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Old 27th May 2007, 08:27 PM   #1
anatech is offline anatech  Canada
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Default Bent HP meter pointer

Hi all,
Well, I saw an HP 334A Distortion Analyzer on Eeeek bay for about $50 USD, so I bought it. The company selling it didn't really know what they had, so I figured I was lucky that day. Shipping was amazing as it arrived two days later.

Okay, so it didn't power up. Fixed that and found it wouldn't respond to any signals (noticed that the meter pointer was bent as well). Traced and repaired that and also repaired the next fault, intermittent operation. So now it seems to work. This leaves me with one last problem.

How does one properly straighten out a bent pointer in a really nice, well built meter? I would hate to break it. I currently have it out on my bench and disassembled. The scale is covered with taped down paper so I don't mark it up.

-Chris
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Old 28th May 2007, 12:17 AM   #2
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I just picked up a 334 that's kinda beat (like I need another distortion meter), and paid twice that for it- you did well. Though I've fixed various meters, never an HP. Ultimately, you'll just have to carefully bend it back, without putting undue stress on the pivots. It will either break or it won't, depending on how much it's bent. A slight bend can usually be fixed with no problem- how bad is it? You'll need to immobilize the pivot side somehow. Armchair idea- use a little block of wood to press the pivot side against, maintaining the height, while you bend the rest back. Caveat emptor. If worse comes to worst, you might be able to pick up another meter of the same type at a hamfest or some other piece of HP equipment on eBay. I've been known to make new scales for meters by scanning the old one and reprinting it on good paper, or drawing a new one in CAD. Though the rest of my 334 is ok, the front handle is broken off on the right side, so I'm in the market for any HP junker in the same size chassis, with a good right side panel.
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Old 28th May 2007, 01:45 AM   #3
anatech is offline anatech  Canada
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Hi Conrad,
I managed to straighten the pointer out. Using some metal blocks to support the meter, and another to support the pointer, I managed to gently get rid of the bends. This is a shot of it now. These pointers are very brittle, so you have to work very carefully. I didn't go for perfection, just much better.

You should have seen this instrument when I got it. It was filthy and banged up. I did strip the entire faceplate and removed the meter (what a pain!). While doing this I scrubbed everything down and lubricated the bearings on all the controls and switches. The frequency dial was seized - no surprise there. My coupling between the frequency dial and the capacitor was lying shattered in the bottom. The capacitor was also seized. After the cleaning was done, the meter was attempted. It had two good sized bends in it. I should have taken a shot of it before I started.

These distortion analyzers are great units. They are easily better than my Leader LDM-171 (bought new). The usable range is up to 600 KHz and the response is up to a couple MHz. I also have an HP 339A that I really like. The 339A doesn't go up that high in frequency, so the 331A ~ 334A are perfectly useful on most benches. Keep in mind that you can float the input terminals to about 400 V pk (remove the ground strap!) on these older units. The analog meters are typically dead on the money with HP gear.

I have one fault left to repair (I hope), then the unit can be checked for calibration. So, one meter saved from the scrap pile.

-Chris
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Old 28th May 2007, 03:37 AM   #4
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Well that sure looks good to me! I haven't had a chance to fool with mine yet. I fixed up a 331 some time back, but really wanted a 334, so I sold it, probably for $20 or so at a hamfest. My 334 has the optional RF input, though I'm not exactly sure what to do with it- still have to find a manual to download somewhere. Years ago I built the Bob Cordell THD analyzer that was published in Audio magazine. It's a remarkable instrument with a floor of about 0.0002% over most of the range. It's also easier to use than most any other analyzers. Right now I'm using an HP 8903B Audio Analyzer that I got a good deal on surplus, because I can talk to it via GPIB. I wrote a simple program that runs response curves, THD curves, gain and signal to noise numbers, all in a minute or so. Being able to do that at the touch of a button has really improved my optimization, but for the best numbers I go back to the Cordell unit.
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Old 28th May 2007, 02:09 PM   #5
anatech is offline anatech  Canada
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Hi Conrad,
PM me please.

