[TenTec] Troubleshooting Argonaut 505 with low output

[Neil Martinsen-Burrell]

The elevator is aligned properly so that both receive and transmit peak at
the same point.

Thanks to everyone for the other troubleshooting tips. The problem turned
out to be much more prosaic: low power supply voltage. Using a 12V
switching power supply from an old laptop, I was only getting 5.5V peak at
the antenna jack. However, the supply only measured 11.5V. With an SLA
battery partly charged at 13.0V I got a much more reasonable 12V peak
output for a power of 1.4W which is at least in the ballpark for this
radio.

Thanks again for the advice, including checking all parts of the station
before blaming the old radio.

-Neil KD0UKC
On Feb 5, 2014 10:49 AM, "Rick - DJ0IP / NJ0IP" <Rick at dj0ip.de> wrote:

> Neil,
>
> The Argonaut 505 did not have a broadband driver.
> It has a tuned circuit that is also tuned by the elevator/pre-selector
> control - the top left knob.
> You have to tune it for maximum during transmit.
>
> PROBLEM:  You also tune that same control to peak the reception.  If the
> radio is not correctly aligned, the peak for maximum received signal does
> not coincide with the peak for max drive power to the final transistor.  So
> if you peak it for best RX performance, your drive level (and output power)
> will be low.
>
> If you did not peak this control during your first attempt on the air, try
> again and this time place the radio in TUNE (or key the Morse code key) and
> rotate the top left knob while watching the output power.
> As I recall, you can read output power directly from the radio's meter.
>
> It may also be that the radio's output transistor is blown.  They are very
> cheap but the originals are no longer available.  You have to substitute
> the
> model.
>
> Here's what you can do.
> Wire up a #47 lightbulb to an RCA phono plug , center connection to center
> connection and the case of the bulb to the case of the plug.  Plug it into
> the antenna jack.  This makes an excellent dummy load AND output power
> indicator at the same time.
> Key the TX again and tweak the elevator for max brilliance on the light
> bulb.
> 3w will light it up pretty bright, but not quite full brightness.
>
> Keep us posted.
>
> 73 - Rick, DJ0IP
> (Nr. Frankfurt am Main)
>
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Neil
> Martinsen-Burrell
> Sent: Wednesday, February 05, 2014 3:25 PM
> To: tentec at contesting.com
> Subject: [TenTec] Troubleshooting Argonaut 505 with low output
>
> I went to try my first CW contact last night on the Argonaut 505 that I
> bought recently and when I measured the power output, I was surprised at
> just how low it was.  I measured 2.5 V across the diode detector built into
> my dummy load for a power output of between 60 and 130 mW depending on what
> I use for the forward voltage drop of the diode.  This seems even lower
> than
> it ought to be on this radio and I'd like to troubleshoot and hopefully
> repair it.
>
> I have the manual and have been getting familiar with the signal flow and
> the voltage measurements in there, but I haven't made any measurements
> inside the radio yet.  I did make sure that the output was peaked on the
> "resonate" control and that the CW drive was adjusted for full output.  I
> did find that if I turned the CW drive down below a certain level then I
> didn't get any voltage reading on the diode detector.  The radio receives
> well, so I believe that the VFO is working.
>
> I am looking for advice about the general approach to this sort of
> troubleshooting.  I have an oscilloscope and a DVM and I imagine that I
> will
> be following the signal flow trying to find the block that is not
> functioning.  Should I start at the beginning of the chain with the
> oscillator or at the end with the RF power amplifier board (90195)?  Are
> there particular components or assemblies that I should be suspicious of on
> this radio?
>
> Thanks in advance for any help you can offer.
>
> -Neil KD0UKC
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