Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

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CrazyFin
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin » 04 Jan 2019, 21:43

Ok so tested with removing the three 13.8VDC cables coming in to the main board at connection point 10 on the main board (near the audio IC 6).

Turned on radio and no white noise or hiss of course. Radio lights on but no other functionality since I have removed incoming 13.8VDC power supply to most of the parts of the RX section.

So I decided to feed 13.8VDC directly (from same linear PSU that feeds the radio on its main 13.8VDC input) and I DIRECTLY hear the white noise / hiss when volume is at zero but a tiny bit lower than normal but still very high.

This is with the radio SWITCHED OFF so I am now feeding 13.8VDC directly in on main board on PCB pin 10.

I guess this must conclude that noise is indeed generated somewhere in the audio section?

And now this: I remove C151 so volume pot should have no effect at all... BUT when I increase the volume to max the noise increases but this seems to be actual receive noise now. Not loud but still I am surprised to hear more than the white noise with C151 removed.
This is making me to think that there is some "leakage" of signal / noise through the AF ALC (Q32) or could it even be leaking noise from the DC Switch (Q31) section?
And how about Q30 (2SD837)?

I think I´ll start with removing Q31 and Q32 while in this "test mode" with 13.8VDC to PCB pin 10 removed and feeding it externally.

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin » 04 Jan 2019, 22:11

Ok, I really NEED to set up the AN7140 on a test board!

I have now removed Q32 (AF ALC transistor), Q31 (DC switch transistor), lifted of the pads for Q30 and even removed diodes D48 and D49 at the end of the AF ALC section and STILL white noise / hiss when feeding in 13.8VDC at PCB pin 10 (while C151 is removed and all the other cables on PCB pin 10 removed).

Ha ha this is getting really annoying... and probably in the end it is something VERY simple... :lol:

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin » 04 Jan 2019, 23:55

Ok so I built up a small test circuit and feeding it with a 9V battery to avoid ANY kind of ripple noise to come via PSU:
AN7140 test circuit - music input.JPG

And guess what? As soon as I enable battery power the white noise / hiss is there!! :roll:

I've even tested with the old AN7140 and both of the 2 that arrived today. Same noise with all of them.

It gets of course completely silent when I ground the input.

If I input a 1.2kHz test tone with 200 uVrms level then the tone is there nice but the white noise / hiss can is present and it does not disappear until I really increase the tone input level quite much.
In the picture above I am playing music from an old Android phone at its lowest output level and music sounds good but the white noise / hiss is there in the background...

So the question is if all AN7140 are very noisy and/or picking up noise very easily and amplifying it?

I need to dig into my radio inventory and see if I have any more CB radios with AN7140 audio IC and see if they also suffer from the same problem. Most of my old CB:s in my storage has TA7205P as the audio amp IC instead of the AN7140 and I can't remember them having that white noise / hiss when volume is at zero... but I´ll test one of those radios tomorrow.

I've kind of given up now... :(
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin » 05 Jan 2019, 10:43

So this morning I fired up a HyGain V AFS that has PTBM121D4X as the mainboard and guess what... same white noise / hiss there as well when volume is at zero and squelch is open.

This board has the TA7205P as the audio chip so I guess these old audio chips used in Cybernet radios are just a little bit noisy...

So lets close this "case" now since most of the times a CB user will not have volume at zero and squelch fully open... :ugeek:

Let's focus on the other problem in the audio section:

My HyGain V AFS is completely silent when squelch is closed and I turn up the volume to 50% or even 100%. The Petrusse Excalibur 2002 starts to leak sound (for example a 27.235MHz AM 30% modulated 1.2kHz and -80dBm test tone on the antenna input) at already 25/30% volume even when squelch is FULLY closed. So the Petrusse has some kind of leakage of the audio signal even when squelch is fully turned on.

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by The Collector » 05 Jan 2019, 13:55

My old nan used to have some kind of leakage, but the rest of her worked ok ;)

Most of us used to listen to our CB's with the volume on low/normal and the squelch turned off anyway, so I'm used to having a bit background noise/hiss/hash. It sounds like you've got an OCD against poor little innocent white noise and need to have complete silence when no one's talking ;) The more modern radios like our little PMR things have auto-squelch and are silent when on stand-by, but I find it strange not to hear that familiar static-hiss, almost like I don't trust the radio is working properly if I can't hear it :)

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by lbcomms » 11 Jan 2019, 22:25

CrazyFin - Haven't forgotten you, been away for a week and will read / reply to your posts over the weekend.

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