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...
Electronic
Member
Last Activity: 12-07-2013, 08:37 PM
Joined: 01-03-2011
Location: UK
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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.Leave a comment:
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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!Leave a comment:
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Re: HP 1955 Inverter Problems
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)...Leave a comment:
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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.Leave a comment:
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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...Leave a comment:
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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...Leave a comment:
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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.Leave a comment:
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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!Leave a comment:
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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...Leave a comment:
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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...Leave a comment:
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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 fineLast edited by Electronic; 02-09-2011, 10:37 AM.Leave a comment:
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Re: New Members - please post your introductions here
Hi, I have a MEng degree in electronic engineering and looking to study for a phd in carbon based electronics. I've come here because of persistent problems with a HP L1955 monitor PSU!Leave a comment:
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HP 1955 Inverter Problems
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...
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