What is telephony? where does the VCC come from?

[fzabkar]

Re: What is telephony? where does the VCC come from?

I wouldn't touch the remote board. The problem is with the RJ11-B voltage on the logic board. Trace it back to its source, probably near the bridge rectifier. Test for continuity with the + terminal of the bridge, or each of the PCB interconnect pins.

[scampo77]

Re: What is telephony? where does the VCC come from?

cant I just put a NPN transistor in place of the 15H and see what happens? should I order new communication ICs for each end?

it makes sense that something in the remote side is causing the 4.5v to drop, when I unplug the remote the output voltage bounces back up to 4.5-5 and when I plug the remote back in, the voltage drops again to 3v

[keeney123]

Re: What is telephony? where does the VCC come from?

The remote PCB is powered from the main board, not vice versa.

Yes, I know this, but I do not know if there is a voltage step down to get 5 volts on the remote board. He verified this when he unplug the R11 connector and got o volt across the transmitter IC on the remote bd. There could still be some type of weird grounding that the designer created or it is just that the voltage is coming from the main board. The voltage could be of a higher amount and a DC to DC converter could be stepping it down on the remote board. The problem still exist on the remote board as not having at least 4.75 volts DC on the transmit IC. The IC will not function with less then this so no communication will occur between the two boards.

[fzabkar]

Re: What is telephony? where does the VCC come from?

AISI, the incoming supply at the remote PCB is regulated as follows:

      "15H"
    .---------. +1.875V .--------. +2.36V .-----------.
RJ11-B |     |  Cin  |    |  Cout  |      |
  o------>|--+-------+----|in  out|----+-----|      |
 -1.1V |   | |  |  |    |  |   |  MCU  |
    |   z |  |+  |    |  |+  |  RS485  |
    |   A |  ===  | LP2951 |  ===  | display |
    |   | |  ---  | reg  |  ---  |      |
    '------|--'  |  |    |  |   |      |
        |    |  '--------'  |   '-----------'
RJ11-C     |    | 100uF |     | 100uF   |
  o----------+-------+--------+---------+-----------'
 0V               |
                ===
                GND

The RJ11-B input appears to be an AC waveform sitting on a -1.1V DC bias.

I'm guessing that the "15H" component provides reverse polarity protection and voltage clamping.

The rectified input is filtered by Cin, but the voltage subsequently decays during the negative AC cycles. It appears that Cin could be leaky or low in capacitance, or perhaps "15H" is leaky.

Cout is charged during the peak of the AC input and appears to retain its charge during the negative AC cycles.

That's the only scenario that I can come up with to explain the observed measurements. It would REALLY help to see what is happening on a scope.

[fzabkar]

Re: What is telephony? where does the VCC come from?

What would be good at this point is to determine on the remote board where the voltage is being generated. We know that the main board it want is generating the voltage on remote board. What we don't know is the link between the voltage on the main board and the remote. By ohming out the trace on the remote board with the R11 connector disconnected you will find out where it is coming from on the remote board.

The remote PCB is powered from the main board, not vice versa.

[stj]

Re: What is telephony? where does the VCC come from?

That bridge rectifier is on the main unit

damn, i thought i was looking at the slave panel!

[keeney123]

Re: What is telephony? where does the VCC come from?

What would be good at this point is to determine on the remote board where the voltage is being generated. We know that the main board it want is generating the voltage on remote board. What we don't know is the link between the voltage on the main board and the remote. By ohming out the trace on the remote board with the R11 connector disconnected you will find out where it is coming from on the remote board. Once you find that then you will be able to find out the link from the main board. So you can physical look at the trace and follow it with the ohm meter. If the other people's input is that it is coming from a particular place that can be ohmed out to verify that is correct. Again the problem is the voltage on the chip on the remote board has to get 5volt min. 4.75 volts DC to work. Following this line of thinking will lead you to why there is no 5 volts DC on that chip. Sometimes, not always, engineers and physic professors know to much about the circuitry. They then think they can skip sets in troubleshooting. The problem happens when they make an error. Instead of a typical error it is complicated because steps have been skipped, so what I would do for your own benefit is when they tell you a particular thing verify that it is the case. I approach a problem like Archimedes. First we do not know anything about a problem, but we must make a decision. Once we make a decision we observe the outcome. Based on that outcome we then make another decision and so forth and so on. We end up getting better and better at dealing with a problem, but we are never perfect at the problem.

