Greening oscillators-Heathkit vs HP - diyAudio
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Old 31st July 2011, 03:13 AM   #1
dgta is offline dgta  United States
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Default Greening oscillators-Heathkit vs HP

Much has been done and written about greening the Heathkit IG-18. But I can't find anything written about greening the HP 650 series (651-654). Is there something inherent in the design of the HP that makes it unsuitable for practical mods and improvements? It does seem to be widely available on the surplus market.
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Old 31st July 2011, 03:40 AM   #2
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The HP 650 is circa 1956 and the manual is available at http://www.hparchive.com/Manuals/HP-650A-Manual.pdf Other manuals might be available via BAMA.

Knock yourself out, but I would think that time investment in an HP200 (I think there are 4 in the series, some standalone, some part of a telecom test set and pretty generally available) would be more worth the time.
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Old 31st July 2011, 04:16 AM   #3
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No, not the old 650. As I said, the 651-654. An entirely different animal. See here:
Audio & Test

They match the looks of the 330 series distortion analyzers.
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Old 31st July 2011, 06:09 AM   #4
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Looks like a perfectly good candidate. My guess is there aren't as many around as you think. I don't remember seeing any at surplus dealers, nor do they show up at hamfests, at least not around here.
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Old 31st July 2011, 05:50 PM   #5
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I've seen plenty of them all over, but that may be a regional thing.

I just downloaded the HP 651B manual and I'm shocked to see the distortion specified at 1%. Yes, one percent, I didn't miss the decimal point. Wow, that's worse than most function generators. Is that a misprint? Or are they really that bad? Hard to believe.
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Old 31st July 2011, 10:55 PM   #6
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The physical size makes it a bit unattractive if it's going to be just a basic test oscillator - though the attenuator, front panel, meter, controls, chassis, and variable capacitor could be salvaged and used as the basis for a home-built high purity oscillator.

I saw one at a hamfest just 2 weeks ago. It was marked $150, and the seller claimed to have no knowledge of its functional condition. That's a lot more than I wanted to spend for a nice box with some potentially useful parts.

Dale
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Old 31st July 2011, 11:58 PM   #7
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I can't see a reason in the world that it would be that bad. The old 200 series with tubes were used by McIntosh for their amplifier clinics back in the day. They claimed to have modified it slightly for lower distortion, IMO this one should be way better than that- unless they lost something by increasing the maximum frequency capability. I'd start by looking at the amplitude stabilization scheme (haven't looked a the circuit). Does it use bulbs? They're really very good- add more bulbs for time constant. Look at what Jim Williams had to do to get rid of them and still get good performance in his oscillator designs. Also look at the oscillator circuit Bob Cordell uses in his distortion analyzer for ideas. Personally I keep a 204 around and rarely use my 200 anymore- it's a pristine rack mount version and I mostly keep it as a display piece.
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Old 1st August 2011, 03:49 AM   #8
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I have a 204 as well, but haven't lain a glove on it. The mechanicals are pretty stunning.
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Old 1st August 2011, 05:11 AM   #9
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I'm hoping someone will chime in who is familiar with the 650 series. I just had a casual look at the schematics. On paper it's an order of magnitude more sophisticated than the Heathkit. Example: Heathkit power supply single rail, one transistor. Very basic. HP, 2-rail, 7-8 transistors depending on model, plus a couple opamps in one model. Fold-back current limiting separately on each rail. Output ripple specified at 1.3-1.7 mV.

It just doesn't make sense that the HP would be an order of magnitude worse distortion than the Heath.
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Old 1st August 2011, 01:34 PM   #10
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These oscillators cost from $20 to $80, perhaps less at hamfests. As I stated, the mechanicals are unbeatable.

One thing I aim to do is use the Linear Tech "true rms" detector LTC1966 with a VCA in one of the HP204's, adjusting the time constant of the LTC1966 with the range switch. One might also consider an ultimate POOGE by implementing the Krohn Hite 4400 VCA. I wrote an article for AX describing the useage of these RMS detectors, and they are very useful indeed.

http://cds.linear.com/docs/Applicati...ote/an106f.pdf
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