E-design
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Tektronix scope repair from hell!
Complete service manual here
I attached the relevant schematics if you want to follow along the troubleshooting process.
We recently got a 485 donated to us. The repair job card said "REJECT-shorted HV transformer". Well, the other day I decided to look at it. I have repaired quite a few of these before.
When I switched it on, I was greeted by the hick-up sound coming from the power inverter. So, I did a few checks to see what was loading it.
Some quick ohm meter checks on the power-supply pcb revealed nothing, so I decided to pull out the A15 transformer board. I made some meter checks but still everything looked fine.
So I decided to check the HV transformer. I hooked up a 20Vp-p, 25kHz square wave on the input winding and scoped the other windings for signs of damped ringing, which may suggest shorted windings somewhere. Everything looked good. At last, I decided to put the module back again. When I fired the unit up, a welcomed trace was on the display. It still had some problems in that there was no intensity control.
I went straight for the DC restorer section and scoped the anode of CR1660. No square waves. I checked the slider voltage, cathode CR1660 for change with turning the grid bias control. It was OK. Then I checked CR1660 and CR1652, which could load the incoming square wave, and found both to be leaky. Once I replaced that the intensity control operated normally. So I then checked all the voltages on the regulator board and proceeded to do a quick cal check.
While I was checking the sweep, I suddenly felt the B delay control going soft. No familiar rotational ratchet sound. First, I thought the set screw came loose, but that checked out. So, out with the horizontal board to gain access to the switch assembly. To my horror, I discovered that the shaft somehow must have broken free from turning the B sweep drum as it was turning quite freely. I thought, well that was it, have to look for another scrap unit on eBay. After studying the mechanism for a bit longer I got the idea to fabricate my own locking mechanism using a nut that would fit over the shaft and loosely fit into the switch drum assembly. Then I took some thin plastic tape and covered the surface of the nut so it would not stick to the epoxy that I was going to use to make this mold with. I then took some JB weld and covered the outside of the nut, and carefully inserted it on the shaft and slid it into the back of the drum assembly. I left it over-night, and the next morning managed to pull the nut out of the switch and was rewarded with a nice nut shaped form on the back of the switch. I then measured the travel by pulling on the B delay switch before gluing the nut in position on the shaft. Did a couple of tests and was satisfied that the switch was again operating as it should. Put everything back together so I could continue with my cal check.
Then I noticed that it wasn't triggering anymore. Not the tunnel diodes, I prayed! After doing a few checks, I saw that on the trigger input fets, the one side connected to R714 measured a positive voltage. Other side measured positive by a few volts as well. I checked the -15V supply at the inter-board connector L1 and found it to be a problem. Cleaned and straightened it out, and I had the -15V as well as a stable trigger back again. So I continued with my calibrations.
The next day while working on the unit, I noticed that suddenly the trace across the screen was very dimmed on the first four sweep positions. I couldn't believe it! It was working the previous day when I was setting the grid bias. I measured the voltage on Q1546's collector (logic board) and found 12V for the first four sweep positions and 5V for the rest of the sweep positions. A couple of checks later and CR1539 turned out to be leaky.
I replaced that and we were back in business.
I wasn't long before another fault appeared. I noticed that if I selected ALT on the horizontal selector bank, it would not do anything, but if I selected B, it worked, but would not switch out of that mode by selecting ALT again. There was no problem with this before as I remember checking all this after repairing the B switch problem. I checked the control signals coming in on U1535A and pins 1&4 was getting the correct logic, but pin 5 wasn't cooperating. Put another 74LS74 in and selector function was active again. Now I could see the ALT trace but trace separation wasn't working. Replacing U1530 (7410) fixed that problem. Oh, and the fan motor driver U1480 had one transistor open and R1499 burned out. That I replaced together with 4 discrete bipolars.
I eventually managed to do most of the needed calibration without further problems showing up. It has been running for a few days now, so let's hope Murphy went off to go and harass someone else!
