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Author Topic: Valiant Audio Problem - Self Inflicted?  (Read 944 times)
KA4KOE
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Posts: 206


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« on: April 25, 2013, 01:05:19 PM »

I have a Johnson Valiant with low/distorted audio. When I first tried to wire up the EV638, something in the mic cord plug came loose, and the area underneath in the vicinity of R61/R62 and L44 got quite hot. Visually, I cannot see anything damaged.

The rig now exhibits the follow behavior:

1. Will work with an EV638 plugged in, but audio sounds distorted/low. The modulator current will swing full scale.
2. Will NOT work with a non-amp D104 plugged in. Modulation current does not swing at all.
3. Modulation idle current is around 50 mils.

HOWEVER,

So, I pretty sure this eliminates the choke L44, as it supplies 300V to other parts of the radio. T4 interstage tfmr is also assumed good.

Checked the secondary of the modulation transformer via two pins on the accessory socket and it appears ok per the VOM.

Not really sure why this problem arose other than someone mentioned an oscillating audio stage. Any suggestions on where I should look to troubleshoot the audio stages?

Any suggestions would be appreciated.

Thanks.

Philip



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KE3WD
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Posts: 5689




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« Reply #1 on: April 26, 2013, 07:08:06 AM »

...So, I pretty sure this eliminates the choke L44, as it supplies 300V to other parts of the radio.

Consider the possibility that a choke that has shorted windings may still be able to pass
dc voltage, but can no longer function as a frequency choke. 

73
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