HP 1955 Inverter Problems

[Electronic]

Hi, Sorry to start another thread on the HP 1955 but I have not found an answer to my problems in the other posts. Initially my problems was that the screen completely died so I realised the fuse must have blown and checked the c5707 transistors and J598 fets. I found some had shorted so replaced them all. The screen then had the usual fault of switching on for a few seconds then going blank which I discovered was due to an npn small signal transistor which had shorted so I replaced that and a few others that looked suspicious. The screen then appeared to work but when I power it down the backlight remains on. Also i noticed in addition to the backlight being on, the J598 fets start getting hot and would no doubt blow if i didnt pull the plug. I have checked over all the components that I would expect to be problematic and have resoldered the transformers etc but still have this problem. Can anybody help?

I have just looked over things again and am wondering if the Schottky diodes protecting the FETS could be dead. With my multimeter on diode setting I get a reading of 137 in one direction and 508 in the other. The other diode is more or less the same. Could the diode make the FETS heat up when the monitor is switched off if it is shot? Also would this force the backlights to stay on?

[retiredcaps]

Re: HP 1955 Inverter Problems

With my multimeter on diode setting I get a reading of 137 in one direction and 508 in the other.

I have a similar Benq style board. When I measure the schottky diodes "in circuit", I get 0.345V and 0L for both.

The only way to know if your shottky diodes are good or bad is to desolder them and test them out of circuit to ensure no surrounding component is affecting them.

[Electronic]

Re: HP 1955 Inverter Problems

Thanks retiredcaps, I have checked the diodes out of circuit now and got around 0.140v drop in one direction and 0 in the other. This seems ok to me but your in-circuit measurements are intriguing and possibly point so a short somewhere else in my circuit leading to the diodes conducting in reverse bias. The capacitors in parallel with the diodes seem to be the obvious thing to check. Any thoughts on what else the problem could be if the diodes are fine?

*Edit - I've checked the caps and they are also fine

[Electronic]

Re: HP 1955 Inverter Problems

I have still not managed to solve this problem and I'm wondering if somebody can offer any advice. To review the situation, the monitor comes on fine but when I press the front panel switch to turn it off, the backlight remains on and one half of the inverter circuitry gets hot (particularly the J598). I have only left it hot for a second or so and the parts are still fine and I have replaced them to make sure.

I have replaced or checked everything I can think would cause a problem but it still has not been solved. I've replaced all the c5707's, both J598s, the fuse, resoldered the transformers and caps, checked the caps in the offending circuit by swapping them with equivalent ones in the other half of the inverter, and I have also replaced the small signal surface mount transistors.

[PlainBill]

Re: HP 1955 Inverter Problems

I have still not managed to solve this problem and I'm wondering if somebody can offer any advice. To review the situation, the monitor comes on fine but when I press the front panel switch to turn it off, the backlight remains on and one half of the inverter circuitry gets hot (particularly the J598). I have only left it hot for a second or so and the parts are still fine and I have replaced them to make sure.

I have replaced or checked everything I can think would cause a problem but it still has not been solved. I've replaced all the c5707's, both J598s, the fuse, resoldered the transformers and caps, checked the caps in the offending circuit by swapping them with equivalent ones in the other half of the inverter, and I have also replaced the small signal surface mount transistors.

The gate of the J598 is modulated by the inverter controller. Obviously for some reason that is not turning it off when the monitor goes into standby. I believe ther are usually a couple of transistors used for a level shifter.

Rant mode <ON> Over and over and over again, I run into the same problem. I can't see the the board, therefor it gets pretty darn hard to advise what area to check. Yes, there are other threads, maybe one of them has a good picture. But if you can't be bothered to supply pictures I'm not going to go looking for them. Rant mode <OFF>

PlainBill

[Electronic]

Re: HP 1955 Inverter Problems

Apologies PlainBill, however I must assure you that it is not because I couldn't be bothered to upload images but more that I thought the circuit was quite well known to members and people may have had similar problems. I have attached a photo of the front and back although the back has a slightly shameful repair on two of the small signal transistors for which I ruined the copper tracks and had to improvise! The red and black wires have no significance other than what was lying on the table at the time.

