576/Repairs: Difference between revisions

Symptom = Low 5V Rail voltage + High current draw on 5V (approx. 425 mA):

Replaced C759  (11KuF 15V)  Shorted, showed high ESR (8 Ohms) and 8K uF capacitance. Filter Cap replacement raised the 5V Rail voltage but did not fix the issue.   

Voltage should be 5.0.  Test readings showed 3.7V before and 3.9V after. Did not check Amp draw after, however, it is most assuredly higher that normal and still loading the supply beyond capacity. Continued troubleshooting, located high ESR (486 Ohm ) C789 5V Filter Cap replaced. 5 V rail came up to 4.2V. Disconnected Step Generator 5V @ “Q” terminal. 5V Rail came up to 5.04V, therefore shorted cap on Step Generator board.

Located  high ESR (426 Ohm) C294 5V Filter Cap where 5V rail enters board. Lifted one end off the board, reconnected “Q” and 5V rail stayed at 5.04V. Removed Unit Right Side Frame rail, replaced  C294 on Step Generator board.   5V is repaired.  

Symptom=  Display trace Running off right side of CRT, cannot center:

Q669 + Q660 Horizontal Display Transistors - Display running off the right side of the display. Horizontal control range insufficient to center. Q669 was bad, but installed a matched pair of 2N3906 PNP. Tek recommended replacement was 2N4250. 40V Vce 250mV @ 500 μA, iC 10mA 200mW F 50MHz TO106 (NLA EOL 02/17). 2N3906 seems to work just fine.

Symptom= Noisy signal 12.5 rails +/- as well as slightly low +12.5 and -12.5 Rail voltages: 

Tested and replaced C698 + C 696 12.5V Filter cap on Display board. Both showed very high leakage. 12.5V +/- Rails came up to 12.55V from 12.35V on Plus and -12.45 from -12.3 on the minus side.   

Symptom= 1-2-5 Horizontal Balance will not calibrate:

Unable to balance 1-2-5 Horizontal Balance within limits of trimmers R 650 – R645. Found that Q631A-B (Dual) and Q633A-B(Dual) had been replaced with pairs of badly matched single components. Found two NOS Q631 and Q633 and installed these. Followed Calibration procedure per 070-0905-01 Sec. 5. Display is now balanced in the 1-2-5 ranges in both Horizontal and Vertical axis. Display is rock solid and dot/traces do not "jump around" when switching HORIZONTAL ranges. Also Checked all Display Amp relays for function.   

Symptom=  No Step Generator Steps:

Step generator not producing 10 steps. Various strange behaviors from step generator. Followed schematics and scope patterns and voltages did not correspond. +1 Amp not functional, found Q226 was bad. Found defective Q229 on Step Gen Board. Replaced and step generator began producing the required steps.   

Symptom= No Single Family sweep:

Step Gen Rate Single Switch is bad. Single family mode not working. Looking for a new switch. Found Q117 N-JFET bad, replaced with know good used component.     

Symptom= Complete Unit shut down:

Power Indicator on, otherwise no indication that the unit is powered on at all. No 225V, Low 4.5V, Low 5V, Low +12.5V, -75V Normal, 15V Normal, 100V Normal and -12.5 Normal

Replaced Q800 in +12.5VPower Supply – 2N3904 Failed and shut down entire machine. Lost +12.5 and therefore 225 as well as 5V and 4.5V were low or lost as well.  

Defects found without any noted symptoms:

Found D507-D708 9V Zener VR TC shorted (continuity both directions) on both Step Gen and Display Board TEK Recommended rep. 1N936, available at MOUSER.

Found shorted D83 -  1N4152. Replaced  Symptoms not noted, but seems to have less distortion in the display.

NOTE: All electrolytic caps which were previously replaced, (except for 1 cap) tested as high ESR to the point of almost being shorts to ground. Undoubtedly loading they power supply and not filtering much at all.   Replaced all remaining Electrolytics that could be found on the Right side. Voltages in all supplies became cleaner and most of the voltages increased as well. Power supply is now well within the normal operating range. Type 576 Electrolytic caps should be your primary suspect when repairing or adjusting these machines.       

Replaced K101 K102 and K103 Relays. NO symptoms or malfunctions noted.