Repairing front panel buttons of Tektronix 7904 oscilloscope.

Front panel interconnect board of Tektronix 7904

Front panel interconnect board of Tektronix 7904

I am a proud owner of several Tektronix 7000-series mainframes. Among them, 7904 500MHz 4-compartment oscilloscope is my bench workhorse. The screen is big enough, the bandwidth is adequate for majority of tasks I do, and absence of cooling fan makes it pleasantly quiet. In addition, the instrument is lightweight comparing to other 7000 mainframes and has legs mounted on the rear panel making it possible to put scope on the floor in a vertical position.

I became annoyed by malfunction of right buttons of both vertical and horizontal mode selectors (they failed to lock in place) and decided that my scope deserves some TLC. Besides fixing the buttons I also wanted to replace some dead illumination bulbs with LEDs. Mode switches are dual, with one switch in the pair dedicated to turning light bulb on/off. Bulbs are powered by 5V and dimming is implemented by means of two diode drops. So the 3V LED restricted to 10ma by a resistor shall work fine and even be able to dim a little. In addition, LED power is comsumption about 8% of a bulb.

The front panel buttons are mounted on a narrow PCB (called “A3- Front Panel Interconnect Board” in the service manual) running across the middle of the front panel. It can be easily accessed after removing side panels. There are several cables connected to the board on both sides, it is good idea to mark them before disconnecting. The title picture shows left side of the board (one close to horizontal bay “B”) with my marks on it. After pulling out cables I moved the PCB carefully towards the rear and to the side. The board bends easily and is somewhat accessible form the top of the instrument.

The button assembly is held in place by screws also used to mount upper plug-in guide bars in the plug-in compartment. Picture below shows location of the left ones (holding vertical mode buttons). After unscrewing four Phillips-head screws I pulled the vertical mode buttons out of the chassis, carefully guiding it around cables, structural members and dead mice.

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Magnetic probe amplifier

Magnetic probe amplifier connected

Magnetic probe amplifier connected

Recently, I was researching low-noise DC-DC converters and while reading Linear Technology Application Note 70 found this clever and useful circuit, designed by Jim Williams. The idea is to sense current in power inductor of the converter with another inductor, placed within short distance from the first one. The sensing inductor is connected to a circuit which amplifies and conditions the signal and generates nice clean square wave pulses which can be used to trigger oscilloscope sweep. The probe is isolated from the circuit preventing measurement corruption. As a bonus, analog output of probe amplifier allows observing current waveform through power inductor.

As is often the case with application notes, circuit description and build details are somewhat brief; I’m posting my notes hoping that the information will be helpful for other builders. Also, since BatchPCB doubled my order, I have extra PCBs; if anyone wants to build this circuit on a professionally made PCB with just couple green wires, e-mail me – the PCB can be yours for the price of postage.

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USB Isolator Most Frequently Asked Question

ADuM4160 PIN pin grounding

ADuM4160 PIN pin grounding

Q. I am getting “USB device not recognized” error message – what do I do? Also, since the isolator is soldered into my circuit, “reconnect peripheral” suggestion seems too difficult to follow.

I decided to write this article after receiving several e-mails from people who bought my isolator. While setups described in those e-mails were different, the problem was the same – a PC refusing to recognize the device connected through the isolator. Here I will try to explain what is happening and also share my ideas how to troubleshoot and possibly fix the problem.

When nothing is connected to USB port, the bus is held at ground level with pull down resistors on the host side. USB device, when connected, pulls one of bus lines up, often times also with a resistor connected to Vbus and data line. Host sees it, sends bus reset and tries to query the device. If device is answering, host keeps querying the device and eventually enumerates it. When device is enumerated, application takes over.

If device is not answering (like for example, when self-powered device is turned off), host will give up and post “Device not recognized” message. To get attention from the host, we need to generate bus event, i.e., disconnect the peripheral and connect it back again.

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Solder stencil for USB Isolator PCB in store

USB Isolator Board Solder Stencil

USB Isolator Board Solder Stencil

I received a batch of solder paste stencils for my USB Isolator PCB. Now those who want to try low-tech reflow soldering technique, can do it. Stencils are laser cut by Ohararp from 0.0035″ Kapton. They are now offered for sale at the store.

