Tektronix tektronix 485 repair free download

File name tektronix_485_repair.pdf

17th June FREE NOTES 2010 ON ELECTRONICS b y d o k t o r p y t a [ a t ] g m a i l . c o m Tektronix 485 repair Fig. 1 Tektronix 485. 2 channel, 350MHz oscilloscope. Some time ago my colleague gave me a faulty TEK 485 oscilloscope. Symptoms: Bright, blue mains fuse explosion after pressing the POWER button. 17th June FREE NOTES 2010 ON ELECTRONICS b y d o k t o r p y t a [ a t ] g m a i l . c o m Repair: I started from downloading the manual from : " http://bama.edebris.com/manuals/tek/ ". The brightness and loudness of the fuse explosion suggested that the failure was somewhere in the mains powered part of the power supply circuit (when the fuse blows off after few seconds or minutes, failure in the low voltage circuitry is more probable). I started with removing the L1825 choke to isolate the inverter circuit and avoid further damages while checking (Fig.3). I connected the oscilloscope to the 230V mains supply. The fuse has been blown off that time also. Then I removed the RT1821 and RT1822 thermistors to determine if the failure was in the mains-bridge part or in the filter part. When I powered up the instrument, nothing bad happened, so I started tracking the filter part. I found that the electrolytic capacitor C1823 had no internal short indeed but it started conducting the current when more than 40V had been applied to it's terminals. I replaced both C1822 and C1823 capacitors with new 470uF/250V snap-in capacitors. Fig. 2 One of these capacitors is faulty 17th June FREE NOTES 2010 ON ELECTRONICS b y d o k t o r p y t a [ a t ] g m a i l . c o m Fig. 3 Mains power supply (partial) 17th June FREE NOTES 2010 ON ELECTRONICS b y d o k t o r p y t a [ a t ] g m a i l . c o m I went further with my investigation. The inverter circuit needed to be checked (Fig.4) Fig. 4 The inverter circuitry I started with checking the power transistors to make sure if everything is working correctly. I tested them using the "diode check" multimeter's function and they seemed to be OK, but I decided to measure the hFE additionally. Q1834 and Q1844 are power transistors in TO-3 metal housing. The TEK marking of these transistors is "151-0368-00" and it's the MJ13015 type indeed. To my surprise the Q1834 has hFE of about 2 and Q1844 has hFE of 10 (it sh