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

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

Post by CrazyFin »

Petrusse Excalibur 2002 that has Cybernet main bord PTBM131A4X

I have a small issue with the radio giving low white noise but still easy to hear even though volume is at zero and squelch is open.
If turn up the squelch the white noise disappears and if I open the squelch white noise is back with volume still at zero.
No matter if mode is AM, FM or SSB. White noise is still there.

I read at http://www.glodark.com/cb/ukrigs.htm that Cybernet PTBM134A0X chassis could have a similar problem giving sound when volume is at zero and that it is squelch setting dependent. Issue in that case is caused by C109 connected between base of squelch transistor Q14 and ground being open circuit.

I am wondering if that could be the case with my PTBM131A4X?

Looking at the schematics for PTBM134A0X and comparing it with PTBM131A4X I would guess that the equivalent of C109 on PTBM134A0X would be C147 on PTBM131A4X?

I removed C147 and measured it with my ESR meter and it looked ok (101 uF and ESR of 0.40ohm) but I replaced it with a new one and with 105 degree C / 16V spec.

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

Post by Mitch »

It sounds like it could be an earthing fault to me. A wire come adrift or a volume pot track issue. It's worth giving it a squirt of switch cleaner and checking the earth connectons (middle pin to ground at zero volume, about 8k ohms in centre). if the squelch cuts audio it's doing what it's supposed to though they can pop which is annoying at low vol, it switches the audio chip off completely when squelched, so no white noise. There is an online cybernet service manual online somewhere but the url escapes me, I have it in png, it's been awhile since I had a play. It could just be background noise from the audio chip.

EDIT: Found the link - http://www.zen96216.zen.co.uk/cb/cybernet-esm.htm

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

Post by CrazyFin »

@Mitch: Thanks for the link to the Cybernet Export Service Manual. :thumbup:
Had it already in my archives here so I was already looking through as well as the schematics for the PTBM131A4X (for example at http://www.cbtricks.com/radios/supersta ... 00_sch.jpg).

I checked volume pot (after squirting in some electronic cleaner) and measured middle pin to ground. I see about 6 kohm with pot in middle position, 0 ohm with pot in turned off position and about 50kohm with volume pot at max so the measurements seems correct according to the schematics which shows volume pot as a 50kohm pot.

I've tested with both running the radio by feeding in 13.8VDC from the DC input connector as well as using the built-in 230VAC PSU and no difference (except for a VERY VERY low hum being present when feeding in from AC but the hum is barely noticeable).

The white noise that I can hear with squelch open and volume at zero is not the same as the white noise when turning up the volume and the "normal" RX noise takes over the white noise volume level when I start to increase the volume. The "initial" white noise is also quite low but still enough high to think that one has not turned down the volume pot to zero.

It is almost like there is a certain level of white noise being produced in the AF circuit around AN7140 IC (when squelch is open).
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by Mitch »

I bet it makes a difference being in a big box with a front facing speaker. I've had a couple of radios with similar boards but they were mobile (Lafayetts 1800 & AFS805 Mk2), I always thought they had a harsh, louder sound to other sets, and used a 4" speaker. They used a different audio IC in earlier sets and with the AF pre-amp it may be just chip noise or could be electrolitic capacitors gone leaky, that happens after 30 odd years. I remember a long time ago, one of the CB dealers used to put an electrolitic cap on the back of the speaker terminals of home base rigs, a 2.2 or 4.7 uF I think, perhaps that damped the stray noise then?
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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 »

I've just been up in the loft and fired up my own 2002. Not that I've ever had it on before with the volume "off", but there is indeed a kind of hiss that's emitted, then the true white noise takes over as the volume goes up. The white noise is slightly different to the hiss as you described.
There is also a low-volume hum from the power supply, again not really noticable when listening at normal volumes.

It sounds like they could all be like it. I've never owned a Jumbo etc, but maybe they suffer the same "character issues".

