Restoring an HP 200CD Wide Range Oscillator
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- เผยแพร่เมื่อ 3 ก.ค. 2024
- In this episode, I need a good oscillator to test our 24V vacuum tube inverters. I’ve had this old Hewlett Packard 200CD Wide Range Oscillator in the garage for years, so before we can get into testing, first I need to clean it up and get it working well!
Thanks for watching!
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Just watched this video. Awesome! I have one of these (in storage for the next 6 months) and my symptoms were IDENTICAL to yours. So I am hoping your discovery will fix mine as well! I have other oscillators that are solid-state, but this thing is just plain cool with the HUGE tuning knob. Just a cool piece of gear! So THANK YOU!
Thank you so much!
This little HP oscillator is a great piece of equipment! Hopefully you can get yours up and going soon too!
Congratulations! Excellent restore of what is still a worthy boatanchor piece of gear... I bought a 200C as part of a university surplus lot nearly 20 years ago (also a Tek 465B and a pair of Heathkit B/C tube bench DC power supplies for $15. The HP and Tek worked and just needed a cleaning. The two supplies became one.
Two years ago the HP died. It was one of the power supply filter caps. I replaced both multisection caps, along with two Sprague 1uf @ 400WVDC cans. The other caps in the unit are axial mica block and rarely go bad--and were in tolerance so they remained. On mine, I dissasembled, cleaned, and lubricated the vernier dial and polished the dial. What a beauty, and ready for another 50 years--long after I will no longer be alive.
If someone is doing a restore, I recommend replacing the two old 'hat style' rectifiers with something like 1N4005 (1A @ 600V). An under $1 proactive fix...
The tubes (all four of them) form a vaccuum-state op-amp with two feedback paths. The circuit is known as a Wein Bridge oscillator and it is what Bill Hewlett based his work on when he created this design in 1937 for his masters thesis. C7 is part of one of those feedback paths. It gives the op-amp some very high frequency feedback to prevent it bursting into very high frequency oscillation, say above 1 MHz. If it works fine without it then you don't need it and can forget about it.
Good work getting this venerable instrument going again.
I notice that the tubes are branded HP so are probably the original ones installed at the factory in the 1940's or 1950's.
Thank you! And thank you so much for the information! I definitely learned something new today as i hadn't heard about the Wein Bridge oscillator before. Time for me to do some more reading on it! That's awesome that it's essentially an op-amp oscillator, and quite a brilliant little design.
So far, I've been playing with it through all ranges and frequencies and haven't run into any problems. It needs a little adjusting on the individual ranges to get it to be spot on, but honestly, I'm probably never going to do anything that would be that sensitive without having an oscilloscope hooked up as well.
I'm fairly certain it's all original inside. It looks like none of the parts have been replaced or even any rework done anywhere. That's super impressive in my book, considering it's getting on to 60+ years old now!
You are absolutely correct. Also, the father of this unit the original HP 200 was their first product, built in the garage. Their first big customer for the 200 was Disney, used in the soundtrack of Fantasia. The company really took off from there.
@@UsagiElectric One note--those light bulbs should never burn out, and don't replace them. They were carefully matched at the factory for their characteristics. Treat them carefully, and don't swap their positions. Those light bulbs are probably the most important and critical component for the stability of the unit, and they are the real magic in the circuit.
Wonderful piece of kit....cheers.
Thank you very much!
It's a really great little oscillator, and those HP guys really were analog wizards!
I bought one of these recently based on this video! Thank you for the inspiration. No dirt dobbers 😊 - I’ll restore based on your video, I got the service manual etc too. 🙌
Nice piece of equipment you have there ! The schematic seems pretty explicit : I think C7 has to be between 1.5 to 7 pF, and you have to adjust it so output is at 24V @ 1kHz. C7 will short R12 at high frequencies, so It may be a gain compensation to counter the output voltage drop or resonance near cutoff frequency
Thank you!
