32" Goodmans TV

[tom66]

Re: 32" Goodmans TV

No need for the attachment, we're on the same manual.

Good.

So what was the problem with yours? Same symptoms? How did you fix it?

[dumpystig]

Re: 32" Goodmans TV

If you're still here in an hour we can continue - I gota take my son to feed the ducks now before he goes back to his mums.

[tom66]

Re: 32" Goodmans TV

If you're still here in an hour we can continue - I gota take my son to feed the ducks now before he goes back to his mums.

Unfortunately I'll be busy, but will you be available later today or tomorrow?

[tom66]

Re: 32" Goodmans TV

Okay...

The main CPU with the weird heatsink thing on it appears to be telling the PSU to turn on. The resistor R497 connects to pin #115. This goes low when I press the button on the remote (it's normally high.)

The signal then travels through the multilayer PCB to pin #6 (not pin #5!) It turns out pin #5 is a protection detect pin. So it going high might be good - but falling low signals a fault with the PSU. The standby signal is optionally inverted by a transistor inverter (presumably to allow for different OEM power supplies.)

The question now is, what is tripping the PSU's protection?

[PlainBill]

Re: 32" Goodmans TV

I've run over this a number of times, but in general this is the process. The power supply has three sections. There is a standby supply that is always on whenever AC power is present. The standby voltage goes to the main board where it powers a control IC (sometimes called the Standby Processor). The Standby Processor monitors both the front panel and the remote control sensor. When the power button is pressed on either, the main supply is turned on by a control line. It may be labeled PS_ON or ON/OFF, or a similar name, it may be labeled 'STBY' or something meaningless.

When the PS_ON line goes high the PFC section and the main supply section of the power supply are activated. On some power supplies there is a PIC which monitors the output voltages and triggers a failure signal if any voltage doesn't reach the specified level within a short time - typically less than a second. On other designs this function is moved to the main board. In either case, the main supply is turned off by the standby processor if the outputs do not reach the proper level.

Troubleshooting these supplies is fairly straightforward. First, check for the presence of the Standby voltage. If that isn't present, troubleshoot just like any power supply. In a case where the main supply turns on, then off again, one simple measure is to disconnect the outputs, and force the PS_ON line high by using a 3 volt supply (two AA cells in series will work), then checking the output voltages.

PlainBill

[tom66]

Re: 32" Goodmans TV

I've run over this a number of times, but in general this is the process. The power supply has three sections. There is a standby supply that is always on whenever AC power is present. The standby voltage goes to the main board where it powers a control IC (sometimes called the Standby Processor). The Standby Processor monitors both the front panel and the remote control sensor. When the power button is pressed on either, the main supply is turned on by a control line. It may be labeled PS_ON or ON/OFF, or a similar name, it may be labeled 'STBY' or something meaningless.

The PFC AFAIK is always active. The bulk cap always has ~330V across it.

The line which turns the PSU on is called STBY. This drives a logic pin on one of the chips on the PSU. It is buffered by a transistor inverter.

There isn't a standby processor (remember, this is a low end TV.) The CPU is always running, although in a low power mode, and is powered by a 3.3V linear regulator, which runs off the 5V or 8V standby line.

When the PS_ON line goes high the PFC section and the main supply section of the power supply are activated. On some power supplies there is a PIC which monitors the output voltages and triggers a failure signal if any voltage doesn't reach the specified level within a short time - typically less than a second. On other designs this function is moved to the main board. In either case, the main supply is turned off by the standby processor if the outputs do not reach the proper level.

There isn't a PIC on the mainboard; I think it would be a bit expensive to include one. The designers included a simple two-transistor circuit which I simulated. It triggers if any rail (of 5V, 8V and Panel_5V) drops below 4.3V or if the 3.3V drops below 2.3V. So something is potentially firing this circuit. The question is, what rail is causing the problem...

Troubleshooting these supplies is fairly straightforward. First, check for the presence of the Standby voltage. If that isn't present, troubleshoot just like any power supply. In a case where the main supply turns on, then off again, one simple measure is to disconnect the outputs, and force the PS_ON line high by using a 3 volt supply (two AA cells in series will work), then checking the output voltages.

PlainBill

There is full standby voltage. I've tried forcing the line high but with no success, but I will try it on pin #6 and not pin #5 this time.

[PlainBill]

Re: 32" Goodmans TV

The PFC AFAIK is always active. The bulk cap always has ~330V across it.

If I recall correctly, your line voltage is 240 Volts. 330 volts sounds like normal rectified line voltage. Typically when the PFC is functioning I expect to see close to 400 VDC across the main filter cap.

The line which turns the PSU on is called STBY. This drives a logic pin on one of the chips on the PSU. It is buffered by a transistor inverter.

There isn't a standby processor (remember, this is a low end TV.) The CPU is always running, although in a low power mode, and is powered by a 3.3V linear regulator, which runs off the 5V or 8V standby line.

So they have incorporated the standby processor into the main processor. The point remains the same - the signal to turn the main supply on originates on the main board.

There isn't a PIC on the mainboard; I think it would be a bit expensive to include one. The designers included a simple two-transistor circuit which I simulated. It triggers if any rail (of 5V, 8V and Panel_5V) drops below 4.3V or if the 3.3V drops below 2.3V. So something is potentially firing this circuit. The question is, what rail is causing the problem...

There is full standby voltage. I've tried forcing the line high but with no success, but I will try it on pin #6 and not pin #5 this time.

That would probably help.

PlainBill

[tom66]

Re: 32" Goodmans TV

If I recall correctly, your line voltage is 240 Volts. 330 volts sounds like normal rectified line voltage. Typically when the PFC is functioning I expect to see close to 400 VDC across the main filter cap.

