President Grant DX PC-999 Noise ?

[Switch Kit]

This radio has been recapped , the power off the dual 1969 transistors is great and the alignment is right on the money . The problem is when you turn the radio on you hear static coming in with the volume turned all the way down with or without antenna or dummy load connected ? As you turn the volume up the incoming receive then appears above the static but is obviously effecting the receive .........any ideas on this one ? Thanks Switch

 

[sp5it]

Was noise present before recapping?

 

[Switch Kit]

Was noise present before recapping?

I honestly do not know ? The power is good , alignment and transmit is good ........on the dummy load you can't even rf gain or squelch the noise out , (although you can hear them working , just not taking the noise out) it's always there and the weird part is you can kind of hear a snap ,crackle ,popping sound within the noise , almost as if it's trying to receive something else ? that's about the best I can describe it . Thanks Sp5it ..........Switch

 

[DoctorAudio]

I have found that a failing audio chip can cause snapping/popping/white noise that's not affected by the volume or squelch. If it has a TA7222AP IC, lift pin 4 from the trace and check if the noise remains. If you still hear it, the audio IC needs to be replaced. If it goes away, it must be coming from a point in the audio chain that is between the squelch switch stage and the audio IC.

 

[sp5it]

Fucked recapping as well.

 

[Switch Kit]

Fucked recapping as well.

I did go over the caps and they all looked like they were correct . Failing audio chip ? interesting , this radio has the UPC 1182H ? It basically sounds like it's trying to receive when turned on (white noise) you can get above that noise with turning the volume up , it tends to knock out the snapping , crackling white noise not so bad during DX but local you can hear the noise much better when there is less noise being received . Almost sounds like bleed over scratch and there's nobody there 40 channels up or down whatever channel your on , the crackling sound seems to be off and on and some times louder at times.

 

[sp5it]

Take audio before UPC 1182H and apply to external amplifier. Than you'll know is it AF amp or not.
Mike

 

[DoctorAudio]

Hard to find a clean datasheet for the 1182H. On the one I did find it looks like pin 1 is the input. Lift pin 1 and check if both the receive audio goes away (verifying that pin 1 IS the input) and see if the popcorn noise remains. If the noise remains the IC is probably bad.
Once in awhile this problem can happen from the DC blocking cap on the output going to the speaker getting leaky. It's the largest electrolytic connected to the output, pin 5.

 

[Handy Andy]

The Heat sink tab on the device - the one that bolts it to the side of the radio, can get compromised by age and dirt, which - since the chassis is insulated from the case, it is NOT insulated from coax ground so it is possible that the heat sink tab and it's spacer to keep the tab from shorting to ground at any point - may have started to fail.

Just unsolder the chip from the board and slide it off and clean the heat sink goo and redo with with same white goo it had only fresh stuff (don't use grey) - the same type as if it was for CPU heat sink goo - white grease and reinstall it.

Another spot to look for in your radio is by the rear panel - if the board has been "strapped" for any reason - the white noise shows up because of a ground formed by that strap and the birdies formed from the IF sections.

Another one that I see a lot is from limiters that get clipped...

IF you have had mods done to the radio - anything that
can affect how the limiter works can affect how well the RX works.​

Now, I'm not saying you have done something wrong - but you are having "white noise" issues when the volume control is down all the way, and the noise then disappears when the volume goes up - something else is affecting the signal line by how it appears to the input of the audio amp.

You may be swamping out a noise by the volume pot moving pushing more signal into the audio amps' input line - so it masks the noise over with itself.

The way volume controls work is they apply signal Across to ground and split it off the rheostat (variable substrate) from the pot - so the load the signal sees is constant, but the other side is what retrieves anything and everything from that substrate. Even if the substrate is intact, the noise can come from another source you didn't expect - further downstream. towards the audio chip - or from an open lead on the potentiometer - most of the time a simple re-assemble is all that is needed to clear that up - but if that doesn't work... time to dig deeper.

You may want to locate your recapping work, there is one that arrives to the audio amp "buffer" a circuit used to amplify the signal. The cap keeps DC separated - so the low-noise level the Audio amp is, is not overriding the signals ambient noise level - you have extra noise coming in - so revisit the recapping work you did and you may need to review your volume control and it's mounting so you can verify those wires are properly routed.

To keep this short, if you have a radio with any mods that disables limiter, so even though the limiter is in there - that line that powers the limiter is also powered by both the TX and RX lines using diodes to steer power to keep the transistor on when in RX so you don't hear the alternator whine or a fluorescent lamp ballast buzzing away in your audio you're trying to find someone while you listen in.