HP 200CD

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CJ

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hey folks! workin on this:

mi1d1d.jpg


just tore into an old Palo Alto version of this classic oscillator.
It puts out more voltage than most transistor generators, especially at 600 kc.
who knows how many pirate radio stations were started with this anchor?
made from 1952 to 1980 something.
one of the longest vac tube product runs, it has to be.

this is the point to point version with the octal 6SH7 (metal) and  6AV5  tubes.

I was wondering if i could swap out the  6SH7 for an 6SG7?

they both have almost identical specs in the tube manual, same base, same everything.

since i have a ton of the SG, which would make the all important tube matching go easier, otherwise i have to order a couple of sh from Antique and hope that they match.

OK, thanks!

just a few notes, check out the 10M resistor across the coupling caps.
Is this to make the leakage due to aging (both my caps were leaky) less of a factor?

also, look close at the transformer output.
there is some clever switching going on.
that is really T1 and T2, two seperate transformers, one big one for low freqs, and one ouncer type for the rf.

pretty slick, might have some applicatins in audio?

also note the weird air core chokes hanging off the plates of the 6SH7.

98xc82.jpg


transformer switching:

j8he05.jpg
 
> the 10M resistor across the coupling caps.

No, look at the 10Meg:8.2Meg voltage divider, which shifts a +100V plate signal down to a -25V grid signal. They are DC coupled. Actually low-low phase shift down to very low frequency. The circuit could be made to work without the caps, but have excess hiss and limited high frequency response. 

The SG gives more current, the SH gives more gain. They are so close that it would probably work with either.

I don't see why matching is important. The general operating points are tightly controlled by overall NFB. There is enough common screen resistance to encourage push-pull action.

> weird air core chokes

Standard Video Amplifier technique. Gain falls when stray capacitance sucks against load resistance. You can pull another ~~ half-octave by resonating-out this capacitance. I estimate this is rung near 1MC. No free lunch: when it finally falls it falls twice as fast. You usually can't do this inside a NFB loop. I'm guessing that this is very low-Q, and that the oscillator runs with small NFB.

H-P has a paper discussing why they did this one this way.
 
hey, thanks on that!

that schematic could use a re draw.

the balance improved after the leaky black molded sprague color banded antiques were replaced with some 1000 volt models.

also replaced all the silver banded resistors with matched pairs, not that many parts, easy to do a total refurb, just rip out the tube sockets with the parts for examples,  then build up some new ones on new sockets and wire them up.

sk7 does have a higher bias of -7 compared to -3 for the sh7.
and the gain is lower, as now only putting out about  3 volts rms.

this thing apparently runs flat out into a load so that any other load attached has no effect, correct?

the 6av5's certainly seem to be doing some work.


next up, wire up some twisted green wires and a hum balance pot for the heaters, instead of this noisy ground return scheme.


hey, which resistor do i change so i can modulate some karoke on KFRC, 610 AM?

 
A few years ago, AI bought about 15 H/P 200 series oscillators for around 26 bucks each off ebay for the purpose of canibalizing them. The 200 has just one output transformer and 6K6 output tubes. A bunch of the CD version had the low freq iron core transformer and the ferite core for high freqs and 6GK5 or somesthing, similar to the EL84 in size. The 200 AB has a giant size output transformer identical to the H/P 600/10K matching transformer in the round enclosure with bananna jacks on the top. A really impressive chunk of iron for the power level it's speced at. Anyway, I used all the iron to build a number of compressor/limiters. Some were similar to the StayLevel, and a couple others using other compressor designs. It was the least expensive way I could think of to get some quality iron. Normally, I hate tearing apart perfectly good equipment, but I have given these parts a second life.
 
> now only putting out about  3 volts rms.

The total gain has to be 1.00000000..... The lamps will only do so much self-trim. Any part or tube replacement, you must diddle R11 to get the designed output (which varies from model to model).

> any other load attached has no effect, correct?

Load affects it, sure. If the Amplitude knob is way down, not enough to notice. I assume the 200-300 ohm series resistors allow you to feed dead-open to dead-short with insignificant error.

> hum balance pot for the heaters

You will waste your time. If the B+ caps are half-good, these things are very low hum, and most of the residual is coupled into the giant capacitor. You can NOT check hum with the case open: they suck all the hum in the county. IIRC you really have to seat and screw the case to keep external hum out.

> which resistor do i change so i can modulate some karoke on KFRC, 610 AM?

Speech-modulated oscillators suck so bad they are illegal to operate and annoying to hear. This particular oscillator is so barely unstable that any modulation will make it alternately faint and fart.

You could try sticking a carbon-mike between the oscillator and the antenna, but I suspect reactance will foil you, and you'd have to talk REAL LOUD!!! to get any audible modulation.

