HP 415E Teardown and "calibration" - Page 1
Forum section: electronics / testgear

author: Tony_G
author_rank: Super Contributor
timestamp: March 06, 2016, 09:12:27 pm
content: Hi All,I say "calibration" in the title because I'm trying to return the instrument to within it's original specifications rather than I'm an actual calibration service - Though if I ever get the gear I may be interested in a part time job in metrology - Working with this gear has been fun.Anyway, my quest here started with getting some 355 series step attenuators and then confirming that they're operating within spec. Once I got the attenuators, the performance check documentation said I needed to get a HP 415E SWR Meter to use. These things seem to be pretty cheap on EBay so I picked one up for $20. Once it arrived I needed to check it out.I don't know exactly when the unit was introduced -- The HP Journal has an entry in 1961 for theHP 415Cand then talks about the meter in theJuly 1966 issuein that way HP did when introducing new products. So I suspect that this was released in early 1966.It has that classic period look to it and the usual HP construction feel - Very solid.The model that I have already has the modern IEC socket on it. Many of the ones on EBay have the older '163' or oval type socket. You can actually use the meter as a high gain amplifier. This is covered in section 3-40 Special Applications in themanual (available online). Also common to equipment of that era was the recorder output. This output provided a 0 to 1V dc value that you could use to "record" the values displayed on the meter.Flipping the unit over you can see the feet attachment (I mentioned this in my other teardownpost for the 651B).Keeping the unit flipped over and removing the bottom lets you see most of the stuff inside. That big rectangular can in the middle is for the battery (you can get it as Option 01). The power switch enables you to test and charge the battery. Mine has the battery but it's dead. The charging voltage is 13.3V but it drops way down to just about 4 V if you try and run the unit from the battery.Up front are the networks for the switches, left to right in this image, Range & Expand, Input, Power. The two 'sealed' pots are for frequency and bandwidth. This is a tuned meter so you need to feed it a 1KHz signal of some format (either direct or as a demodulated signal via a crystal detector or bolometer). I had to give these things a generous helping of contact cleaner to get them to work smoothly.Flipping the unit back over and removing the top cover gives us a nice set of test points. well actually it's the edge socket for the PCB but someone really thought it through and made it a super convenient test point section for working on the unit. You can also see the top of the battery there in the middle.Standing the unit on it's side lets you take the right cover off - You can see the solid aluminium frame that HP used in a lot of their gear. I love the box construction that allows you access to all the parts. I guess it's easy to do that when you aren't trying to stuff 10 lbs of electronics into a 5lb bag.Going to the other side gives you access to the PCB for calibrating the unit. Like the 651B I was expecting TO-5/TO-18 packages but instead they're TO-92. Actually I'm not sure when they introduced the TO-92 package so maybe I'm just wrong about my expectations.Like all HP manuals of the era, the HP 415E manual has a detailed set of performance checks. I was able to bring the meter back into specification for all of the tests with the exception of "3B - Expanded Range Accuracy". The actual test setup says to use a 200CD Audio Oscillator which can drive 10V into 600 ohm load whereas I'm using my 651B Test Oscillator and I ended up a dBm or so at full range from being able to zero it to conduct the test. Here are you results I got:I think my next step is to look into an amplifier for test equipment - a few dB would have gotten me a complete set of performance checks.Hope you enjoy the look into this.TonyG

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 21, 2016, 07:59:50 pm
content: Just wanted to tell you I really appreciate your YouTube channel, your video about this meter is the only one on the web i was able to find on the subject. I would call it priceless.Really great stuff.I have two of those meters I would love to restore, one had a problem with the movement so I had to fix it first and the second one is "erratic".I would love to pick your brains about how to approach them and how to try to restore them.I was searching for the attenuator to be able to check the accuracy but they are expensive so I had to put the project on hold till I get one of those.I really like your videos about the power meters and the sensors.I would like to encourage you to share more of your knowledge, I know it takes time and effort, both of those in high demand, lol, but ... I'm counting on you.Thank you.

author: Tony_G
author_rank: Super Contributor
timestamp: June 25, 2016, 03:18:31 pm
content: No problem - Glad you like the channel.I now have 3 of the 415E's sitting here but I had to shelve them for a while as I acquired a bunch of stuff from a DoD auction and I'm working through that now. Will get back to them soon I hope.There is a 355D on eBay right now with no bids for $35 -http://www.ebay.com/itm/371665722753They're usually pretty stable and accurate so it's generally a safe buy. I wouldn't pay the hundreds of dollars for this or the 355C - I bought several of these in the $30-50 range.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 27, 2016, 09:35:51 pm
content: I was wondering about this deal with DoD.... how did you find it.. was it any good... the 415 you got.. besides of missing top plate was immaculate, wonder what else did you get. Do they sell those things in a lot or you have to bid on every single item.One more time about your youtube videos. A lot of information in them, usually I have to re-watch them couple times.I mean, nothing wrong with it, great source and I like the way you quantify things and cover the methodology of your "experiments".Keep them coming.

author: Tony_G
author_rank: Super Contributor
timestamp: June 28, 2016, 02:51:21 am
content: W.r.t the DoD Auction - It depends - There is a lot of stuff that is sold by the pallet load (e.g. 175 pieces on 5 pallets) but you can rarely get that because the guys that run eBay businesses drive up the prices. Mostly I've been buying a few devices in a lot at a time to round out my home lab and to grab some of those classic components - I just bought a 435B again because it's just such a nice meter.Search for Gov Liquidations and see how you go - I use Fast Track Logistics a lot for shipping and they're pretty good - Just one ask, don't bid against me

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 30, 2016, 06:09:15 pm
content: Just watched your new video, congratulations on bringing this HP Receiver back to life. Very nice piece of gear.... very nice.What about the other one? Wasn't it identical? Did you try it, is there anything wrong with it?It's horrible that those devices were thrown out of service for such a trivial reasons... but again and again, you have to know how to fix it.I've got some attenuators from ebay and I will try them out sometimes next week when they arrive. I hope I didn't bid against you.But you know, ebay is bellum omnium contra omnes... lol.

author: Tony_G
author_rank: Super Contributor
timestamp: June 30, 2016, 08:08:55 pm
content: The second 8902A had been damaged at some point (the connectors are bent) - I haven't had a chance to put it on the bench and bend things back out. I might do a new post thread on that when I get to it.TonyG