Digging into the HP manual a little more, it seems that C7 is only used to adjust the x10k range. You set the dial to "60" and then adjust C7 until you get exactly 600 kHz. Fortunately, the x10k range seemed to be pretty spot on with C7 removed, so that's some good news!
I blame your video. I just bought one! Have it home and yeah pretty dirty inside but works! Now to clean :)
Отличный аппарат!
Great machine!
Thank you!
I own and regularly use a 200CD. It gives a very clean, pure sine wave, better than any multi-function generators I have used. If you find one, best to replace the filter caps.
From the Wikipedia article on Hewlett-Packard::
Of the many projects they worked on, their first financially successful product was a precision audio oscillator known as the Model HP200A. Their innovation was the use of a small incandescent light bulb (known as a "pilot light") as a temperature dependent resistor in a critical portion of the circuit, the negative feedback loop which stabilized the amplitude of the output sinusoidal waveform. This allowed them to sell the Model 200A for $89.40 when competitors were selling less stable oscillators for over $200. The Model 200 series of generators continued production until at least 1972 as the 200AB, still tube-based but improved in design through the years.
One of the company's earliest customers was Bud Hawkins, chief sound engineer for Walt Disney Studios, who bought eight Model 200B audio oscillators (at $71.50 each) for use in the animated film Fantasia.
I'm still blown away at just how good this oscillator is! The BK Precision 3010 Function Generator I have on my desk is nice, but the sine wave is just no where near as clean or rock solid as the old HP 200CD.
The little incandescent lights in there threw me for a loop the first time I saw them, but once I saw that they were using them as temperature dependent resistors, I couldn't help but smile at the absolute brilliance of the old HP engineers.
That's awesome that the 200B was used for Fantasia, I had no idea about that! I'm totally going to use that bit of information every time I talk about the HP oscillator now, haha.
I also own one sitting on my bench, and use it from time to time (as it's the only LF generator I own). It's a Model 200CD tough, but looks identical. The range switch is just getting a little dodgy but still workable. The big frequency wheel is much cleaner than yours. Also still have the original paper service manual for it.
The dial today would be $80.00 - 😊
The light bulbs do stabilize the output. But you are missing the main point. The bulbs control the feedback to a level where a very low distion sine wave can be obtained. It was a novel approach achieved by Bill Hewlett.
Consider without the bulbs, you would not be able to obtain a sinusoid!
The earlier approach used by RCA ond others to obtain a low distortion sinusoid was the heterodyne approach: beating two rf carriers, the difference would be the desired frequency out.
That HP 200CD is a Music Analog Synthesizer instruments To Me
It would certainly be excellent at that too!
HP made one amazing little oscillator and there's not many out there that produce such a clean sine wave (or have such a brilliant huge control wheel on the front).
@@UsagiElectric 😎
Did you do a part 2 of this..? I can't find it...
When you clean the internal parts with water and detergent or similar cleaning agent, it is always recommended to thoroughly flush it all off with distilled water. That removes any residue that might crystallize and pit the surfaces, especially aluminum and other porous metals.
I didn't do an official "Part 2", but I did put the oscillator to work in testing out some tubes: th-cam.com/video/VtWRT-VIW54/w-d-xo.html
I didn't show it in the timelapse, but I actually carried the whole unit into the garage after washing and used compressed air to blow dry everywhere I could before letting it sit in the hot Texas sun to 100% dry out. Not quite as good as distilled water, but at the time I did this restoration, the local grocery store was out of stock of just about everything (even rice)!
My guess is that the HP 200CD Wide Range Oscillator is using a circuit called a Wien-Bridge Oscillator circuit, and that broken variable-capacitor you removed is basically a trimmer-cap used to align the circuit so that the Vernier-dial markings are properly calibrated across the entire range of frequencies that the oscillator is capable of running at, I could be wrong though, incidentally, the reason why HP chose to go with a Wein Bridge Oscillator design for the HP 200CD, is because it generates a Sine wave with relatively low THD (Total Harmonic Distortion), which is important if the HP 200CD is going to be used as a piece of audio test equipment, HP could have gone with a Phase-Shift Oscillator design.
