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Old 3rd Aug 2021, 9:28 am   #1
vinrads
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Default AWA 38a Australian radio volume fault !

I have just rebuilt this radio, having painted the chassis, and it works which is always a relief, but it had this problem before, the volume is flat out, by bridging the 500k R12,with a 56k resistor I get it to a reasonable level and it's controllable. I read somewhere this is a reflex circuit. I can't get my head round the circuit and how it works.

Mick.
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Last edited by vinrads; 3rd Aug 2021 at 9:41 am.
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Old 3rd Aug 2021, 10:15 am   #2
ms660
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Default Re: AWA 38a Australian radio volume fault !

Quote:
Originally Posted by vinrads View Post
I read somewhere is is a reflex circuit , I cant get my head round the cct and how it works .
Yes, it's a reflex job for sure, the 2nd IF amplifier also functions as the AF amplifier, the output from the 3rd IF transformer is fed to the detector diode(s) in the usual way and the DC voltage produced is used as the AGC voltage to control the gain of the Mixer and 1st IF amplifier, the detected AF is fed back to the 2nd IF amplifiers control grid via the volume control, C29 and the secondary of the 2nd IF transformer T6, the amplified AF is developed across R17 and is fed to the control grid of the output valve via C35.

If that's any help.

Lawrence.

Last edited by ms660; 3rd Aug 2021 at 10:27 am. Reason: extra info and correcting an editing error
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Old 3rd Aug 2021, 1:20 pm   #3
vinrads
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Default Re: AWA 38a Australian radio volume fault !

Thanks Lawrence I have just re drawn the diagram , with your description it is making a bit more sense now ,I still don't understand why I have max volume , having said that it has been worked on before I think I need to go through the component layout , Cheers Mick.
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Old 6th Aug 2021, 10:31 am   #4
vinrads
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Default Re: AWA 38a Australian radio volume fault !

Having been round and round in circles ,chasing the odd fault I ordered a detector valve a 6b7 ,It arrived yesterday fitted it guess what didn't cure it ,still full volume and the control did next to nothing!, so in desperation I decided to do a re alignment 460 kc/s the third i.f tx was quite a way off , to my surprise it cured the fault , I guess something to do with the reflex cct I notice there is an gain switch at the front end which also now works , I never did like reflex circuits.
I will be doing a full write /up on restoring the chassis and cabinet on this radio . Mick.

Last edited by vinrads; 6th Aug 2021 at 10:36 am.
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Old 6th Aug 2021, 11:13 am   #5
joebog1
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Default Re: AWA 38a Australian radio volume fault !

ALL Australian radios used 455 kHz IF frequencies. I have never seen 460kHz here. JUST an observation.

Joe

P.S. Communications sets here had a variety of oscillator / IF frequencies, some above 30 MHz, some around 1.8 kHz, followed by the standard 455 kHz.

j.b.
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Old 6th Aug 2021, 11:32 am   #6
ms660
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Default Re: AWA 38a Australian radio volume fault !

Quote:
Originally Posted by joebog1 View Post
ALL Australian radios used 455 kHz IF frequencies. I have never seen 460kHz here. JUST an observation.

Joe

P.S. Communications sets here had a variety of oscillator / IF frequencies, some above 30 MHz, some around 1.8 kHz, followed by the standard 455 kHz.

j.b.
The AWA 38's IF is 460kHz in the service data:

https://www.kevinchant.com/uploads/7...8231/38_41.pdf

Lawrence.
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