Gericom S1902D - flashing Power LED

[teslacap]

Hallo there,
I got this defective Monitor with a LM004 inverter board for repair.
After a detailed diagnosis I found out, that one of the inverters
do not work correctly. This causes the controller IC switches off
after a few seconds and the Power LED blinks. After measuring
the transistor pairs I checked two transformer drivers as broken.
This are Q11 and Q12. Because I had no NPN 2SC4672 I used 2SC5707
for replacing (they have nearly the same hFE and are a little stronger
with current and voltage). BTW: I changed all important capacitors also.
Now the high voltage occurs on every transformer (measured with an
inductive coil via oscilloscope) but on this side where the transistors
were changed I got a much higher amplitude for seconds, then the
monitor switches off again and the LED blinks. Also the one transformer coil,
switched now by the 2SC5707, makes a little noise . Could it be that
the (not original) 2SC5707 pair drives to much and the IC switches off in this reason now?
Or does anybody have another idea?

[Scenic]

Re: Gericom S1902D - flashing Power LED

It might just go into standby mode after a few seconds, regardless of what the inverter is doing.
I've had a crappy Medion LCD to fix that did just that. It displayed a "no signal" message for just 2-3 seconds (wtf?) before going into standby mode.. not really helpful :P

Try connecting it to a signal source (i.e. computer) first..
Also, when changing inverter transistors I usually try to replace them all at once (especially if they're not the original types) to keep things balanced.

The only photo I found online suggests that this uses the dreaded royer oscillator design for the inverter. Check if the capacitors next to the inverter transformers (the two brown-ish blobs) have bulges or are cracked. If they are, the replacement transistors will self-destruct..

http://reparaturprofi.ucoz.de/_fr/0/0915457.jpg

[selldoor]

Re: Gericom S1902D - flashing Power LED

Can you check the secondary windings of the two tranformers and post the exact results - T1 - xxxohms T2 = xxxohms

Do you have any spare ccfl lamps or another panel you can use to test the lamp circuit.

[teslacap]

Re: Gericom S1902D - flashing Power LED

Hallo,

thank you for these hints. I tested the tranformers before, with a simple
ohm measure-test. The results were symetrical similar.

But it was in fact the missing signal-cable which leads to the same
symptom, as it were, while the bug still was existing.

Now the monitor works fine in test state since half an hour.

Thank you!

[budm]

Re: Gericom S1902D - flashing Power LED

as scenic suggested,
I have a feeling that this monitor uses Royer oscilator output set up, you need to check the tuning caps for value and resistance leakage, otherwise the transistors will run hot (due to running at the wrong frequency) and short again.

[teslacap]

Re: Gericom S1902D - flashing Power LED

Hi budm, I already have changed the two tuning caps (150nF) against new one. Thanks..

[jetadm123]

Re: Gericom S1902D - flashing Power LED

In addition to changing the tuning caps, there should also be two coils/inductors, L1 and L2?, as part of that royer circuit. On other brand boards, they develop bad solder joints over time. Try resoldering them. Also, transistor substitution is always a crapshoot and the two 2SC5707's you used may not be compatible. Maybe they can't handle the switching frequency?

[budm]

Re: Gericom S1902D - flashing Power LED

"Also the one transformer coil,
switched now by the 2SC5707, makes a little noise"
It may not be running at the correct frequency, you may want to swab the transformer or the transistors with the other section. High frequency Oscillator can be picky on the components being used.

[teslacap]

Re: Gericom S1902D - flashing Power LED

Hi jetadm123 and budm, thank you for those hints.

The 2SC4672 provides a transit-frequency from about typical 210 MHz.
This is a walk-over for the 2SC5705, because its transit frequency is much higher, around (290) 330MHz.

I've heard a similar little "coil-noise" even in original-running ciruits.
But the level is such minimal, you won't regonize it, near sitting by.

This monitor is already closed now. I will test for one day before shipping.
And the customer will get 1 year warranty on the reparair. That is a deal
guys isn't it?

[JonathanAnon]

Re: Gericom S1902D - flashing Power LED

Hi there, just had one of these on my desk... An important point, as mentioned above, is that the monitor shuts down almost instantly when there is no video present... So if you think that it's inverter shutting down, hook it up to a PC first...

I replaced the two 25V caps (which had ESR>3ohms) on the PSU, and it seems to have fixed mine.