Commentary on the HP 200 which everyone should own one of.

Hewlett-Packard 200CD Wide-Range Oscillator modifications as posted to
the Ampex Mailing List in 2000

Usual disclaimers. Follow instructions at your own risk. For use by
qualified personnel only. To avoid injury or equipment damage, take
usual and customary precautions.

Selling or modification of this information prohibited but you may
distribute if quoted as-is, with no changes, edits, modifications,

Dave Dintenfass
Full-Track Productions, Seattle, Washington
March 2006


From: David Dintenfass
Sent: Wednesday, February 02, 2000 10:13 AM
To: Ampex List
Subject: [AMPEX] OT: HP 200CD low-distortion modification

About ten years ago I did the low-distortion modification on one of my
HP 200CDs. Since I can't find the specific service note, I opened up
the oscillator to see what I had done (at the time, I followed the
procedures listed in the official HP service note).

To reduce distortion, change R12 from 3000 ohms to 2700 ohms. With a
600 ohms load on the oscillator output, crank the front-panel
"amplitude" knob wide open (fully clockwise), then adjust R11 until
you get 7.5 Vrms across the output.

Keep in mind that HP 200CDs can be divided into two broad categories
-- those with octal tubes and point-to-point wiring and those with
miniature tubes and a mix of point-to-point wiring and a small pc
board. I think the change occurred around 1962 going by the changes
reflected in various service manuals I have for the thing.

On older units, R11 is not identified but you can adjust it from the
top of the unit if you mesh the tuning capacitor. On newer units, R11
is on the pc board and is marked "output volts adj" with silkscreen
lettering. You will have to turn the unit on its side to adjust R11 on
newer units.

If you have trouble finding R12 on older units, look for R11 (which is
a pot) and R12 is the 3K resistor connected to the wiper of R12. With
newer units, R12 is marked on the pc board.

Note that after you make the mod, the oscillator will take longer to
settle when you change frequency. This is particularly noticeable at
low frequencies. This is a normal consequence of making this

THD (at full output) on my particular unit went from:

0.068% at 1 kHz before the mod to 0.024% at 1 kHz after the mod

Note that this is THD, not THD+noise as it was measured on a spectrum
analyzer, not a THD meter. You results may vary. Newer oscillators
(those with miniature tubes) tended to have better distortion specs
due to the better linearity of the newer tube types.

While you are in there, make sure the two incandescent lamps are tight
in their sockets. That can make a difference in performance.

If you're wondering why there are so many 200CDs, that's because it
was made from approximately 1952 to 1985. I'm not sure of the exact
introduction date but there was an article introducing the new HP
oscillators in the December 1952 issue of the _Hewlett-Packard
Journal_. The earliest units were made in Palo Alto. Production was
then moved to Loveland, Colorado, then finally to Everett, Washington
in the early 1980s.

Disclaimer: Operate on your 200CD at your own risk. Hazardous voltages
are exposed when you have the cover removed.



From: David Dintenfass
Sent: Wednesday, February 02, 2000 3:05 PM
To: Ampex List
Subject: [AMPEX] OT: HP 200CD lamp aging procedure

By popular demand, here is my original "HP 200CD lamp aging procedure"
post from my private archive. I'll post the other one (reducing 60-
cycle hum) shortly.

Note that although the lamps are described as "250V, 10 W" in the
parts list in the 200CD manuals, they are also described as
maufactured by General Electric (Nela Park division) with the GE part
number being "8A/S6-12V."

For some reason, these lamps are labeled as 12-volts lamps though they
are in fact 250 volt lamps. I have not been able to determine why this
was so. As far as I know, General Electric at one time made these
lamps special for HP (very early 200CDs may have off-the-shelf lamps
with 250V markings).

Some years ago I obtained a bunch of these lamps (new old stock) at a
scrap auction. If you need a replacement set of lamps, e-mail me
privately. As Howard mentions, these lamps rarely fail.


From: David Dintenfass
Sent: Thursday, July 16, 1998 9:46 AM
To: Multiple recipients of Ampex
Subject: AMPEX: HP 200CD lamp aging procedure

Although this has minimal Ampex content, there's been considerable
discussion related to HP 200CD oscillators on this list, and I know
that many of you own HP 200CD oscillators and use them to align your

The HP 200CD was a popular product and was made from approximately
1952 to 1985. Like any product made for over 30 years, there were
incremental improvements but the basic design remained the same. Even
if you have an older unit, you can trick it out a bit and it will
perform nearly as well as the more recent ones. When I find my stash
of HP Service Notes, I will post the various procedures, including the
low-distortion modification. And although I summarized the hum-
reduction modification the other day, I will post it as well since
there are a few minor details that I didn't mention (including
installing a braided shield over the twisted filament wires underneath
the chassis).

I should also mention that while I do have modest electronics
technician training, I am not an HP 200CD expert and am not prepared
to troubleshoot your HP 200CD. However, I was fortunate in having been
a technical writer at one of the Hewlett-Packard plants where the HP
200CD was made. Since I had an older HP 200CD at the time, I was
inspired to browse the service file.

