Re: Tektronix 2445 calibration issues


Mark Vincent
 

Dennis,

Resistors: R710, R910 https://www.mouser.com/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/_/N-7gz41Z1yzvvqx?P=1z0vko0Z1yzbppy&Keyword=1%25+resistor&FS=True (2 in series. Pick which two parts you want) The additional meg will not hurt. R1871 279-H4P150KFDA, R1872 279-H4P180KFDA, R1853 (2) 660-MF1/2CC2213F (put in series) , R1854 71-CCF60-332K, R1880 279-H4P430KFZA, R1991 588-SM102031006FE or 588-MOX-300001006FE, R1018, R1019 approx.100,000 ohm 1W 1-5% (bleeds off filters faster. Precision not necessary), R1020 71-CCF60-133K-E3 and 71-CCF60-137K-E3 or a single 2W 270,000 ohm, R1352, R1353 to one 588-45F7R5E (original pair of 15 ohm 1/2W resistors run hot), RT1696 parallel 10,000-13,000 ohms across thermistor to speed fan up, R1220 and R1240 check and replace if out of tolerance, R125, R225 279-H4P360RFZA , R811 279-H4P330RFCA. If you want to raise the 1/8W resistors in some places on the power supply board to 1/4W, go ahead, it is your item. Some of the resistors will give problems, especially on the high voltage board to the collectors of three transistors. The others should be raised in wattage and mounted off the board. The 270,000 ohm 1/2W resistor in the power supply should be 2W. The original is stressed hard.

Electrolytics and film: The main filter 290mfd to 390mfd Nichicon LGR. See which diameter will fit and get a pair to replace them. The 47mfd decoupling throughout Nichicon ULD, The 4,7mfd tantalims to 80-R82CC4470AA30J or 505-MKS2B044701KJC00. The caps on the A5 board can be the ULD 47mfd types The 47mfd electrolytics will be 25V. The RIFA types must be replaced! These are on the power supply schematic. The mains filters C1016, C1018 594-F339X126848MDA2B, C1020, C1051 594-F339X122248MDA2B, C1065 871-B32912A5563M. The 10mfd on the fan board can be a ULD 50V type. There are no lover voltages in that series at 10mfd. The 3.3mfd C1072 in the power supply 647-ULD2G3R3MPD.

Items listed as you progress down the schematics. Items listed are ones currently in stock. This is how I listed the Mouser part numbers.

Recapping and new resistors may cure your problems. The electrolytics I listed are low ESR, high temperature and long life. The film replacements for the three 4,7mfd tantalums is advised. These tantalums are frequently bad. Film is not polarized and have lower ESR than tantalum. The resistors I listed are 1% types that have very low resistance drift with temperature, PPM/C.

Check the diodes in the power supply. Some of the Schottky diodes can get leaky. These are CR1110, CR1113-6. If bad, replace them and use compound under the TO-220 rectifier. I use MX-4 compound.

I have done these things in my 2440 and 2465/B. The 2440 is fine now. That one was redone, calibrated with the d-c and a-c voltages according to the instructions along with the other instructions for calibration. The battery was installed last. I should have done it first to get rid of the fail messages. It took a few power cycles to get them to pass. I did it over a few days, one power cycle a day then a few days wait. I doubt it takes as many days to change. I had other work and waited to power it back on when I had the time. I made sure to oil the fans in the scopes. They were dry. I did add the above compound to the heatsinks on the CCDs in the 2440. That helped cool them better. I did have a small heatsink I added to U800 in the three scopes using the same compound. I used a thicker solid wire from the screw threads to hold the heatsink down and used another nut on the wire ends to hold it down. I bent the wires around the threads as though it would be an electrical connection even though it is not for electrical but mechanical. Some here do not believe in the U800 heatsink, some do. See if the higher voltage lead to the PDA is black/dusty. If so, I recommend taking the tube out and cleaning it off. The crt removal instructions are in the manual. I find that ammonia works well. That black stuff can be conductive causing leakage paths.

I hope this has been of help to you. I know it will be some work to get it overhauled. I find the work is worth the effort and cost of new parts. Cleaning makes it appear better visually and can prevent leakage pats and heat buildup.

Mark

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