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Re: Help to repair my HP654A Test Oscillator
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Bob Albert
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:
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