Question about HP 71 17 in.CRT Conventional Monitor

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HP-71 D8901a with fuzzy video.

I have an HP-71 with fuzzy video. I have adjusted the focus on the flyback. There are no pots on the board for fine focus adjustment. I will start testing the caps and trans. Does anyone know the specific component that should be replaced? Either on the neck board or the mainboard? Thanks for any help.

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Re: HP-71 D8901a with fuzzy video.

A weak or defective Picture tube can also cause a crt to get fuzzy or out of focus.

Posted on May 19, 2006

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Image not focused

Possible worn out tube. The focus adjustment is on the flyback transformer and has to have the back cover removed to access it. The flyback will be a black and have a red wire connecting to the crt tube. This is very high voltage and can be dangerous. With a plastic screwdriver you can turn the focus adjustment knob on the transformer. Do not adjust the screen control knob. If the focus does not clean up, then the tube is probably worn or the flyback is bad.

Nov 27, 2013 | Samsung SyncMaster 1100MB 21" CRT Monitor

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Monitor display is fuzzy and pixelated


Focus voltage on the CRT is usually in the range of 2-8 kV DC and should be controllable over a fairly wide range by the focus pot - usually located on the flyback or a little panel in its vicinity:

If adjusting the pot results in a position of acceptable focus, you may be done. It is not unusual for the focus setting to drift a over time.

If the setting is already as good as possible but not really good enough, the CRT may be tired. Alternatively, the filament voltage may be too low. Check for bad connections in the filament circuit.

If the optimal setting is out of range of the focus pot, the problem is likely leakage in the focus divider in the flyback or one of the components on the CRT neck board.

Also see the sections: "Focus adjustment" and "Focus drifts with warmup".

The focus wire usually comes from the flyback or if the general area or from a terminal on a voltage multiplier module in some cases. It is usually a wire by itself going to the little board on the neck of the CRT.

If a sparkgap (a little 2 terminal device with a 1/8" gap in the middle) is arcing with power on, then the resistive divider has shorted inside the flyback, focus board, or HV multiplier - whatever you TV has - and the this unit will need to be replaced. Ditto if the SCREEN control affects focus and/or vice-versa.

Using a suitable high voltage meter (range at least 10 kVDC, 1000 M ohm or greater input impedance), you should be able to measure it connected and disconnected. The ground return will be the outside coating of the CRT which may or may not be the same as the metal chassis parts. If the voltage is very low (less than 2 kV) or too high and the pot has little effect:

When measured right off of the source disconnected from the CRT neck board, then the problem is probably in the focus network in the flyback (or wherever it originates). Sometimes these can be disassembled and cleaned or repaired but usually requires replacement of the entire flyback or voltage multiplier. Note: you may need to add a HV (10 kV) capacitor between the focus wire and DAG ground to provide filtering so you get a DC level for your meter.

When measured with the focus wire attached to the CRT neck board with the CRT connected but reasonable with the CRT unplugged, there is probably a short between the focus and another electrode inside the CRT. See the section: Rescuing a shorted CRT.

When measured with the focus wire attached to the CRT neck board with the CRT unplugged, there is likely a component on the CRT neck board that is leaky or breaking down. Also, check for decayed (tan or brown) glue which may turn leaky with age.
Good Luck...

May 30, 2010 | Computer Monitors

1 Answer

LG 700E monitor gives a blurr vision

open unit, adjust focus pot on flyback, may have 2 focus pots.
do both until satisfied.

If the problem varies with temperature (on time), then you may have a failing flyback.

Oct 02, 2008 | LG Studioworks 700S 17" CRT Monitor

1 Answer

Blurry screen

There is a focus adjustment inside on a part called the flyback transformer. It's located toward the back of the circuit board, and you'll know which part it is by the red cable going from it to a connection on the picture tube. The flyback generally has two small adjustment knobs on it. The focus knob is the upper one. Slightly turning this one way or the other may improve the focus a bit, but usually a sudden loss of focus means either the flyback or the picture tube has failed. Either of these is cause to treat yourself to a new monitor. You can buy 17-inch flat screens now for less than $150 at regular prices, and sometimes you'll catch a good sale at the office stores.

If you choose to try the focus adjustment, BE VERY CAREFUL!!