I'd like to pick up the 8903A sometime. I have a lot of respect for HP instruments from all the ones I've come into contact with over the years. I find the 339A pretty easy to use also.

I did get a 331A to rebuild. I think they are worthwhile compared to the Leader I have. I also have a 652A and a 654A to rebuild.

-Chris
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Old 29th May 2007, 01:42 AM   #6
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You should be able to find a lot of downloadable HP test equipment manuals at http://bama.sbc.edu/hp.htm .

I got an HP 334A on ebay, a couple of years ago, for about $180 including shipping. But it appeared to be in perfect condition, and even well-calibrated. (I had intended to pay more to get a working one, since I needed it for a job at the time.)

I wish I still had more of the HP stuff I used to buy, refurbish, and resell, up until a few years ago. I mostly dealt with Tektronix scopes. But I probably also had about a hundred or so nice pieces of HP gear go through here. Some of them were exquisite.

You guys REALLY ought to check out http://www.govliquidation.com .

It's not quite as good as it used to be, from a reseller's point of view, since it's not all sealed bids and larger lots, any more. But you might be able to get three of something for the price of one on ebay. It's riskier, in many ways, but, I think, better in many ways. Some stuff is even "unused". Even just going to one of the larger test equipment sale sites during the pre-sale "inspection period" is a very memorable experience!

Try their Advanced Search option and select warehouses within a few hours of you, if possible, for the next big monthly test equipment sale, and see what comes up.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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Old 29th May 2007, 01:56 AM   #7
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Default Question

In the RF bissnes we use whats is call a Bird meter to check real watts out of a radio trasmiter amp . another word it checks it out the raw watts not peaks like most other cheap meter , With audio who built a similar thing? , for checking true RMS .
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Old 29th May 2007, 02:18 AM   #8
anatech is offline anatech  Canada
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Hi Tom,
I'm a Canadian! I'd have to buy sight unseen. An audio signal analyzer and RF spectrum analyzer would be (more!!!) than nice. I'll bet buying there is less risky that buying from some of the hacks on Eeeekbay.

Hi 70s music,
Most of HP's meters have been of the true RMS type for years. I use a 34401A, and could not have made a better choice for what I do. It's also a very good idea to check what you are measuring with an oscilloscope. Also, know the character of your meter. For a handheld multimeter, it's pretty hard to beat a Fluke. They are accurate and hold their calibration (unlike most other meters out there, especially Extech). I'd like to see the new one Agilent just released.

If you count on your test equipment for the correct answers at all, don't buy cheap meters. If all you need to do is see if +12 VDC or 120 VAC is there or not, it's not that critical. At least check a new meter against a known good one before trusting it.

-Chris
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Old 29th May 2007, 02:02 PM   #9
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Chris thanx . But something like this is more what i meant. http://www.chambonino.com/construct/const3.html
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Old 29th May 2007, 02:13 PM   #10
anatech is offline anatech  Canada
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Hi 70s music,
Okay, all that is, is a dummy load. With a known resistance (I use some Dale 250W, 8R, 1% non-inductive units) you will know the current draw given a measured voltage. If you were to measure this voltage with a meter like an HP 34401A, you would very fairly certain how much power you are dissipating.

If you want to make things easier, use a four quadrant multiplier and sense both voltage and current (they will be in phase for a resistive load). Monitor the output with a meter, like the one suggested. You will have to scale the voltage and current functions first. If you wish to build your own, continue along this line. Run the output into a true RMS converter chip, such as an AD536 (I think). Use a digital panel meter (available on Ebay) to monitor the output.

Now, if you wish to buy this commercially, it will cost you a fortune. Bet's are that most devices will not respond to the waveforms using a true RMS conversion.
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