got a problem with my scope, need some advice

[Trileru]

Hi all. I come with a problem. I got for free a broken oscilloscope and it's rather unknown, made in Germany in 1966 it seems, ofc it is a analog tube machine. First problem it had was with the switch-on pot, the switching part was broken so I did a direct circuit (temporary until I install a sepparate on/off switch somewhere) as the pot is still ok (pot for display intensity). After first fire-up the dot showed no problems, I attached a 100hz sine wave generated by my computer through a M-Audio Fast Track Pro soundcard so everything ok on this end. Attached the probe and sinewave came up on the tube display. The problem is that the upper part of the sinewave is "clipped", first I suspected the soundcard but luckily I have (only for a few days) a digital portable scope (Velleman) that showed a perfect sinewave, same as triangular and rectangular. But my scope seems to clip the upper part, at least distort it. I forgot to mention, my machine is a RIM Rog 7A and I can't seem to find the manual/schematics anywhere but radiomuseum where I don't have an account (I would like to ask for anyone who could send me the manual/schematics for my scope, I would very much appreciate as I don't have the means/possibilities to pay for the membership there) And they seem to be the only ones who have it online. I could only check the tube heaters, and as well I tested about 2 or 3 of the 5 E88CC's on my (now laid out on a breadboard) tube amp as it uses an equivalent for input stage/PI stage, and they seemed ok. I also discovered that the problem might have a thermal character, as when I opened the case to check the transformer voltages I discovered that the transformer has 43-45 degrees C and in about 5 minutes the shape improved a bit. After I put the cover back on I checked the temp on transformer and it only rose for 1-2 degrees. But the shapes got distorted again after 5 min or so. I also changed a 100uF/25V electrolytic as it was bloated, I think there's another one there and I got 3 big ones as well, but they seemed ok, I measured with a DMM and are in spec (I realize it isn't the most conclusive test for caps but hey, least I could do anyway. no leakage at least).
I also attached pictures with case closed and case open, you will see what I mean. (last 2 pictures are taken with open case)

[RJM1]

It sounds like it might be that the B+ capacitors for the vertical amplifier are breaking down under voltage (probably one of the three big ones).
But in any electronic equipment 35 years old when the electrolytics start going I’d just replace all of them at once.

[Trileru]

Yes, I have one more 100uF/25V that I'm going to swap tonight and hopefully the 3 big ones, if I find them today somewhere... How does the scope react when one ore more tubes are out of spec? Does it still work or when they slide out of their spec it just shows garbage? I will try and source all the tubes anyway as I love the scope, as I appreciate very much vintage devices made to last! Nowadays everything is built to break down as fast as possible :/ Or you need to pay premium to have something for more than few years.

[thaumaturge]

Check your PM

[Trileru]

Ok, I got the schematics, and I made an easy check, I changed the EF184 between them and the problem got worse. In fact, one has 105 degrees Celsius and the other 57, and it seems that the heating problem is socket related not tube related, nevertheless seems that one tube is really screwed up. I attached a photo with a sinewave with swapped ef184

[Trileru]

I would post the schematic but I don't know if it's ok with respect to RIM, although I think it was a kit kind of package, so you could put it together yourself. At least that's what I got out from it's manual translated online from german

[Trileru]

I changed the 100uF/25V cap and it seems to be more sensible in a bad way, I can't seem to keep the line in place, it shakes badly now and then. I also checked the B+ rails and they are 5V lower than in the schematic. But I got different tension on the 2 100uF/25V cap, 102.5V on one and 143,1-143,9 on the other (relative to ground)
If I switch the tubes I read 87V on one and 133V on the other. These caps are on G2 of the tubes paralleled each with a 3.3k resistor.

[Trileru]

weird stuff going on, i got molten solder at a resistor junction. I got the whole board down, measured the resistors and seem ok, and will change all the caps, hope I find ones that fit as it is a tight squeeze. tubes are getting change as well, all of them.

[thaumaturge]

Originally Posted by Trileru

But I got different tension on the 2 100uF/25V cap, 102.5V on one and 143,1-143,9 on the other (relative to ground)

Unless the south end of that cap is floating high, 102-143 volts will definately punch a 25V cap. The voltage ratings on caps are absolute maximum they should ever see. I usually run at least 25-30% headroom above what I plan to use them at. What's the reading directly across those caps?
Doc

[Trileru]

Well I didn't even bother to measure how high they float as they look very good. They would have popped instantly! I suspect a bad ef184+caps. I took the board down and will fit new caps today and hopefuly I find 2xEF184 as well