Author Topic: [SOLVED] Compaq V55 CRT monitor failing  (Read 1294 times)

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Offline DarkChronos

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[SOLVED] Compaq V55 CRT monitor failing
« on: November 17, 2019, 08:45:49 pm »
Hello everybody,

I need some advice regarding a failing Compaq V55 CRT monitor.

I got this monitor from an attic sale and plan to use it with some old computers. When I got the monitor I turned it on and it worked on the first time (green power light, tube warming up) and after a while the light turns orange and it goes to sleep, which is normal. So I connected the monitor's VGA cable to the computer and when turning on the computer, the monitor light went green again, tube started to warm up and "clic" the monitor shut down at this point and would not turn on again.

As soon as I plug the monitor to the AC I can hear the power coming in and the green power light will briefly turn on then I think that's the noise of the relay who is shutting down. I also noticed that the large 400V capacitor near the AC input remains charged even several hours after unplugging the monitor, I don't know if this is normal or not (I disassembled a PC PSU before and noticed the HV caps discharge quickly after unplugging it)

I checked the values of most capacitors of the board and also tested the power transistors but I did not find any issue there. I did not notice any physical damage to any component on the board either.

Attached are the board pictures. I also recorded a short video of what happens when I plug the monitor to the AC: https://www.mfsite.fr/vid/VID_20191117_193240.mp4

Any advice and suggestion is welcome.

Thanks in advance.
« Last Edit: December 17, 2019, 12:04:47 am by DarkChronos »
 

Offline sean0118

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Re: Compaq V55 CRT monitor failing
« Reply #1 on: November 18, 2019, 09:36:15 am »
If your multimeter has a low impedance mode you can use that to discharge the 400V cap. Just make sure you measure it again on the high impedance V DC mode to check it has discharged completely before working on the monitor.

Repair wise, first thing I'd check is for shorted power rails. With the monitor off measure the resistance to gnd. I know you probably already checked this when you checked the capacitors, but one shorted rail would cause it to shutdown due to over current protection.

Also CRTs can produce X-rays if they are supplied with a voltage that's too high. So manufacturers design in X-ray protection circuits which shutdown the flyback if the voltage sensed is too high. The circuit is normally a winding of the flyback, which is proportional too, but much lower than the CRT cathode voltage.  This then feeds some zener diodes which set the threshold that the circuit will trigger at. The problem is a faulty zener diode etc will cause the CRT to shutdown even if there is no over voltage.

It could also be the flyback itself is shorted.

Lot's of things can go wrong with CRTs, but they are interesting, you have high voltage, digital and analogue circuits all in the one device.  :D



edit: I should also mention the lead coming from the flyback or the pins holding it to the board could have 10's of kV on them. But I'm guessing you might already know to discharge that.  ;)
« Last Edit: November 18, 2019, 09:41:37 am by sean0118 »
 

Offline PKTKS

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Re: Compaq V55 CRT monitor failing
« Reply #2 on: November 18, 2019, 12:57:00 pm »
I don't have the schematics...

Hard to say "guess"  (even w/schema)

But, usually Multisync CRT are highly dependable
on the proper SMPS sync.

Vast majority of last generation multisync monitors
use an MCU unit to regenerate the synch and that
only happens when the SMPS can synch with the
incoming signal.

Otherwise the SMPS may eventually trigger some
sort of fault protection

Several reasons possible.
Start by inspecting the proper SMPS synch
procedure and if it is really entering protection..

Protection mode may also be caused by overloading

Paul
 

Offline shakalnokturn

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Re: Compaq V55 CRT monitor failing
« Reply #3 on: November 18, 2019, 05:48:09 pm »
A fast and efficient repair is a detectives work, unfortunately an art that I still don't master...

Considering it is quite old, that PCB is single sided with many heavy or hot components and that it has recently traveled being submitted to vibration. I would start by looking at the solder side, I'm quite sure you'll find some very doubtful. Take note of where the worst ones are and look for shorted semiconductors "in the area".

Take this with a pinch of salt it's been a long time since I've repaired a CRT monitor but seem to remember LG or Liteon were very common OEM's for Compaq.

Edit: Checked your photos again, considering the microprocessor is stamped "Phillips" I'd say the PCB is an LG job...
« Last Edit: November 18, 2019, 05:53:27 pm by shakalnokturn »
 

Offline sean0118

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Re: Compaq V55 CRT monitor failing
« Reply #4 on: November 19, 2019, 12:31:53 am »
I found a schematic that's meant to be for a Compaq V55. But seems to be a slightly different version, your IC202 has been replaced with IC282 for example. But it most of it looks roughly similar.

