May 26

HP 53131A Hard Power Switch Modification

Following on from my previous post in relation to the HP 53131A Frequency Counter Teardown I have devised a reversible modification that replaces its soft power switch (where the switch mode PSU and the fan are running continuously when the unit is plugged into the wall outlet) to a hard power switch which properly turns the unit off from the front panel. The modification is simple to do, uses inexpensive parts and is completely invisible, it looks and feels like factory behaviour. It is will worth applying if you have one of these for bench use so you can leave it plugged into the wall without having the fan running and switch mode noise being generated when the unit is not in use. It’s hard to understand why HP did not include a hard power switch in the original design, you will see from my modification there is plenty of room, and it seems such an obvious thing to do. Anyway, I have created a video showing the details of the modification, if you have one of these frequency counters I hope you find the modification interesting and/or useful.

UPDATE: Having re-read the schematic again it appears that the unswitched +/-12v from the switch mode PSU are provided on the OCXO option connector J9. This would explain the design approach, the PSU will run the xtal oven even when switched off so when you are ready to use it, its instantly available. Thats fair enough except that many of these counters exist that do not have an OCXO option, and even with one fitted, I personally would be happy to know I have to switch it on in advance of using it.

NOTE: Please forgive the under-exposure on the first part of the video and the audio noise in a few places through it, I have a new camera setup and am getting used to it. The audio noise I believe comes from the phantom power not being liked by the wireless mic (I hope its that at least). The under-exposiure is just my misinterpretation of the zebra exposure indicator, you will see I get it right (or at least much better) in the second half of the video.

See you next time, and thanks for watching….

May 25

HP 53131A Frequency Counter Teardown and Spillage Clean-Up

One of the instruments I have is a HP 53131A Frequency Counter, and putting the positives of this unit aside, its by far the most annoying bit of test gear I have! Why? well for some reason, when you apply power to the thing the fan runs even when its switched off. This is because the power switch is a soft switch and with just the AC cord plugged in the fan runs and makes the annoying fan noise. This is a really poor design and certainly not one of HP’s shining examples of engineering….I like to be able to switch my stuff off without reaching around the back and having to pull the power chord or unplug from the wall outlet – just plain crappy! HP what where you thinking?

I thought I would tear it down and have a look and see why this is, and perhaps see if there is any possible modification I could come up with to improve on this. When I took it apart I found a real mess of dust and some kind of substance spillage so a full clean-up was needed which somewhat distracted me from the power switch problem.

While its in bits, I also do a quick run through of the major components of the circuit and have a quick look at the power supply and power switch circuits in some detail.

UPDATE: The clean-up also appears to have resolved the strange trigger problems I had seen previously with this counter. Well pleased…

Here is the Agilent 53131A Component Level Manual including schematics for the frequency counter

Thanks for watching…

Apr 20

My E3634A Mystery Explosion – WTF!

This is a short follow-up to a video where I was recently testing a Mayyuo M9711 DC Electronic Load (See Here) and I was using an Agilent E3634A Power Supply (which I previously fixed) as a power source. When I put the DC load into pulse mode within a few seconds the E3634A PSU exploded. What I heard was a loud pop, and a flash followed by a loud vibrating 50Hz hum, by which time I was able to reach the power switch and shut it down. I switched to another supply and continued with the test of the M9711 but today I thought I would open up (once agin) the E3634A and see what damage was done and what I found is a real mystery!

Did I imagine it all? Was there beer involved? Who knows, its a mystery!

Hope you enjoyed the video, catch you next time…

UPDATE – I found the smoke source

While tidying up I thought i would have a look at the two power MOSFET’s that failed and found the IRFP260 has a very small but visible smoke vent! I have added the photo’s below, I had to take these under the microscope x10 and x30 to see it. With the naked eye it looks like a light scratch.

Apr 16

HP/Agilent E3646A Power Supply Teardown & Repair

So I present the third in what is starting to feel like a long line of Agilent PSU repairs, but before you read on you should know that I have now depleted my collection of broken Agilent E36xx series power supplies so I will need to find another subject to blog about next time! That being said, even though I have blogged about two other PSU;s from the same family, I thought it would be worth videoing the repair exercise and at the same time try to demonstrate how I go about identifying and resolving a fault on this kind of circuit. The E3646A is a different model to the other two PSU’s I have repaired and blogged about – this one is a dual output 0-8v @ 5A / 0-20V @ 2.5A for each channel and the internals are different so I hope this will be of some use for anyone who may need to repair one of these.

Notes and Schematic Parts

  
  

You can download the Agilent E364xA Service Manual which include the schematics

I hope you find this useful or interesting. If you do please give the video a thumbs up and if you have any comments or suggestions please post them here and I will do my best to answer them.

Thanks for watching and catch you next time.

Apr 06

HP/Agilent E3634A Power Supply Teardown & Repair

Over a year ago now I repaired a HP/Agilent E3631A triple output PSU and have been using it very often ever since. Recently I bought an Agilent E3634A PSU which is a single output but high power supply, I bought it in non-working condition for not very much money (about $90). It turned out to be a non-trivial task, with physically burned out components and a difficult to track down and understand secondary problem. The components to repair the supply cost less that $20 and the repair exercise was challenging and educational so well worth the effort.

