HP 1955 Inverter Problems

[Electronic]

Re: HP 1955 Inverter Problems

To update the situation - There were indeed two wiring errors (I feel stupid now!) and the FET is no longer getting extremely hot but I am back in the situation of the backlight lighting for a few seconds and then going off. I suspect I must have blown some parts trying to solve my previous problem.

[alexanna]

Re: HP 1955 Inverter Problems

Try unhooking all the ccfl’s.And then hook one up at a time then power on, you will still get 2 sec to black But do all of the bulbs light

[Electronic]

Re: HP 1955 Inverter Problems

Thanks for the advice alexanna - I would have given it a try but I believe the problem has been solved now. It turns out my dodgy wiring was too much for the J598's. I've replaced them and so far so good. It turns on and stays on and turns off completely without going up in smoke... fingers crossed.

Only other problem that is common with all 4 of these monitors and gets worse over time is the high pitched whine. Its only when the monitor is 'soft off' using front panel switch, or is on standby. I thought a bit of glue on the transformer windings might help but there is already some there and it doesn't appear to help much. I've also heard it could be the caps.

But thank you to everybody for helping me solve the problem! I'll post back if it goes up in smoke...

[jetadm123]

Re: HP 1955 Inverter Problems

Thanks for the advice alexanna - I would have given it a try but I believe the problem has been solved now. It turns out my dodgy wiring was too much for the J598's. I've replaced them and so far so good. It turns on and stays on and turns off completely without going up in smoke... fingers crossed.

Only other problem that is common with all 4 of these monitors and gets worse over time is the high pitched whine. Its only when the monitor is 'soft off' using front panel switch, or is on standby. I thought a bit of glue on the transformer windings might help but there is already some there and it doesn't appear to help much. I've also heard it could be the caps.

But thank you to everybody for helping me solve the problem! I'll post back if it goes up in smoke...

Good work! Glad you were able to fix it. As for the whining sound, many folks run their monitors at 100% brightness. Try running the monitor at say, 90% brightness to see if that reduces the whine. The transistors may run a little cooler too.

[Krankshaft]

Re: HP 1955 Inverter Problems

He's referring to the sound of the main power supply switching transformer with the monitor in standby.

Nearly all of my 1955 monitors do that a complete recap didn't fix it.

It's either as you say a vibrating winding or the switching frequency when in standby is within the high range of human hearing. I hard power off the monitor either via the rear switch or with a surge protector when I'm done using it anyway so it doesn't bother me.

[PlainBill]

Re: HP 1955 Inverter Problems

He's referring to the sound of the main power supply switching transformer with the monitor in standby.

Nearly all of my 1955 monitors do that a complete recap didn't fix it.

It's either as you say a vibrating winding or the switching frequency when in standby is within the high range of human hearing. I hard power off the monitor either via the rear switch or with a surge protector when I'm done using it anyway so it doesn't bother me.

I think you are correct. It might be possible to change the timing cap to shift the frequency upward, but this might have undesirable consequences. I'd say best to leave well enough alone.

PlainBill

[1lostsock]

Re: HP 1955 Inverter Problems

Hello All,

Hijack alert! My problem seems to share a lot of background with this thread so I thought I should tack this on the end.
Let me know if a new thread would be better and if you have a few suggestions please help me save this back-burner project from the landfill.

I have a dead HP 1955 monitor with a Benq 48.L1A02.A31 inverter board (a stirling piece of work). I have repaired 3 others of the same type with the same failure in almost identical ways and apart from the HF noise when powered down I am very happy with the results.

This one however...

Originally I replaced the 5707s and driver Fet on the blown inverter and the schotty diodes on both channels. In this case no tracks needed rebuilding.
This is marked as 'First Repair' in red in the image.

As with all the other monitors this did wonders and it lived again.
As standard I resoldered all transformer legs and the HV output caps, all caps appeared clean and stable, or at least not obviously damaged.

At this point I was smug.

I noticed that the lower (the previously damaged) inverter would not turn off.
I searched and found this thread and researched the cause.
It turned out that Q811 was shorted base to collector(ground plane)(Q811 legend obscured by TO-92 in images) but do you think I could positively identify what it should be? No of course not, that would be too easy!

Looking at the BENQ Q7C3(FP767) circuit diagram which has striking similarities I decided that Q811 on my board is Q742 or Q752 (depending on inverter) and thus a A733 SOT-23 PNP BJT transistor (which Farnell has never heard of).

I procured some TO-92 2SA733 transistors to use instead and eventually installed it (see graceless soldering in the images). Since pinouts are not consistent I am only really sure that the base is correct but the hFE in the installed position should be about 350 out-of-circuit so it should switch OK.
Maybe I should swap my assumed collector and emitter?

Feeling dubious at this point - I have hack installed a part from a different circuit, what could go wrong?

Power up, backlight and screen appear - for about 2 seconds. Reproducible, nay unavoidable. Back to where everyone else seems to start at.

Now reading the 2-seconds black screen guide it would seem that capacitors are a problem but this screen ran, and ran well, with a faulty Q811 so many parts of the system seem to be good.
I could even restore it to that state with the old SOT-23 which I kept (and hopefully did not destroy further).