Mine was fettled by Nick, "Black Spirit" for its previous owner and had some PIC work done on it, so if it'd been something bad, I'm sure Nick would've probably noticed and done something about it at the time.
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin »

@Mitch: I have done a complete re-cap in the audio section already as well as in most other parts of the radio. Replaced all electrolytic caps with higher voltage ratings as well as 105 degree C ratings instead of 85 degree C ratings.

@The Collector: Great, thanks for taking the time to test your Petrusse Excailbur 2002! :thumbup: Not so many of them out there... :shock:

I will try to fire up one of my Kraco 2555FM stations that were made for the Nordic countries back in the 80´s (PTBM058COX mainboard) and see if they have the same issue. My feeling is that these Petrusse 2002:s have somewhat slightly higher "hiss" / "background" white noise than most other radios.

Time to enjoy some Christmas time now though so I will be back in few days. :D :wave:

Hopefully we will be able to find out the root cause of this "issue". :thumbup:
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by Mitch »

The '058' board will be quieter (no AF pre-amp), but the squelch pop will be more noticable (there's a fix for that, but you'll get some white noise with the squelch on afterwards).
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by lbcomms »

Take it step by step to track it down:

1) Remove Q29, is the noise still there? If so, it can only be the audio power IC (IC6).

2) Replace Q29 and remove C151. If the noise disappeared in step 1 but is still there after removing C151, then Q29 is noisy.

If it is Q29, try a few (by tacking them onto the top of the board) and choose the quietest one. Or better still, replace it with a low noise transistor such as a BC109C - but you'll need to look at the datasheets as the pinouts are different.
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by Moreupsthandownz »

I use a Lafayette AFS-805Mk2 on pretty much a daily basis in the car, it shares the same board as the Petrusse.

I have just put it on with the volume at zero threshold and have no white noise through the speaker. It only appears logically as I increase the volume. Hope this gives you another datum to go by.

I echo Mitch’s comment about the audio being quite loud and harsh on the set, which makes it very sensitive to over deviation on some stations who are a little enthusiastic with their audio.
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin »

lbcomms wrote: 1) Remove Q29, is the noise still there? If so, it can only be the audio power IC (IC6).
Removing Q29 and noise is gone! :-)

lbcomms wrote: 2) Replace Q29 and remove C151. If the noise disappeared in step 1 but is still there after removing C151, then Q29 is noisy.
Putting back Q29 and now removing C151. Noise is still there so Q29 must indeed be noisy!


Time for Christmas dinner now but tomorrow I will go through my component storage and see what low noise transistors I have that can be used as a replacement (BC109C as you mentioned).

@lbcomms: Thank you very much for pointing me in this direction. :thumbup:

@Mitch: I have done the squelch pop mod on some of my PTBM058COX boards and it has really softened the squelch. I have seen two different mods circulating on the web and I did the one with an 16 (or was it 22) uf / 25V electrolytic cap across the collector and base of the squelch transistor Q33 (on the PTBM058COX board). Works really fine.
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin »

@lbcomms:
Alright, did not have any BC109C:s at home but I tested with BC549B and BC239C and unfortunately no success with those. Same noise.

Are the BC109C:s much lower in noise compared to BC549B and BC239C?
I will order some of the BC109C:s as well as some KSC1845.

I am a little bit concerned when comparing data between the original 2SC945 and for example BC109C and KSC1845.
Collector current for 2SC945 is 150mA and collector-emitter voltage is max 50V while for example BC109C has only 25V as C->E voltage (but it has 200mA collector current though).
KSC1845 has only 50mA as max Ic but 120V CE-voltage.

Other substitutes I am considering is the BC337 and 2N5210BU.

I guess BC109C will indeed be the best substitute for 2SC945 in this case?