It's a pretty confusing schematic. The pages are in some weird order, I'm pretty sure the bottom of page 3 joins to the top of page 6 for example. If you have a printer, print it out and try to join the lines up.  :D

But having any schematic will make it much easier to find power rails to check for shorts to GND. If you have a look on page 4, there's a whole bunch coming out f transformer T601, pretty much every pin on the transformers secondary is a different rail (-12B, 75B, +13B, 7B, 56B, USB). Then there's some regulators like IC604 (KA78R12) and IC603 (KIA7005P), you can check the outputs of these for shorts too.

Probably worth checking the horizontal output transistor (aka HOT) and the components / rails around it for shorts. I'm pretty sure the HOT transistor is Q402.


That X-ray circuit I mentioned is made up of R503, R506, C502, D519 (24V zener), C503, R544, D502 coming from the flyback T501. But best to check for shorted power rails etc first and to check what others have mentioned.   ;)
« Last Edit: November 19, 2019, 12:33:48 am by sean0118 »
 

Offline DarkChronos

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Re: Compaq V55 CRT monitor failing
« Reply #5 on: November 20, 2019, 04:46:46 pm »
Hello,

Thank you very much for your replies about this issue, and thanks to sean0118 for finding a schematics.

My multimeter doesn't have a zlow mode, I had some spare 10K 5W ceramic resistors for discharging the 400V cap, and a good old screwdriver attached with a wire clipped to the ground for discharging the tube :)

I did not find something wrong in the SMPS components, and I checked if there was any cold joint but I did not find one.

I'll take a look at the rails coming out from T601, the ICs, and also the X-ray circuit, and also the components around the Q402 (which has been already previously tested)

I'll let you know my findings.
 

Online james_s

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Re: Compaq V55 CRT monitor failing
« Reply #6 on: November 20, 2019, 06:04:49 pm »
The first thing I would do is look for shorted semiconductors. Check the horizontal output transistor as well as diodes and transistors in the power supply. Back when I was repairing a lot of monitors I often found shorted transistors, a lot of bad electrolytic capacitors and occasionally an open resistor.
 

Offline DarkChronos

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Re: Compaq V55 CRT monitor failing
« Reply #7 on: November 21, 2019, 06:15:48 pm »
Hello,

The power rails, HOT and power supply diodes/transistors seem to be fine.
X-ray protection circuit components are good too.
I'll do further testing. I also pictured the backside of the PCB.

An interesting point, I found an article about flyback transformer testing and one part of it is stating this:
Code: [Select]
Capacitor shorted

a. No display (No high voltage).
b. B+ voltage drop.
c. Secondary diode (UF5404) will burned or shorted.
d. Horizontal output transistor will get shorted.
e. Power blink.
f. Sometimes power section will blow, for example: MTC 15 inch monitor.
g. Power section shut down for example: Compaq V55, Samtron 4bi monitor.
h. Sometimes the automatic brightness limiter (ABL) circuitry components will get burned.
This circuit is usually located beside the flyback transformer. For example: LG520si

The g line mentions about power section shut down and Compaq V55 as example, it is matching my issue.

I will test the flyback transformer internal capacitor, in case it's shorted, do I need to replace the whole FBT?

Thanks in advance.
 

Online james_s

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Re: Compaq V55 CRT monitor failing
« Reply #8 on: November 21, 2019, 07:05:41 pm »
I've never seen a flyback transformer with an internal capacitor, if the flyback does have any kind of fault you pretty much have to replace it though. Fortunately in my experience flyback failures are rare although they can happen.
 

Offline DarkChronos

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Re: Compaq V55 CRT monitor failing
« Reply #9 on: November 22, 2019, 12:39:50 am »
Hello,

This flyback transformer is a Samsung FKD-15A001, I attached the schematic and I pictured it after taking it apart from the PCB. The capacitor is located inside the plastic under the sticker.

I checked the capacitance between the anode and pin 4 of the transformer and I don't get any capacitance reading (a correct reading is supposed to be between 1.5 nF and 3 nF).

I guess the FBT is good for replacement?
 