So my questions to all of you out there are:

Does anyone know what the Q811 transistor for a BENQ 48.L1A02.A31 really should be?

Any other useful observations or suggestions?

Thanks.

[jetadm123]

Re: HP 1955 Inverter Problems

Did the original Q811 have any markings on it?

[retiredcaps]

Re: HP 1955 Inverter Problems

Does anyone know what the Q811 transistor for a BENQ 48.L1A02.A31 really should be?

Likely FU9024. Q811 and Q812 are likely to be the same component.

[1lostsock]

Re: HP 1955 Inverter Problems

Hi,

The markings are illegible without a microscope but it's identical part on the other inverter channel is marked:
W0452
with the 52 sideways

Hope that helps, my limited SMD experience was unable to make much sense of this marking.

Thanks for your interest.

[budm]

Re: HP 1955 Inverter Problems

I think yours looks like a match to this diagram, I looked at the designators and they seems to match. You did have the TO-92 2sa733 pin out correctly, the Japanese center pin is Collector, the US center pin is Base.
The parts with 'open' mean they are not used.
If you bypass the MOSFET 811/812 with 1 Ohm 5W resistor B+ will be supplied to the ROYER OSC which will make the OSC run at full mode and it will stay on. The MOSFET is used to vary the amount of B+ thus increase/decrease the lamps brightness.
You should try test lamps also.

[1lostsock]

Re: HP 1955 Inverter Problems

Hi Budm,

Thanks very much for the schematic - it is a much better match that the previous one I was using and shows Q811 as a 2N3906 rather than a 2SA733.
The two are superficially similar though so I am not sure if replacing my T0-92 hack with the right part will help. For the cost of 2N3906 is is probably worth a go. I will track one down in the next few days.

My apologies, I believe the short was a collector-emitter short on the Q811 which makes sense given the original path of silicon destruction.
This would force Q812 (FU9024N) on and keep things running.

Similarly your suggestion of 1 Ohm bypass (Q812 Drain to Source) should power the inverter in an open-loop i.e. at full power?

It would be nice to get this turning off the way it should so I will try a 2N3906 as soon as I can lay hands on one.

Cheers.

[budm]

Re: HP 1955 Inverter Problems

I jump the MOSFET with the resistor to test and see if the Royer Osc is working or not when I work on this kind of output setup. Be sure to check the 0.22 tuning cap, if the value is off, the Osc will run at the wrong frequency and will take out the C5707, they will run hot.

[1lostsock]

Re: HP 1955 Inverter Problems

That is a good idea - I am pretty sure that the oscillator runs as I have had the screen running recently but at what heat I have not checked. I will get some part temperatures (via thermocouple, too many volts for my fingers) next time I have it up for more than a few seconds.
I have some more of the 0.22 caps so I could just straight out replace them.

[1lostsock]

Re: HP 1955 Inverter Problems

OK.

As luck would have it I had some 2N3906's - they were even SOT-23.
One was clumsily soldered in but no change - 2 seconds to black.

I started looking at components on the feedback path but came back to the driver circuit for Q812 (FU9024N) where I had the original Q811 (2N3906) collector-emitter short (1 Ohm).
I found that Q810 (2N3904) also had a collector-emitter fault (30 Ohm 'short').
Why I had not seen this earlier I don't know, excessive stupidity or worse, a new failure perhaps.

Theory:
When Q811 was a short, Q812 was fully on and the LCD was bright and could not turn off.
With Q811 fixed and Q810 a 'short' the story is less clear. The LCD would go because Q811 was dragged on by a faulty Q810 as part of a bad DC bias (?). Pin 7 of TL1451 sees a fault and shuts the inverter down.
Fixing Q810 may do the trick.

Unfortunately I do not have any so a couple on order might arrive mid week...
__

Test: 1 Ohm Q812 source-drain bypass test.
Result: LCD backlight on and stays on (15 second test).

Cheers.

[1lostsock]

Re: HP 1955 Inverter Problems

OK installed a 2N3904 where it should be (Q810).

Operational test successful - the light turns on for more than two seconds and stays off when off.

Unfortunately Q812 (FU9024N) gets quite hot (+30 Celsius degrees in ~10 seconds) so I turn it off quite quickly when I can smell it.
As the screen turns on there is a quiet squeal that climbs in frequency (sort of sounds like heavy loading in some components while the voltage is slowly topped up, lasts ~5 seconds until stopping or passing the edge of audiblilty).

Nearly there...

[budm]

Re: HP 1955 Inverter Problems

It sounds like Q812 is not switching on and off with PWM. You will need the scope to look at the Gate drive frequency. Or either that the Royer Oscillator is not running at the right frequency.

[jetadm123]

Re: HP 1955 Inverter Problems

Did you have a chance to replace C801 and C802? If they drift off too much in value, it can cause problems.

[budm]

Re: HP 1955 Inverter Problems

I thought he also replaced the two tuning caps already.

[jetadm123]

Re: HP 1955 Inverter Problems

I got the impression that he had additional .22uf caps, but had not replaced the originals? I was also wondering if he had replaced the two electrolytic caps (470uf?), C801 and C802.