Author Topic: Need help: CRT B/W monitor too bright  (Read 1993 times)

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Online Bicurico

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Need help: CRT B/W monitor too bright
« on: July 30, 2017, 09:32:15 pm »
Hi,

I purchased a second hand Unoahm EP 2500 field meter. It is in working condition and was overall a nice deal and cool add-on to my collection.

I cleaned it, stiched the softbag and was pretty happy, when I noticed that suddenly the CRT brightness increased. I cannot specify the exact moment.

Anyway, even with the brightness toggled (per software) to minimum value, the screen is too bright.

I opened the case (once again) and toggled the miniature poti, but none allows additional brightness control.

The field meter can be operated in three modes:

- just by 12V Ni-MH battery, which is not a real option, because the battery is virtually dead and the image on the CRT screen does not get to its whole width, probably due to lack of voltage?
- just connected to mains through a 16V DC transformer and battery removed - image is too bright
- connected to mains and charging the battery - image is too bright

On the power input, a sticker says 12-18V operation, 15-18V charging. 12V is currently not enough to drive the CRT as it should, unless I reconfigure the three poti, but then the image gets screwed up when I attach the power source.

I tried feeding 12-16V on both the power input and battery connector and with a voltage <15V the image is a little darker, but not filling up horizontally, >15V it fills the CRT horizontally, but is too bright.

From what I read, the cause could be the 250V capacitor on the CRT board, but visually it looks OK. Actually all capacitors look fine.

I know about the dangers and risks regarding handling a CRT and despite this one being just 4.5", I do have respect of the charge contained in the tube and capacitor. I have discharged a CRT a couple of times in the past, also of a field meter (I exchanged the CRT tubes between two field meter).

My questions:

Where should I start to find the culprit?
What is likely to be the cause?

Other issues of this field meter:

1) CRT -> too bright
2) Memory operation like storing channel or logger data will produce a timeout error of the eeprom/flash. I ordered new ICs, as I want to upgrade the firmware, but the uploader software requires a key code based on the serial number, which I don't have.
3) Power source has a bad contact somewhere in the cable - needs to be cut and resoldered to the connector: It *may* have been due to a few reboots caused by sudden power loss that the CRT suddenly became to bright. The power source is working fine, though, as I measured it and used a programmel power source, too.

As it is, the device is fully functional and the screen is usable. It just annoys me, because I know it should be a much darker picture.

Mind me: the whole fun is this cleaning, repairing, upgrading firmware, finding all files, etc. Once all of this is done, the field meter goes to the cupboard where all the otehr field meter are stored... -->TEA thread calling... For regular use I have modern field meter which are light years ahead of this one.

Regards,
Vitor


Online Bicurico

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Re: Need help: CRT B/W monitor too bright
« Reply #1 on: July 31, 2017, 10:47:33 pm »
Attaching two pictures to raise intrest...

Not good pictures, will try to get better ones.

No doubt the pictures is too bright, I bet it is the 250V 3.3uF capacitor, but it might be something else. Thus I am eager to hear the opinion of experts, before I start to replace components randomly.

The small PCB with flat cable on the first picture is not related to the CRT. It is the RS232 + USB connector board.

The three poti on the first picture (lower left corner) adjust picture dimension XY, picture height Y and picture width X. A different pot (next to the black capacitor on the left lower side) adjusts the focus.

Sadly no poti to control brightness... Brightness is controlled by software setting. Even at 0% it is producing a too bright image.

Another possible culprit is an IC with 16 pins (I think), which has a heat sink on it (visible between the two blue potis on the right side, above the red whatever component). I am not sure what that IC does or if it is a kind of amplifier.

Regards,
Vitor


Thanks,
Vitor
« Last Edit: July 31, 2017, 10:52:49 pm by Bicurico »
 

Online Paul Moir

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Re: Need help: CRT B/W monitor too bright
« Reply #2 on: August 01, 2017, 12:46:49 am »
Are you sure there isn't a screen brightness control on the back of the flyback transformer? 
 

Offline IanMacdonald

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Re: Need help: CRT B/W monitor too bright
« Reply #3 on: August 01, 2017, 06:10:28 am »

I'd start by having a look at the cathode waveform, using a x10 probe of course. A healthy waveform will be all within the range between the supply voltage and 0v, if either end is clipped then there is a problem with the DC operating point. (Lower voltage = brighter)  Since the video stages can be DC coupled quite far back, that doesn't necessarily mean the problem is in the output stage of course. Have you checked the video amp supply voltage? Typically this is 100-200v fed from a rectifier on the flyback trans, and if it's too low then the screen will be overbright because the cathode voltage cannot be raised to cutoff point.
 

Offline Armadillo

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Re: Need help: CRT B/W monitor too bright
« Reply #4 on: August 01, 2017, 07:10:34 am »
Likely the 250V 3.3uF capacitor.
 

Online Bicurico

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Re: Need help: CRT B/W monitor too bright
« Reply #5 on: August 01, 2017, 08:27:08 am »
Can someone tell me what the component where the red arrow is pointing at is?

Can it be that this is supposed to be ajusted (rectangle on top of the cylinder, which I imagine is ferrite or so)?