Offline PKTKS

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Re: Compaq V55 CRT monitor failing
« Reply #10 on: November 22, 2019, 11:00:06 am »
Hello,

This flyback transformer is a Samsung FKD-15A001, I attached the schematic and I pictured it after taking it apart from the PCB. The capacitor is located inside the plastic under the sticker.

I checked the capacitance between the anode and pin 4 of the transformer and I don't get any capacitance reading (a correct reading is supposed to be between 1.5 nF and 3 nF).

I guess the FBT is good for replacement?

Hard to say.. always is with flybacks which may have
some sort of HV  leakage not easily spotted

That part actually is the  "TRIPLICATOR"  which in several
models is a separate component.. that will double the
price of the flyback per si.

It is just an HV filter in which the CRT ACQUADAG capacitance may
also suit as well.. as a filter..

IF it is not shorted it should not pose a problem
question is how much HV leakage it has now..

And how to check without a new one.
Typical flyback repair problem

Paul
 

Offline andy2000

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Re: Compaq V55 CRT monitor failing
« Reply #11 on: November 22, 2019, 05:55:16 pm »
Troubleshooting shutdown problems can be difficult since everything shuts down before you're able to observe the symptoms.  Start by checking for obvious problems like bad soldering, shorted power transistors, and physical damage.

It could be caused by excessive voltage from the SMPS, or HV, or both since excess B+ will usually lead to excess HV.  Some monitors shut down if the vertical deflection fails (to prevent a line from burning into the CRT).  It could be over current shutdown of the SMPS.

Be sure that you're sending it a signal that's within it's range.  Most monitors will shut down if you send them a signal with too high of a horizontal sync frequency. 

If you have a service manual, there are often instructions for figuring out what is causing shut down.  If not, you may be able to reverse engineer it from datasheets for the SMPS and deflection controllers. 

Sometimes the shutdown circuit itself fails and causes it to shut down even though there's nothing else wrong.

As a last resort, you can try disabling the shutdown circuit.  This will give you time to observe symptoms and make measurements.  Obviously, there's a risk of causing irreparable damage from excess voltage or current. 
 

Offline DarkChronos

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Re: Compaq V55 CRT monitor failing
« Reply #12 on: November 25, 2019, 08:38:45 am »
I looked online for replacement flyback, I found one but the cost is approximately 30 euros ...  :-\

I will stick for now on checking SMPS, transistors and other components I did not check before. I will also resolder them at the same time to make sure no cold joint problem.
The HOT is not shorted, and I have a spare vertical deflection IC that I can also try later.

If all components are alright, I will have to go for a new flyback ...

I'll let you know the results.

Thanks again :)
 

Offline PKTKS

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Re: Compaq V55 CRT monitor failing
« Reply #13 on: November 25, 2019, 09:27:13 am »
I looked online for replacement flyback, I found one but the cost is approximately 30 euros ...  :-\


Yep .. with integrated triplicator they are not cheap...

Exhaust all possible candidates first ...
the flyback check is always a 50% chance unless
you can spot a dead shorted one.

Typical problem with CRT flybacks
Paul
 

Offline sean0118

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Re: Compaq V55 CRT monitor failing
« Reply #14 on: November 27, 2019, 07:56:35 am »
If you are looking for a new project you could make your own flyback tester?

http://bobparker.net.au/lopt_tester/fbt.htm

Might not be easier/cheaper than a new flyback. It only tests at low voltage, so won't detect high voltage breakdown. But it's a cool project nonetheless.


Out of interest, when you tested the zener of the x-ray protection circuit, did you take it out of circuit and measure it's zener voltage? It may have drifted even if it's 0.6V forward voltage drop is correct. Probably not that likely it's failed, but worth testing before buying a new flyback.  ;)
 

Offline m3vuv

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Re: Compaq V55 CRT monitor failing
« Reply #15 on: November 27, 2019, 12:17:12 pm »
a couple of things spring to mind,can you hear the high pitched whistle from the lopty also what about cap esr values?
 

Online Renate

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Re: Compaq V55 CRT monitor failing
« Reply #16 on: November 27, 2019, 04:00:27 pm »
I dunno, if it's shutting down and you don't know if it's overcurrent on the HOT or overvoltage on the HV, why not pull the transistor and see if it stays up?
 

Online james_s

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Re: Compaq V55 CRT monitor failing
« Reply #17 on: November 27, 2019, 06:21:53 pm »
30 Euros sounds pretty cheap for a flyback transformer, I remember paying $50-$100 for some of them back in the day and that was more money then than it is now too. Frankly I'm surprised you can still get one at all for a random PC monitor. I have a ring tester that I built from a kit many years ago, I think it's a Dick Smith thing. I've also used a 555 driving a mosfet to test (and play with) flyback transformers. If it is indeed an internal capacitor that is shorted you may be able to cut it out and replace it.
 

