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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 28th Aug 2019, 9:33 pm   #1
G4YVM David
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Default Tentec Argonaut 515 restoration

well...

I've had a play with the tentec...it receives and transmits perfectly except that it's down on power...i think from cursory checks its giving a few hundred miliwatts rather than five watts. It has many faults and I suspect dry joints, which might be the cause of the low power too...unless the finals are fried.

It's an interesting radio...a nice receiver but the feel and quality of the American product is dreadful. It feels.like a toy. And this from the 80s I believe...still, cultural differences I suppose.

David
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Old 30th Aug 2019, 10:44 pm   #2
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Default Re: Tentec Argonaut 515 restoration

I had one of these, bought new (serial #557). I agree completely about the flimsy build quality.
Mine was OK on receive but a bit deaf. This turned out to be a poor (but not dead) MC1496 receive mixer. It's about the last thing I checked - after chasing my tail for months, A new mixer brought it up by about 10-12dB. It was still a bit below spec on 10, but usable (0.5uV for 10dB S+N/N, handbook figure of 0.35uV).

I had low output also, following a replacement of both MRF476 PA transistors which cooked themselves one night (no idea why). I set it up to the standing current recommendations in the handbook (10mA). That turned out to be flat wrong. I couldn't get anything like a decent output. Eventually I discovered that the correct figure should have been 30-40 mA. Adjusted PA bias to 35mA and full output restored.
The moral to that tale is - beware of TenTec handbooks. There's quite a lot of mistakes, and I've been bitten by this more than once since then.

There's a build problem which hits most of these - VFO instability. I was getting quite large frequency jumps and warble - apparently mechanical rather than electronic in source. This turned out - eventually - to be lots of poor grounding paths caused by the use of small pop rivets to join chassis components together, rather than good honest nuts and bolts. The rivets didn't give a good low resistance connection in the first place, and a few years of surface oxidation just finished the job off.
The fix : Drill these rivets out and bolt the joint up good and solid. (I wrote this up in detail in SPRAT 98 (spring 1999)).
I'll bet at least some of your problems are caused by this chassis grounding thing. I didn't have any problems with dry joints, though.

All in all, I was a bit disappointed by the Argonaut. My faith in Ten Tec was restored when I replaced it by a Corsair 2, which is a magnificent machine . Very different price range and market though, to be fair.
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Old 1st Sep 2019, 5:38 pm   #3
G4YVM David
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Default Re: Tentec Argonaut 515 restoration

Thanks John, some handy tips there.

Well one of my mrf476s is deceased so I'm replacing them both with 2sc1969s. The boarding resistor, R1, is fried black...whether the cause of it caused by the dead 476 I don't know. Thanks for the tip on the bias current.

I'll let you know how I get on

D
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Old 7th Sep 2019, 9:27 pm   #4
G4YVM David
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Default Re: Tentec Argonaut 515 restoration

Sorted. I fitted new 2sc1969s amd it now gives me 7 watts. A bit low for the devices do maybe the drive is down or the bias. Another job for another day
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