 Id curve

Interesting failure

Schematic for Argonaut 505 Audio Amplifier Audio Amp voltage table

When I recently exhumed my Argonaut, the audio was dead. I ended up measuring the voltages around the audio amplifier, and nothing made sense. The biasing was way off, which lead me to suspect Q3 but that wasn't it.

Audio output transistor Q4


I ended up removing and testing Q4 and Q5 -- both tested OK. Replaced them and the fault was gone. Bolted the transistors back down on the heatsink and the audio was gone again.

Turns out there's an internal short -- the 2N4105 base had shorted to the case. So for now, my one output transistor is hanging in the air (note: after a few tens of minutes, it will start to smoke, since there's no longer a thermal bond to the NTC. This messes up the biasing. Ask me how I know. (And BTW, after smoking quite a lot, the transistor is still OK. I'm guessing it's the silicon grease that generates the smoke)).

I couldn't easily find a replacement 2N4105/6, but it's easy to get AC176K/AC128K matched pairs on eBay. These are only rated for 1W, so we will have to see how it goes. Otherwise I might end up fitting BD139/140 silicon transistors in there (this will need resistor changes to fix the bias I'm sure).

Argonaut 505 crystal filter

Argonaut filter bandwidth plot with hand-written notes, dated 25/11/91

I have absolutely no recollection of what I did here. I found this plot in my Argonaut black book. It's my handwriting, and it looks like a Matlab (which I used extensively at the time) plot. The date is right at the end of the seven years I spent at the University of Stellenbosch, and I don't think I was ever again as bright as I was then, so there might actually be value here.

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