Get a 200AB. Figure a plate tank circuit for 600KC. Put it with one of the 6K6 in the 200AB, bias it "off", drive its grid HARD with the 200CD. Cap-couple 10'-100' of wire to the plate. Tune-up like any transmitter. At 600KC the ideal antenna is almost a half-mile, so any domestic antenna will be several K reactance with just few ohms radiation resistance. Which sucks, but no worse than any loudspeaker. You build a 4 Watt Champ-amp, drive it hard, get 0.040 Watts actual radiated signal, it's fine.

Now take the other 6K6 and a Champ output transformer, wire just like a Champ SE output. Best use a fatter OT like they sell for SE 6L6. Use your 6SN7 as voltage amp to bring your iPod/CD's 1V up to the 15V the 6K6/6V6 grid needs.

Now move the B+ end of the RF tank over to the plate of the Champ-Amp.

Stable oscillator, over-driven amplifier, and plate modulation is great AM.

It does not make a lot of sense. The 200CD is low-low distortion. In radio transmitters, distortion does not matter, you can filter it. And high-distortion stages are more efficient, and can be neatly modulated. See any ham manual from the 1940s. Take the basic 160 Meter transmitter and up-scale the tanks into the AM band. Typically you use a 6C4 and a tank to oscillate, a 6V6 or 6L6 and tank to make the big RF, a 6SL7 to boost mike to a level to slap a 6V6/6L6 SE audio stage, and a common choke to couple the AF onto the RF's B+. Simpler than your plan. And with a few refinements (including a 1/4-mile tower and a salt-swamp ground) you can be heard far off your property. And with your expertise in audio, it could be a clean loud signal.
 
ok, the guy hacked in a heliopot instead of the T, so he changed the output wiring, now i am getting 30 volts!

i put a LCR 50/50uf  can in there, the hp three section can uses 2 10uf in parallel so it just fell into place.

wondering about the weird cap across the output primary.

so a little contact cleaner and just call me wolfman jack.

i saw some of these anchors going for 400 to 600 on evilbay, i hope you did not hack all 26, a few posts up.
???

adhw9d.jpg
 
> anchors going for 400 to 600

$15 for the anchor, $85 for cap-job labor, $100 "working". More for a good clean mechanical carcass; one of mine the tuning-shaft was frozen solid with old grease, had to drive it out with a hammer, couple hours of brutally lazy labor there.

edit: Woah! Prices on 200CD and even 200AB are WAY up from last year. Probably Jim doing a pump-and-dump to get rich. I better de-dust and sell my anchors before the bubble bursts.

CYA reserve for "Warranty", debatable, but another $100 is fair. Warranty fulfillment is annoying, and you can't use low-pay staff.

The $650 one is "NIST", which should mean significant time (very significant on this mechanical contraption) checking and trimming every spec against Standard bars and buckets which have been compared against that meter-stick in Paris or the current comparable stick, coil, battery (or atom-clocks and Joey-junctions). NIST-traceable calibration is usually done on new-fancy gear, $6,500 boxes, and a $500 annual calibration is part of the cost of owning such a machine.

0004sixview.jpg
 
all three shafts were frozen with black crud, one day just to refurb the cap assy.
and i chipped the bakelite fine tune grommet thingy.

when you remove the cap assy, you can remove the 90 degree sheet metal then scotch brite the chassis.

now i get to bend the cap plates so the dial matches the freq counter, we are talkin old school tweakin.

found a 6fw5 that likes the 6av5.

i always wanted an area heater, the sun is sinking as we speak.


 
Wow! I had no idea I had such control of the boatanchor business. That me buying several old H.P boxes could have such an effect and drive those prices to such stratospheric levels. I still have a couple left. Maybe I should think about dumping them before PRR sends the market crashing!
I just retired last year from AT&T Alascom, after 36 years mostly working in the satellite communications field. One of the perks was having our own cal lab where all our spectrum analyzers and tons of other equipment were brought to NBS tracable accuracy. I have a very substantial workbench here at the house, and I used to run my stuff through the cal lab once a year where our very capable technicians would check it out and make sure it was up to snuff.
As for the H/P 200's The first to be used for other projects were the ones with the most cosmetic defects. I still have several that look almost new and even the handles are perfect. I was paying around 25 bucks a pop for them, and usually the shipping was the most expensive part of the deal.
On the 200CD, it uses a grounded plate scheme in the output stage, with the OT connected to the output tube cathodes. I had thought about trying a cyclotron type design, but instead kept the circuit pretty close to what H/P had done.

