Hey guys, I've lurked around here for a while but am officially new to posting here. Earlier today I picked up a Tek 475M in great visual shape from a guy on FB marketplace who didn't know anything about it. He said, "the lights come on when you pull the switch". Okay, I'll bite... I get her home and plop it on the bench to discover that it does in fact turn on but it isn't quite right.
First I was having issues with what appeared to be a very out of focus trace. almost as if it was hitting the side of the tube and spraying the entire viewing area green. really odd. Beam find button did nothing. I turned her on the side and pulled it out of the case to start taking a look at blown components and also test the voltage rails on the bottom board. Once I turned it on I noticed I had a short trace (still out of focus but MUCH better) and then went black. no more "green screen". after hitting the beam find button at this time I now get a single dot on the screen but nothing else. Not sure what happened but it seems that the original issue hasn't come back so I'm not too worried about it at this time.
The current issue is still that I have no trace. only a spot when the beam find button is pressed. good news is that is closer than before. after doing some reading this appears to be an issue with the horizontal amplifier. the TEK manual that can be downloaded for this thing is huge and can be difficult to find certain things. My expertise is 50s/60s tube audio amps and are much easier to follow than something like this.
Okay, on to what I know so far. all voltages on the LV power supply appear to be spot on with no issue. after doing a more thorough search for blown components I run across what used to be a resistor. it seems to have blown a lot of the outer layer off. I have no idea what resistor this is or what section it is in. I believe it is a 10 ohm resistor judging by watching youtube videos of these things and reading the values on non-blown ones when I pause the video. I have included a couple of pics of my fried one. Measuring the resistance of this component shows 68 ohm off and infinite resistance with the unit turned on. after switching off the unit the value fairly quickly returns back to 66-68 ohms. very odd.
So obviously I replace this resistor, but can anyone who is more familiar with these help point out what section of the circuit this belongs to? what does this do? Also, with a resistor this fried I can only imagine something else nearby is also bad, maybe one of those tants I keep reading about in these things. any input on what to look out for while I'm in this area would be greatly appreciated.
I picked this unit up to replace my Tenma 20MHz scope. the Tenma works okay aside from needing a switch cleaning, I just saw this thing and had to take a chance on some vintage Tek gear.
One last thing. Does anyone understand the difference between the variations of the 475? I have seen 475 and 475a, but mine is a 475M which is much harder to find mention of on the internet. What is the difference?
Thanks again guys I really appreciate it. I will update with my findings.