Tristar 747 Alignment advice

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26fb1964
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Tristar 747 Alignment advice

Post by 26fb1964 »

Hi Chaps,I have a Tristar 747 that has a small issue on AM in that I don't get any positive swing.If I speak
into the mike in a low voice the modulation is ok ,however increase the volume of the voice a little and AM briefy cuts out.I have a feeling it may be an alignment issue but could do with some advice as to where to tweak.

Cheers
Phil
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Re: Tristar 747 Alignment advice

Post by koberk »

These big Cybernets use a special transistor to handle the AM, so I would suspect that this tranny has failed, and cannot modulate the carrier.
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Re: Tristar 747 Alignment advice

Post by bigpimp347 »

back the power down to about 4w turn the AMC up to nearly full and you should see some swing, but not a lot, upto maybe 10w or so if the power in the rig is all set right..
even then the limiter may still kick in and limit to about 7w..

got a Cobra 29GTL does it, swings from 3 to 10w then the limiter kicks in and limits for a short while, but i'm not chopping the limiter out as it's fine for the driver two pill.
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Re: Tristar 747 Alignment advice

Post by koberk »

Best use a Modulation meter, and make sure the Mod never exceeds 100%.
Over-modulated transmitted audio sounds distorted and leads to interference on other channels, as I'm sure you know.
This 'swing' business is a meaningless S-Meter phenomenon.
Power and modulation can only be accurately measured by using a meter or 'scope.
How much power does your Tristar produce on FM?
26fb1964
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Re: Tristar 747 Alignment advice

Post by 26fb1964 »

Thanks for all the replies, gives me something to work on.The output on FM is about 12w which seems a little high to me for this type of radio.It possibly may have been modded by someone in the past although looking inside the radio everything looks original and not abused.

Regards
Phil
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Re: Tristar 747 Alignment advice

Post by koberk »

12W on FM is standard on a HyGain V, and is an easy mod for the 747.
Basically it by-passes the Darlington Transistor I
was alluding to previously.
Doesn't half give the TX strip a hard time tho but.
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Re: Tristar 747 Alignment advice

Post by Dragonfly »

I had a 747 aligned by an engineer years ago and when i collected it from the shop he told me that it is a cracking radio and doing 16 wats on FM .
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bigpimp347
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Re: Tristar 747 Alignment advice

Post by bigpimp347 »

Dragonfly wrote:I had a 747 aligned by an engineer years ago and when i collected it from the shop he told me that it is a cracking radio and doing 16 wats on FM .

screaming to save it's life after the 18w maximum transistors were being screwed to death :)

i never realised that hy-gains and other similar rigs had the FM maxed out, yet AM was variable..
which seems a pointless act, but a simple mod you can make the FM variable off the AM VR (i had to hassle nick BS for the info on it)

now my hy-gain boarded uniden 300 is 10w and 4w switchable on FM :)
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163TM847
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Re: Tristar 747 Alignment advice

Post by 163TM847 »

Dragonfly wrote:I had a 747 aligned by an engineer years ago and when i collected it from the shop he told me that it is a cracking radio and doing 16 wats on FM .
You would not believe how many Hygain 2795 and Tristar 747's i have seen where the enameled wire around L12 has cooked and output dropped to almost nothing because of this , Not alot of people notice the L12 problem and go straight to the final and wonder why it still wont work after its been replaced.

Regulating the FM output on the 747 is dead simple, It only requires moving one wire on the mode switch, The Hygain 2795 is a little more tricky.
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Re: Tristar 747 Alignment advice

Post by koberk »

Oooh! 16W continuous on FM is probably beyond the
capacity of the standard heat-sinks to preserve the O/P
transistors. The coils will also suffer from heat stress,
and I bet the enamel will break down...
16W yes, but not for long, I'll wager. :^(
I used mod Concorde 2 rigs to give 10-12W,
but set their low-power to 1W, for local traffic.
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Re: Tristar 747 Alignment advice

Post by Dragonfly »

The big Cybernet radios that i have had always stayed cool at the back plate but i was never a music player so probably never tested my radios to destruction .
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Re: Tristar 747 Alignment advice

Post by Dragonfly »

On the AM problem i seem to recall that if the AM power is turned up to high the modulation suffers .
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Re: Tristar 747 Alignment advice

Post by koberk »

Heat-sink is down one side...
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Re: Tristar 747 Alignment advice

Post by bozzy »

Sorry to bring up an old post, but does anyone have details on the wire that can be moved on the mode switch as below?

I have just got my 747 running but want to reduce the FM power, it is too high in my opinion.

Regards
Bozzy
163TM847 wrote: 22 Aug 2012, 10:48
Dragonfly wrote:I had a 747 aligned by an engineer years ago and when i collected it from the shop he told me that it is a cracking radio and doing 16 wats on FM .
You would not believe how many Hygain 2795 and Tristar 747's i have seen where the enameled wire around L12 has cooked and output dropped to almost nothing because of this , Not alot of people notice the L12 problem and go straight to the final and wonder why it still wont work after its been replaced.

Regulating the FM output on the 747 is dead simple, It only requires moving one wire on the mode switch, The Hygain 2795 is a little more tricky.
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Re: Tristar 747 Alignment advice

Post by bozzy »

Forget the above, I managed to work it out as below picture for others that may want to do this.

Basically remove the green link wire between FM and LSB,

Then link from AM to FM.

Regards
Bozzy
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Last edited by bozzy on 04 Aug 2021, 23:09, edited 1 time in total.
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