HP 200CD Valve (tube!!) oscillator problems?

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Swedish Chef

Well-known member
Joined
Jun 7, 2004
Messages
351
Location
London
we have just scored one of these:
hp200cd.jpg

for not too much on eBay.
It's a balanced valve oscillator with a pretty good amount of poke!
It "Sounds" like its working fine but when you scope the Hot and Cold o/p on a dual trace, as you wind up the amplitude on the osc, the amplitude of the Cold trace seems to fall in level to zero and then come back up with the polarity flipped...! This may be a scope annomaly (which I suspect as when you add the traces the amplitude of the trace rises smothly, and similar on the balanced module!!) but if it is, what on earth is causing it?

Baffled chef :?
 
Do you know if it's transformerbalanced output? In that case, it could be just poor high-frequency cmrr from the transformer?

Is it ment to be used balanced, or just "floating"?

Jakob E.
 
Manual/schematic on the HP200AB:

http://bama.sbc.edu/hp.htm

Looks pretty much as the same unit..

According to the schematic, the output transformer has no internal static shielding - so you will have poor balance on high frequencies due to electrostatic transfer between windings.

note - these manuals are in the .djvu graphics format, you need a plugin to view. Also the FTP server is working too hard - keep pressing refresh till you connect..

Jakob E.
 
Yea,

Bear in mind that one side of the scope input is grounded to mains ground. Try connecting it to the scope input via a 1:1 transformer with a load resistor of 10k (or 600R) depending on tranny.

:thumb:

Mark
 
I have the manual for the 200CD somewhere here, if you need a copy.

Best regards,

Mikkel C. Simonsen
 
You got ripped-off. There is supposed to be a shorting strap to (optionally) ground one side of the output. Demand a 3 cent refund for missing part.

The truly floating output is nice in some situations. But for general bench work, leave one side of the output winding grounded.

> scope the Hot and Cold o/p on a dual trace, as you wind up the amplitude on the osc, the amplitude of the Cold trace seems to fall in level to zero and then come back up with the polarity flipped...!

Stray capacitance here and there. Dunno why you are using a dual-trace on a single winding. Ground one end and use a single-trace.

Clean the switches. Oil the tuning shafts. Do NOT fiddle the trim-pots and trim caps unless it is working perfectly but a little out of spec. If it is a lot out of spec, there is something sick: don't mess with trimmers, fix the problem and then it will probably be in-spec again.
 
This isn't directly related to your question, but I wanted to mention a very common problem with those old oscillators, as well as the cure for it. The problem is that the output will not settle, it will just "pulse" as if being put through a tremolo. Or, it might not start at all, particularly on the low frequency ranges. In most cases, I've found the fault to be with the capacitors (the fixed ones, not the big air-variable) in the frequency-determining Wien bridge circuit. Replace these, and the problem usually goes away.

There's a 200CD schematic (albeit a not very legible one)here.
 
The HP200CD I got has the strap from ground to one of the 600 ohm terminals, for unbalanced operation. The strap is not used for balanced operation. Keep in mind, that model was produced for a lot of years, and has more than a few revisions.
 
> transformer output; one for the HF, and one for the LF.

Just one output. H and P were not that savvy at transformer design.

If you have one of the 2-terminal models, the black post is always grounded.

If you have a 3-terminal model, ground is just a ground, the other two posts are a transformer winding not connected to anything (floating).
 
As mentioned previously, there are at least two major versions of the circuitry, with the newer version using small tube, I believe, and providing better distortion specs. I had one of each for a while, and the newer one definitely measured better.
 
I think you also get a fairly constant 600 ohm output impedance. Keep this in mind since that isn't what you will see from most audio sources, which are typically less than 150 ohms.
I cut my teeth on a 200CD, and I still really like that design.
There is a light bulb in the oscillator circuit to act as the non-linear device to control the amplitude of the oscillation. DON'T mess with it!
 
> I think you also get a fairly constant 600 ohm output impedance

Dim memory?

The 200's output runs about 90-110 ohms across the audio range, rising above 10KC or 20KC. It is a feedback amplifier, so basically low-Z, but in most of the several models the feedback is taken from a separate winding so the output winding resistance and leakage inductance are not cancelled. Leakage inductance is non-trivial, since to get good 20CPS response and low distortion they put many-many turns on the core.

Notice that a pair of 6F6 or 6AQ5 are good for 8 or 10 watts; the 200 "underloads" them with a "too-high" primary impedance so it "only" delivers 24V or 1 Watt in 600 ohms.

At most audio frequencies you can get around 2 Watts in 150-200 ohms: I have seen smoke from a 1W 220 ohm resistor left bridged across the high-power low-THD audio-only (20CPS-40KC) 200CD model (rated 40V in 600 ohms).

Any decent audio shop of the period would have a "Gain Set": a box of pads and meters that you would put between your wanky signal source (before the HP-200, most audio sources were wanky) to set your exact levels (and build-out impedance). Sadly all the Gain Sets that once existed have vanished; we saw one in the Worst Bench contest at the old place, and that may be the only one I have seen since the 1960s.

