HP 200CD oscillator restoration question

[Randall]

Ok, so this isn't about amp per se, but it is about a vintage hand wired, point to point piece of tube gear. I picked up what I think is an old Navy HP 200CD 5k - 60K HZ generator to restore. I have two questions about it. The small fine tuning knob that turns the large wheel is hard plastic, spring loaded to contact the back of the aluminum tuning wheel. Surely, there must be something missing, as hard plastic doesn't grip very well. Maybe a fiber, felt or rubber washer of some type. What would be a good way to get this function to work again, and to quiet the hard grating noise it makes when turned?

And, what are the two 12v lamps next to V1 and V2? I can't imagine they are for lighting inside a metal enclosure. So, are they current limiter maybe? And if so, why would this piece need them?

Thanks!

[Enzo]

I think that is 5Hz, not 5k.

Hewlett-Packard HP-200CD audio generator


Light bulbs have a filament that changes resistance with temperature. They are used here for stability.

[nashvillebill]

Here's a very technical explanation of the lamp's function: Bill Hewlett and his Magic Lamp

[Randall]

Yeah, and it's also 600K Hz, not 60K Hz. But, I really want to know about the vernier dial situation.

[nashvillebill]

Mine is just the hard plastic pressing against the aluminum dial and it doesn't grate at all, it's quite smooth. Have you lubed the bushings in the tuning-capacitor drive mechanism per the manual?

[Randall]

Ah, so it's a cathode resistor that shape shifts, a cornerstone of the Hewlitt original design. Very cool.

And, hmmm... interesting that now it seems mine is stock as was intended. I have lubed some things, but not as per manual, so thanks I will follow that lead. This is right up there with totally not getting how my press fit chuck fit into my new drill press was supposed to work, but it does!

[nashvillebill]

Manuals are on the BAMA site http://bama.edebris.com/manuals/hp/200cd/

[Randall]

Here's where I am: HP 200CD Photos by Randallissimo | Photobucket

I like it.

[Tom Phillips]

Randall,
That 200CD looks to be in really nice condition. Note that there is good info to be had on HP equipment by joining the Yahho group at hp_agilent_equipment : HP/Agilent Test Equipment . Another one of my favorites is http://www.kennethkuhn.com/hpmuseum/ which has a lot of great HP information and good links to more.
Tom

[Steve Conner]

Nice! The HP 200 series are classics but still useful in a modern workshop. I think the dial is supposed to turn freely enough that the little vernier knob can move it even with a modest amount of friction.

[Randall]

Well, I got it cleaned and lubed, and all is well in vintage test gear land. The vernier is now glitch free. I hooked it up to my Fluke Hz setting as well as my scope, and although it isn't exactly calibrated according to those measurements, I don't mind. I may take a look at the calibration procedures for fun, and see if I can get it closer, but I do not have access to a frequency counter other than my Fluke meter.

However, I am very satisfied with my new piece of bench gear. It is so smooth, looking at it on the scope. I love that. Everything is working properly with the minor exception that the dual sealed AB Amplitude pot is a little jumpy, but only when being adjusted. Maybe someday I will look into replacing it, but it doesn't really bother me. It even looks good on the original cap cans, AFAIK. Thanks guys for helping me along with this. You're the best.