Are there any other sources around the audio IC6 AN7140 that can cause noise?
I have replaced all electrolytic caps around this area with high quality and 105 degree C rated and higher voltage ones (10V has been replaced with 16V or 25V and 16V has been replaced with 25V)
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by lbcomms »

How it works (reply is below the pic):
Image

The low level audio from the microphone and volume control enters at "IN". C152 removes anything above audio frequencies and C151 blocks the base bias voltage from going back into the previous stages. The signal enters the base of Q29 and is amplified, leaving at the collector. C155 blocks the collector DC supply and passes the audio.

Q28 acts as a switch to enable or disable the audio path. If its base has more than about 0.5 volts present, it is a short circuit collector to emitter. This bypasses the audio from C155 to ground, killing all audio. With the squelch turned all the way down and during transmission there is no voltage on the base of Q28 and the transistor is open circuit, allowing the audio to pass to IC6 via R150.

The audio at that stage is a few hundred millivolts at best with an impedance of kilohms, not enough to drive a speaker. The signal is amplified to about 10 volts p-p at a few ohms impedance inside IC6. C162 then blocks the DC voltage that would otherwise burn out the speaker. From there (at "OUT") it goes to loudspeaker when receiving, and also feeds the AM and SSB modulators during transmission.

When current is drawn from the 12 volt supply, the voltage fluctuates a bit. If these fluctuations get into the preamp, they will be amplified and cause feedback. To prevent this from happening, a low pass filter (R149 and C156) blocks the fluctuations, giving Q29 a clean DC supply.

> I am a little bit concerned when comparing data between the original 2SC945 and for example BC109C
> Collector current for 2SC945 is 150mA and collector-emitter voltage is max 50V while for example
> BC109C has only 25V as C->E voltage (but it has 200mA collector current though).
> KSC1845 has only 50mA as max Ic but 120V CE-voltage.

The worst case scenario (i.e. if the transistor was shorted) would be 2.2mA in the above circuit.
12V divided by 5520 ohms (the 1000, 3300, 220 and 1000 ohm resistors in series) = 0.002174A, i.e 2.2 milliamps.
So even a 50mA transistor would be well within its ratings.

The C-E voltage is a maximum rating before the transistors break down, in a 12 volt circuit like this a 25V rated component will be well within its limits.

Both the collector current and the C-E voltage ratings on the datasheets are just the absolute maximum values before the component will break, similar to the speed rating of a car tyre. Components will be used in a circuit with values a lot lower than the maximums in most cases.

> Are there any other sources around the audio IC6 AN7140 that can cause noise?
Yes, but highly unlikely. It would be a very rare fault to have a resistor or ceramic capacitor go noisy.

The squelch gate transistor (Q28) can't be the culprit, in your previous test you found that removing Q29 made the noise go away.
If Q28 was the source of the noise, it would have still been present when Q29 was removed as Q28 connects to the circuit after Q29.
It could be intermittent though, remove Q28 for 100% certainty. This will disable the squelch control but let you know for certain that Q28 is not the source of the noise.

If C156 was bad, noise from the power supply rail will enter the preamp. But that would most likely cause a rumbling or "motorboat" sound, not a high frequency hissing.

Possibilities but extremely rare are the other resistors and capacitors that connect to Q28. We would see one or two of these a year at a shop that does 20 to 40 repairs a day. For a one off repair like this, it's easiest just to swap them out. But odds are swapping those out won't make any difference, those components are very reliable even in a 40 year old radio.

Other possibilities are broken foil traces or open solder joints on components such as the low pass filtering electrolytics.

It's a process of elimination to weed out the culprit...
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin »

@lbcomms: Thanks for a very good explanation of that part of the circuit. :thumbup:

So I did the following tests in that part of the circuit (with a BC239C in Q29):

Removed Q28 = Still noise so I put it back
Removed R150 = No noise, value was ok 2.2k but I replaced it with a new one. This must also conclude that AN7140 is not noisy? (Unless it gets noisy only with a signal on its input?)
Removed C155 = No noise, value was ok 47nF. Tested with 10nF but still noise so I put back the 47nF (did not have any 47nF ceramic caps in my storage)
Removed C157 = Still noise, value was ok 6.8nF but I replaced with a new one.
Removed R148 = No noise, value was ok 3.3.k but replaced with a new one.
Removed C153 = Still noise, value was ok 680pF but I replaced with a new one.
Removed R144 = No noise, value was ok 100k but I replaced with a new one.
Removed R149 = No noise and value was ok 1k. I even tested with feeding in 12.6VDC at the left of the R149 but then the noise came back.

