Troubleshooting mic amplifier circuit

[JKKDev]

Hey, I'm working on a somewhat simple unit this time.

It's an external house door bell/communication unit. The problem with it is that the sound level of the microphone is low on the internal phone (lower than what we remember it to be).

I've desoldered and checked all the capacitors and they are all within the spec. The mic and the physical hole for sound are clean.

Now I wonder if a deteriorated BJT could cause a lower volume level. Is this reasonable?

Below is the circuit diagram of this unit. Input connector goes to the distribution unit inside the building. The speaker is loud and clear so I don't think there is anything wrong with the distribution unit.


Thank you for any help and have a nice day/night

[Armadillo]

With the Mic not connected, measure the AC voltage at P2 Pin1 and Pin2, and DC voltage at v+ and v-.

[JKKDev]

With the Mic not connected, measure the AC voltage at P2 Pin1 and Pin2, and DC voltage at v+ and v-.

I'll try and do that tomorrow however here are a few measurements I did today:
-line voltage while no call is connected: 2VDC
-line voltage when call is connected: 4.65VDC
-line voltage when call is connected but the external unit is disconnected from said line: 5.66VDC - and a loud and clear buzz was heard on the connected phone
-phone line voltage when loud and clear connection is established (between two phones connected to the same distribution unit): ~5.2VDC - this one might not be applicable as it doesn't share circuitry with the external unit and the idle line is at 25VDC

What I believe based on this measurements is that the DC offset of the line voltage isn't high enough to properly amplify the mic. As the sound got louder when the external unit was disconnected (and the line voltage rose) I would assume this proves that the external unit is at fault or that the distribution unit's power supply for the DC offset deteriorated to the point that the length of the cable is too much for proper amplification. I will get a short cable to connect the unit closer to the distribution unit to see if the line voltage drop plays a significant role (I believe there is about 20m of cable between the outside unit and the distribution unit).

This is a Panasonic unit with proper Panasonic and Rubicon caps and as I said all that I've checked were still within spec. It is just about 20 years old so something had to give but I do hope I'll be able to restore it as everything else works like a charm

Is there an easy way to test out the mic itself? Is it reasonable to assume that the mic could have gone bad? If this is the case how can I find a compatible replacement?

Thank you for your help

[Armadillo]

You can connect your mic to the PC mic input with proper adapter, for example, record your sound and playback.
Probably also check your telephone line cords/extension wires by using a known good one for example.

[JKKDev]

You can connect your mic to the PC mic input with proper adapter, for example, record your sound and playback.
Probably also check your telephone line cords/extension wires by using a known good one for example.

If I understand you correctly you're suggesting spicing the mic to a 2.5mm jack and testing it on a pc? I'm not sure how I'd know if the mic deteriorated since it does still work even on the external unit it's just a lot quiter.

[StillTrying]

The mic looks like an electret type and they don't like any moisture at all.

[JKKDev]

The mic looks like an electret type and they don't like any moisture at all.

I don't think the moisture is the problem as the mic itself was wrapped in foil and caked with some sort of goo at the back. But I'll keep it in mind

With the Mic not connected, measure the AC voltage at P2 Pin1 and Pin2, and DC voltage at v+ and v-.

I was able to do some further measurements today:
- DC voltage across V+ and V- is about 3.3ishV
- DC voltage across AC side is 4.5ishV
- DC voltage on P2 is 4.67V
- shortening the line from distribution unit to the outside unit by a factor of 10 didn't make any difference on the line DC voltage

This tells me that the diode bridge is working (~0.6V drop per diode). The drop across half of the coil (which is ~50\$\Omega\$) would indicate a current of 5-10mA which is reasonable.

Summing everything together it appears the "AC" side of the diode bridge is fully working and the "+-" isn't working properly because the DC voltage on the line isn't high enough for proper amplification. I am leaning towards a conclusion that this unit is working properly but the DC voltage PSU in the distribution unit is degraded.

Does anyone see a flaw in my thinking?

Thank you for any help and have a good day/night

[StillTrying]

The DC voltages look they could be about right to me.
Have you checked the transistors are working, if there's 0.6V base to emitter, and around 1V collector to emitter they're most probably working.
The DC side is powered by the voltage on C1. Q2 and R8 are probably the components with the hardest life.

[JKKDev]

The DC voltages look they could be about right to me.
Have you checked the transistors are working, if there's 0.6V base to emitter, and around 1V collector to emitter they're most probably working.
The DC side is powered by the voltage on C1. Q2 and R8 are probably the components with the hardest life.

The base to emitter is 0.66V on both, collector to emitter on Q1 is 1.3V and on Q2 is 2.2V. I believe this is somewhat normal.
R8 is spot on the value (150\$\Omega\$).
Voltage on C1 is 2.5V. I would think that this value can only be changed by increasing the line voltage right?

[JKKDev]

After putting together a SPICE simulation of the external unit I've realised that the DC components of the voltages in the amplifier are as expected but the AC component is amplified from what appears to be 5mVAC on the mic to about 50mVAC on the output of the unit. 10x amplification seems a bit low. I'd expect somewhere in the range of 20x to 100x.

The transistors used are C3311. Since the unit is made by Panasonic I'd assume it's a 2SC3311A or similar.

Here are the values of the components (capacitors are all on the lower end of +-20% (a bit over 0.8uF, 8uF, and 80uF respectively) except C3 which is spot on, resistors are all spot on):


Can anyone comment on the amplification factor of this unit (what they'd expect to see in this case)?

Thank you

[SilverSolder]

Capacitor C4 is important for good AC amplification -  if it has dried up / lost capacitance, the DC voltages will all be correct but AC will be weak. 

Worth a try! :-)

[StillTrying]

"5mVAC on the mic to about 50mVAC on the output of the unit."

The output of that circuit will be a varying current through R8, as a way to get the DC supply and audio down the just 2 wires.
Plot the current through R8, but I don't know what the AC current variation should be.

Why didn't you draw it the way up I did in doorbell.gif, I don't like wrong way up analogue circuits.