SMOKIN’ HOT…..

I decided that for this repair I would try doing a video teardown and repair which I have not done before. The repair took quite a few hours in the end, although I carried out the repair over about 4 weeks elapsed.

This is a really useful addition to my work bench, the high power nature of this supply makes it very useful for working in low voltage systems like audio amplifiers communications equipment and automotive applications that have high power requirements.

The Agilent E3634A Service Manual with schematics is available on the internet for download, I have provided a link here for convenience.

See you next time…

May 17

HP/Agilent E3631A Power Supply Teardown & Repair

I recently bought a faulty Agilent E3631A bench power supply on e-bay which I thought would be a nice addition to my *slightly excessive* electronics hobby workbench.  These power supplies are really nice; they are engineered and built like military equipment, good high quality materials and mechanically very robust.  An great indication of how good these things are is the second hand values, these things cost $900-$1000 to buy in reasonable condition so they are not cheap.  I bought this particular one faulty and thought I would have a go at repairing it.

My first impression of the electronics in side was not great, it seemed very seriously over engineered for what it was trying to achieve.  It seemed like the designers had a field day adding all sorts of crazy circuits because they could.  The ADC is made up of discrete IC’s, there is a custom logic chip in there as well as a CPU, ROM and RAM, there are numerous power supplies for bias and control circuits all floating around each other and most things seemed much more complicated than they need to be.  The one real surprise though was the opto isolation in the analogue domain. The CV and CC reference signals from the DAC for the +6v supply are isolated through high linearity opto couplers type HCNR200, this is something that would be crazy to do today when the cost of micro-controllers are so low and have all the goodies like DAC’ ADC’s and PWM’s making isolation in the digital domain a far more sensible design choice.

6vIsolation

In fairness though, I was making my initial judgements based on what’s possible with today’s components, things were very different 20 years ago so given its age it’s a pretty sophisticated piece of kit really.  Once working it does appear to work very well so my initial thoughts are not really founded on anything other than my own instinct to want things to be easier to understand and better as a result.

On with the repair….

First things first, after a quick check of the obvious big components like the series regulator transistors etc, I very quickly needed a schematic diagram. Agilent were less that helpful here, the manuals they put out now days specifically have the detailed schematics removed from the documents despite there being a reference to them in the index. When I contacted Agilent and asked for a schematic I was told in no uncertain terms (after a 4 day response time) that they no longer make the schematics available, but they do offer a £450 exchange repair service – come on HP/Agilent, by all means offer the service but don’t stop those of us who want to hack around from doing so.  The solution was to buy an original printed service manual which did include the schematics; e-bay and $10 got me what I needed.  As luck would have it, while waiting for the manuals to arrive in the post, I also managed to find a manual on the net which still had the schematics present – not from any official Agilent source I might add…

I set out to work on fixing it and found I had to strip it down completely, removing the two boards, front panel, transformer and wiring from the chassis and spread it out on the bench. If you find yourself needing to repair one of these, be prepared to commit serious bench space to the exercise.  I have taken a bunch of photo’s if the teardown so you can see what all the bits look like.

Home » HP/Agilent E3631A Power Supply Teardown & Repair » HP/Agilent E3631A Power Supply Teardown
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There were various faults with the PSU, numerous op amps and some CMOS logic IC’s were faulty as well as two open circuit 33k resistors. At a guess I would say there was some kind of big static or high voltage discharge into or across the outputs that caused the original fault. I had to isolate the various areas of the circuit and work on them individually, making assumptions about what should be present in terms of voltage levels and feed in lots of external signals to get to the bottom of each fault.  I struggled with the configuration of some of the analogue circuitry – fortunately for me I have a good friend who understands much more about analogue electronics than I do so some exchange of e-mails and sections of circuits with measurements kept me on track and expanded my own knowledge too – cheers Span.

Here are all the components I ultimately had to change…
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While fixing it I also managed to introduce some faults of my own. Specifically I managed to blow two of the HCNR200 opto couplers, easily done just with a slip of a multi meter probe shoring out pins 2 & 3 puts 15v with no current limit straight into the internal LED rendering it open circuit instantly.  I managed to blow four of them like this before I figured out what I kept doing – doh!

After working through these problems I finally got it working except — when placing a dummy load on the +6 output, the voltage I was measuring went up!  A bit more inspiration from my friend Span and a scope on the output and voila – it was bursting into oscillation under load, probably due to the 4 ohm wire wound load resistor. It turned out this was down to the fact that the electrolytic capacitors soldered onto the back of the binding posts on the front panel are actually there for stability reasons – obvious once you know. I had removed the output wiring from the front panel to make it easy to work on. Strapping 1000uF across the output solved the problem.

You can download the Agilent E3631 Service Guide which include the schematics

Having worked on this I have been inspired to have a go at designing my own programmable PSU from the ground up to see if I can match the specs but use more modern components and design approach – I will post info on progress if I get around to it.

[UPDATED:] I am getting around to it… http://gerrysweeney.com/fully-programmable-modular-bench-power-supply/