Correct Connection Setup - FR Signal Generator to Oscilloscope

Discussion in 'Test & Measurement Forum' started by btvarner, Jan 10, 2019.

  1. spinnaker

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    Congratulations!

    You need to play with the trigger to get it to stabilize.

    Increase the frequency of the generator. Then increase the frequency of the scope. It should not flash as much. I am assuming what you are seeing for flashing is the trace being painted. It won't be so apparent on higher frequencies.



    Is that the flashing you are seeing like in the video?


    .
     
  2. spinnaker

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    The frequency control knob might be broken. It does not even change the trace if you turn to B,C,DE etc?

    What connector are you using on the generator?
     
  3. spinnaker

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    If you look at the Band switch, it has 2 screws. This does not seem consistent with the rest of the generator. Looks like someone replaced that switch. Something may have been wired up wrong or maybe it isn't even connected. You might want to pop the cover and take a look.

    And I will bet that frequency control in the center is broken. It might be connected direct to a pot or variable cap. Or it could be connected via a string like the older radios. They would break a lot. And a pain to restring. ;)
     
  4. btvarner

    Thread Starter New Member

    Jan 7, 2018
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    Yes, I am sure that the video describes the flashing I am seeing. I will attempt to correct again tomorrow

    None of the knobs feel like they are broken. No, it did not change, even when I changed to B, C, D, or E. I was connecting the generator to the scope as suggested earlier in the thread. Just attaching the scopes probe to the microphone positive center point. Tried both AF and RF. Either way provided the sine-wave, but neither way produced any changes in the wave.

    Also, see the two hole, key-way looking holes above and below the words SG-2? Wonder what they do?

    Will break into the generator tomorrow and see what I can find out. Will let everyone know.
     
  5. Reloadron

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    Jan 15, 2015
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    Well getting interesting, sorry as I fell asleep. :) Post #20 is interesting as it looks almost like a delaying time base setting or an A intensified by B setting. What is the horizontal time base setting anyway? I am wondering what you are seeing and why it won't change? Post back when you can.

    Ron
     
  6. btvarner

    Thread Starter New Member

    Jan 7, 2018
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    Alright. A good night’s sleep clears the mind. Several details to address.

    Spinnaker, thanks for the video! This guy is providing just what I need to learn scopes, and this oscilloscope in particular. I Even watched another one of his videos “Analog Oscilloscope Basics: Making a Frequency Measurement” which is an end result I am trying to achieve. That video is here:



    I will try to re-open the generator today if possible & trace down problems.

    Reloadron, my settings when I captured the image in post #20 were:
    10x = Aligned Probe. All output responded the same way, whether the probe was connected to: center post of AF, RF, or bare wire connection point.
    CH1 = AC, 5v
    Time Delay = 10ms
    Trig Mode = Auto
    Trig Holdover = Norm

    I was able to stabilize the trace better by slowing to 5ms, but it still flashes. Just does not travel from left to right. Instead it just flashes. I can get the trace to totally stabilize, but only by slowing to such an extent that I only get about ½ a wave on the screen. Manipulating other settings could not bring a full wave onto the screen at this stabilized rate……

    Here is a quick video of the trace when I set the device at a point where I can see multiple waves. Several points:
    1) Settings are as stated above with the Time Delay on 5ms.

    2) I used an alligator clipped lead, attached to the bare wire connection on the generator. (I used that point because I did not have to hold the probe onto one of the center points of the RF or AF output. Sine-wave was exactly the same when held against any of these three points.)

    3) After watching the frequency measurement video above, I followed their example and measured the same frequency as being output from the generator during the video. If I did it correctly,it was 62.5khz. That does not seem right?

     
  7. btvarner

    Thread Starter New Member

    Jan 7, 2018
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    Tested tubes, all good. Looks like I have some parts replacing ahead of me! Selenium rectifier, wax caps, electrolytic, testing of switches...........

