Chassis was fitted with new HOT and LOPT in 2017 then shelved. Brought out of storage, it ran okay for some hours but then the brightness started to tail off and whilst increasing the screen pot restored the brightness the black level seemed to fluctuate depending on what was on the screen with retrace lines often showing.
I was also having issues with vertical sync (hold) I could lock it but it would not be locked after a power cycle. After removing the chassis to investigate the sync issue chassis became dead with 170 (ish) volts B+ but no HV.
I won't go into painstaking detail but after reading some old posts from here/klov/an excellent French (google translated) repair guide and one from a website run by a gentleman called Mark Cotsford I found the following issues:
HV was totally dead due to a previous clunky looking doubled up parallel resistor at R120 (6R8) which ties TH1 to the HOT TR15 (BU508) having cracked traces.
Brightness issue was caused by R90 going from 270K to several Megaohms which probably messed up the brightness preset. Nearby capacitor C45 looked toasty and testing it suggested it was a lot less than 1 microfarad so it was replaced.
The sync issue was likely user error and connecting the sync to 'H' sync from V sync seems to have solved the problem.
EDIT:
Further issue:
Monitor ran for a few hours, smelt of burning, with partial V field collapse, and wavering image in H field. F1 then blew. The HOT had shorted, which is odd since LOPT and HOT had few hours on them. Replaced fuse and hot and measured B+ on power up (expected value=130v), it quickly ramped up to ~150 volts and rising.
Fix:
Disconnected yoke connector, attached 60watt light bulb as dummy load between chassis GND and pin 3 of yoke connector, initially watched B+ rise from 120 upwards to around 140 and then mysteriously drop back down to 120. After a few minutes running it was ramping upto around 130 and then dropping down to near zero (light bulb dimming/going out as one would expect).
I attached a second voltmeter to one end of R99 and got a steady reading of 160v so I suspected the voltage reg (TR20). BUT, and it's a big but.... it was down to bad/cracked traces on pcb between R99 and TR20 and also another nearby track. The cracking was caused when the large metal heatsink was refitted after various repairs and the lugs were bent over to far or too hard and it caused the pcb to flex and break the tracks.
karlcdoe2022-11-10 03:12:21
I was also having issues with vertical sync (hold) I could lock it but it would not be locked after a power cycle. After removing the chassis to investigate the sync issue chassis became dead with 170 (ish) volts B+ but no HV.
I won't go into painstaking detail but after reading some old posts from here/klov/an excellent French (google translated) repair guide and one from a website run by a gentleman called Mark Cotsford I found the following issues:
HV was totally dead due to a previous clunky looking doubled up parallel resistor at R120 (6R8) which ties TH1 to the HOT TR15 (BU508) having cracked traces.
Brightness issue was caused by R90 going from 270K to several Megaohms which probably messed up the brightness preset. Nearby capacitor C45 looked toasty and testing it suggested it was a lot less than 1 microfarad so it was replaced.
The sync issue was likely user error and connecting the sync to 'H' sync from V sync seems to have solved the problem.
EDIT:
Further issue:
Monitor ran for a few hours, smelt of burning, with partial V field collapse, and wavering image in H field. F1 then blew. The HOT had shorted, which is odd since LOPT and HOT had few hours on them. Replaced fuse and hot and measured B+ on power up (expected value=130v), it quickly ramped up to ~150 volts and rising.
Fix:
Disconnected yoke connector, attached 60watt light bulb as dummy load between chassis GND and pin 3 of yoke connector, initially watched B+ rise from 120 upwards to around 140 and then mysteriously drop back down to 120. After a few minutes running it was ramping upto around 130 and then dropping down to near zero (light bulb dimming/going out as one would expect).
I attached a second voltmeter to one end of R99 and got a steady reading of 160v so I suspected the voltage reg (TR20). BUT, and it's a big but.... it was down to bad/cracked traces on pcb between R99 and TR20 and also another nearby track. The cracking was caused when the large metal heatsink was refitted after various repairs and the lugs were bent over to far or too hard and it caused the pcb to flex and break the tracks.
karlcdoe2022-11-10 03:12:21