Hi John,
It's good to hear that you are making progress on your 475.
Those large capacitors for the low voltage supplies in the aluminum cans often fail internally and do not show any bulging or other obvious failure.
When you find a large ripple on one of the low voltage supplies and take that capacitor apart, you will often see one of the connections to the outside has corroded and completely disconnected the capacitor internally.
Work on whatever seems to be the biggest problem you see and you will gradually get it all fixed.
I have not worked on a 4xx scope in a very long time so have no idea about how things come apart.
tom jobe...
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On 8/15/2018 3:25 PM, JR wrote:
It turns out that C965 (Board A9), a tantalum bypass capacitor on the +15 rail had gone bad; removing brought the +15 rail resistance to ground back to where it should be. Most of the capacitors look to be in pretty good condition; I have not come across any bulges, deformities, leaked electrolyte or burn marks on any of them, even the larger axial lead supply capacitors.
I powered it on, and checked the supply voltages at the test points; all were within specification, although I haven't checked the ripple on each yet. I haven't checked any of the high voltage test points yet, as I'm currently lacking the equipment to do so safely.
A couple minutes into testing the powered on scope (I couldn't find the test point for +105/160), some other component let go, with a loud pop, accompanied by a whistling sound and small quantity of smoke. So, the hunt is on for the culprit. I have removed boards A3, A7 and A8 so far, and have found no obviously failed components on any of them. I thought at first that something on A7 (timing board) failed, given the location where the smoke seemed to come from, but that doesn't seem to be the case. Is there any component in the recess along with the CRT known to fail in such a way?
Interestingly, after the component failure, all the supply voltages were still within specification.
Regarding the removal of the two attenuator boards/assemblies, is the best way to get at the nuts holding them to the front panel with a small spanner? I'm having a little difficulty getting them free.
Thanks,
John
