I’ve gotten a lot of questions on the blog about the new version of the MHS5200A function generators available on eBay. Viewer Tolga was kind enough to send one in to me to review and tear down. Although some improvements have been made over the older models, there are some concerning issues with these new models too!
I got a sick Tektronix 485 oscilloscope for cheap on eBay; here is the first of several troubleshooting/repair/restoration videos, in which I attempt to troubleshoot and repair the input attenuator boards.
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.
It’s time for another analog IQ test, courtesy of Jim Williams! Can we identify and stop the source of oscillation in a simple voltage reference circuit?
Today we revisit the MHS-5200A function generator distortion issues, this time examining the slew rate limitations of the AD812 output amplifier, as suggested by the viewers!
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:
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:
You’d think by this point in my life I would have learned to stop making assumptions… 😛
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:
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:
(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:
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.
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.