It is Q805 which is getting particularly hot

[seanc]

Re: HP 1955 Inverter Problems

Have you tested Q805 out of circuit? I believe it may be D10PF - it may be shorted. The one on my BenQ board is blown.

[Electronic]

Re: HP 1955 Inverter Problems

Thanks for your reply - I have tested Q805 out of circuit and it seems fine. I can't seem to find D10PF anywhere. Could you roughly direct me to it!

[seanc]

Re: HP 1955 Inverter Problems

http://uk.rs-online.com/web/search/s...duct&R=4865592
I've had these on back order since October 2010 - I'm in no hurry.

Apparently FU9024 are compatible http://uk.rs-online.com/web/search/s...duct&R=5411657

[Electronic]

Re: HP 1955 Inverter Problems

I have a few FU9024 but my circuit uses the J598 which I have already replaced and it doesn't make any difference unfortunately - same symptoms and still getting hot.

[PlainBill]

Re: HP 1955 Inverter Problems

How the h**l did you know how I like to label pictures? I was all set to rename it to my standards and it had already been done!!! Threw me off for a minute.

Excellent quality pictures, and arranged as I like them. It's easy to identify where the problem is. Exactly what is wrong may be more difficult.

The IC circled in red is of course, the inverter controller. Just to the right of it I've identified two lines, one of these is the drive line to Q805. The last point circled in red is the gate of Q805.

The service manual for the Benq Q7C3 gives an excellent explanation of how this circuit works. Search for benq_q7c3_fp767_[et]

PlainBill

[Krankshaft]

Re: HP 1955 Inverter Problems

Are you 100% sure when you tore up the SMD traces that the jumper wires are ALL going to the proper places. That's a lot of spaghetti and a lot of places for stuff to go wrong.

Also recheck the jumper wired transistors with a diode check if they're fine wrap them with electrical tape if they were to tap nearby solder points when the monitor was running they can cause all kinds of havoc not to mention destroy themselves.

Those signaling transistors (two per side) signal the two halfs of the inverter on and off by driving the base of their respective FET. The fact that the one side won't shut off and the ON / OFF FETs are fine tells me there is a wiring issue on that sides signaling circuit.

I have a spare repaired 1955 inverter board in the parts bin I can take pics of but it's a later revision with SMD ON/OFF FETs so the trace routing may be different.

Look at these pics and see if the routing on your board is similar if so I can take better detailed pictures so you can compare your signal transistor routing:

https://www.badcaps.net/forum/showthread.php?t=8598

[EGuevarae]

Re: HP 1955 Inverter Problems

Apologies PlainBill, however I must assure you that it is not because I couldn't be bothered to upload images but more that I thought the circuit was quite well known to members and people may have had similar problems. I have attached a photo of the front and back although the back has a slightly shameful repair on two of the small signal transistors for which I ruined the copper tracks and had to improvise! The red and black wires have no significance other than what was lying on the table at the time.

It is Q805 which is getting particularly hot

Things like discoloration (from excessive heat) and bad solder joints can be present in your board (and not in others) or in different places.That's why even if the forum already have pictures of your same model/unit, it is advisable to post good pictures of your own boards.

[Electronic]

Re: HP 1955 Inverter Problems

PlainBill - The labelling of the photos seemed like a logical method of describing them and distinguishing one from the other. I'm glad you approve! The Service manual you suggested is an excellent resource and will come in handy tracking down the problem. It's not immediately obvious to me where the problem lies though since I have checked most components related to J598 or FU9024 in the service manual.

Krankshaft - I can never be 100% certain it is all correct but I rerouted the jumpers so that they went to the same points as originally and also checked the wiring against a circuit diagram and it looked correct. I also have 3 other identical monitors and it agrees with those too. The replacement of those transistors solved my intial problem of the classic couple of seconds backlight flash. Overall I am reasonably confident they are OK, but its a good idea to put some electrical tape on there.

The wiring issues you suggest sound plausible and it's where I have been looking for mistakes but I've rechecked on many occasions now and can't see any problems. The diode tester yields results which look ok too. Nevertheless I'll go over it all again and look for any touching joints or suspicious readings.

Would the transistors not shutting off properly cause the FET to heat up? I couldn't figure out why that was getting so hot.

