Audio Oscillator Project - Hp 200c Restoration

Discussion in 'The Projects Forum' started by Mckayman, Aug 20, 2009.

  1. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Okay well I just got my hands on a Hp 200c from the dawn of the 50s! My goal is to restore it in to working condition again. So far I have replaced the power cord, fuse and have used a variac to slowly power it back up. All of the tubes are warm when on and the correct heater voltage is being applied from the transformer. (6.3v) I have not been able to use contact cleaner on the switches or controls yet. For a brief period of time I was able to hear it work when hooked to the stereo. It didn't sound too good though so I connected to the oscilloscope and saw the problem. The waves were faint and not fully oscillating. (Maybe a bad output cap?) Sketch is included below. Shortly after though the oscillator stopped producing a signal and hasn't produced one again.

    I am still very new to vintage electronics and I think that working with something like this would be a good start to get to know them. If you have any ideas or suggestions to help me out on fixing this I would appreciate it.

    Manual --> http://www.hparchive.com/Manuals/HP-200C-Manual-sn-50000.pdf

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  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    What a prize! We had one in the shop. Nothing like 2 watt resistors and point-to-point wiring.

    You should be suspicious of any electrolytic capacitor. I don't think you can find the chassis mount kind any longer, which is really too bad. That Aerovox cap may be a wax type, and may need replacing. They don't age well. The smaller yellow cap up at the top of the chassis is another electrolytic that need checking.

    Your waveform clipping on the lower excursion may mean that B+ is a bit low, or that some bias is off. Check/replace the electrolytic caps and then - using a Simpson 260 - check voltages. A glowing heater doesn't mean the tube is working to spec.

    Good luck with restoring the oscillator.
     
  3. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Alright well i'll start with some contact cleaner and replace the electrolytic caps. When checking the caps am I just looking to see if they have the right farad using a regular test function on a digital multi meter? I don't have access to tube tester (have no idea how I can get my hands on one) but I may be able to find tubes that haven't been used. (Last Resort).

    Do you know of any places that would possibly carry the types of caps for this device such as Mouser? Mouser had some of them but not all.
     
  4. flat5

    Active Member

    Nov 13, 2008
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    IMO

    Replace all the caps unless you care if the unit looks exactly the same as original. Except perhaps the mica ones.

    Test all resistors to see if they are still within their tolerance.

    Then do voltage/current checks on the tubes.
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    There's an incandescent lamp in the feedback loop. I thin it's a 6s6 or something like that. Be sure that's in the socket and glows dimly.
    eric
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    It's also a good idea to measure all of those carbon resistors. They had a habit of increasing in value over time with exposure to heat, especially the ones with high resistance values. I still love those old girls though! It certainly won't hurt your eyes while working on it! ;)
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    The lamp must be good, as it's producing a partially clipped sine wave.

    Wow, 10% resistors. I see 47K and 4.7K, so the circuit has to be designed right.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yeah, and it was made long before the boneheads at the EPA forced wisker growing solder on us!
     
  11. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    UPDATE: Well good news! I got some good De-Ox-Id contact cleaner, applied to the range switch mechanism, amplitude control, and parts of the variable cap for the frequency knob. This must have cleaned a lot of stuff up, the oscillator now operates 100% of the time now. However, I am still getting a clipped sine wave, so I am going replace the electrolytic and wax caps next.
     
    Last edited: Sep 19, 2009
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Since you're getting an output, it's most likely that the resistors have shifted in value.

    Those old cylindrical brown-body carbon resistors really tend to shift quite a bit with age/heat. I have a number of them from the 60's in various values that were still hermetically sealed in factory packaging when I opened them up. They had increased in value anywhere from 5% to over 210% their markings, and they weren't even in use!
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    That's the truth but ya gotta admit that the color codes were so damn easy to read against their brown bodies! The colors that were used were far more vibrant than what they use today. I still have 10s of thousands (1/4 to 2W) of them buried in a storage bay... somewhere. Probably close to where all my Tubes are!:)
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Heck yeah! Well, the newer "dumbell" style carbon film resistors sure are a heck of a lot more stable; I have had some stored for 20+ years and they still read within tolerance. Yes, they're harder to read, but there is less space to apply paint than the old cylindrical bodies. And yes, the dark brown made the colors seem to "jump out" a lot more. I can still read the tan resistors OK, but the blue metal film resistors are a real bear! :(

    LOL - well, I don't have anywhere near that much of the really old stuff; maybe 500 or so resistors and zero tubes. I never bought any tubes that I didn't have an immediate use for. Transistors and ICs... well, that's another story... :rolleyes:
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    I think all of the old hands are following this thread with interest. I'm hoping you get this classic in full working order. I haven't had much to contribute, so I haven't chimed in before.
     
  16. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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  17. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    UPDATE: Well good news! I went and pretty much replaced all of the caps in the oscillator. All except for one that I felt was probably okay. After firing everything back up, I got some good results! The sine wave was no longer clipped and the frequencies were accurate! I am really pleased that I have gotten this old thing to work. I have another one a HP200B model that I will begin working on for another project.

    Quick question, if a cap stats that it is a 3x10 µf 450dc (it has 3 sections) could I replace it with a cap that is 33µf 450dc?
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    I doubt it because while multi uF electrolytic's shared a common (-) case ground, the individual sections were usually connected to different nodes. For instance they were used in pi filter configurations. I thought you had the print? They're not wired in parallel on the print are they?

    EDIT: Are those photos real? I don't know if I could work in such a clutter free environment! :rolleyes::D:)
     
    Last edited: Sep 25, 2009
  19. SgtWookie

    Expert

    Jul 17, 2007
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    A neat workbench is the sign of a sick mind. ;)

    Good that it's working better. Don't toss out those caps that you removed. Even though they are ancient, you may be able to revive them (once you get the proper equipment) using a process known as "re-forming". This won't work if the caps are physically damaged or the electrolyte has dried out, but it's certainly worth a try.

    Even if the caps can't be re-formed, you can gut them and use the original cases over the replacement caps to give that original look to your equipment.
     
  20. Mckayman

    Thread Starter Active Member

    Jun 22, 2009
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    Ahh yes I figured it out now. I realized that the last capacitor (on the HP200B not the same oscillator as shown above) that I had not replaced yet had 3 different nodes for 3 different 10uf sections. Two were connected together while one was by itself. Once I understood this, I went ahead and replaced it with 2 capacitors.
     
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