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 AWA 453-P Battery portable oscillator problems
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 Return to top of page · Post #: 1 · Written at 2:33:00 PM on 24 July 2016.
Simplex's Gravatar
 Location: Bathurst, NSW
 Member since 7 August 2008
 Member #: 336
 Postcount: 297

Having another attempt at getting this Dual band battery radio to work on the shortwave band. Operation on broadcast seems quite reasonable, all resistors and capacitors have been replaced.

Only the black AWA rectangular black tar silver micas in and around the osc and r.f coils have not been replaced.

Shortwave operation is virtually non existant except for a few strong stations at nightime. And then very, very faintly with a longwire aerial.

The mixer/osc is a 1R5 valve and have put in a 95% tested one, as well tried a couple of other known tested 1R5's.

There is a 1T4 R.F amplifier stage so by rights this radio should be a lively performer on both B/C and S/W.

At grid 1 (osc) have measured the following voltages using a digital multimeter which is the closest I have to a VTVM.

Shortwave lowest end of band 0v, middle of band -2.4v, upper end of band -5.6v

Broadcast lowest end of band -5.48v, middle of band -8.7v, upper end of band -10.94v

Even the broadcast band could do with an improvement at the lower end I think.

Have tried re aligning the oscillator trimmer caps with no improvement at all.

Any suggestions on how to sort out this problem would be welcomed.

The anode and screen voltages match up pretty well with the service sheet in the 453-P manual.


 
 Return to top of page · Post #: 2 · Written at 4:52:16 PM on 24 July 2016.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3786

I have never had trouble with the black encapsulated Mica caps not the later open wafer types. The valves can stop oscillating if they are tired, or have not got enough volts & that is critical on SW. The fact that BC is working does implicate components associated with SW.

The alignment is a critical phase and the oscillator trimmers only set the frequency and the issue is not that unless the oscillator is stopping?

As that sounds like it has a TRF stage before the mixer. and that is where the issue is liable to be, notwithstanding that SW stations are disappearing. The IF transformers should be set first with a Signal Generator with some semblance of accuracy. If too much signal is used & AGC cuts in, you will never get them right. Any adjustable padder should be set first at around 600KHz for BC band.

Then perhaps using WWVH 15MHz; 12MHz 10MHz or 2AY (1495) & 3SR (1260) get the oscillator trimmer/s set. Only after that lot has been sorted out do we bother with the antenna coils.

Normally their trimmers are adjusted with appropriate antenna connected. What I am suspecting with SW is that the two SW antenna coils are not on the same freq. If they are not, they will attenuate the signal.

Marc


 
 Return to top of page · Post #: 3 · Written at 8:24:05 PM on 24 July 2016.
Simplex's Gravatar
 Location: Bathurst, NSW
 Member since 7 August 2008
 Member #: 336
 Postcount: 297

Thanks for the observations so far and I should have mentioned the 1R5 is tested 95% and have a couple of others around 85% with good Ia, gm.

The voltages to the anode and screen are as per the maintenance sheet with a bit of variation due to my more sensitive multimeter.

Thing gets me is why the oscillator stops completely about midway across the band on S/W or lower.

Have aligned the I.F's ok but will re do the alignment again, re read the alignment instructions which are a bit hard to follow paying more attention to the front end alignment of the 1T4 r.f amp.

The bulky loop aerial on the back panel is a awkward thing to manover at times. Almost have to be a Houdini.

Agree a lot of S/W broadcasters have disappeared, still Radio Romania, BBC, VOA, scores of Chinese, and some others are still about which can make interesting listening.


 
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