This is sooooo strange! I have even resoldered all solder joints all over the board even though it looked pretty neat before I did it.

I guess I will have to wait for the BC109C:s to arrive.
According to datasheet for 2SC945 they have a typical noise figure of 4dB (max 6dB) at 100Hz with Vce = 6V
BC109C seems to have MAX 4dB (no typical NF given) according to this datasheet: https://www.mouser.se/datasheet/2/68/bc107-32245.pdf
Not so much difference according to datasheets but lets assume that the BC109C will have 2dB noise figure at 1kHz and the 2SC945 has 4dB noise figure at 1kHz, would this cause the noise I hear when squelch is open and volume at zero?

Noise is still there when I turn the squelch pot into PA mode as well.

I have uploaded to sound recordings.
The first one is with squelch open all the time. It starts with volume at zero and then I increase volume slowly and then back to zero.
The second one is where I FIRST have squelch closed and volume zero and then I open the squelch with volume still at zero. It sounds almost like I open the squelch AND have volume turned up a bit... :o

Ah! Another observation I did now:
If I close squelch and move up the volume to say 50% I can start to hear sound as if the squelch would be open...
So I removed Q28 again and simulated squelch closed by connecting R150/C155 intersection to ground and still sound when moving volume up to 50%. I can actually here the receiving signal noise already at like 20/25% volume... Almost like squelch does not ground audio path completely even though I now connect the R150/C155 to various pcb (and chassi) ground points...
This is most clear in SSB mode compared to AM and FM mode.

I am actually thinking of leaving this as it is now since this is not typically an issue... it is not often that one has the radio at zero volume and squelch open. Normal setting is "normal" volume" and squelch closed and in this situation the noise will not be there...

I do have both an analogue (Tektronix 2465A) and digital oscilloscope (Rigol DS2272A). Can I use it to try to trace down from where the noise comes in?
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by 163TM847 »

Out of curiosity have you tried lifting the centre wire from the volume control and connecting it to ground to see if the noise is gone, Ive had this problem with various Cybernet radios and it turned out to be the volume control damaged so with volume down it wasn't being grounded enough for volume to be fully off, Maybe something you could try to rule out bad volume control..
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - Low white noise even when volume is at zero BUT squelch open

Post by CrazyFin »

Black Spirit wrote: 26 Dec 2018, 20:25 Out of curiosity have you tried lifting the centre wire from the volume control and connecting it to ground to see if the noise is gone, Ive had this problem with various Cybernet radios and it turned out to be the volume control damaged so with volume down it wasn't being grounded enough for volume to be fully off, Maybe something you could try to rule out bad volume control..
If I lifted the centre wire and ground it the Petrusse is completely silent when squelch is closed but the same "hissing noise" (I would call it "normal" receive noise without antenna connected) is still there when squelch is open and volume at zero.

I fired up one of my HyGain V AFS (PTBM121D4X) and it actually has the same background noise when squelch is open and volume at zero BUT much lower than the Petrusse. However, there is NO sound at all even with volume at full and squelch closed. On the Petrusse I start to hear sound already at 20/25% volume even though squelch is fully closed and this happens even if I remove squelch transistor Q28 and ground that collector point to gound (= squelch closed). SSB mode is worst on the Petrusse.

I removed cables from volume pot and:
When I connect middle cable to ground it is silent until I open the squelch and the hissing/background noise is back.
When I connect middle cable to left pin (=0 ohm) and have squelch fully closed then I have so much sound that it is as if squelch is open...
I measure the pot as well and it shows 46kohm on the position equal to zero volume.
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