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I have had this problem with a few 485's I have refurbished and every time it was the X6 voltage multiplier U1600 located on the A15 Transformer Board.? It is supplied by Voltage Multiplier Inc., 8711 W. Roosevelt, Visalia CA? 93291, their part number VM176 and sells for $175 in small quantities.??
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You can test if U1600 is working but you must be very careful or you will burn out U660, the vertical output IC.? To test U1600 proceed as follows: 1)? Unsolder the connections from the A7 vertical board going from U660 to the CRT.? This removes part of the arcing path which might be present in the next few steps.2)? Pull out the connections to the horizontal plates present near the rear of the A11 Horizontal Amp board.3)? Unplug the two wires going from the hybrid board to the forward connections of the distributed vertical plates. You have now isolated the CRT and eliminated any chance of damage to any semiconductor components due to a large current surge.? Now start your 485 up and run it for about a minute.? Turn the power off and immediately pull out the high voltage connector going from U1600 to the CRT.? If you get a nice fat spark as the connector clears the plastic socket then you know the X6 multiplier is working.? If you get a spark the problem is in the horizontal output circuit.? Replace all the CRT connections removed in the above numbered steps. If you get no spark replace U1600.? I always replace R1602, a 15K, 1/2W resistor but do not use a film resistor, use a carbon composition one. Reed Dickinsonreed714@... On Monday, December 23, 2019, 09:16:12 PM PST, Harvey White <madyn@...> wrote:
The horizontal input has a calibrated deflection factor.? Adjust the vertical to the same factor, put a sinewave into both, and you should get a 45 degree line. If the amplifier is ok, and the problem is in the sweep, then you should be able to center the horizontal dot (no sweep, x only). It should have about the same travel (L and R) with the positioning control. If you have another scope, you can check for linearity by borrowing a ramp from it and using it for a horizontal sweep. Good function generator will also work. Harvey On 12/23/2019 1:26 AM, stephen white wrote: How can we narrow this short sweep to the sweep circuit or to the horizantal amp circuit ???? I am back at this problem child of mine again and am trying to figure which board to chase around on...I have a few more things to check per all the good inputs I have had here but seems like it would a good thing to? determine which board the problem is on.. |