Offline andy2000

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Re: Compaq V55 CRT monitor failing
« Reply #18 on: November 27, 2019, 09:51:18 pm »
Honestly, I wouldn't put much time, or money into a run of the mill 15" CRT monitor unless these are exceedingly rare where you are.  While it's tempting to blame a part that can't be conclusively tested (like the flyback) that's just a shot in the dark.  It could be any one of 20 other things causing shutdown. 
 

Offline DarkChronos

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Re: Compaq V55 CRT monitor failing
« Reply #19 on: November 29, 2019, 08:33:40 pm »
Hello,

Thanks for the replies, I went through testing most components of the board, all SMPS area and around the flyback too.
I found the solder started to be ugly on the big red ceramic caps lined up near the HOT, I resoldered properly and I tested all the components around too.
I also tested the small ICs for shorts but they're all good.

The LOPT tester looks nice to try. I'll check if I can build one.
The 24V zener diode has been replaced already but no luck.

An interesting thing about the "high pitched whistle" is that I actually not hear anything when plugging the monitor but after 10 seconds suddenly it comes.
I made an audio recording of it attached below, you can hear after I plug the monitor nothing happens until 10 seconds passed the whistle comes.
Any clue about this?
I have no ESR tester, should definitely get one.

I don't know if the internal capacitor can be easily replaced because I've seen the plastic has been glued all around the capacitor area. I may try but I want to clear out all other possible elements that could trigger the shutdown, I think I already checked most of them, maybe I'll test the remaining components I did not test yet, except if any of you has a specific idea :)

Thanks in advance.

[Edit]: And also, as Renate said, I will try to remove the transistor and plug the monitor to see if anything changes.
« Last Edit: November 29, 2019, 09:40:28 pm by DarkChronos »
 

Online james_s

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Re: Compaq V55 CRT monitor failing
« Reply #20 on: December 02, 2019, 04:54:54 am »
That Bob Parker LOPT tester is exactly the one I have, I built it from a kit years ago, it's been a very handy device. Several years ago my friend needed one so I reverse engineered mine and he built his on perfboard. That's pretty cool that you can download the design now.
 

Online Renate

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Re: Compaq V55 CRT monitor failing
« Reply #21 on: December 03, 2019, 08:34:02 pm »
Well, I can't hear anything anyway.
It could be that the circuit is not doing anything, then it oscillates at frequency X.
It could be that it works correctly at frequency Y (which you can't hear/don't notice) then goes to frequency X (which you can hear).
 

Offline DarkChronos

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Re: Compaq V55 CRT monitor failing
« Reply #22 on: December 17, 2019, 12:03:29 am »
Hello,

I have been off a while because I had some vacation and also because the happenings in my country.

However I really wanted to clear out of my mind that internal capacitor thing.
I found a website (am I allowed to share it?) located in Eastern Europe where they sold a replacement FBT (Diemen HR 46117) for only 10 EUR. I placed an order there before I left for vacation.

When I came back home yesterday the package was already delivered. The first thing I did was to test the capacitance between the ground pin 4 and the anode cup and this time I got a result of about 3nF, where the original FBT was giving nothing.

So I installed it and tested, and yes it is back to life!  :clap:
I went on adusting screen and focus and now it's really perfect.

Here is a picture of the monitor plugged to the computer it originally came with
890902-0

And I want to thank again everybody here for involving and advising me to get it fixed  :)

Have a great day!
 

Offline Rasz

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Re: [SOLVED] Compaq V55 CRT monitor failing
« Reply #23 on: December 17, 2019, 11:08:10 am »
Afaik its a common practice to always preventatively(sp?) replace HOT/snubber when fitting new flyback, and go over all solder joints in the area.
Dont know if its applicable to cases when flyback is simply worn out due to age, plus might be hard getting your hands on a non fake transistor replacement part.
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Offline PKTKS

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Re: [SOLVED] Compaq V55 CRT monitor failing
« Reply #24 on: December 17, 2019, 12:11:49 pm »

Lucky found.

Vintage stuff here sells for absurd prices.
This whole monitor would cost may be $50

But bit and pieces for it can go as high as well

Paul
 


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