Hello World (And discussion of my new HP214A) - Page 1
Forum section: electronics / repair

author: W1ngs
author_rank: Contributor
timestamp: August 18, 2019, 10:09:36 pm
content: Hello all. I have been mulling over joining EEVblog for years... I suppose it's time I did.For my first entry, I'd like to share a recent acquisition: an HP 214A pulse generator from 1963. I have a penchant (and a use) for test equipment and I enjoy collecting and restoring these items when possible.This unit came to me for $30CAD and was in non-working condition. It's beautifully built (oh wait, it's HP from '63... there is no question about build quality).I did exactly what you're -not- supposed to do and powered it up and looked for smoke (the previous owner had already done this recently so I figure if any damage was to be done, it had already happened). I indeed got a little smoke but it was, thankfully, moisture and dust burning off a glass power resistor (R341).. no harm done.After determining the source of the excessive current in the resistor to be a shorted electrolytic (C318), I replaced that and carried on sorting it out. I could get action in all the stages except the output itself. The bias voltages were all over the place and it turned out the shorted cap destroyed all the Zener/Avalanche diodes (CR320,321,322,326) in the chain and also the hold-on transistor (Q301), and CR327, 328 in said transistor's emitter circuit. I was able to find reasonable substitutes in my collection except for the transistor itself which is specified as HP number 1901-0441... great...Looking through an updated copy of the HP manual which indicates design updates and part changes, HP had changed to the 2N2190 and this shed a bit of light on things. It's a low-voltage high-speed (for the day) PNP Ge diffused-base Mesa transistor. Not something I have handy. The NTE substitute NTE160 for some reason has lower voltage, current, and power specifications than the 2190, so forget that (I hate using NTE anything (or ECG if you're an old fart) anyhow). So in went a 2N2907A! In the end this worked out just fine.It took me hours to get my head around how the whole thing works and find its various troubles but I eventually worked it out... normally these things go fast but this one was interesting and different. On final calibration I was able to bring everything into spec but the 10V output range. C317 was acting up which is interesting as it is in parallel with the originally-shorted C318 but probably only a coincidence. The ceramic cap here was holding the bias to about 6V and letting it go no higher. Maybe a crack in the dielectric that had oxide making a diode or MOV-ish effect? I'm not sure. Either way, with that sorted it works now and I have a nice pulse generator to play with!You can see in the scope photo I'm getting somewhere in the 10ns ballpark rise on 10V into 50ohms. A bit more on the lumpy side than I want. The big lump turns out to be a cable reflection (calculated nicely for the length and type of coax I was using) so maybe my terminator isn't so good or I had a bad connection, who knows.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2019, 10:22:54 pm
content: Nice!Tim

author: W1ngs
author_rank: Contributor
timestamp: August 19, 2019, 06:16:33 am
content: I decided to mess about with this nice Pearson current transformer. Here it is measuring the current in a transmission line from the 214A. Pretty reasonable bandwidth although a bit of preshoot there which I presume is a stray-capacitance thing going on as it changes a fair bit on moving the wiring about.

author: factory
author_rank: Super Contributor
timestamp: August 20, 2019, 09:03:53 pm
content: Quote from: W1ngs on August 18, 2019, 10:09:36 pmYour 214A is about ten years younger than you think, I can just about see some 73 & 74 dates codes on various components inside, HP test gear made in the 1960's usually have red lenses for the front panel neons and the older (pre IEC) PH-163 power connector.The updated part numbers for components can also be because parts became obsolete in equipment with long production runs. Also note that they sometimes redesigned parts of the circuitry to use silicon NPN transistors instead of germanium PNP types.David

author: SilverSolder
author_rank: Super Contributor
timestamp: August 21, 2019, 12:28:12 pm
content: Nice old HP test gear! - Today's electronics is just not as tactile, when most of the functions of big boxes like this have become available on 3mm square ICs...

author: W1ngs
author_rank: Contributor
timestamp: August 22, 2019, 06:03:17 pm
content: I agree entirely! Obviously we can do more now but less thought, in many cases, is paid to good ergonomics. Reading about some of the HP pulse generators of the old era, it turns out the designers went so far as to design one model with use-by-feel in mind. Controls that needed quick regular access were laid out as sliders and arranged such that a horizontal slider would affect something time-related like pulse width, position, rate, etc. and vertical ones would be used for amplitudes for example. It was nice to see such thought put in whereas now we are sometimes stuck going deep into sub-menus to find a setting that was once trivial to use without much effort.Quote from: SilverSolder on August 21, 2019, 12:28:12 pm

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2019, 06:31:40 pm
content: Quote from: W1ngs on August 18, 2019, 10:09:36 pmIf you have a 50ohm terminator on a Tek 24x5 scope input, don't forget that there is also a 15pF capacitor in parallel with it. That's why I like my Tek 485, which has two attenuators, one conventional 1Mohm and one pure 50ohm.

