This has appeared in various places around the Internet. Feel like I need to consolidate it all on the best resource for such things, which is EEVblog forum by the looks!
I scored an apparently dead Tektronix 475 with DM44 module the other day for a respectable £29.
I'm going to enumerate the identified problems with it and what has been done and resolved so far.
Problem 1 - 15v supply out. RESOLVED!This was a duff tant on the interface board on the output of the 15v regulator. All rails are in spec now. I will do a wholesale tantalum swap out job over the next few days. There are quite a few to replace!
Problem 2 - Occasional HV drop out. RESOLVED! NOT RESOLVED AGAIN FINALLY RESOLVED!Every 2-3 minutes the display jumps in brightness and then returns to normal as per the following video.
Cleaning it appeared to make the problem go away for a bit but it's back again suddenly. I've opted to replace all the film capacitors and diodes in the HV section as a couple of caps look slightly mottled/cracked (like shitty RIFAs about to explode). They are being replaced with ceramic equivalents.
Note: postponing HV refurb as the issue may have been cured by another tant on the 50v rail going short periodically and self-clearing.
Confirmed it was the 50v rail tant that was doing this. It doesn't do it now that has been replaced with a low ESR electrolytic.
This occurred again and I found another dead tant on the 15v rail that might have actually been doing it to start with. Grr. Replace them all!!!
Problem 3 - chop doesn't work. RESOLVED!As yet undiagnosed. Shows channel 1 in chop. Hopefully not the channel switch IC gone phut and just a dirty switch or something! Not a big priority at the moment. Drifted resistor in chop oscillator bias circuit and duff ceramic decoupling capacitor causing VCC to be around 4v which broke the startup condition for the oscillator. 7400N's are possibly hooky so I'm ordering a couple as replacements anyway.
Problem 4 - fine horizontal pot broken - IGNOREJust going to try and replace this if I can find one or live with it. There's enough resolution in the coarse pot for now.
Problem 5 - at fast sweep speeds, the trace starts moving to the right - RESOLVEDThis one is a real PITA. I've had another scope on the timebase and that appears to be working fine. Horizontal amp seems fine as well as it has full deflection and bandwidth in X-Y mode. However as you increase sweep speed from 1us to 50ns/div, the trace compresses and moves right. Scoping the timebase shows that the sweep return is much shorter for these timebase settings than others and if you turn the brightness up fully you can see the retrace so I'm wondering if it's Z-axis related. Also looks a little blurry in delayed sweep mode when the trace is intensified. This is possibly CRT blooming or on it's way out (is it?). Any hints welcome!
After much arguing I spent until 3AM working this out. Unplugged the z-axis cable to poke a scope in the hole, plugged it back in again and gone. Ahha we have a problem. So I resoldered each end of the z-axis cable's sockets and hey presto it works. Looks like one of the terminations was iffy and had an accidental filter in it.
Problem 6 - dirt and corrosion - RESOLVED!Slowly working on this. Corrosion is minor and appears to be limited to the front cast panel. Everything else is cleaning up nicely. All the controls are in really good condition and the bits that are cleaned leave it looking like it's brand new!
Any help or suggestions on any of these issues would be appreciated as this is a fine cosmetic specimen; would be nice to get it up to spec again.
State after I fixed the power supply: