APX2000 background speaker whine issue

[Eaton]

Hi there!
I used to have a VHF Motorola APX4000Li (M 1.5, tagless) from Ebay which worked like a charm.

A couple of days ago I was offered to swap it for the APX2000 M3 and this offer was too tasty to dismiss...
Here's what we have now:
Host R12.05.00
DSP R12.05.00
Secure R01.05.12
Model number H52KDH9PW7AN
Flashcode 9A206Q-J31E94-5 (oh yes, it's been whoreflashed, just like the above mentioned APX4000Li).
Genuine unit right from the official Motorola dealer with all tags and approvals.
Bluetooth is manually disabled, as well as GPS.

Now, here's what troubles me. When in a quiet place, I can hear a little constant whine from the radio speaker, as if an inverter cellphone charger is working somewhere. Now, with my desktop PC (or a kitchen fridge) running, I can't hear anything annoying. But in a quiet environment this whine really drives me crazy! I always hated these whining chinese power supplies and never expected such abug from a Motorola radio:wtf: The whine is still there even if I disable the internal speaker (Surveilance profile). there's no whine with the radio turned off.
Otherwise it works just fine!

Basically, APX4000 and APX2000 are the same radios intended for different markets. My previous APX4000Li didn't have a full-featured keypad board (no Bluetooth, no hardware encryption) and ran firmware R13.00.00. It has never exhibited such a whine!

Does anyone has any suggestions? Has anyone experienced that before?

Thanks in advance,
A

 

[duggerd]

I have not heard of this, but this sounds like a hardware fault.

If you turn the volume all of the way down (this is a single knob unit, right?), does it still make the same intensity of whine?

Edit: This is not normal behavior, I have 8 of those radios (APX4000 UHF and 800 flavors), none do this.

 

[Eaton]

Yes, the whine is there regardless of the volume level (anywhere from 0 to 15).

it might as well be not a speaker, but some hardware part whining. Unfortunately, I have no other radio to check it out, they are too rare as of yet here.

duggerd, which radios do you have? Full or limited keypad? What firmware do they run?

I've seen some MotoTRBO DP4400 with very old firmware do unwanted sound artifacts, but these effects were sporadic, not constant. Firmware upgrade to R02.30.13 always put an end to these inconveniences...

 

[duggerd]

Yeah, I would say that there is a problem with the hardware if it happens even if the volume is at min (even though minimum is not off, I have observed that you can still hear traffic when the volume is at the minimum setting).

I have most of them on R11 but one 800 and one UHF R1 are on the latest R14.00.03 FW. All of them are model III, but the only difference is that model 1.5 and 2 radios do not have as many buttons on the keypad board. The speaker and display assembly is the same (confirmed via service manuals). Some have MACE keypad boards / BT boards and some do not.

You can try entering service mode and see if it happens there.

Can you try bumping the FW up to the latest (if you have the CPS for it).

 

[Notarola]

sounds like the mute circuit is floating (bad cap) and allowing internal noise into the amp. try a full quieting signal with no modulation and see if the wine is still there. This will let you know if the noise is internal to the audio circuit or if its in the muting portion.

 

[Eaton]

Notarola, thank you for the input!
I've just tried applying an unmodulated signal and the whining stops for the time the radio is receiving it. As soon as the signal dissapears, the whine begins once again

How do I rectify it?

 

[Alpha]

Think of a guitar amplifier humming with no input, when you plug in an instrument the input is no longer floating, so the hum goes away.

It sounds like the audio circuits have a problem with the muting hardware. When there is no RX signal, it clamps the audio output (normally) so if there is any internal noise from the DC-DC converters, etc. on the audio lines, it will mute it. This can be done with an "enable" type signal to the Audio PA, or it can be done before the PA circuit by something like a 4066 CMOS SBS or even a transistor switch gating the signal to ground.

You will need to look at the muting hardware circuit to analyze it, but at least you have a go/no-go condition now to troubleshoot; you can put a scope on the speaker outputs of the Audio PA and you should see the "whine" audio present, coming and going when you provide receive signal. Then, look further back at the enable and power inputs to the PA and check if the whine signal is present (probably at much lower level due to the amplification of the PA). It is possible it's coming from the DC supply to the PA circuit, not on the input audio line - you need to figure out where it's coming from. You should check if the PA enable line is toggling on/off like it should, if that's how the circuit is muted. If it's muted further upstream, you can again trace back and find the "clamp" circuit and check the enable line going to it (the mute signal), and again, it should toggle on/off (or high/low) with the presence of the RX signal. If the audio going through it does not mute or clamp like it should, the gate is probably defective. You can test this easily by grounding the audio line momentarily with a probe and a .1 - 1 microfarad cap in series to ground to see if the noise goes away. This will route the AC to ground but not ground the line w/respect to DC in case there is a DC bias voltage present on the line.

A scope and a schematic are your best friends at this point, if you have the ability to component-level troubleshoot this one.

 

[Notarola]

Alphas has good advice. One futher check you can make is to see if the whine appears suddenly or if it 'ramps' up. if instantly then its a bad control line or internal to the chip. if it ramps up then look for a bad filter cap between the output loop of the AFIC and the audio amp.