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576/Repairs: Difference between revisions
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==Symptom: Low 5V Rail voltage + High current draw on 5V (approx. 425 mA)== | ==Symptom: Low 5V Rail voltage + High current draw on 5V (approx. 425 mA)== | ||
Replaced C759 (11,000 uF 15V). | Replaced C759 (11,000 uF 15V). | ||
C759 was leaky and showed high ESR (8 Ohms), also low capacitance @ 8,000μF. | |||
Filter cap replacement raised the 5V rail voltage but did not fix the issue. | Filter cap replacement raised the 5V rail voltage but did not fix the issue. | ||
Voltage should be 5.0. | Voltage should be 5.0. | ||
Test readings showed 3.7V before and 3. | |||
Test readings showed 3.7V before and 3.9-4.0V after. | |||
Did not check current draw after, however, it is most assuredly higher than normal and still loading the supply beyond capacity. | 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. | Continued troubleshooting, located high ESR (486 ohms) C789 5V Filter Cap replaced. | ||
5 V rail came up to 4. | 5 V rail came up to 4.2- 4.3V. | ||
Disconnected Step Generator 5V @ “Q” terminal. | Disconnected Step Generator 5V @ “Q” terminal. | ||
5V Rail came up to 5.04V, therefore shorted cap on Step Generator board. | 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. | 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 | Lifted one end of the cap off the board, reconnected “Q” and 5V rail remained at 5.04V. | ||
Removed Unit Right Side Frame rail, replaced C294 on Step Generator board. | Removed Unit Right Side Frame rail, replaced C294 on Step Generator board. | ||
5V is repaired. | 5V is repaired. | ||
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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.). | 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. | Found shorted D83 - 1N4152 not aware of symptoms related to this defect. This would have affected the DAC Function. See Page 3-9 for Circuit Description. | ||
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. | 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. | ||
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Replaced K101 K102 and K103 relays. | Replaced K101 K102 and K103 relays. | ||
No symptoms or malfunctions noted. | No symptoms or malfunctions noted. | ||
==Symptom: Complete Unit shut down #2== | |||
Bad HV transformer. Unit #2 quit working after a short time of operation. Very low S/N B010169 unit is very old and very early production model. Had brown epoxy potted HV XFMR. No luck with finding a functional new or used part. Found a manufacturer who was willing to make a small batch of these transformers, contracted for a batch to be made. Replaced old part with new one, unit works again. There is copious discussion on the TEKScopes Group regarding this repair. 250+ hours of operation proves this part is durable. I have very few of these transformers remaining available for sale. (Contact Me OFF-LIST for details.) | |||
[[Category:Instrument repair reports]] | |||