[TenTec] Troubleshooting Argonaut 505 with low output

[Bob McGraw - K4TAX]

This is not at all meant to be critical of Neil and his power supply 
utilization, but more in reference to past comments about switching power 
supplies.  Frankly, I view things such as this which most likely and 
incorrectly gets into the ham community and off goes the misplaced 
statements and rumors that switching power supplies cause issues.  FYI - a 
linear supply of the same character would have behaved about the same.

Thanks Neil for bringing the topic to the discussion group and reporting the 
successful findings.

73
Bob, K4TAX





----- Original Message ----- 
From: "Neil Martinsen-Burrell" <nmb at wartburg.edu>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Friday, February 07, 2014 5:17 PM
Subject: Re: [TenTec] Troubleshooting Argonaut 505 with low output


> 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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