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Oscilloscope does not display the correct waveform RIM Rog 7A

Discussion in 'Troubleshooting and Repair' started by bob808, Nov 8, 2011.

  1. bob808

    bob808

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    Nov 8, 2011
    Hi all. I'm new to this forum, and 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)
     

    Attached Files:

  2. davenn

    davenn Moderator

    13,342
    1,773
    Sep 5, 2009
    hi Bob,
    welcome to the forums :)

    for something of 1966 vintage, I would suspect pretty much every electrolytic capacitor in the scope. They are all probably getting pretty dried out and well past their use by date

    swap a few more and see how ya go you may well be pleasently suprised

    the only way to test the caps properly is with a capacitance meter or an ESR meter nothing else will give you a sensible indications as to their health
    seriously I wouldnt even waste my time testing caps of that age, they would just get the boot instantly :)

    cheers
    Dave
     
    Last edited: Nov 8, 2011
  3. bob808

    bob808

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    Nov 8, 2011
    Hey, thank you for your help, I will search for those big caps. Hope I can find similar ones or I will just adapt the old case and put new ones inside. I will post back when I'm done. Btw, I got the schematic, anyone who needs it can pm me.
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Don't replace only the big ones (they have more electrolyte to seal ratio so they take a lot longer to dry out than the small ones).
    Replace even the small (1uF) 'lytic's. Some non-electrolytics (paper) may also have degraded.
     
  5. bob808

    bob808

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    Nov 8, 2011
    I changed the second 100uF/25V electrolytic and things got bad, the line shakes now and then quite violently, and I can hardly keep it steady. The electrolytics are on G2 of each tube in parallel with a 3.3k resistor. And I got uneven voltages, on one of them 104V and the other 143.1-143.9 (relative to ground). If I switch the tubes I read 87V and 133V on the caps. Another interesting thing is that the shape distorts even worse when I switch the EF184 with each other. And another thing is that one tube is 105 degrees C and the other 57. It is socket dependent, not tube. I'd post the schematic if it's ok.
     
  6. bob808

    bob808

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    Nov 8, 2011
    while checking for tensions I got molten solder at a resistor junction. got the whole board down, checked ok every resistor, all the caps are going down, all the tubes are going to be changed. I suspect 1/2 from e88cc malperforms, especially the PI one. I got some weird readings on one coupling cap, and I think there was a drainage somewhere on the cathode line, that's the weird cap, and that's the ef184 line from the 2 that had the problem with hi temp resistor, i'm suspecting the temp of the tube derived from that resistor.
     
  7. davenn

    davenn Moderator

    13,342
    1,773
    Sep 5, 2009
    Hey Bob

    chances they werent 25V rated caps in the unit more probably 250V
    check you old caps again maybe the text has been smudged ?

    Dave
     
  8. bob808

    bob808

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    Nov 8, 2011
    They are 25V it is just that they are floating above ground.
     
  9. davenn

    davenn Moderator

    13,342
    1,773
    Sep 5, 2009
    yes please post a schematic that would help lots :)

    I'md still worried about your comments of 104V etc on a 25V cap there's something very wrong there... anyway lets see the cct diag

    Dave
     
  10. bob808

    bob808

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    Nov 8, 2011
    Ok, attached
    I suppose that if swaping the tubes made it worse (at least made a change) than the tubes are not equal then they both must be changed. I'm talking about EF184.

