Jeremy, I couldn't find anything on 200CD in EL34 website.? But the other one yielded essentially the same info as you gave on level setting.
I am resisting the temptation to get into these oscillators.? I don't have bench space and too many things going on.? But soon.
Bob
On Sunday, December 15, 2019, 01:34:00 PM PST, Jeremy Nichols <jn6wfo@...> wrote:
Matching tubes: I didn¡¯t want to repeat myself. For my 200CD, I found a matched pair of 8425A¡¯s (the industrial version of 6AU6A) for V1 and V3. (Please don¡¯t tell HP they¡¯re Tektronix tubes!) For V2 and V4 (6CW5) I found a nice pair of Made-in-Holland Amperex tubes in my spares box.?
I agree the prices on eBay are steep and that seems to be so for much older HP equipment. Clearly, sellers don¡¯t understand that the market is limited for this stuff. The ¡°sold¡± listings on eBay range from $15.50 to $150. Plus shipping, of course. Flea markets and estate sale prices are much more attractive.?
There is some good 200CD information in <> search for ¡°HP 200CD oscillator repair.¡±?
Another good link is <>. Under ¡°General Vintage Technology/Success Stories¡± search for ¡°Hewlett Packard 200CD Wide Range Oscillator.¡±
Jeremy? N6WFO
On Sun, Dec 15, 2019 at 10:35 AM Bob Albert via Groups.Io <bob91343= [email protected]> wrote:
I will have to check my serial numbers.? Your most recent message didn't mention matching tubes.
Anyway, this is interesting.? The easiest mods would be to lower the amplitude and improve the lamp sockets.? I will do those first and see what I get.? If results are promising I can get rid of the function generator.? Matching tubes can only work if both units use the same type, or I can find a source of them.? Buying tubes in hopes of finding a good match and then seeing if that helps seems a waste of effort and money.
I have two of these, one is rack mount and I don't want to preserve that one.? So I will concentrate on improving the cabinet mounted unit.
Perusing ebay shows that you can't buy a working unit for less than about $100.? I am amazed that their perceived value is that high.? Once this is done I may put one or both on ebay depending on distortion.? Of course I can brute force lower distortion with a filter but that makes for another project.
Bob K6DDX
On Sunday, December 15, 2019, 10:02:20 AM PST, Jeremy Nichols < jn6wfo@...> wrote:
*** Distortion of the HP-200CD ***
I used my HP-330B Distortion Analyzer to check the 200CD. The 330B is a vacuum tube unit; 0.04% is definitely pushing its limits; it probably would be more fair to claim my 200CD's distortion is "less than 0.1%."
*** Additional information on the HP-200CD Audio Oscillator. ***
1. The later versions using miniature tubes have lower distortion than the earlier versions with octal tubes. The split appears to have taken place at serial prefix 103, which also introduced Dynamic Balance Adjust pot R50 and Hum Balance pot R51.
2. The 200CD was listed in HP catalogs beginning in late 1951 ($150) and was last in the 1985 catalog (un-priced but I have been told it sold for $1250). This 34-year production run is probably a record for HP. The 200CD was HP's second-to-last vacuum tube instrument. One of this nice things about this long run is that there are a zillion of them still floating around for those of us who like to play with the descendants of HP's very first product.
3. The internal pot that sets the maximum output is R11 (1000 Ohms), which is normally set for a 24 Volt no-load output at 1000 Hertz (12 Volts into a 600 Ohm load). If the output is reduced below 10 Volts (loaded) the distortion is reduced; below 7.5 Volts is even better. Clearly, if your application requires higher voltage, you have to live with more distortion.
4. The manual sold by Artek (CD or download) includes in the second volume a group of HP "Service Notes" that are especially valuable for those owning early models using octal tubes. 5. The last revision appears to carry serials prefixed "960-" or "0960A." These final versions use solid-state rectifier diodes and have a standard IEC plug-in power cord.