Very excellent guess! It is indeed a Wien-Bridge Oscillator circuit, and this particular circuit really put HP on the map!
The vernier dial markings are slightly off, so you may be on point with your thinking that it's to adjust the markings to be accurate. If I ever get around to taking this thing apart again, I'll take a look at getting a replacement cap in place!
Yes, but remember they started building the first versions of this (200A) in a ramshackle garage!
@@AE1PT I have three of them there all different inside. The oldest one is the strongest. I can't remember all faults but one was the light bulbs were not making contact and a fresh dab of solder on bottom the of bulb fixed it. The unit in this video is a newer model.
Good video; maybe I'll go ahead and take mine apart to see why the X1 and X10 ranges don't work
I stumbled across this video while anxiously waiting the update on your Centurion minicomputer project. This is a great video about one of the most iconic products of Hewlett Packard and Silicon Valley. Thanks for sharing your restoration. The Hp 200A, which eventually turned into the 200CD was HP’s first product. It was also William Hewlett's 1939 Stanford thesis. Hewlett was also awarded patent 2,268,872 in 1942. I can recommend a great read “Max Wien, Mr. Hewlett, and a Rainy Sunday Afternoon”. It can be found in chapter 7 of the EDN book, “Analog Circuit Design” (ISBN 0-7506-9640-0). The patent and Chapter 7 can both be found on the Internet.
Thanks for the video! Could you please give us a link to that manual and schematics? Many thanks in advance.
Thank you!
And sure thing! Here's the manual for this 200CD Oscillator, the schematic is about 2/3 of the way through:
www.hparchive.com/Manuals/HP-200CD-Manual-SNP_605.PDF
@@UsagiElectric Thank you very much! That's exactly my model! Now, where is my screw driver...? 😆
That broken capacitor looks like an E F Johnson PC mount air trimmer. I have one in the junk box, you want it?
Thank you so much for the offer, but I've got a few that I can use if I ever get around to taking this thing apart again!
14:52 Enter the attack rabbit.
The cutest attack in all the land!
Con ésas mismas válvulas se puede construir un amplificador de audio de baja potencia
The 200CD oscillator actually uses two 6AU6 tubes in it, which happen to be my favorite tube. I've used it for just about everything, but notably, I'm building my entire vacuum tube computer using the 6AU6. It's a great tube for low voltage stuff!
(I used Google Translate for your comment, so if my reply doesn't make sense, I blame Google!)
So am I correct that you use the water hose to clean the variable capacitor?
old equipment are tough.. I'm surprise he doesn't use power washing to get the job done
I indeed did use a water hose to clean the inside. The unit hadn't seen power for at least five years, so there wasn't any residual power in the capacitors for certain. After washing, I actually blew most of the water off with light compressed air and then dried the rest by hand with rags before putting it in the hot Texas sun for a few hours to properly dry out. Then, it came inside and dried for an entire day before power was applied to it. I didn't really take any time lapse footage of that because it's super boring, haha.
@@flmag88 You're absolutely right! Most of the electronic equipment I come across from the 50's and 60's is built like a tank!
What is a 200 cd wide range oscillator worth
0:10 What lathe is that?
At first glance at the schematic, I see the two rectifier diodes are shown in the wrong polarity to supply positive plate voltage. If this is a genuine HP schematic it would be unusual to see any errors.
As far as I can tell, at that time diodes were always drawn the other way around like that. The schematic symbol is literally a picture of the current being blocked in the direction of the bar, and before they worked out that conventional current wasn't actually representative of the electrical flow, it made sense to draw it that way around. I dimly remember, many decades ago, being taught that diodes were correctly drawn that way around; when I began to re-familiarize myself with electronics, a few years ago, not realizing that had been changed caused me no end of confusion.
There is a company that makes 18w tube amps out of the chassis of these. I am 50/50 on wether or not that's OK. :)