> at least two major versions of the circuitry, with the newer version using small tube, I believe, and providing better distortion specs.

The 200-series were in production about 20 years, very "exciting" years for electronics. So there are several feature sets (wide range, low distortion) and internally several generations of parts. Both Octal and Miniature tube-sets were used, and I think each type was produced in most of the feature-set models.

The 1950 models did not get the very low THD levels of the later models: no demand for sines much cleaner than tenth-percent. I'm amazed at how clean the 1960- models got, considering that the "lamp stabilization" won't work without large nonlinearity in the oscillator's amplifier.

> There is a light bulb in the oscillator ... DON'T mess with it!

AGREE AGREE AGREE AGREE !!! (Though if waveform is large and distorted, gently check that it is tight.) It will never burn out, it isn't supposed to light, the filament hardly gets warm, but is critical to getting the level right. Also do not fiddle with the trim-pot near the lamp: unless you have to replace the lamp, the factory trimmer setting is good forever.

> In most cases, I've found the fault to be with the capacitors (the fixed ones, not the big air-variable) in the frequency-determining Wien bridge circuit.

I have not seen those go bad. But they are the kind of part that would vary in production, and they could have got a bad design. Mine are ceramic body with mica insulator, lovingly built, and will probably last a lifetime.

You can get similar no-start trouble when the coupling caps dry up, especially the 20uFd(?) can-cap that couples the oscillator 2nd stage plate back to the Wein bridge. The exact value for this cap is not critical: I bridged mine with 47uFd.

A HP-200 will always bobble when set near 60CPS with the cover off. I once spent days trying to fix this with better power supply filtering. It goes away when you put the cover on. D'oh, that huge air-cap is super high impedance at 60CPS and sucks all the hum from the room to beat with the oscillator. The case shields it excellently. The frequency and level-vs-freq also shift when you put the cover on, so do your final check and calibration with the cover or you are wasting your time.
 
Excuse me, but the 1970 HP catalog states:
"The 200CD give a maximum sinewave output of at least 10 volts across its rated load of 600 ohms and at least 20 volts open circuit... A special feature of the 200CD is that its waveform purity does not depend on load. The output impedance is nominally 600 ohms. The output tranformers are balanced within .1% at the lower frequencies and within approximately 1% at the higher frequencies." $275" Option H20 is a standard 200CD modified to have an extremely low distortion. Add $55

Output of the H20 option dropped from >10V to 7.5V into 600 ohms.

As I recall, a 600 ohm load produced the expected 6 dB drop in output voltage.

The output is balanced to ground at maximum output setting, but not necessarily at lower output settings. There are indeed 2 separate output transformers for high and low frequencies
 
Erm, so after all the sabre rattling...I am still an idiot who bought this thing as i wanted a balanced oscillator for the workshop. Can I not run it like that? :cry:

chef
 
If you need it "real-balanced", hook it to an audio transformer.

For most workshop purposes, you won't need a balanced output though. I never had balanced on any of my generators, and never missed it..

I think this HP is a good little box, that will serve you well..

Jakob E.
 
Cheers Jakob. But what is the point of having the 2 600ohm outputs on the front, and a Gnd post and strap? Why would you want to use this? How would you? :?
 
Steve,

Here's a clip from the 200CD schem that NYD linked to:

HP200CD_output.jpg


There are two output transformers for high an low level outputs, and you can see the balanced output-attenuator is a "bridged-tee" type. The blue squiggle shows the grounding strap.

Here's a simplified view of all that:

HP200CD_simpleout.jpg


So basically it's just a floating transformer winding (with no connection to chassis ground on either side of the winding). Essentially this will give you a "floating balanced" output when the ground-strap is not in place, and it would drive another balanced input with no problem. But for most testing and development work on the bench you can strap the lower winding to chassis ground/mains earth to give an unbalanced output. I use an unbalanced output signal generator feeding a 600R stepped attenuator feeding a unbalanced>balanced converter if I need a balanced output.

The reason the strap is there is that sometimes you will want a floating output to break a "ground loop". If you have a circuit under test which is connected to a PSU which has its 0V rail connected to mains earth, and you connect your scope probe to a place in the circuit along with its ground lead (also connected to mains ground), and you're feeding it from a one-side-grounded signal generator, there is a possibilty of a ground loop- easily spotted by a "wobbly" 1k trace on the scope. Sometimes just disconnecting the ground strap on the generator can solve it. I don't lift the mains earth connection on my scope for safety reasons, so having a floating-output generator is a very useful option. But leave it in for now (just use a piece of thick tinned Cu wire or fab a "strap" out of a drilled and filed bit of copper strip).

Have an experiment with it connected to different inputs and try different strappings.

It's incredible in this "yesterdays-technology-is-trash" culture (in the eyes of the average commercial/industrial world) that HP still honours its older products:

HP pays homage!

:cool:

Mark
 
Cheers Marky! Once again you excel yourself!! :green:
I'm still not sure I have explained meself correctly but need to go back to my mates, Horowitz &Hill, and Mr Pease... :roll:

Cheers again all who took the time!

chef
 

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