It's interesting that the 200CD does not have a regulated power supply, only a pi-network filter.
The solid-state rectifier does not make much, if any improvement in distortion. (My "best" 200CD carries serial prefix "333-.")
6. Some excellent information, copied from the old Ampex mailing list, is posted in . This includes detailed information on the 7.5 Volt modification (which I did not do to my 200CD). 7. In the last versions, the sockets for resistor lamps RT1 and RT2 are mounted on a small pc board (with other components). Ensuring good contact between the sockets and the traces can be challenging due to the riveted construction.
Jeremy
On Sat, Dec 14, 2019 at 11:38 PM Bob Albert via Groups.Io <bob91343= [email protected]> wrote:
Okay Jeremy I appreciate it.? I have two 200CDs and am sure I can glean a matched pair of tubes from the four.? In any case I am interested in taking this far enough to at least approach the amount of distortion you have.? How did you measure it, and under what settings and conditions?
I have a 331A but don't use it.? A VNA I have does go down into the audio, as well as the FFT in one of my scopes and I can measure distortion that way.? I never measured anything as low as 0.04% though.
Bob K6DDX
On Saturday, December 14, 2019, 11:29:55 PM PST, Jeremy Nichols < jn6wfo@...> wrote:
There are a number of tricks that have been published over the years. I¡¯ll send you copies tomorrow. In the?meantime,? ? balance the two pairs of tubes that make up the push-pull oscillator. The pairs of tubes don¡¯t have to be new/perfect but should be as close as possible in terms of gain.? ? make sure the two lamps in the oscillator feedback have clean bases, are making tight contact with clean sockets, and the sockets have good solder connections. Sockets that are mounted with rivets to a pc board are especially susceptible to poor contact.? ? use the internal gain control (tomorrow I can give you the R-number) to reduce the maximum output to 10 Volts, perhaps even less depending on your needs. Reducing the output voltage lowers distortion.?
I¡¯ll add more in the morning when I¡¯m more awake.?
Jeremy?
On Sat, Dec 14, 2019 at 10:31 PM Bob Albert via Groups.Io <bob91343= [email protected]> wrote:
What's your secret Jeremy?? I have a couple of 200CDs but they are nothing to write home about regarding distortion.
Bob K6DX
On Saturday, December 14, 2019, 10:19:52 PM PST, Jeremy Nichols < jn6wfo@...> wrote:
My650A did fairly well distortion-wise but my favorite is the 200CD. With a few tricks and a little tweaking I was able to get it down to 0.04%, well below spec.
Jeremy
On Sat, Dec 14, 2019 at 9:43 PM Bob Albert via Groups.Io <bob91343= [email protected]> wrote:
I know this doesn't help but I did want to say that, in all my years of working with HP gear, this circuit is about the worst design.? I have repaired my 651A a couple of times and have never been able to get it to meet its distortion spec.? Mine works okay but I got tired of messing with it and it's no longer in line for use.? My main repair seemed to center about that FET in the middle of the diagram.? I tried a few different types without much success.? I ddi finally get it to oscillate on all ranges.? The distortion runs a bit over 0.5% worst case and the frequency calibration isn't as good as it could be, but it remains a useful instrument.
Perhaps you'd like to play with it.? The main thing I like is the nice attenuator.? Back in the day, I had a 650A that was a marvelous piece of gear.? Too bad this one didn't match that.
Bob
On Saturday, December 14, 2019, 08:35:49 PM PST, Don Bitters via Groups.Io <donbitters= [email protected]> wrote:
Almost certainly 50Hz AM modulation - 20msec per cycle.? Does touching the 645 chassis change the amplitude of the modulation, if so check the chassis ground at the power cord input, you may have an open ground pin.? If not check the power supplies for 50/100 Hz modulation, percentage of voltage of the supply, the worst looking supply is probably the culprit. Don Bitters
-- Jeremy Nichols
6.
|
