Problem with intercom PS

[CHexclaim]

I need to repair Philips door intercom, specifically the unit with the monitor that goes on the wall. It is an old system, those that have a right angled CRT screen for the camera.

I narrowed it down to the power supply which is a small board with a switching type power supply, based on a KA3842B IC. I do not have a service manual or schematic so I'm finding it difficult to solve the issue.

I think it is a common problem because I was asked to fix another one with the same symtom

The label on the back of the unit says:

PHILIPS
Monitor for LHM 9000/00
NC 8958 900 00001

Any help will be appreciated.

Thanks, Charlie.

[Whales]

On the KA3842B datasheet there is an example circuit.  Yours may be very similar.

What are the symptoms?  What have you tried or investigated?  Can you post good pictures of the front and back of the board?

[CHexclaim]

Thank you for your prompt reply.

I am attaching photos of the PS board.

The symptoms are:

- The unit works partially and with lots of noise
- The CRT does not turn on
- There is a constant tic-tic-tic.. noise that comes from the inside of the unit and the red led on the front blinks at the same rate.
- Voltage on the PS output, measured with a DMM, is around 5V, but the scope shows a horrendous triangular waveform, at the same frequency of the tic-tic noise.

I powered the unit with an external PS and the CRT works fine although I could not test the intercom system as a whole as I only have the monitor here.

Thank you.
Charlie.

[poot36]

I would check all of the electrolytic capacitors in the power supply.  The ones on the 5V output as well as the small bootstrap capacitor for the switching chip.

[mij59]

Check R4 and R3 looks like they are burned.

[CHexclaim]

I did replace the caps but I am suspecting that there is a problem with the IC itself.

I have two options to solve the issue, one is fixing this and the other is to externally power the unit with another PS. I will ask if this is a valid option.

The problem is that getting the parts here (in Uruguay) is difficult, so replacing the active components is simple but logistically difficult.

I'll see what I can do. Thank you.

Charlie.

[Richard Head]

If you have an isolated bench PSU use it to power up the chip so you don't have to rely on the bleed-on circuit. The turn-on threshold is 16V.
This will allow you to test the gate drive circuitry without mains connected. Wide open gate drive will be about 97% duty cycle.
If all is ok then connect the mains. It should produce an output and work as expected. Then disconnect the DC supply and see if it continues to operate. If not then the feedback winding is not powering the IC for some reason.
Common problems are capacitance reduction of small electrolytic bleed-on cap, bleed-on resistor going high in value, shorted output diode.

Dick

[CHexclaim]

Thank you for your reply.

The power to the IC if fine and directly derived from the DC high voltage via a resistor divider. I came close to the conclusion that the problem resides in the transformer itself, although I did not investigate further as the owner accepted the use of an external PS to substitute the whole board and he will upgrade his intercom system soon (better late than never).

Thank you for your help which is always useful.

Charlie.