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Old 2nd Dec 2023, 9:52 pm   #1
audiomm
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Default AWA ST964 Receiver

I have a NZ made AWA ST964 (AM Only) Receiver on the bench and have it running ..but the AM Radio section has a problem. The first coil in from the Aerial connection ( T1 on attached cct) is open so needless to say the radio wouldn't pull a hen off its nest.
A bit of history........the ST964 is basically an AWA ST96 with the AM4 Tuner built in. Earlier models e.g. ST953 was an ST95 with the AM3 Radio. I have the service manual for an AM3 but I have never seen a service manual for the AM4 Tuner or for that matter the ST964. I don't think one was ever released.
Coil is a Toko YXR 30823 and I reckon my chances of finding a replacement are likely close to zero. However, I wonder if maybe some of our NZ and Oz members might be able to help as this receiver was sold in both countries. Back in the day (c. mid '70's) AWA made AM Tuners with variable selectivity and these were extensively used in local broadcast stations for off air monitoring - they were that good. Any suggestions (polite ones ) greatly appreciated.
Thanks

Mike
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Old 2nd Dec 2023, 11:28 pm   #2
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Default Re: AWA ST964 Receiver

The attached snippet looks to be an FM tuner front end. Dual gate MOSFET RF amp, Dual gate MOSFET mixer, bipolar LO. Pretty standard fare.

David
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Old 3rd Dec 2023, 1:16 am   #3
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Default Re: AWA ST964 Receiver

It would be nice if it had an FM Tuner, but it ain't - it is solely AM.
FM never hit NZ until mid 1980's which should explain that away.
I've attached the full cct. diagram for the Tuner part and the advertising blurb for the ST964 that I got sometime last century.

Mike
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Old 3rd Dec 2023, 3:54 am   #4
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Default Re: AWA ST964 Receiver

If it's the primary of T1 which is open, then you could bodge an antenna connection through a small value capacitor to the tap on the secondary which is G1 of the RF amp MOSFET.

That MOSFET will be rather open to damage from voltage transients like connecting a static charged antenna or even touching the antenna. Some back to back fast diodes (Schottky) after another small capacitor on the way from the antenna connector, might help preserve things. But you'll need another C on the way to that tapping point becase it has DC bias on it.

There is a risk that something took out the primary winding, so it would then be likely that the first FET would be toast.

Definitely an unusual set!

Plan B might be to open that RF transformer and wind a new primary.

If it's the secondary that's open, then careful unwinding and counting turns will be needed to find the right tapping point.

David
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Old 3rd Dec 2023, 11:13 am   #5
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Default Re: AWA ST964 Receiver

It will be a 10mm coil and I think any tuner/receiver from that era would have an equivalent antenna transformer. Can you find a scrap donor unit? I have several scrappers here but that would cost a fortune in money and time to get to you. Here's the front end from something, probably LW+MW
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Old 3rd Dec 2023, 9:38 pm   #6
audiomm
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Default Re: AWA ST964 Receiver

Quote:
Originally Posted by Radio Wrangler View Post
If it's the primary of T1 which is open, then you could bodge an antenna connection through a small value capacitor to the tap on the secondary which is G1 of the RF amp MOSFET.

That MOSFET will be rather open to damage from voltage transients like connecting a static charged antenna or even touching the antenna. Some back to back fast diodes (Schottky) after another small capacitor on the way from the antenna connector, might help preserve things. But you'll need another C on the way to that tapping point becase it has DC bias on it.

There is a risk that something took out the primary winding, so it would then be likely that the first FET would be toast.

Definitely an unusual set!

Plan B might be to open that RF transformer and wind a new primary.

If it's the secondary that's open, then careful unwinding and counting turns will be needed to find the right tapping point.

David
Thanks for your thoughts. Yes I have considered doing a bodge, but would ideally like to replace that transformer T1(White colour code). The primary is open, the secondaries are OK. I'll be replacing the 40841 Dual FET too.
Plan B is not feasible as I have eyesight difficulties i.e Macular degeneration in one eye and those 10mm Toko coils are beyond my capabilities.. I am on the lookout for a 'parts mule' as this might be my best bet.
So I'm in no panic to resolve the fix - something will turn up -eventually

Mike
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Old 3rd Dec 2023, 10:52 pm   #7
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Default Re: AWA ST964 Receiver

I understand and sympathise. I've just had a 14 month wait for a cataract op with no useful vision in one eye and a second cataract beginning in the good eye. I have a list of things to tackle now I can see things again!

David
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