Tek RM17 repair update
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So the voltage divider is bad then. I checked both resistors, they are good. Eh ?! 
Only explanation I can come up with is : schematic shows a little 5nF cap, C166, across R168, the bottom leg of the divider. So if that cap was say, leaking a bit, it would indeed lower the voltage of that divider. So let's find this cap... uh..... can someone tell me where it is ? For the life of me I can't find it !
There is a similar cap on the anode of the tube, it's a small brown Z5U disc ceramic cap. There are a few of these in the area but... I just can't find the one connected to that voltage divider ! 
Maybe it was not implemented on my particular scope but... I doubt it. It serves a purpose here... I think. I am not sure Tek would have deemed it wise to remove it.
Also, when I measured the voltage on the divider, hence across that cap, assuming it's indeed there somewhere.... I noticed that the voltage is NOT stable. The 300V rail is stable, but not the divider output.
I also noticed some capacitive effect maybe : when I put my probe on the divider/cap, I get say 115V, but it immediately starts dropping, slowly, and jerkily, and wonders about. It can go down to 112V every now and then. But... if take my probe off, wait say 3 seconds, then apply it again... I read 115V very briefly then it drops again... Like the impedance of my DMM was stealing some current from that cap and playing havoc with it. Something like that. Just a gut feeling really
So... I do need to take this scope apart until I can find this cap, hoping it is there indeed. My only hope : the output of the divider is on a ceramic strip, from which two wires bugger off. One goes to the tube, but the other I can't see where it goes, it merges into a little harness and I can't follow it, don't know where it goes.
So... my only hope is to persevere and try hard to trace that wire... it's my only hope... I MUST find where it goes !!!! 
OK, fixed !
However just like yesterday, it got fixed quickly in the first few minutes, but then I wasted 4 hours pulling my hair trying to fix a scope that didn't need fixing because I thought I had not completely fixed it in the beginning... when in fact I had. More below, just so you can laugh at my expense.
So I executed my "plan" exposed last night : I followed that mystery wire to see where it would lead, and if it would lead me to that 5nF disc ceramic mystery cap, C166, that I could not find anywhere.
Using side cutters, which were not easy to manoeuver in there, I managed to cut some of the lacing of that wiring harness. This partially freed the wires, and using tweezers I was able to wiggle my mystery wire to trace it... and I did see where it went. It goes to a cap, so that's cool, but NOT the expected tiny ceramic cap. No. Instead it goes to a big 10uF electrolytic metal can !!!!!
It's a 3 section can, 3x10uF . It's C146. This cap is no stranger to the pre-amp, as you can see two of its sections, C146A and B, are used to decouple the 175V supply of the preamp. Fine. But the third section, 146C, according to the manual is found.... in the POWER SUPPLY section, where it filters the output of the 300V rail (the one used by our voltage divider in the preamp).
So... if they decided to repurpose that cap to filter the output of the voltage divider instead... then WHAT on earth is filtering the 300V rail, eh ?!
Also, replacing that 5nF ceramic cap with a huge 10uF electrolytic makes no sense does it ? A small ceramic is for HF stuff, big electrolytic is for low mains related ripple... makes no sense !
So could it be a brain fart from the guy who designed the wiring harness for that scope ? He connected that 10uF to the wrong end of the resistor ?!
Then this means that the 5nF ceramic cap has either not been implemented, by choice (maybe it was added later and the manual does not reflect the early models ? Possible I guess...).... or maybe they did mean to put it, and the girl at the factory simply forgot to solder it in.
At any rate, the electrolytic being removed from the 300V rail and instead filtering the voltage divider... seems fishy at best !!!
So, I disconnected that cap, and hey presto as I hoped, voltage divider now outputs proper voltage ! 165V (not quite 175V but much better than 120 for sure ! ).
So that cap is a bit leaky...
So, did that fix my power supply ? Nope... still 114 Volts instead of 175V !
So I spent the next 4 hours trying this and that, pulling wires, testing and / or replacing resistors, disconnecting caps, pulling all my hair, disconnecting/isolating the 175V supply/regulation from the pre-amp altogether, to divide and conquer, still no joy....and eventually ran out of option, and concluded that the only possibility left is that the tube/triode, that I had already replaced last night... somehow already failed, open circuit, well, not conducting at all any more I mean. So, I flipped the scope to get to it from the top, for the first time in many hours, go to grab that tube and..... eh ? WHERE is that tube ! There is NO tube in there, its socket is empty !!!
I must have removed it at some point and forgot to put it back ! 4 hours wasted !!!!
Put the tube back in, works like a charm now ! I get 170Volts (a bit more than the voltage divider then), good enough !
I have plenty enough gain, even needed to use the gain adjust trimmer to crank the gain way down actually !
But now, looks good !
Only problem left is the trace moving up and down a bit. With now zero filtering on the voltage divider, plus crappy caps decoupling that pre-amp supply... no wonder. So I will replace these x3 10uF caps and I guess it should get rid of that problem !
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Topic: Test Equipment Anonymous (TEA) group therapy thread (Read 18121407 times)