    [​IMG]
    [​IMG]
     
  8. spinnaker

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    I jealous. :(

    Where on earth did you get a tube tester???


    Sounds like you know more about this stuff then you are letting on. ;)

    I would start with the switches. Of course if they have been replaced before, there is no telling if it was correct. I would look at that Band switch. See if you can trace it out and figure where is is going.

    I would also be interested in the function of the switch in the lower left (as looking from the front). Did you turn it to see if it has any affect? How many positions does it have?
     
  9. Reloadron

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    While I am thinking about it there is something you may wish to consider. That large green selenium rectifier has a few schools of thought behind it. I am not going to jump up and down screaming replace it but also won't say don't replace it. Literally a few dollars will buy you a handful of 1N400X diodes rated for 1 Amp and up to 1,000 volts (1N4007). I doubt the existing selenium rectifier even comes close to a PIV of 1.0 KV or 1.0 Amp forward current but take a look at the 1N4000 series data sheet. Page 3 of the linked data sheet shows the curve for forward voltage drop verse forward current. At 200 mA which is likely close to your existing selenium rectifier the silicon diode has a forward voltage drop of about 0.8 Volts. Therefore in for example a full wave bridge configuration you will have a forward voltage drop of 1.6 volts on alternate half cycles of the AC voltage in. The old school thinking on a selenium rectifier was about 1 volt per plate per rectifier and since the plates were stacked you could have several rectifiers in a single stack. The end result of all this would be the DC voltage going to the filter caps will be higher with the new diodes. How much higher I really don't know and I really don't know what effect the increased B+ voltage will have on overall circuit performance. Just something to consider.

    You didn't mention the tube types but at that point in time tubes like the 12AU7 were popular as the heart of the oscillator. The power transformer generally had two black leads for the primary, two yellow leads which were about 6.3 VAC secondary for tube heaters and two red leads which was the high voltage AC to the rectifier followed by a Pi configured filter of a few capacitors (or a single dual capacitor) of 20 to 30 uF rated at 250 to 350 VDC depending on the B+ voltage used. If yours used a 12AU7 The tube has a dual filament which can be configured in series for 12.6 volts or parallel for 6.3 volts operation. The 12AX7 and 12AT7 were also popular for the era.

    The guts look super clean which is really nice to see and in your first image I see the small trimmer cap on the top right which might be the internal modulation oscillator but I really don't know but it is where the front panel tweaks are.

    I still don't get that what looks to be intensified portion of the scope sweep? That scope offered a delaying sweep function or intensified sweep but in the pictures the Time/Div knobs looked to be locked with no delaying sweep functions in use. On your Horizontal Display functions, buttons beside the CRT on the right the A button should be depressed and the sweep time knobs locked.

    Well in the time it took me to type this I see Spinnaker has already left some good advice. :) I did let the dogs out and fielded a phone call from an intoxicated neighbor which reminds me it's time for a beer. :)

    Ron
     
  10. spinnaker

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    Wow, you have a REALLY good memory. I couldn't remember a tube number to save my life! And all the details of configurations. That is impressive. Not sure if I would have remembered all of that back in the day without looking at a schematic. ;)
     
  11. Reloadron

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    It's pretty much what I grew up with and learned in school. When I started at the place I retired from we still had plenty of stuff using tubes and they loved me. People who were into music using tube amps were always at my office door. I just have this wealth of absolutely useless information. Matter of fact, this scope, the Tektronix 465 was a best seller and one of the first totally solid state (transistors) scopes Tek made with only the CRT being a tube. When I was on medical once they gutted a store room I had and among the things trashed (literally into a dumpster) and a Tek 465M as well as a mint Tek 575 Curve Tracer went in the dumpster along with other great stuff. The bitch was the new program they were making room for was the then new B&W M-Power Program. Today, may it rest in peace and I am glad I was never sucked into it. :)

    Ron
     
  12. spinnaker

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    Well I learned tubes (transistors were going to be just a fad ;) ) first too but I sure don't remember any of it. ;)
     
  13. ModemHead

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    Nov 1, 2010
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    The waveform period is a little over 3 divisions at 5ms/div, ie. about 16.67ms. That's a 60Hz signal. Possibly just some mains leakage coupled into the entire device. Is the chassis grounded with a three-pin plug or is it all floating?