[budm]

Re: What is telephony? where does the VCC come from?

That bridge rectifier is on the main unit so more than likely that the two ~ pins are connected to the transformer winding.
What ACV do you get between those two ~ pins?

[stj]

Re: What is telephony? where does the VCC come from?

that 5ohms sounds very wrong,
does it read 5ohms wih the cable disconnected?

[scampo77]

Re: What is telephony? where does the VCC come from?

On the main logic board at the rectifier, I have 5 ohms between the 2 pins on the right hand side, all other combinations have very high resistance.

the picture enclosed is the remote PCB with voltage measurments

[stj]

Re: What is telephony? where does the VCC come from?

I have no idea what this piece is or what to look for, I just see that there is a "+" and a "-", so I checked for ACV and found some. I have never seen ACV on a circuit board before

see if any of the 4 pins on that are shorted together.

[fzabkar]

Re: What is telephony? where does the VCC come from?

The LP2951ACM (on the remote PCB) is a linear voltage regulator. It comes in adjustable and fixed voltage versions. I suspect that yours has a 5V output.

The 3-terminal component (15H) near the edge of the remote PCB could be a rectifier diode plus a zener clamp (?).

[scampo77]

Re: What is telephony? where does the VCC come from?

I have no idea what this piece is or what to look for, I just see that there is a "+" and a "-", so I checked for ACV and found some. I have never seen ACV on a circuit board before

[scampo77]

Re: What is telephony? where does the VCC come from?

Bud, that photo you linked is the main logic board, I will try my best to label all the photos moving forward.

[scampo77]

Re: What is telephony? where does the VCC come from?

if I measure for ACV on pins on B&C then I get about 2.5VAC and if I reverse the probes I get 3.5VAC, but its really bouncing around on my meter, It fluctuates by.25v up and down.

I tried to get some photos of the capacitors and also the ICs on the remote. the other 8 leg IC is labeled MTAK 2951 ACMC. sorry I tried to get as many photos as I could but they did not come out very well. These are all photos of the remote board, when I remove the phone cable, everything goes to 0

[budm]

Re: What is telephony? where does the VCC come from?

Measuring A+B together with my meter on ACV I am getting 2.5VAC. I have no way to scope this reading but I suspect you are onto something.

If you look on post #6 there is a photo of the remote board, I don't think there is any rectification circuits here, but I'm not 100% sure what I am looking for.

Reverse the meter probes to see if you are still getting 2.5 VAC.
is this the remote board or main board? https://www.badcaps.net/forum/attach...5&d=1451312753, based on post 6 showing the remote unit.
So you do have bridge rectifier on the remote board. So you are getting AC between the two ~ pins of the bridge? You indicated 0V, and 5V and 7.5V, diode check OK for the bridge? 2.5Vac is not enough to produce 5VDC from the bridge.

[fzabkar]

Re: What is telephony? where does the VCC come from?

The two middle pins (B & C) would be the power pins.

What is that other 8-pin IC on the remote PCB? Where do those electrolytic capacitors connect?

[scampo77]

Re: What is telephony? where does the VCC come from?

Measuring A+B together with my meter on ACV I am getting 2.5VAC. I have no way to scope this reading but I suspect you are onto something.

If you look on post #6 there is a photo of the remote board, I don't think there is any rectification circuits here, but I'm not 100% sure what I am looking for.

[fzabkar]

Re: What is telephony? where does the VCC come from?

Is the supply to the remote unit actually AC, and are you actually measuring a DC offset of -1.1V on this supply??? An AC supply would resist galvanic corrosion.

Is there a rectifier and regulator on the remote PCB?

[scampo77]

Re: What is telephony? where does the VCC come from?

so after throwing my hat in the air and kicking my cat across the room out of frustration, I came back and realized that my main logic board is not totally dead. I have power to most of the ICs I had before. I now have 4.25v at terminal D leaving the logic board and arriving at the remote board, The remote board is still not working.

I still have power to the 483ECN on the logic board, the only difference now is that I have 4.25v output to the red D circuit.