576/Repairs

TYPE 576 Repairs (Components replaced)

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

Replaced C759 (11,000 uF 15V). It was shorted, showed high ESR (8 Ohms) and 8,000 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 current draw after, however, it is most assuredly higher than normal and still loading the supply beyond capacity. Continued troubleshooting, located high ESR (486 ohms) 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. After capacitor replacement, the +12.5V came up to 12.55V from 12.35V, and the -12.5V rail came up to -12.45V from -12.3V.

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. This occurred after a considerable period of seemingly "Proper" operation. In other words, the unit functioned one day and then the next time it was turned on, the step generator ceased to function correctly. 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

Single family mode not working.

Cause: Step Gen Rate Single Switch is bad? (Wrong "assumption" on my part! Never "assume" anything)

Looking for a new switch. (see above)

Found Q117 N-JFET bad, replaced with known-good used component. Not certain that this had anything to do with the malfunction? (Still no function)

This is a work in progress as this switch is not critical for me at the moment. (REPAIRED 07/26/2019)

UPDATE:07/30/2019 This issue is resolved. Further (more rational) testing indicated that the switch was, in fact, good. When operating the "SINGLE" push button (SW78B), the switch portion of the circuit was producing the expected momentary "Low" at the wire connected to PIN "H" and through to Pin#5 of U69B (part of 156-0030-00 or SN7400N). I was able to clearly see this pulse on my scope when scope probe was connected at the end of previously disconnected wire at Pin "J" of the Step Generator board. Referring to the schematic, this lead me to swap out the IC's between U69 and U20 (same part). U20 seemed to be fully functional, since the related push buttons all produced their assigned function when operated. This part swap resulted in the single step family working, but the .5X Rate ceasing to function. This confirmed that the issue was indeed U69(B). I had new SN7400N IC's in my repair parts, swapped in a new one to the U20 position, the entire circuit now functions as intended.

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 +12.5V Power Supply. This 2N3904 had failed and shut down entire machine. Lost +12.5 and therefore +225 as well. Also affected were 5V and 4.5V which were both about half the required voltage.

Some Defects without symptoms. NOTE: Just because I did not see "symptoms" does not mean that they were not there, simply that I was not experienced enough to find any such symptoms.

Found D507-D708 9V Zener VR TC shorted (continuity both directions) on both Step Gen and Display Board. Could these be related to other previously discussed problems or repairs?

Tek's recommended replacement is a 1N936, available at Mouser Electronics. D507 was replaced with a TC Diode, D708 was replaced with a standard 9V ZENER (not TC, did not have the proper part on hand).

RESULT: -75V drifts badly as the machine is turned on and heats up (High @ -73.5V when turned on and drifting slowly to -75.05V when warmed up.).

Found shorted D83 - 1N4152 not aware of symptoms related to this defect.

Replaced D708 with proper 9V TC ZENER, -75V Supply is now rock solid @-75.015V, no "drift" as the machine is turned on and as it heats up.

NOTE: All electrolytic caps which were previously replaced, (except for one cap) tested as either high ESR or to the point or 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 and Power supply Filter caps. 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 a primary suspect when repairing or adjusting these machines.

Preemptive strike: Replaced K101 K102 and K103 relays. No symptoms or malfunctions noted.