    5ms/div is a relatively slow sweep rate and will always appear to flash. That's just how analog scopes work. The sweep rate knobs are locked and the buttons are set for single-timebase operation, so the "intensified" portions of the waveform is likely just an artifact from a camera shutter speed that is faster than the sweep rate.

    I wouldn't expect an RF signal output to be on a binding post. Seems like that should be a signal ground.

    On a side note, the scope pictured in the very first post is actually mine, before it was restored. Still in use, BTW.
     
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  14. nsaspook

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  15. spinnaker

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    Excellent observation. Looks like the generator might not be working after all. :(
     
  16. btvarner

    Thread Starter New Member

    Jan 7, 2018
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    Spinnaker: I am new to all this. I decided a couple of months ago that I wanted to restore a few old tube radios, but I have taken a different approach than most others. Rather than starting to repair old radios & purchase equipment as needed, I have purchased equipment while reading up as much as I can on the subject and processes. I now have:

    Tektronix 465 Oscilloscope
    Realistic SG-2 RG Signal Generator (Trying to find manual)
    Global Specialties Frequency Counter Model 6022 (Trying to find manual)
    Jackson 648A Tube Tester
    RCA VoltOhmyst WV-77E VTVM

    But as you can see from my questions, I am trying to learn the subject and all this equipment BEFORE I restore my first radio. May be a couple of days before I get too deeply into the generator, tomorrow is the Chiefs playoff game……… Plus, I take care of a disabled son full-time, so I get distracted often. I will check the band switch & that function switch first. It only has 2 positions by-the-way.

    Reloadron: thanks for the tips on the selenium rectifier. I was just starting to look that up next. I will then work through your whole paragraph and decided how to proceed on that front. The tubes in the RF generator are: 6CB6, 6AV6, & 6BA6.

    I am still figuring out the oscilloscope. I will try your suggestions on the scope and see what happens.

    ModemHead: Thanks for joining the discussion. That makes sense. No 3-prong plug. Looks to be grounded to the chassis. I guess that pretty much explains the reason why the sine-wave would not deviate. Please explain “binding post”? Yes, on the scope in the first image. I just grabbed one off the internet for the same scope to describe what I had……..

    Nsaspook: I obtained an already made adapter on eBay from one of the earlier contributors of this thread who makes them. Seems like it will work. I have also received 2 new BNC to chassis connectors. I may just replace the microphone connectors with the BNC during the process………..

    Sure wish I did not have to learn this by testing/fixing the test equipment themselves. Was hoping to start with an easy AM radio. Now to figure out how to go about testing the components.
     
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  17. Reloadron

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  18. ModemHead

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    Yeah, I think you're just picking up a 60 Hz signal, maybe from a faulty power supply or just capacitive coupling.
    I think you called it a "bare wire connection". Commonly called a "5-way binding post" because it accepts banana plugs, spade lugs, bare wires, and tip jacks through the crosswise hole. Not normally used as a high-frequency connector though.

    BTW, I forgot to mention I like the Mickey D's soda straw used as a wire loom. :)
     
  19. spinnaker

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    Binding post

    Noun

    Little black thing in the lower right of your generator.
     
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  20. btvarner

    Thread Starter New Member

    Jan 7, 2018
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    Ok, I am hopefully going to do two things this morning.

    1) Use a DMM to see if the chassis is hot
    2) Trace out & create a schematic of the RF generator (Wish me luck because I have not done this before)

    I may open a new separate thread for those posts since we are getting so far off of the original subject of this thread. Think that's a good idea?
     
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