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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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...
If the colors appear to be aligned and the only isue is focus, than you can adjust the focus on the flyback transformer. Carefully remove the rear cover and locate the flyback transformer (part with large red wire coming out the top). There should be 2 controls/adjustment points on it. One is for screen and the other for focus. Try adjusting the focus using a mirror to see the front of the set. You should adjust the control back and forth a ferw times to "loosen" it up a bit before settling down to the focus point.
Is that diagonal thin line about 10 to 20 degree slant? And then you have a very bright raster?
Then you set the flyback(for tube tv), I think its "screen" voltage is set too high adjust it slowly to lessen the brightness. It is commonly a small pot on the side of the flyback together with the focus pot.
If you have raster turn the tv on, use insulated screw diver, then slowly turn the knob to the direction(CC or CCW) where your raster go dim.
Be careful your dealing with live and high voltage. If your not inclined on electronics better not to.
Take a wire ground it put it in a srew and screw it to the frame somewhere for grounding pull out the video board out of the CRT tube must plug in for this job stick wire it the square hole one crt pin u hear a pop u done it,or u hear a guruing sound ur crt not very good it still work.unplug,put back the video board After u put back the video board to Readjust the focus on the flyback transformer.U just rejuvenate ur crt pic tube.
Inside the set you should see a large part with a red wire coming out the top. That is the flyback transformer. There will be 1 or 2 controls on the back of it. One should be labeled screen and one focus. Carefully adjust the focus one until the picture is again in focus. Please note that in that area are high voltages (33,000V) that can be dangerous. Be careful where you touch. IT is best to have a mirror so you can see the picture while adjusting the focus control.
retrace lines usually means screen voltage is to high . that could mean bad picture tube.or flyback.
But there is something you could try. Be careful high voltage.on the back on the flyback transformer there to adjustments screen voltage and and focus go slightly counter clockwise or in your case clockwise that pot addustment will make it darker or lighter.If this does not fix it you have a flyback oy tue problem.
I hope this helps thanks doctor video leadingedgeelectronics 1987
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.