    [​IMG]

    Uploaded with ImageShack.us
     
  11. bob808

    bob808

    15
    0
    Nov 8, 2011
    I think the cap was floating. I took ground from case. Since I had some other more important things to do last year here I am again. Winter is coming and I'm trying to fix my scope so I can go ahead with fine tuning the tube amp.
    I got a friend to test all of my tubes but I can't remember what he told me. I found some notes on a piece of paper, one EF184 165% and the other EF184 92%. The rest of the tubes are about 67-90% each triode. I have no idea what that means :) but the EF184 seem way out of balance. I will buy two of those to replace.
    Now on the board, I replaced some np caps, about 4 with wima mks4 0.22uF/630V because that is what I had laying around. I saw that almost all np caps in the amp are vintage Wima, mostly fluorescent yellow. I have no idea what kind they are as most of them don't have the type written on them. I found in the manual that there's some Wima Tropyfol M, metallized polyester (the 125V ones) and Wima MKB 2 (250V) and some higher voltage spec Tropyfol (400V).
    As I read about the Tropyfol line many people still use salvaged ones with great success. They said they didn't find one leaking one until now. So I kind of trust those. Large lytics are within spec. I replaced the smaller lytics.
    If you think that the new MKS4 are wrong please mention why and what to replace them with.
    I became interested in the part that was real hot when I first tested the amp. It's about the connectiong point between R28 and L3. I connected the MM clip there and when I wanted to take it off it was soldered. I made some more tests and indeed that part had molten solder with the scope on. Must have been real hot. I unsoldered the inductor and resistor (they are connected directly one to another, not un pcb). I measured the resistor and it is within spec. I don't have and inductance meter so I cannot say anything about the state of the inductor only that it has a resistance of 3 ohms. I did the same for the other pair of R25+L2 and the resistor and inductor tested the same as the first pair. Those are mentioned as "red point" on the schematic. The only other inductor in the schematic (minus the choke) is refered to as "white point" and that is something tricky as well. I attached some pictures with the parts. The manual makes reference at the inventory of parts as 2 Hf Drosseln (inductors) - red point and 2 Hf Drosseln - white point. On the schematic there seems to be only one L1 inductor but on the board there are actually 2 inductors connected to a variable resistor as show in photo. Those inductors seem identical (the 2 L1 ones) and L2 and L3 seem identical between them. I have no idea why L1 consists of 2 identical inductors connected to a variable resistor. I also don't know the values of these inductors. The values aren't mentioned in the parts inventory nor in the schematic. They have colors on them but I couldn't find something online to decypher them. If you could help I would appreciate. I'd like to replace all 4. Also I'd like to know how to operate that variable resistor with the inductors and understand it's purpose in that layout. Curious enough the R28 resistor looks like nothing I've seen before. It is all black with a white spiral on half it's body. I can only read .7K on it like the 4 from 4.7 would be under the white spiral. I'd replace the whole resistor-inductor L2 and L3 pairs as they seem like they've been under a lot of heat. What could cause that? Could it be that one of the tubes (especially the one marked 165%) drew too much current?
    After I receive the new EF184 tubes I will connect everything back and resume tests.
    Also it would mean a lot if I could find NOS Wima caps to replace the ugly new ones. Those oldies look incredible!
    I've never seen the type of resistors on board, I'm curious to what type they are. Wirewound or carbon or...? They are 1/2W or 1/8W spec in the inventory.
    Also the on-off switch is busted, that's actually a on-off potentiometer (50K). I had to bypass the on-off part of it. I couldn't find it at my electronics stores in town but I'd like to get a NOS one if possible.
    Ah, one more thing. What kind of connectors are the ones on the faceplate (for the probes). I couldn't find probes with connectors like that.
     

    Attached Files:

    Last edited: Nov 14, 2012
  12. bob808

    bob808

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    Nov 8, 2011
    I've wasted one inductor trying to measure it with the 6.3VAC from the filaments :) thought I'd get 50hz from there. Does anyone know the value of this inductor from the color code?
    Also what's the word on the tubes? Can I just insert any EF184 pair if they are closely matched? Does it matter who made them? I had originally Valvo tubes but they are out of spec and waaaay too expensive.
     

    Attached Files:

  13. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    The connectors are SO259 for PL259 plugs as commonly used for aerial connections to amateur transmitters.

    The resistors look to me to be normal carbon resistors except the green job which is likely to be wire wound in a ceramic mould and will withstand higher temperatures than the carbon type.

    I see no inductor.

    Fitting old capacitors is likely to ruin reliability, use new good ones.
     
