Heath GC-1A Mohican alignment.....fixed

[majoco]

Hi guys - I built one of these back when I was in the Merchant Navy in the early 60's. Went very well except often popped the front RF amp when I neglected to disconnect the wire that I ran up to the bridge deck and then started keying the main antenna with 700watts or so!

Anyway, I sold that one when I left the sea. One came available locally for a modest sum in "working" condition so I grabbed it. Replaced electro's and a good clean up and goes well, although not up to the modern stuff.

But I have a problem with the highest frequency band - it won't align. The handbook says that the oscillator freq. is below the signal freq and I found it crossed up - Ok at the low end 20Mhz but reversed at the 30Mhz end - dead in the middle of course. Using a digital receiver I set the osc slug to 21.545 with the dial set to 22Mhz - then tried to set the osc to 29.545 with the dial on 30Mhz but the trimmer wound right out. Tweaking again back and forth makes no difference, then noticed that the osc stops going below 21Mhz!

Also noticed that the two slugs for the RF amp are wound right out for max response - there's definitely something funny going on here!

Anyone who has played with one recently and has any information it would be gratefully received.

Thanks
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Regards - Marty ZL2MC

[majoco]

Ok guys - stop fretting - I fixed it!

A 33pF silver mica cap in the feedback loop of the oscillator had a crack in it which was not readily apparent until I unsoldered one end and it fell off! Aha - I thought - this may be a clue!

Temporarily replaced with a ceramic and seems OK - have to go shopping tomorrow.

Having re-aligned the whole beast now, I must reinforce what Dave Doughty said back in 2007 - carry out the Heath alignment instructions to the letter - not a job for the faint-hearted!
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Regards - Marty ZL2MC

[Ed in SoDak]

Hi Marty,

Glad you got it sorted out and thanks for posting the cause. A good thing to watch for that one would normally assume to be infallible.
-Ed

[radiorich]

Hello majoco,
few years ago while cleaning out a basement of house that was going to be tore down I found one of those old .

Heath GC-1A Mohican's sitting in a old metal looker .
So when I get time I'll restore it .
Rich
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[majoco]

After getting it all going as above, I "modernised" the RF stage.

(AAAAH , a heretic, burning at the stake, etc etc )

I replaced the 2N1396 germanium with an ordinary 2N3907 silicon transistor (just plugs in to a socket) and added 100k across the original 68k base bias - now more sensitive and less noisy. May treat the IF strip the same - every mod will be 'reversable'!

Also the BFO injection was too high for weak signals, so removed the 4.7pF cap and replaced with a twisted gimmick - much better.

Also noted that the UK version has a low impedance antenna socket with an extra winding on the antenna coils - would be handy for desktop use.
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Regards - Marty ZL2MC

[Dave Doughty]

I'm wondering if the AGC action was affected when receiving strong signals after the RF upgrade.

Dave

[majoco]

Hi Dave - it's actually better - with the Ge transistor on local BC stations, the signal strength meter would go right up to '10' on the scale and then the audio would sound distorted. Backing off the RF gain improved the audio. I guess this means that the RF stage was actually driven into cutoff. With no signal, the meter sat on 3 to 4.

Now with the Si, the meter stays on scale and with no signal, the meter has dropped right down to 1.

Of course, your mileage may vary. I didn't select the 2N3907, just grabbed it from the junkbox!
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Regards - Marty ZL2MC

[OZ132HOME]

I like it, Marty. Can't help doing mods myself. try to keep them easily reversible for the Purist who may follow...
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-OZ