Samsung 172N different problem

[smason]

Initial symptom was no power.
Caps looked ok, but were Capxon so I replaced all power supply/inverter caps with Panasonic FMs.
While testing, all voltages were fine, just wouldn't power up. Touched scope probe to one of the inputs from the control pcb, and it came on.
Monitor works fine except none of the controls work, including the power button.
I don't get how it's supposed to work. The control pcb has 6 switches on it, and a few LEDs and a bunch of resistors. No chips or transistors.
There are only 2 inputs from the control pcb to the MCU. (part of main board schematic attached) I don't have a schematic for the control pcb.
The switches all test ok, and continuity from the control pcb to the
mainboard is good. I will probably test the zeners out of circuit this week.

Will post some photos tomorrow or Tues.
Any ideas?

[PlainBill]

Re: Samsung 172N different problem

Initial symptom was no power.
Caps looked ok, but were Capxon so I replaced all power supply/inverter caps with Panasonic FMs.
While testing, all voltages were fine, just wouldn't power up. Touched scope probe to one of the inputs from the control pcb, and it came on.
Monitor works fine except none of the controls work, including the power button.
I don't get how it's supposed to work. The control pcb has 6 switches on it, and a few LEDs and a bunch of resistors. No chips or transistors.
There are only 2 inputs from the control pcb to the MCU. (part of main board schematic attached) I don't have a schematic for the control pcb.
The switches all test ok, and continuity from the control pcb to the
mainboard is good. I will probably test the zeners out of circuit this week.

Will post some photos tomorrow or Tues.
Any ideas?

This is a fairly common design. There are several resistors in series on the control pcb. The form a voltage divider network between +5V and Gnd. So, for example, if you press the 'menu button', 3.6 volts will appear on KEY2. The input button might produce 1.8 volts on KEY2, etc. So you troubleshoot by pressing buttons and seeing if any produce voltage changes. If none do, you start looking for broken wires, cracked traces, etc.

PlainBill

[smason]

Re: Samsung 172N different problem

This is a fairly common design. There are several resistors in series on the control pcb. The form a voltage divider network between +5V and Gnd.

Ahh (cue the light bulb going on) Thanks Bill!! As usual you are a fountain of useful knowledge.

I'll do some measurements tonight and report back.

[smason]

Re: Samsung 172N different problem

I tried measuring the voltage of the key1 and key2 inputs while pressing buttons, there was no change.

After a magnified visual inspection of the control board I spotted questionable solder on one of the resistors, so I touched up a couple.
I managed to accidentally remove one (when you WANT to remove one it's not easy, go figure) I've always prided myself on having good soldering skills but this tiny surface mount stuff is nasty. I also used to pride myself on my excellent eyesight, but 50 is in the rear-view and I cant se s41t up close any more...
When attempting to re-install the resistor my tweezers went "ping!" and there it was - gone.
So I'll have to acquire a 150 ohm resistor before I continue. None of my donor devices seem to have that value. I s'pose I could graft in a regular 1/4 watt for test purposes....

[smason]

Re: Samsung 172N different problem

Well I have to clean up my temp work area, so this one goes on the back burner for a while. Besides there's only so much time I can waste on a 17"....
If I can get my normal work area cleaned up, there are more on my pile that deserve more attention, particularly the Gateway 2185w which *should* just be 8 caps, it looks to be in pristine physical condition.
And a couple BenQs which *should* be solder joints and transistors.
If these next few turn out to be a "tough dog" like the last 2, I'll smell a rat.
I.E. some other wanna-be monitor repair guy tried fixing these before giving them to me.

[lucky13]

Re: Samsung 172N different problem

Aren't we all wanna-be monitor repair guy in the beginning?