| Electronics > Repair |
| Fixing a HP200AB Audio Oscillator, where to start? |
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| factory:
The SN won't tell you the date it was made, only the date of the last major design revision (for post 1960 units only). David |
| ardiesse:
The heavily clipped waveform at the output terminals means that the Wien bridge is out of balance - the amplitude limiting function is not working. The original HP manual will have a description of the theory of operation of a Wien bridge oscillator. Pay particular attention to the section on amplitude stabilization - the HP 200s use a light bulb for this. I'd also suggest that you try to track down a copy of Fred Terman's textbook "Electronic and Radio Engineering". As well as a detailed explanation of the Wien bridge oscillator, Terman's book has much material on the fundamentals of vacuum tube circuits. You need to understand how a Wien bridge oscillator works before attempting repair work. In equipment like this, the valves (sorry, tubes) are in general the most reliable components. The oscillator produces an output, so you can assume they're all in working condition. And hold off replacing any capacitor, including the electrolytics, until they are shown to be bad. If you've never worked on valve equipment before, beware of the high-tension supplies. Rob |
| rwgast_lowlevellogicdesin:
I read through the manual http://hparchive.com/Manuals/HP-200AB-7725-Manual.pdf and it says for waveform distortion bulb to investigate bulb R11, so ardiesse is right, it is a problem in the wien bridge oscillator. The manual says it should be a 250v 10watt, the bulb I pulled out is a 120v 7w bulb made in China and looks fairly new. the thing is I don't know how to get a replacement bulb. I mean I know I just need a 250v 10w incandescent bulb, but I don't know what kind of socket that is, it is much smaller than a regular household fixture. I'm in the US on 120 mains what is the likely hood of finding the right kind of bulb at home depot or something? The current bulbs filaments are fine and it measures about 250ohms, it does not light up when installed in the circuit, I'm guessing it is not suppose to though? |
| ardiesse:
A 250V, 10W light bulb will have a nominal resistance of 250x250/10 ohms, which is 6.25 k. But the "hot" resistance is much higher than the resistance at room temperature - this is how the amplitude stabilization works. In operation in a Wien bridge oscillator, the filament will glow red or dim orange, and in a well-lit room, you might not be able to see it glow. Your 120V, 7W light bulb will have a "hot" resistance of 120x120/7, about 2 k. That's a factor of eight higher than its resistance at room temperature (250 ohms). At a guess, the bridge will probably balance for a filament resistance between 500 and 1000 ohms. You're in luck. Digi-Key part number 10S6-230V (and in stock in the USA, too) is a 230V 10W candelabra screw light bulb. I bet it'll be a drop-in replacement. Maybe even your local lighting store has them. |
| rwgast_lowlevellogicdesin:
Thank you! Candelabrea bulb is exactly the information i needed! |
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