Friday, October 2, 2009

The Acme-825 ESR Meter

Acme Enterprises currently has a version of the Dick Smith ESR meter designed by Bob Parker. The price is $89 with free shipping. Acme is a hard site to link to so you will have to navigate a bit via Test Equipment to see the meter. Here is a photo of the Acme-825 next to an original Dick Smith kit that Mike built years ago:


Dick Smith is an Australian electronics retailer that marketed the meter on the right as a kit for many years, but it was recently discontinued. The Acme meter on the left is made by EVB out of Portugal.

ESR stands for equivalent series resistance. The Wikipedia definition:

"Equivalent series resistance (ESR) is an effective resistance that is used to describe the resistive parts of the impedance of certain electrical components."

Basically, components such as capacitors add resistance to a circuit. Film caps should add very little, electrolytics (when young and fresh!) and tantalums should add more, but still very little. But we all know the passage of time is not a good thing for electrolytic capacitors and this often shows up as an increasing ESR. Check the little ones: they dry out quicker and make that tone control circuit or that phono section sound terrible. Or worse, they blow up those rare output transistors.

Here is the Acme-825 manual (pdf) for a good description of both the meter and ESR. Here is a very nice web site that provides a good explanation as part of a DIY ESR meter project. Here is another explanation using an original Dick Smith meter.

These ESR meters are actually small computers powered by a Z80 chip (please wait while I flash back to a software project in 1983... ah the joys of CP/M). The meter essentially pulses the cap with a very high frequency, very low voltage alternating current to measure the resistance. The low voltage prevents transistors from turning on and allows testing in circuit (but not on!)

Since the lion's share of electronic sleuthing involves the electrolytic capacitor it should come as no surprise that Mike's ESR meter ranks behind only his soldering iron and Fluke DMM in importance. He has been prodding me for years to get one. Well, I did.

My initial impression of the Acme-825 is that it works well, but is very much a lightweight compared to the original Dick Smith kit. Mike's original is cosmetically whipped, but it still works well. The plastic case is very heavy duty plastic which no doubt helped it survive in a commercial setting. Fortunately, my situation should be less demanding, but I think I will put a bit of thinking towards shock proofing the case a bit.

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