Yet Another Tek 475 Question

[HeadResto]

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.

[JohnnyMalaria]

Here's a comparison of the 465, 475 and 475A: https://www.testequipmentdepot.com/usedequipment/tektronix/oscilloscopes/tek465seriesspecs.htm

The M might mean it's a military version.

The VR956 in your photo is on PDF page 207 of this version of the service manual: https://www.tek.com/manual/475-instruction-manual

[HeadResto]

Thanks for the help Johnny,

the resistor in question does is next to VR956 and also R956 when consulting the board layout image a couple of pages prior. that being said the layout does not call out the one resistor in question with a name...

I still cannot figure out what this resistor name is unfortunately, and I'm really just making an educated guess on its value. One thing I haven't done is try continuity testing to components nearby to see what it is connected to. I will try that once I get back to it.

my guess is that it belongs to a different section of the circuit. there are no traces on this side of the board connecting it to anything. it's traces appear to be on the other side. Hoping I won't have to pull that board but it looks like I might.

any other guesses?

[JohnnyMalaria]

Here's mine

[HeadResto]

Thank you! so it is in fact a 10 ohm. awesome.

After doing some more digging, I ran across this thread on a different forum. Author is asking what item that resistor is in between VR956 and R956 not called out in the board layout. Other members noted that it was likely R963, and is a filter for the 15V supply along with tant cap C963. I haven't located C963 on the board just yet but I'm not too worried about finding it. I don't have it here in front of me to look over ATM. funny thing is my 15V supply is working great, unlike mentioned in the other thread. I have pasted that thread link below. hopefully it might help someone out down the road.

https://groups.io/g/TekScopes/topic/tek_475/33067505?p=Created,,,20,1,0,0::,,,0,0,0,33067505

not sure if replacing this will fix it, but we will see. this resistor is certainly toast.

Thanks again guys!!

[HeadResto]

trying to document as best I can for future nerds.

Good news! I replaced the blown 10 ohm resistor (R963) and also its mate C963 (blue 2.2uF tant shown in photo) and bam! pressing beam find now shows a trace instead of a dot. I found both of these components listed clearly in the 475A manual but not in the 475 manual FYI.

C963 was dead short so I was lucky enough to find a 4.7 radial electrolytic 35V in town for $1.49 and popped it in. The original tant appeared visually to be fine but was certainly a dud.

Now onto the next issue. I only get a trace/sweep when pressing the beam find button. as seen in the other photo attached when hooking to the cal lug I do get a square wave but again can only be seen with beam find pressed. I understand this is all outlined in the troubleshooting area of some of the manuals but does anyone off the top of their head know where I should look next?

Thanks again you guys for all of your help. I feel like I am on the cusp of bringing this girl back to life!

[HeadResto]

GENTLEMEN!

it turns out the latest issue with this scope was my own ignorance. after replacing C963 and R963 I noted that scope appears to function okay only when the Beam Find is pressed.

after scratching my head for the last few it hit me that this thing must be working fine... all of the troubleshooting algorithms don't show a scenario where the sweep is present with beam find but not on screen. then I wanted to do a triple check on the settings. Surely I was missing something.

Turns out I had the intensity set all the way down after replacing the 2 faulty components. She seems to be working tip top now! 

So I suppose final thoughts on this, was that the blown R963 resistor and shorted tant C963 were the culprits. after replacing those and then removing my head from my ass she is up for another few years of good service.

Thanks again for the help. hopefully someone will run across this in the future and help them save another Tek from the dump. all in all I fixed this thing for less than $2.

[JohnnyMalaria]

Excellent!

I love my 475. There's something innately satisfying about turning mechanical knobs quickly that repeatedly pressing up/down soft keys on a DSO just lacks. Plus the XY mode gives you the true 50s sci-fi Lissajou experience