PRC320 tx woes. - UK Vintage Radio Repair and Restoration Discussion Forum

thx1138 · 21st Mar 2022, 1:10 pm

Hi,
I have two 320s presenting identical faults. Weak almost no tx out, I can see carrier on spectrum analyser when I key on any modes (on HP it's a little stronger) , but no meter reflection or tunability. When I switch to ANT needle shifts to 3 o'clock, but no other reaction. If I attempt to tune. It's as if the final PA is not being switched in or something is o/c. Relays are clicking fine and PSU voltages are all ok, one PSU is recently rebuilt, so the obvious done. Also I checked the D connectors that they're engaging properly. Just both "backs" not kicking out decent RF.

Anyone point me to the obvious I'm not seeing.
I have wiring diagrams and the service manual for the set, but need a pointer to look in right place. any help appreciated.

Thanks, David.

Sparky67 · 21st Mar 2022, 5:34 pm

David,

Is there any RF output direct from the ‘radio’ side of the coax link on the rear panel? That will bypass any issues in the antenna tuner. The meter needle does still move on Tx if the link isn’t present.

Martin

thx1138 · 21st Mar 2022, 7:28 pm

Hi,
yes rf is v weak on t/r side. Also atu on both sets is OK , rx comesup as I tune to best resonance. Have checked also the caps on the switch on D&E and all is ok. VHF filter wiring also intact. I did a dry joint marathon reflow on both sets to no avail, but no harm.

So its beyond atu section. Am thinking c3 on rf board , have yet to tackle the tants but nearly every one I have done is either o/c or shorted. ...

21st Mar 2022, 1:10 pm	#1
thx1138 Tetrode Join Date: May 2007 Location: Bratislava, Slovakia Posts: 65	PRC320 tx woes. Hi, I have two 320s presenting identical faults. Weak almost no tx out, I can see carrier on spectrum analyser when I key on any modes (on HP it's a little stronger) , but no meter reflection or tunability. When I switch to ANT needle shifts to 3 o'clock, but no other reaction. If I attempt to tune. It's as if the final PA is not being switched in or something is o/c. Relays are clicking fine and PSU voltages are all ok, one PSU is recently rebuilt, so the obvious done. Also I checked the D connectors that they're engaging properly. Just both "backs" not kicking out decent RF. Anyone point me to the obvious I'm not seeing. I have wiring diagrams and the service manual for the set, but need a pointer to look in right place. any help appreciated. Thanks, David. Last edited by thx1138; 21st Mar 2022 at 1:18 pm.

21st Mar 2022, 5:34 pm	#2
Sparky67 Heptode Join Date: Feb 2008 Location: Great Barr, Sandwell, West Midlands, UK. Posts: 589	Re: PRC320 tx woes. David, Is there any RF output direct from the ‘radio’ side of the coax link on the rear panel? That will bypass any issues in the antenna tuner. The meter needle does still move on Tx if the link isn’t present. Martin

21st Mar 2022, 7:28 pm	#3
thx1138 Tetrode Join Date: May 2007 Location: Bratislava, Slovakia Posts: 65	Re: PRC320 tx woes. Hi, yes rf is v weak on t/r side. Also atu on both sets is OK , rx comesup as I tune to best resonance. Have checked also the caps on the switch on D&E and all is ok. VHF filter wiring also intact. I did a dry joint marathon reflow on both sets to no avail, but no harm. So its beyond atu section. Am thinking c3 on rf board , have yet to tackle the tants but nearly every one I have done is either o/c or shorted. ...