[PlainBill]

Re: HP 1955 Inverter Problems

PlainBill - The labelling of the photos seemed like a logical method of describing them and distinguishing one from the other. I'm glad you approve! The Service manual you suggested is an excellent resource and will come in handy tracking down the problem. It's not immediately obvious to me where the problem lies though since I have checked most components related to J598 or FU9024 in the service manual.

Krankshaft - I can never be 100% certain it is all correct but I rerouted the jumpers so that they went to the same points as originally and also checked the wiring against a circuit diagram and it looked correct. I also have 3 other identical monitors and it agrees with those too. The replacement of those transistors solved my intial problem of the classic couple of seconds backlight flash. Overall I am reasonably confident they are OK, but its a good idea to put some electrical tape on there.

The wiring issues you suggest sound plausible and it's where I have been looking for mistakes but I've rechecked on many occasions now and can't see any problems. The diode tester yields results which look ok too. Nevertheless I'll go over it all again and look for any touching joints or suspicious readings.

Would the transistors not shutting off properly cause the FET to heat up? I couldn't figure out why that was getting so hot.

Good question, the answer is yes.

The inverter is a constant current, high frequency, high voltage AC source. The current through the CCFLs is fed back to the inverter controller to limit current. It is further reduced to limit brightness. The 2SC5707's, poly cap, and transformer form a Royer oscillator. The FET turns on and off to limit the average current. If the FET is always on it would overheat.

PlainBill

[Electronic]

Re: HP 1955 Inverter Problems

I have checked the small signal transistors again with a diode tester and there are no shorts. I'm not entirely sure what unit the diode tester works in but the lowest reading I get is around 700 and the rest are completely open circuit which is consistent on all transtors in circuit. The 700 reading is when the negative lead goes to the base and the positive is on either of the other pins. Reversing the leads is gives a reading of 1 which is open circuit.

I've attached a close up image of the area around the suspect circuit. Not sure if it is of any use. One of the transistors looks like its touching another joint but it isn't and there is insulating tape there now to make sure.

I've also attached a circuit diagram of a similar board which might aid proceedings if the same information is available.

Referring to the diagram, and looking at the top half of it (because its easier to see), if Q751 is getting hot, and Q753 and Q752 appear ok... I have checked D751 and the parallel cap and since the other half of the inverter is not getting hot, i believe the problem is unlikely to extend beyond Q761. Indeed the conclusion of wiring errors would again be logical but I can't find any... going round in circles!

[jetadm123]

Re: HP 1955 Inverter Problems

Did you try checking the bias resistors for Q752-753? From the schematic, they are R766,767 and 755. Check their resistance to see if they are within specs.

[Electronic]

Re: HP 1955 Inverter Problems

Thanks for the advice - I've just checked now and they are within spec. The furthest out is supposed to be 4.72k and mine is 4.69k.

[Electronic]

Re: HP 1955 Inverter Problems

The inverter is a constant current, high frequency, high voltage AC source. The current through the CCFLs is fed back to the inverter controller to limit current. It is further reduced to limit brightness. The 2SC5707's, poly cap, and transformer form a Royer oscillator. The FET turns on and off to limit the average current. If the FET is always on it would overheat.

I'm starting to think that it might be the polycaps which are no good which is resulting in an incorrect resonant frequency for the transformer which is in turn passing too much current through the FET. I have not really considered these so far because the general view seems to be that they are rather robust. Unfortunately I don't have any way to test them other than looking at the frequency through an oscilloscope but I don't think that would do the circuit much good (being left on when the components get extremely hot almost instantly) and it's difficult if not impossible to get under the circuit when the monitor is operating.

A counter argument to the polycap problem would be that the monitor appears to work ok when it is switched on and images are displayed.

[Electronic]

Re: HP 1955 Inverter Problems

I've been checking over the circuit once again and the conclusion of wiring errors/problems seems to be becoming apparent. One track which should be short circuit has a high resistance and the transistor jumpered with the red wires appears to be wired wrong but I need to check this against the circuit diagram.

It's crazy how many times I can look over a circuit and think it looks fine... Having said that i've also convinced myself things are wrong when they aren't. Time to take a break!