Jim
 
i hooked the optoelectronics 2 ghz freq counter, and the darn thing stays stable to 5 digits on the 100K range!
and when you dis connect the 600 ohm load, it's like it does not even notice.
these hp guys were pretty sharp .

i wonder how the power line rejection scheme works, i know the thought of that.

this is the perfect box for me, as i can now saturate some transformers that used to make me run a variac down to 10 hz, and if you want to hear sometin ugly...
 
ok, i had to rebuild the range sw, this thing has the #10 steel binding posts, so it was from

one of the first production runs, every joint has to be hit.
which ain't a bad thing.
;D
the end cap fell right off one of the 50 meg resistors when i unsoldered it.
thats why the X1 range did not work.
i tried some hexfreds instead of the 5y3 but they kept popping.
then som 1n4004's and everyyhing is fine.
weird.

now i have 100 extar volts because the thing quit resonating,, do i have to re balance the power supply with the 10k pwr resistor to make this thing swing?

so every part has been replaced, you can not only replace half the parts on these anchors, because something else is gonna break because you fixed a related part.

once this thing starts working again, what is my max out gonna be then?

i did not want to drill this chassis, but since we are going green, i just save 12 watts of heat in the 5y3, that should cool things down a bunch.


 
ok, everything working, bad wire on the wafer switch.

question is, what to do with 84 volts p-p audio generator?

solid state pwr supply equals beastatron

running a lot smoother after all the rework.
riveted tube sockets for grounds, pretty sketch, but still works.

the bridge does not touch ground anyway, ok, next up, the vtvm anchor.

should be duck soup compared to this thing.

that r11 is real touchy.
i yanked all the trim caps and the pot and am paralleling resistors to calibrate.
the silver was toast on the trim caps.

 
Hello all, I am currently tinkering with a 200CD that I am making into a mic preamp. It works except for a loud hum. I suspect the caps. Actually I measured 94 volts dc between the outside of the can of C13 (the multisection cap) and ground. There shouldn't be voltage on the outside of the can should there? In the manual this cap is covered with a cardboard cover, but on my unit this cover is gone. Is this for insulation purposes? It looks like it used to be there, some of the sticky gunk is on the top.

Thanks, David
 
one of the caps is 150 volts above ground, so be careful.

i would replace all the lytics, people use to leave these on 24/7 becuase they had a goverment job and did not like to wait for a 30 minute warm up.

even the pink 0.5 coupling caps i measure from a 1964 vtvm today had massive leakage.
and one section of a multican was bad, so it is best to replace the whole thing.

the good news is lytics are a lot smaller for the same value, so you have plenty of real estate if yo want to discard the can and lay down some seprate units on a tag board like i did.

funny the quality difference on the chassis eras, the 1959 chassis was a pain to take apart, you had to cut the resistor leads in 6 places in order to get them off the tube sockets, they were crimped so tight.

this 1964 box, they just stick the lead thru the socket pin, no bend or nothing, and solder it.

so it comes apart real quick, but i trust the old gals more.

plus, the newer box had a screen cap missing! hp, do you believe it?

you learn a lot of tricks from these guys, like shielding, heater circuits, balancing chassis leakage with trim caps, trick output switching,

it only takes about two nights to completely gut the chassis and switch out all the silver band c comps, unless you want the funk and the 10 percent upward drift in value. some of the leads rip right out of these resistors, ...

 
be careful with the 100 uf, it is un polarized, it sits across the output winding to bypass signal to the neg feedback winding.

you can either buy an un polarized cap, leave the orig, and pray, or make your own by using 2 back to back + outside, 200 uf caps.

voltage rating un sure.

111qow2.jpg


i found 2 more leakers in  this vtvm, another pink 0.5/400, and a lytic cathode bypass cap.

hook an ohm meter to the cap, let it charge, if it charges enuff to read infinite ohms, it may be ok, depending on how long it takes to get there.
put the black lead on the can or cap ground term.

one of these metal cans is in real good shape, 25 uf instead of 20, and no leakage.

the ohm meter trick may not work on the 100 uf un polarized because of the weirdness inside.


you can clip a dc pwr supply into the un plugged chassis, charge the pwr supply caps at 30 or 40 vdc if the unit has not been pwred up recently, let it sit over n ight, it might reform the cap if it is not dried out, preventing a high voltage break down from being shocked back to life.
 
howabout a photo phinish to this thread>

i got a couple of ge 6av5 for cheap, andy;s garage of course,

so a nice marshall cap and the output is tight,

2zdodoo.jpg


dywww1.jpg


run all day, run all night, no 1/8 watt allowed, everything shiny and new,

b85wti.jpg


those freaky dual output transformeres,

16k0q4g.jpg


steel 6sk7 

new nylon mounts, old ones cracked apart, re-solder everything on  those lamps!

1zv3beo.jpg
 
My 200CD has started acting up - it now has a nice built in tremolo effect... The output frequency is stable, but the volume changes at a steady rate. So I guess something is oscillating that affects the volume.

My 200CD is the latest/last version with EL86 output tubes and silicon rectifiers. It's made in '74, judging by the data codes on the parts. Any idea where I should look for a fault first?

Best regards,

Mikkel C. Simonsen
 
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