I'm still not 100% certain it is a PFC circuit. Some PFC circuits use a low duty cycle to maximise efficiency, but at lower power factor.

So they have incorporated the standby processor into the main processor. The point remains the same - the signal to turn the main supply on originates on the main board.

Yes, it is called STBY_ON_OFF and it goes to pin 6 after being inverted.

[PlainBill]

Re: 32" Goodmans TV

I'm still not 100% certain it is a PFC circuit. Some PFC circuits use a low duty cycle to maximise efficiency, but at lower power factor.

Trust me, it's got a PFC front end.

Yes, it is called STBY_ON_OFF and it goes to pin 6 after being inverted.

PlainBill

[tom66]

Re: 32" Goodmans TV

Trust me, it's got a PFC front end.

It looks a lot like it. It has a PFC MOSFET and PFC diode in the right positions.

According to this thread (http://www.tv.quuq.org/forum/index.p...e;topic=2334.0), the slow start FET can be causing issues. Might be worth me looking at it.

[dumpystig]

Re: 32" Goodmans TV

Ref: Hitachi No. 0234 Power Board Circuit

- Sheet 1 - IC802, IRF7314, Pin5 > PFC_OFF

- Sheet 2 - PFC_OFF > Q823, BC548C

During RUN C819 (main elect.) = 386VDC, STBY = 327VDC; note -V test lead connected to heatsink, not chassis. All other V's should be taken with -V test lead connected to chassis.

When I initially got this set a couple of weeks ago it was as though it had several faults. After doing a lot of resoldering of heavy joints on the PSU I managed to get it (the PSU) running as it should. But then I had a problem where the set wouldn't come out of STBY and power on. I did some resoldering on the Mainboard related to the PDP_GO/BL_ON_OFF components and the set started working fine. Now the problem I have is when trying to turn on with HDMI connected; sometimes the set comes out of STBY, the backlights power on for a split second and then the set goes into the STBY loop again - but this doesn't happen all the time. If I disconnect the HDMI the set will start and run just fine. Have to admit that the combination of these two Vestel boards is a right pain, but I'm finding it a nice challenge trying to fix these probs and hopefully getting the set working 100%. Also I don't think the comparison between my particular model of TV and the Hitachi data should be taken as being entirely the same.

[dumpystig]

Re: 32" Goodmans TV

I'm also not as advanced in my electronics knowledge as you two obviously are, but please bear with me, every day I'm learning something new.

[tom66]

Re: 32" Goodmans TV

I'm also not as advanced in my electronics knowledge as you two obviously are, but please bear with me, every day I'm learning something new.

Heh, all my knowledge (what little there is) is gained from the internet, I have no formal education in this stuff.

[dumpystig]

Re: 32" Goodmans TV

I'm self taught but my knowledge is limited - maybe it's time for me to start swotting up on stuff.
So how are you getting on with your TV, where are you up to now?

[tom66]

Re: 32" Goodmans TV

I'm self taught but my knowledge is limited - maybe it's time for me to start swotting up on stuff.
So how are you getting on with your TV, where are you up to now?

Well, I've found that the TV is signalling a fault condition by pulling down the PROTECTION_DETECT pin. But this could be because the PSU is shutting down...

[tom66]

Re: 32" Goodmans TV

So:

Pin #6 is on/off for PSU. Off is 1.8V, on is <0.4V. It's at 0.02V when switched on and 1.8V when off, so that seems good.

Pin #5 is an error flag from the signal board. It's supposed to be high when there is no error. But it's often low. Only when the set is working is it high (2.5V.) The pin is low if the rails are out of spec (too low.) However, the rails will always be out of spec if the supply is too low because it's not starting...

The voltage across the PFC cap is always ~330V while in standby or while trying to start (not yet had an opportunity to measure it during operation.) So something with the PFC circuit is not working. However, 330V should still be enough for it to work. I don't care that much about the PFC to fix it unless it's crucial.

So why isn't it starting... could be the fet Q837?

What was the problem with your set dumpystig?

[tom66]

Re: 32" Goodmans TV

Hmmm

Pin 8 (of SG2525A) is looking like it might be the problem

So it's making my meter go crazy on the DC range, reading random values from -9V to +10V. On the (True RMS) AC range it reads 46V AC.

This pin definitely shouldn't have these kinds of voltages on it, but how are they getting there?

EDIT:

Okay, so this time I did it with the meter grounded to a heatsink (the FET heatsink.)

Now during start-up it measures 0V, it should be above 0.4V, preferably 0.7V. It is charged with an internal 50µA current source.

[tom66]

Re: 32" Goodmans TV

FIXED!



Pin #8 was the problem... I broke the trace between Q837 and pin #8 of the SG2525A and it now starts up, turns off and does that all many times including between pulling the plug out, which it never did before. I'll leave it overnight to see if I can turn it on in the morning, but it's looking good .

I still left the slow start caps connected to the pin because they provide the slow starting (the set takes 5 seconds to come on, approximately) whjch might be helping to reduce the load on the supply.

I ordered some of the secondary side caps for it, but I don't think they will be necessary any more.

Oddly, the PFC bus voltage is still only 328V - which means PFC isn't working. But I guess if it ain't broke don't fix it, it seems to still be happy with the ~330V.

[tom66]

Re: 32" Goodmans TV

Oh no, it just died after I wrote that post... I was a bit too quick.

Hmm... possibly my track break wasn't good enough. Will look into it. It was definitely working better than before...

EDIT: Yep, the pin is touching something. I will have to remove all the solder, as I twiddled with the pin and it started working. So it looks to definitely be this pin causing all the troubles.

[dumpystig]

Re: 32" Goodmans TV

Hi Tom,

I'm gona read thro' this thread again, compare with my findings/results, then I'll get back to you, OK.

Mark