  14. bob808

    bob808

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    Nov 8, 2011
    The socket is actually SO-239. I cleaned them up and I could read it on them.
    In the schematic there seems to be an inductor exactly in that spot. Connected to a resistor. The black one that is attached to the inductor is as well a resistor. They were pairs but while testing one pair broke appart. So in the picture above there is a black resistor of 4.7k connected to an inductor. The rest of the 2 pieces are a green resistor of 4.7k and the other one is an inductor. Identical to the the other one connected to tha black resistor. I tested the so-called inductors and both show a resistance of 3 ohm. I don't know why in the schematic there would be an inductor connected to a resistor and then to find on the board a 3 ohm resistor connected in series to another 4.7kohm resistor. there is nothing else connected between them so that must be an inductor. The color code also doesn't match to any resistor color table I found. The colors are black-red-blue-silver. Or in reverse. There is nothing starting with black. And the last band is silver not grey. I think it's some vintage notation style. I attached again the full view of one of these "inductors". It just doesn't make any sense to put 2 resistors in series... Also in the manual there are stated 4 Hf inductors. The four inductors seem identical two by two. On the schematic there is L1 (Actually 2 inductors connected on one variable resistor, picture attached) and L2 and L3 that are equal.
     

    Attached Files:

  15. duke37

    duke37

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    Jan 9, 2011
    Sorry I got the number wrong.

    A resistor and inductor in series is used to create the correct impedance to expand the frequency response. If you have broken the inductor, then you could replace it with a short but the high frequency response will be compromised.
     
  16. bob808

    bob808

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    Nov 8, 2011
    Yes, I know. I'd like to replace it anyway. Is there any chance that I could find out what it's value is without measuring the other one? I'm pretty sure I've wasted one because smoke of death came out of it :D Live and learn right?
    If I were to short it, what exactly will I loose? I'm looking to use the scope for audio use so I'm interested in 20-30kHz max.
     
  17. duke37

    duke37

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    726
    Jan 9, 2011
    I think the inductor would expand the range by 50% or so, rough guess. Find out what the specification of the scope is.

    I looked up the plugs and sockets on Wikipedia. The SO-239 socket matches the PL259 plug.
     
  18. bob808

    bob808

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    Nov 8, 2011
    5hz to 8mhz. I should be fine. Should I short only the inductor or the resistor as well?
    I will buy a set of adapters to BNC. Something like this:
    http://www.powerwerx.com/_images/products/RF-8312_xlrg.jpg
    I have no probes so I will get a set of normal ones.

    Can you tell me if Vishay MKP1839 series would be ok for some caps used for coupling? the 0.15uF and 0.22uF ones. I like that they are axial as the ones that I am replacing.
     
  19. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    8mhz seems a very low frequency, 8MHz is more likely.

    The resistor is there to develop a voltage in the amplifier. No resistor, no amplification.

    I could not tell you the best capacitors to get. I only know that vintage radio enthusiasts will often change all the old capacitors and will not use similar ones that have been stored from new (NOS) even if they test OK.
     
  20. bob808

    bob808

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    Nov 8, 2011
    I meant MHz but my lazy finger forgot to hold shift :)
    I made a list of Vishay/Panasonic caps. Should be just right for my purpose. I will go with the other inductor (if that's what it is) and have it measured by a friend that has the right equipment. So far all the remaining electrolytic caps tested off spec of about +30-50% . I know that is within electrolytic spec but in all my amateur electronics experience I've never measured more than 5-10% difference in new electrolytic caps and they were the cheapest. The 50uF+50uF can type caps measured ok between 50-53uF. I only have a capacitance meter so I don't know the other values. I will go from here with new tubes and see how it turns out.
    One single problem remains. On the schematic there is L1 inductor that on board is actually that weird setup with 2 inductors connected to a variable resistor. Can anyone tell me what that is and what it is used for? It is clearly meant to be adjusted for something.
     

    Attached Files:

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