author: W1ngs
author_rank: Contributor
timestamp: August 22, 2019, 06:34:32 pm
content: Indeed, this is a 2247A and it has 1M || 20pF input and I'm aware of it. Thankfully, working into a 50Ω transmission line the 15pF is damped rather wellQuote from: tggzzz on August 22, 2019, 06:31:40 pm

author: SilverSolder
author_rank: Super Contributor
timestamp: August 22, 2019, 10:03:24 pm
content: Quote from: W1ngs on August 22, 2019, 06:03:17 pmI think I have one of the kinds of pulse generators you are talking about (an 8012A), lovely to use. - Today, the need for manually controlling pulse trains is probably less than it used to be... but once in a while, it comes in really handy to be able to control every aspect of the pulses.The smartphone is the culmination of the trend of everything being made a cheap and compact as possible - now, one device is expected to do everything. "Jack of all trades, master of none"?The trend of putting touch screens in cars (instead of tactile buttons you can use without taking your eyes off the road) seems directly dangerous. Why is it OK to fiddle with a music system on a car touch screen while driving, but not your phone?Sooner or later, there will be a trend back to simplicity and it will become a "thing" that things are easy to use again.

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2019, 10:21:48 pm
content: Quote from: SilverSolder on August 22, 2019, 10:03:24 pmTo their credit, Mazda have said they won't in the future. I hope that spreads to other manufacturers.

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2019, 10:35:50 pm
content: Quote from: W1ngs on August 22, 2019, 06:34:32 pmA quick spice simulation indicates that with a 1ns risetime there would be a transient 30% of the amplitude at the receiver. Your waveform looks like a 4ns risetime, and with that you would expect a 10% "glitch".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2019, 11:08:48 pm
content: Quote from: SilverSolder on August 22, 2019, 10:03:24 pmBut to finish the quote -- "often better than a master of one". Modern phones are far better at communicating, in general, than any of their predecessors, among all the other things they can do. Some of which they probably don't do all that well, it's true.The downside in regards to test equipment is, you're probably programming an arb gen through a clunky keypad interface, to do similar things these days. Lots of bother for not a lot of action.But if you're using an arb gen for much arbitrariness, it's probably that you're programming it through a computer anyway, which gives you all the power of a computer proper. If you're generating test sequences for example, they can be designed once, then fired off in a fraction of a second, far faster (and more accurately) than an operator of the present equipment could've.Tim

author: W1ngs
author_rank: Contributor
timestamp: August 23, 2019, 12:55:46 am
content: The rise was close to double, around 8ns. The transient (I assume we speak of the same one) was almost exactly the same when the generator drove a 50Ω load directly and was sent to the scope via a 10:1 compensated probe.Quote

author: tggzzz
author_rank: Super Contributor
timestamp: August 23, 2019, 06:53:10 am
content: Quote from: W1ngs on August 23, 2019, 12:55:46 amIn your photo IMG_3378.JPG, the risetime is very close to one minor division, i.e. 1/5 of 20ns = 4ns. You could get a better estimate with a 10ns/div sweep rate.The scope probe has a similar input capacitance to the scope's input, i.e. 15-20pF. Hence I would expect a reflection transient caused by that capacitance to be of similar amplitude. The location would change with changing length of cable between the sig gen and the 50ohm load.If the ripples do not change as the cable's length changes, the ripples are either part of the sig gen's output or part of the scope's response.

author: W1ngs
author_rank: Contributor
timestamp: August 23, 2019, 10:36:22 am
content: I was referring to the risetime of the first image I shared, not the second. I see now that you are referring to the latter.As to the TDR, it was affected by cable length in the tests I did.In any case, we're getting pedantic... The point is that it was fun to restore this item and it is useful in my development work.EDIT: You explicitly stated 3378, which is the image I was referring to. I'm counting two minor divisions to the nearest trace-crossing at the 10% and 90% points, so 8ns. But again, it's immaterial.Quote

author: tggzzz
author_rank: Super Contributor
timestamp: August 23, 2019, 11:58:49 am
content: Quote from: W1ngs on August 23, 2019, 10:36:22 am<expletives deleted>, you are right about it being 2 minor divisions 8ns. How the ... did I get that wrong. I hope it is too little sleep or caffeine, rather than senility!But yes, the objective should be fun, and the reason for my post was to avoid chasing after unattainable perfection

author: W1ngs
author_rank: Contributor
timestamp: August 23, 2019, 05:44:22 pm
content: It's not a problem! Although I would love to get 4ns rise out of this thingI wasn't too concerned about unattainable perfection; I just slapped a load on this and set it to the shortest range to give a quick picture to show that it does its job. A perfectly square wave at this speed would be a pipe dream with the setup I was using!Mainly I'm glad to have saved a gorgeous piece of gear from being dismantled for parts; notwithstanding, it is still a highly useful item (albeit a big and very power-hungry one).When I get around to doing some useful things with this, I'll be sure to share. I did already muck about with some acoustic and EM delay lines (although you can do these with a slow old function generator).Quote

author: SilverSolder
author_rank: Super Contributor
timestamp: August 23, 2019, 06:03:59 pm
content: Quote from: T3sl4co1l on August 22, 2019, 11:08:48 pmYes, I wouldn't want to go backwards... A lot of good things can be done with switches and rotary encoders to make new equipment as pleasant to use as the old stuff.As you said, often you need a computer (which still has a tactile interface: keyboard, mouse) to do serious work anyway.Time waits for no man!