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Old 27th Nov 2017, 10:05 pm   #1
Ilikevalvesme
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Default Hewlett Packard 200CD Wide Range Oscillator

This 200CD was an ebay purchase. The seller packed it well & it arrived well protected & in good order.
HP introduced the 200CD in 1952 & produced it all the way up to 1985 - a 33 year production run with a few minor mods along the way, e.g. the original octal valves became miniature B7G/B9As & the last variant ditched the 5AR4 (GZ34) for semiconductor diodes. This one has the miniature valves & the 5AR4 rectifier so ticks the boxes for me. The date code on the three can electrolytics indicate it was built in 1960.
Well first job is to uncase it & see what we have. The chassis upper & power transformer were quite dirty, something had clearly been spilled on the top & dripped through the vents. The underside was clean & unmolested while the variable capacitor gearing was stiff with (hopefully) old grease.
So clean up & sort the mechanical bits first. Removing the shield around the capacitor & also the gear train made enough access to clean the chassis top & front panel with foam cleaner. The big tuning dial cleaned up nicely with Flash kitchen surface cleaner & gentle use of a mildly scratchy sponge (after having tried it first on the dial rear - always test first on a non visible surface!) I put penetrating oil in all the shaft oil holes which did free them up a bit overnight but I did have to pull the shafts out & wipe them off to really free everything up. Reassembled it all & now we have a nice smooth tuning action. Took a little fiddling to get the variable cap aligned with the gear train but the cap's on threaded adjusters. Good old HP.
Now to start on the electronics....
To be continued...
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Old 28th Nov 2017, 12:35 pm   #2
Andrew2
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

Lovely. I've long regarded the 200 series as the prettiest piece of test gear ever. So beautifully neat and compact, and the workmanship inside is super. There were a number of look-alikes around in the 60's IIRC - my dad bought me a TEC (not TEK!) one that looked nice but was pretty rough and drifty.
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Old 28th Nov 2017, 9:15 pm   #3
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

The HP manual was downloaded from the internet & printed out, with
this in hand the electrical restoration begins. The circuit is basically a balanced push-pull Wien bridge oscillator. The active devices are a pair of 6AU6 pentodes driving a pair of 6CW5/EL86s operating as a cathode follower output stage. Frequency range is 5Hz - 600kHz. Well on inspection the power transformer mounting was slightly bent due to the unit having been dropped on its side at some point in the past. This was soon fixed by putting the chassis on its side & applying some gentle muscle to the transformer. The power switch had been replaced by a pushbutton type, a rummage in the spares box turned up a Bulgin toggle which matched the original style. The rubber power cable was perished where it entered the chassis, this was removed & a new rubber cable the same diameter was installed. I took the opportunity to rewire the transformer primary from 115V to 230V, a
simple job of swapping tagstrip jumpers.The only paper capacitors are a pair of 0.5uF 400V caps. I had some 0.47uF 1200V polypropylenes, a bit overkill on voltage but they were the same size as the originals so I used them. Apart from the close tolerance ones on the range switch, the resistors were all carbon composition. I checked them for drift but all were nicely within spec.
The valves (2 x 6AU6A, 2 x 6CW5/EL86, 1 x 5AR4) all tested full spec on an AVO Mk4. As well as the valves there are two candelabra screw lamps on the chassis. These are connected in series & form the dynamic resistance for the feedback loop. These are listed in the parts list as 250V 10W which I thought may have been a typo; however checking the current/resistance against a 240V 7W lamp I had handy, they are fairly similar (470 ohms cold, 8mA at 12V or 1500 ohms). A resistance check across the HT line read 2.5 Meg so I applied power & checked nothing's amiss. the oscillator started OK, the can filter cap seemed very slightly warm after a few minutes so I kept an eye on it, it seemed OK for a while but then suddenly started to heat up. OK, switch off & let's see about a replacement. In the end I decided to fit two tubular electrolytics under the chassis & leave the can in place for appearance & as a binding post for the new caps' negative leads. With power back on, all was OK & a check of the DC voltages agreed nicely with the manual.
I ran through the alignment procedure, setting up the tuning dial tracking & also the output voltage, which is set by adjusting the negative feedback level. The maximum output is specified as 12V RMS into 600 ohms. Distortion is specified as 0.2% from 20Hz - 200kHz. I measured it at 0.22%. The output level is more than I need, it is possible to adjust the negative feedback to lower both the output level & distortion. I tried this; with the max. output set to 6V into 600 ohms -as low as it would go - I could get a distortion figure of 0.06% at 1kHz, however the output level would take a few seconds to stabilise when changing range. Eventually I set the max. output to 10V, stability wasn't compromised & the distortion level was then a respectable 0.12%.
It runs pretty warm; as well as the two power pentodes & the rectifier there are five 10W wirewounds bolted to the chassis. However HP thoughtfully put all the hot bits on one side of the chassis & the oscillator valves & the frequency determining components on the opposite side. The casing is well ventilated, too.
With the electronics completed, it was time to give the casing a final clean before reassembly. It didn't look too bad but on starting cleaning with some Flash & a sponge, the brown wrinkle paint went about 3 shades lighter as a stream of brown water went down the plughole. It seems HP chose the cabinet colour wisely.....
The last picture shows it reassembled & ready for work.
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Old 28th Nov 2017, 9:46 pm   #4
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

Looks really good. We had a couple of these still at university in the 1990s. I wanted to crack one open and take a look. At least my curiosity is now satisfied as well
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Old 28th Nov 2017, 10:35 pm   #5
kalee20
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

What a super oscillator!
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Old 28th Nov 2017, 11:30 pm   #6
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

Well done for saving such a lovely bit of kit- this era of test gear from the Big American Duo must be some of the nicest stuff around. So often, I've found that even scruffy and neglected stuff can come up surprisingly well with a bit of elbow grease and common-or-garden household cleaners.
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Old 29th Nov 2017, 10:45 pm   #7
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

Thanks for the comments folks - I was in the fortunate position of being able to upgrade some of my test gear so this is a replacement for a little Russian oscillator I bought a few years ago from a radio rally. I wanted a good one, preferably valve & I thought it was a great piece of industrial design.
Oh and it was built the same year I was born.
The lamps in the negative feedback loop are Bill Hewlett's patent which was used in HP's first product, the 200A. The HP lamp part number cross references to a GE part rated at 12V, 8mA. They do actually take 8mA at 12V but they don't light at that current of course. It seems they actually are the 250V 10W lamps stated on the schematic.
The output voltage is a bit too high for my needs so I'll probably build an off board switched attenuator as it's a little difficult to accurately set an output much less than 0.5V using its own rotary attenuator.
Cheers
Mark
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Old 29th Nov 2017, 10:49 pm   #8
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Default Re: Hewlett Packard 200CD Wide Range Oscillator

THe 200C/D oscillator stayed in production for a very long time. One reason was because they were written into a lot of companies' test procedures because of their large output voltage swing.

I bought one from a trader at the G-QRP doo many years ago and I put it in the display in the foyer of building 5 at HP South Queensferry where I worked. There was a collection of local developments but nothing earlier. I'd have loved to have found a 200A, but the 200C/D whas what I came across. Then hit by redundancy I left it there but made sure it was understood that I owned it, not the firm and it was on loan. The building's been demolished and houses built. I heard a rumour that the 200C/D got taken somewhere down South. I guess I've lost it.

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