Fixing the MHS-5200A

Today we make some simple and cheap modifications in order to address the two biggest problems in the MHS5200A function generator: sine wave harmonic distortion, and square wave ringing/overshoot.

If you want to make the same modifications, here is the Digikey BOM. Note that the capacitors and inductors I used for the Bessel filter were 0402, while the pads on the PCB are 0603; either size fits, though 0402’s are obviously a bit harder to work with.

See the pictures below for more details.

Board Overview

Board Overview

Replacing the AD812 with the THS3092

Replacing the AD812 With the THS3092 (Be Sure to Replace the Heatsink!)

Modifying the DAC Filter(s)

Modifying the DAC Filter(s)

Bessel Filter Components

Bessel Filter Components

Beware the P2N2222A!

I was working on a simple 16MHz Pierce oscillator recently, built around a 2N2222 transistor. Nothing special, should work without any trouble, right?

Nope. The darn thing just wouldn’t oscillate! I found that it would oscillate below about 10MHz, but not above that. After some testing of the transistor, I found that it only had a beta of about 3, and its base-collector breakdown voltage was 8 or 9 volts!

Not only that, but I had 5 such transistors, all exhibiting the same behavior, and all with the same part markings:

P2N2222A E02

P2N2222A E02

Now I’ve bought a lot of 2N2222 transistors, so I wasn’t sure who the actual manufacturer was, but I was sure that somehow I’d gotten a bad batch of transistors. I was just about to Widlarize them all, when I decided to put it to the EEVBlog forums.

Several of the commenters asked if I was sure that I didn’t have the transistor in backwards. Well I’m quite familiar with the standard 2N2222 pin out, so I hadn’t even considered that to be an issue, and I was sure that wasn’t the problem. That is, until Kevin.D pointed out that what I was seeing was in fact indicative of swapping the collector and emitter pins: low beta, and low base-collector (which, if swapped, is actually the base-emitter) breakdown voltage.

Doh!! That was exactly the problem, and had I bothered to look it up, Wikipedia would have told me that too:

2N2222 pinout variants; note the “PN2” vs “P2N” part prefixes

You’d think by this point in my life I would have learned to stop making assumptions… 😛

Comparator Basics

Comparators are quite handy devices that are often used to detect when a certain voltage threshold has been crossed. They are basically like an open-loop operational amplifier, but unlike op amps, are specifically designed for driving their output to “rail to rail”.

The simplest comparator just compares an input voltage to a reference voltage, setting its output high when the input voltage exceeds the reference voltage, and setting its output low when the input voltage falls below the reference voltage:

A basic comparator circuit

A basic comparator circuit

Note that the only additional components are the pull up resistor on the comparator’s output (the LM393 uses open collector outputs), and the resistor divider network to set the reference voltage. This works as expected, with the comparator’s output switching between its high and low states whenever the input signal crosses the 2.5V mark:

Comparator output (blue) vs input (yellow)

Comparator output (blue) vs input (yellow)

(Note that the input signal here is actually exceeding the maximum negative input voltage for the LM393 when run from a single supply, which likely accounts for the distorted waveform on the input pin; so yeah, don’t do that!)

However, noisy or slow-moving input signals can easily cause false triggering, resulting in many rapid pulses at the comparator’s output:

Noisy inputs cause false triggering

Noisy inputs cause false triggering

Continue reading

Repairing an HP3466A Multimeter

My main bench multimeter is an HP3466A that I found at a hamfest a couple years ago. Mind you it’s older than I am, but it looked to be in good functional order, and $30 seemed like a fair price for a 4.5 digit bench meter, so I picked it up.

The HP3466A

The HP3466A

After a while though, I noticed that its DC voltage readings seemed to be low when probing circuits containing larger resistances; anything around 100K ohms caused a noticeable discrepancy in the expected voltage reading, and it worsened with larger resistances.

Continue reading