1) You have to remove the back of the monitor and work with the power on. Parts inside, especially the picture tube, can give you a very nasty shock AND EVEN KILL YOU.

2) Be very careful where you are putting your fingers when working on a live set. Make sure you have a safe grip on the adjustment knob first, THEN look at the screen while you adjust it. Don't go blindly reaching around.

Aug 31, 2008 | HP Compaq FS7600 17" CRT Monitor

1 Answer



I have posted this response to similar problems.

Your display being fuzzy as you described is indicative of either:
a. weak picture tube; or
b. component failure or deterioration.

Even if it were a weak picture tube or deterioration, it is still possible to "squeeze" a few more usable time out of the monitor. You simply have to adjust the focus control. Normally, this is accessible from the rear of the electronic circuit board and is a part of the Flyback transformer (FBT). Below is an image of a sample FBT. Pls notice the two (2) protrusions, one of them is marked as Screen and the other as the Focus. You need to adjust the focus till it satisfies your visual requirement or preference.


A word of caution. This procedure is not recommended if you have no TV/Monitor repair experience. It is not a fix. It is as earlier stated, squeezing in just a few more usable time. The unit would have to be adjusted "live" with the cover off. You need to be familiar with electronic components, tools and circuitry as well as working with powered devices and extremely High Voltage. The monitor may have to be operated with insulated support since the back cover will not be attached. Additionally, you may need to use a mirror to check if you adjusted it as you want it.

As you try to adjust it with an insulated screwdriver, you will notice that there is mid point where the focus will just be about right. You may have to adjust a couple of times to get the setting as you want it.

Should you feel uncomfortable doing this, perhaps you may need to seek the assistance of a qualified TV/Monitor technician.

Hope this be of some idea/help. Pls post back how things turned out or should you need additional information.

Good luck and kind regards.

Jun 26, 2008 | NetTV DTV 34X 34 in.CRT Conventional...

1 Answer

Horizontal/Vertical Postion of Image on LCD Monitor

the controls for focus adjustment are on the flyback transformer, (that big black thing on the left hand side (usually) of the monitor with the thick red wire that goes to the CRT.) the flyback may have 1,2,3 or even 4 knobs on it. 3 of these would be labled SCREEN (bottom knob of flyback. handles brightness)
FOCUS 1 + FOCUS 2 (focus adjustments. these are what you need to play with) and the 4th knob (some old mitsubishi workstation monitors had these) i do not know, it wasnt labled and HV adjustment was on the deflection board

Apr 25, 2008 | Gateway FPD2275W LCD Monitor

2 Answers

Fuzzy, unstable display

This sounds like a high-voltage power supply problem, which would need to be diagnosed by an experienced computer or electronics technician.

Check to see if there is a small focus knob on the back. If so try turning it back and forth three or four times to "clean" it....then adjust for correct focus.

Symptoms like this can also be caused by accumulated dust inside the monitor.

Jan 15, 2008 | Sony GDM-C520K (Dark Gray, Gray) 21" CRT...

1 Answer

Unclear images

Remove the rear cover and adjust the focus control on the flyback. Use a non-conductive probe, available as adjustment tools from any radio shack. On the FBT (Flyback Transformer) you have two protruding adjustments - one marked screen, the other focus. The screen adjustment increases the G2 voltage to the crt and brightens the picture by increasing the voltage exponentially. You don't want to adjust the screen voltage. The other is the focus, you can easily turn that clockwise or counter clockwise and watch the screen to set it at your preference.
Hope this helps.

Nov 11, 2007 | E-Machines 17F2 17" CRT Monitor

1 Answer

My samtron 55 monitor becomes hazy and out of focus

Focus and brightness (sometimes called screen control) are usually adjusted in the flyback transformer. Check for a bad connection where the flyback is soldered to the board. This could also indicate a problem with the CRT.

Sep 06, 2006 | SAMTRON 55V 15" CRT Monitor

3 Answers

Samsung 753S focus problem

Focus controls are on the back of the flyback transformer. You may have bumped one while the back was off. Due to dangers working on live circuits I recommend you have a service professional adjust the focus.

Sep 04, 2006 | Samsung SyncMaster 753S 17" CRT Monitor

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