The two 250-volt incandescent lamps used in the oscillator circuit
proved to be one of the more troublesome components. These lamps were
used as nonlinear resistors to stabilize the oscillator output. Since
the lamps operated at very low current they should, in theory, last

But because the lamps operated at very low current, manufacturing
defects that would have been trivial for a lamp operating at 250 volts
created headaches for this application. The problem here was
intermittent or high resistance between the filament leads and the
base of the lamp. Another problem was poor contact in the lamp socket,
caused either by a loose lamp, dirty contacts, or defective lamp
sockets. Around 1965, HP began using a lamp socket with welded

At various points, lamp problems were blamed for a number of
complaints, including failure to meet published distortion specs and
60 Hz modulation. Before doing anything else, I would suggest adding
twisted (and shielded) heater leads, a hum-balance pot, and shielding
on the twisted wires to the front-panel power switch if you have an
older unit.

If you still have a problem, you might try the following procedure
(quoted here from a 1972 memo). Note that this procedure assumes that
you have a newer unit, which incorporates the improvements I just

<begin quote>

1) Age RT1 and RT2 for 30 seconds to 1 minute at 110 volts ac. While
the lamp is being aged, check the filament to see if it oscillates
(defective lamp will oscillate). Also, the filament should appear to
be tight (no sagging). Filament resistance is 480 ohms cold, 6.25 ohms
hot (at 250 volts).

2) Solder bulb socket rivets.

3) Tighten light bulbs.

4) Adjust hum-balance pot.

5) Use 6AU6A, not 6AU6. As the tube ages, 60 Hz feedback from the
filament is possible; the only cure is a new tube.

) Move a filament wires close to the chassis and all B+ leads away
from the chassis.

<end quote>

-dave dintenfass (usual disclaimers apply)


From: David Dintenfass
Sent: Wednesday, February 02, 2000 3:27 PM
To: Ampex List
Subject: [AMPEX] OT: reducing 60 Hz hum in HP 200CD oscillator

Here's another one from the archive. I'll summarize for those who
aren't interested in the original context of the discussion.

To reduce powerline hum in the output signal of the HP 200CD (either
60 Hz or 50 Hz, depending on your powerline frequency), you can do the

1) See if your 200CD has a hum balance pot. Early units do not.
Install a 1K hum-balance pot across the 6-volt filament winding of the
power xformer. Each side of the pot goes to the winding, the centertap
goes to ground. But before doing anything, see step 2 (below).

2) If you don't have a hum-balance pot, chances are you don't have
twisted pair to each tube filament. Early units used one side of the
chassis for filament heater return. Replace all filament winding with
twisted pair of suitable current-carrying capacity.

3) If your front-panel lamp socket is grounded to the chassis, replace
it with a type where both sides of the lamp are electrically isolated
from the bracket. Then install twisted pair to the isolated socket and
then install a braided shield over the twisted pair (ground the braid
shield to the chassis but take care that it doesn't short to the panel-
lamp wires). I didn't insulate the braid shield but if you are picky,
you can insulate it and ground it to the chassis at one end only -- I
have no idea if that sort of care is necessary.

4) Install a braided shield over the leads to the front-panel power
switch. Use extra care here to ensure that you don't short the leads
at the toggle switch. I used heat-shrink tubing to "service" the end
of the braid shield at both ends. Ground the shield to the chassis.

It's a lot of work but this mod really does work and it will make very
old units nearly as good as the newer HP 200CDs (note that distortion
figures for the very oldest units may not be as good as newer ones).

Hope this helps. Good luck. Usual disclaimers.


From: David Dintenfass
Sent: Wednesday, July 15, 1998 11:24 AM
To: Multiple recipients of Ampex
Subject: AMPEX: HP 200CD and driving Ampex capstan motor

If you're thinking of driving an Ampex capstan motor off-speed using
an HP 200CD oscillator and a power amplifier, keep in mind that the 60
Hz residual hum in the oscillator (not 120 Hz power supply hum, but 60
Hz pickup) can cause beat-notes that will swing the output voltage
quite a bit. If you hang a voltmeter on the 120 Vac output from the
power amp, you will see this clearly, particularly as you center the
oscillator around 60 Hz.

I ran into this problem a few years ago when running my old Telex/
Viking three-motor transport with a huge Dowty RFL power oscillator/

The 60 Hz beat-note problem was a known issue at Hewlett-Packard many
years ago. They issued a service note that described steps you could
take to reduce it. This was more of a problem for the older run of HP
200CD oscillators -- these are the ones with the octal tubes and point-
to-point wiring (later versions had miniature tubes and a small pc
board which reduced the amount of point-to-point wiring).

The older HP 200CD oscillators did not have twisted filament leads (in
fact, the chassis was used for one side of the filament). They also
did not have a hum-balance pot, nor was the wire to the front-panel
power switch shielded. Adding all three things reduced the 60 Hz hum
quite markedly. Some people even went to far as to put a metal shield
on one side of the large tuning cap, but I didn't find a need to go
that extreme.

I performed HP's recommended hum-reduction procedure and it worked
like a champ after that.

Another interesting service note for these units involved changing one
feedback resistor to lower the oscillator's distortion. This reduced
the output level (and also increased the settling time, which means
the output tends to wobble for a bit when you change ranges). At one
time HP even sold a special version of the 200CD with this mod in
place. If there is interest, I'll post the procedure to the list.