USB Isolator kits in store

USB Isolator parts

USB Isolator parts

Immediately after announcing USB Isolator circuit I received several suggestions to put together a parts kit. Indeed, users of such devices are usually not afraid of small parts and generally aware of which side of soldering iron is better suited for holding on to. Also, I’m having hard time trying to find a supplier, where one can buy all the parts to build this circuit; usually, you need to buy from 2-3 suppliers, which significantly adds to shipping expenses. Considering this, I’m now offering not one, but two parts kits to build an USB Isolator based on Analog Devices’ ADuM4160.

The kits are added to “ADuM4160 USB Isolator bare PCB and kits” dropdown. One kit contains all the parts necessary to build an isolator – PCB, ADum4160, capacitors, resistors, USB connectors, pin headers, and jumpers and costs $25. Another kits contains all that plus all parts for 5V buck converter – LT1376-5, inductor, caps, etc and costs $33. Enjoy!

I’m not writing a build manual at this time; I’d like to get some feedback from builders first. Any questions you have, please don’t hesitate to ask.


Building USB Isolator.

USB Isolator

ADuM4160 USB Isolator PCB panel

USB Isolator PCBs has arrived. In this article I will be talking about building and configuring one. The PCB was designed with hobbyist-friendly large size SMD packages and 10mil clearances and I hope it won’t be difficult to made one at home. The layout files are available from downloads section.
Let’s talk about parts procurement. This is a BOM at Mouser sans ADuM4160, LT1376-5, and PCB. This is BOM at Digi-Key, which includes LT1376-5. Last time I checked, Digi-Key was way more expensive. As far as ADuM4160, since no one sells it in single quantities right now (check this using FindChips), the easiest way to get it is to ask Analog Devices for a sample, they are generous folks.

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USB Isolator.

USB Isolator

[ Update ] I now have bare PCBs and parts kits of this design for sale in store, as well as assembled and tested boards.

We all love USB. It is well supported across many platforms, easy to work with, and even able to provide a little power to the peripheral. However, the quirk of USB is that peripheral must share ground line with the host. The host is usually a PC and very often a desktop, which means it’s USB ground is electrically connected to earth ground in the wall outlet. With PC-based test instruments, like oscilloscopes, logic analyzers and such, It works fine most of the time, but not always.

There are situations when we prefer our ground separate. It happens when PC/earth ground is too “dirty” and we don’t want our circuit to pick up this dirt. Sometimes our device’s ground is not too good or even dangerous if connected to earth ground. Sometimes we are trying to overcome ground loops. Sometimes, we want our oscilloscope to behave like a multimeter, i.e. being able to show voltage drop between any two arbitrary points of the circuit. In any of this cases we want our USB data and ground isolated from the host.

Isolation improves common-mode voltage, enhances noise rejection, and permits two circuits to operate at different voltage levels. It also tends to save test equipment, as well as PC itself. It is also very useful in industrial setting, that probably why industrial USB isolator devices cost between $200 and $400. While looking for a solution for my lab, I found interesting USB isolator part, recently released by Analog Devices, and decided to give it a try.

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Spring cleaning of AM503.

AM503 with replaced capacitors

AM503 with replaced capacitors

The single most valuable instrument in my possession is Tektronix current measurement system, which consists of AM503 probe amplifier with accompanying A6302 AC/DC probe. While probe itself doesn’t need much attention besides occasional wiping the dirt off the jaws, the amplifier recently became quite noisy in several attenuator positions so I decided to give it a good spring cleaning.

I adopted the procedure described on a web site dedicated to restoring Tektronix oscilloscopes; the guy was talking about fixing fuzzy trace by cleaning input attenuator contacts of Tek475. I am unable to find this site anymore; if anyone knows the link, please let me know so I can give him proper credit.

The places in need of cleaning are attenuator contacts and range resistors R206, R208, R212, R214, as well as terminating resistors R204 and R216. For cleaning I use Deoxit from Caig Laboratories and a piece of clean printer paper. A match or a toothpick could be helpful in tight places; don’t use anything metal though; you don’t want to scratch your old Tektronix gold.

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