Yoke Inductance vs resistance-identify your monitor- the right replacement

[Rickn]

Hi all, I have noticed a few postings on the forum regarding the resistance of the yoke to determine the operating resolution/frequency and to find a suitable board replacement.

Not that resistnace means nothing, but the real determining factor is Horizontal inductance.

Here are some guidlines on determing a few things about your monitor as well as determing operating frequency/resolution...

Measure the inductance as follows:
 - Make sure there is no power to the monitor.
 - Unplug the yoke from the chassis, this plug usually has 4 wires 1
 plug,  but in some cases may have 2 plugs with 2 wires each.
- The Horizontal windings on the yoke are usually the red and Blue
wires, the vertical is usually Green and Yellow. - Measure and record the horizontal readings by setting the meter to the 200mH (milli henries) scale. Now make contact to the red and Blue leads ( if  the solder joints where the leads connect to the yoke are accessible connect there- if not put a couple of paper clips in the plug to make the contact)
- This should tell us what type of chassis should match the yoke.
-If the readings are in the .9 to 1.3 range it is a CGA (standard res)
yoke.
- If the readings are .6 to .75 it is EGA)mid res or switchable CGA/EGA
 - If the readings are .24 to .3 it is high res /VGA
- If the readings are .17 to .20 then it is SVGA or higher.
        These readings are based on reasonable design practices, in some case
like the Neotec 2701, they have not used the correct yoke. the readings indicate
a match fo a VGA monitor and they have used an EGA chassis. This requires
the yoke to run hotter and cause early failures.

 Repeat the reading on the vertical windings (green and yellow). In this
case you may need to set the meter to 200 range, readings will be in the 6 to
30 range. These readings are useful in identifying the yoke and hopefully
avoiding vertical size problems.

 In any case where you are looking for a replacement board We need to know
 the> following if possible:
- Tube manufacturer (such as Thompson, Phillips...)
- Tube number such as a68..., The 2 numbers are the size of the tube as measured Diagonaly from 1 corner (top left) to the other (bottom right). It is measured Phosphor to Phosphor (the actual lighted are of the screen< not glass to glass. This is where the confusion cones between say 27V (27 viewable) and 29", they are the same. The 27V is the viewable area of the screen, the 29 inch glass to glass.

- In some cases the tube may be dual focus- this can be determined by looking into the white plastic socket. You will notice a "triangle like shape, if there is 1 pin inside this triangle- it is single focus, 2 pins inside means dual focus.

 - If available the part number and manufacturer of the yoke (if there is a label) as well as the horizontal and vertical inductance.
 - Also if available the make and model of the original chassis.

One more thing to consider is the signal input connector. If it is a 15Pin Sub-d, it is likely VGA/SVGA... or tri-res.
Molex type (white plastic) usualy indicates standard or Mid res. In this case count the wires used...
-if 5 wires then we have Red, Green, Blue, Ground and Composite sync ( the vertical and Horizontal sync signals are on the same wire)
- if 6 wires then the Horizontal and vertical sync are on separate wires.

Armed with this information, it should be possible to get a replacement board, but you must be sure your tube is good!!
By the way the meter you would need for this is about $40, please see the link below

http://www.circuitspecialists.com/prod.itml/icOid/3394


Hope this helps...

Rick Nieman

Rick@Niemandisplays.com