Regards,
Vitor

Offline IanMacdonald

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Re: Need help: CRT B/W monitor too bright
« Reply #6 on: August 01, 2017, 12:36:04 pm »
Horizontal linearity. A saturable reactor in series with the scan coils. The magnet can be turned to as to bias the saturation point to one current direction or the other. When the core saturates, the scan current increases.

You should use a square plastic tool. A metal tool may break the ferrite.

BTW, re "Are you sure there isn't a screen brightness control on the back of the flyback transformer?  "

A1 voltage pot, normally only found on colour CRT types. About 1kV on the pot. The b/w A1 voltage is lower, and often not adjustable. Too high an A1 voltage will give a very bright screen, but probably not the issue here.
« Last Edit: August 01, 2017, 12:41:27 pm by IanMacdonald »
 

Online Bicurico

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Re: Need help: CRT B/W monitor too bright
« Reply #7 on: August 01, 2017, 07:42:05 pm »
@Ian, you were correct, but unfortunately that did not solve anything, either.

@all: Thanks for the help.

I guess the problem is within the CRT circuit board, but I have not enough expertise to repair it myself. I could try to randomly replace all capacitors and I am pretty sure that would probably fix the problem, but then it might not. Also, I am not fully at easy with CRT high voltage. While I know where not to touch, it's been a few years since I discharged the last one.

Finally, I don't want to risk creaking the CRT itself, which is in good shape.

So I decided that after the summer holidays I will get a real TV technician to have a go on this. One has to know when one reaches his limits.

Regards,
Vitor

Offline IanMacdonald

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Re: Need help: CRT B/W monitor too bright
« Reply #8 on: August 01, 2017, 08:29:11 pm »
Random component replacement is definitely not a good idea. One guy who insisted on that technique had a cap hit the ceiling and cause a dent in a title, ricochet off and land on another bench. People were ducking for cover, and the room was full of smoke.  :scared: The screenprint had the polarity wrong, a favourite trap for parts jockeys.  ;)

Before consulting a TV guy you might save some trouble by (a) checking the grid voltage, which should be relatively low or zero, and if that is OK, (b) powering up with the cathode lead disconnected (and obviously, insulated as it still has voltage) If this gives  a completely dark screen then likely the CRT and its circuitry is OK, and the problem is in the video stages.

If you still have a milky glow with the cathode disconnected then it's likely an internal leakage (insulation failure) between cathode and grid or heater. It could be other CRT voltages being way high, but it's usually leakage in that case.

 

Online Bicurico

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Re: Need help: CRT B/W monitor too bright
« Reply #9 on: August 29, 2017, 07:48:21 am »
Update:

Hi,

I want to share some update on this subject.

It turns out, that the CRT is perfectly OK.

The problem was something else, which I have sorted out to an acceptably, but not perfect, manner.

The way the device works (Unaohm EP-2500 field meter) is to provide several modes:

1) Analog TV (inluding FM/AM radio)
2) Digital TV
2.1) QAM
2.2) OFDM (emulated)
3) Analog SAT
4) Digital SAT

Each mode has its own video render engine and the resulting image get's the overlay OSD for menus and softkeys.

What I noticed was that some modes, like analog TV would have a perfectly OK image (crisp and sharp, good contrast and normal brightness), while other modes did not.

On top of it, the brightness and contrast setting (softmenu) work in percentage and simply turning down brightness to 0% would not work as expected.

This is what I did:

1) Reset the Audio/Video settings from the softmenu
2) Adjusted the image shape on the CRT board
3) Found a level potentiometer for the analog satellite board (this mode has good contrast now)

That left me with a not so good contrast on the digital modes. Here the image gets generated by the MPEG2 board, but neither any ELKO seem bad, nor is there any poti to adjust. I configured the image shape as good as possible for this mode on the CRT board and ended up with an acceptable image. Not perfect, but OKish for a >10 year old field meter.

I would love to replace the 4.5" CRT with a modern 4.5" TFT screen, but I have no clue how to figure out were the video signal going to the CRT is. Also, I am not sure if the field meter uses some "CRT tricks" to render the live spectrum - possibly it isn't even possible to just use a TFT! I say this because the device has a SCART connector, which can act as an input (to use the CRT as a video monitor) or to output the CRT image. This can be done in two modes ("monitor" or "mirror"). The first only outputs the live decoded video image, while the latter outputs the menus, too. However, this mode does NOT show the spectrum and digital measurements screen.

Very strange!

One a side hack, I managed to upgrade the firmware from v3.2 to the lastest v4.0. This is a feat, because Unaohm actually charged customers for the upgrade: the upgrader software requires a serial number dependant activation key to upload the firmware. Well, I bought me a suitable adapter for my Genius G540 programmer (great device for little money, btw.) and read out the flash (M29F040B). Then I took the v4.0 firmware file and "glued" it to the dumped v3.2 image, in order to get a new image with v4.0 and the original boot loader. Programmed it to a new IC (keeping the original, just in case) and... success! I will document the process in my blog.

Cheers,
Vitor

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Re: Need help: CRT B/W monitor too bright
« Reply #10 on: August 29, 2017, 08:30:06 am »
My blog entry rearding the firmware upgrade: http://vma-satellite.blogspot.pt/2017/08/unaohm-ep-2500-upgrading-firmware.html

Might be usefull for other owners of slightly older Unaohm field meter.

Regards,
Vitor


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