Complete service manual here
I attached the relevant schematics if you want to follow along the troubleshooting process.
We recently got a 485 donated to us. The repair job card said "REJECT-shorted HV transformer". Well, the other day I decided to look at it. I have repaired quite a few of these before.
When I switched it on, I was greeted by the hick-up sound coming from the power inverter. So, I did a few checks to see what was loading it.
Some quick ohm meter checks on the power-supply pcb revealed nothing, so I decided to pull out the A15 transformer board. I made some meter checks but still everything looked fine.
So I decided to check the HV transformer. I hooked up a 20Vp-p, 25kHz square wave on the input winding and scoped the other windings for signs of damped ringing, which may suggest shorted windings somewhere. Everything looked good. At last, I decided to put the module back again. When I fired the unit up, a welcomed trace was on the display. It still had some problems in that there was no intensity control.
I went straight for the DC restorer section and scoped the anode of CR1660. No square waves. I checked the slider voltage, cathode CR1660 for change with turning the grid bias control. It was OK. Then I checked CR1660 and CR1652, which could load the incoming square wave, and found both to be leaky. Once I replaced that the intensity control operated normally. So I then checked all the voltages on the regulator board and proceeded to do a quick cal check.
While I was checking the sweep, I suddenly felt the B delay control going soft. No familiar rotational ratchet sound. First, I thought the set screw came loose, but that checked out. So, out with the horizontal board to gain access to the switch assembly. To my horror, I discovered that the shaft somehow must have broken free from turning the B sweep drum as it was turning quite freely. I thought, well that was it, have to look for another scrap unit on eBay. After studying the mechanism for a bit longer I got the idea to fabricate my own locking mechanism using a nut that would fit over the shaft and loosely fit into the switch drum assembly. Then I took some thin plastic tape and covered the surface of the nut so it would not stick to the epoxy that I was going to use to make this mold with. I then took some JB weld and covered the outside of the nut, and carefully inserted it on the shaft and slid it into the back of the drum assembly. I left it over-night, and the next morning managed to pull the nut out of the switch and was rewarded with a nice nut shaped form on the back of the switch. I then measured the travel by pulling on the B delay switch before gluing the nut in position on the shaft. Did a couple of tests and was satisfied that the switch was again operating as it should. Put everything back together so I could continue with my cal check.
Then I noticed that it wasn't triggering anymore. Not the tunnel diodes, I prayed! After doing a few checks, I saw that on the trigger input fets, the one side connected to R714 measured a positive voltage. Other side measured positive by a few volts as well. I checked the -15V supply at the inter-board connector L1 and found it to be a problem. Cleaned and straightened it out, and I had the -15V as well as a stable trigger back again. So I continued with my calibrations.
The next day while working on the unit, I noticed that suddenly the trace across the screen was very dimmed on the first four sweep positions. I couldn't believe it! It was working the previous day when I was setting the grid bias. I measured the voltage on Q1546's collector (logic board) and found 12V for the first four sweep positions and 5V for the rest of the sweep positions. A couple of checks later and CR1539 turned out to be leaky.
I replaced that and we were back in business.
I wasn't long before another fault appeared. I noticed that if I selected ALT on the horizontal selector bank, it would not do anything, but if I selected B, it worked, but would not switch out of that mode by selecting ALT again. There was no problem with this before as I remember checking all this after repairing the B switch problem. I checked the control signals coming in on U1535A and pins 1&4 was getting the correct logic, but pin 5 wasn't cooperating. Put another 74LS74 in and selector function was active again. Now I could see the ALT trace but trace separation wasn't working. Replacing U1530 (7410) fixed that problem. Oh, and the fan motor driver U1480 had one transistor open and R1499 burned out. That I replaced together with 4 discrete bipolars.
I eventually managed to do most of the needed calibration without further problems showing up. It has been running for a few days now, so let's hope Murphy went off to go and harass someone else!
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