Thursday, December 25, 2014

Converting a BK Precision 2831A bench multimeter to 220V supply

I just came across a BK Precision 2831A bench multimeter (for free). I do not know the age or the general reputation of the unit, but it seems to be a rather low-end 3-1/2 digit multimeter. Nevertheless, another multimeter might come in handy.

An old BK Precision 2831A (on top of something else)

The device is supposedly only meant for the US marked and is labeled 120V, so I could not test it right away. First, I thought of buying a 120V/240V transformer, but good quality ones does not come for free.

It is labeled 120V on the back side

Based on the labels on the circuit board (GDM-558D), it seems like this unit is a relabeled GW Instek multimeter of some kind. It is probably ins the same family as the GW Instek GDM-8034 (or the GDM-8135, although it has a circuit board marked GDM-625A). The accuracy of those two devices (DC volt) are 0.5% and 0.1% respectively, whereas the BK is reportedly 0.1%.

The circuit board also has some markings indicating that the transformer has two primary windings that can be coupled in parallel for 120V usage (marked as 114V on the circuit board) or in series for 240V usage (marked as 234V on the circuit board).

Typical configuration for switching between 110V and 220V with two primary windings.

Normally, units with such a transformer has a switch on the back, making it usable for both 120V and 240V mains supply, but this multimeter has no such switch. However, there are some resistors on the board that can be replaced to allow for the higher voltage setting.

Zero-ohm resistors configured for 120V (117V)  mains supply

Zero-ohm resistor configured for 240V (234V) mains supply

All there is to do is to remove the two resistors (and resolder one of them), and voila, the multimeter is ready for european voltages. In addition, the main fuse should be reduced to about 2/3 of its original size.

As indicated on the first image, the multimeter now works on 230V (or 220V/234V/240V or whatever). I cannot report on the quality on the device itself, such as the accuracy in taking measurements, but it seems to be fairly close to my Fluke 8050A so at least it is not damaged or anything. Hurray!

Monday, December 1, 2014

DIYcrap audio mixer #1


During the last year I have built four synthesizers: a MFOS Noise Toaster, a x0xb0x, a Shruthi, and a Sonic Potions LXR drum machine. Hence, now I have an urgent need for an audio-mixer, and I have decided to build one, DIYcrap-style.


The mixer is based on the MFOS Stereo Panning Mixer. This circuit board uses TL071 and TL072 opamps and gives four mono input channels each with panning two effect send loops. In addition there are two stereo inputs, a headphone amplifier and stereo out to drive an external amplifier or recording unit.

I also need some built-in effects. The first effect is the MFOS ECHO FXXX. This is a PT2399-based delay module, and I am going to use two of these. Hence, they can be used in parallel (for awesome stereo effects), in serial (for super-long delays), or individually on two different mono-sources.

The second must-have effect is Reverb. I will use the SKRM-C8-R02 Reverb/Delay from This unit is based on the Spin Semiconductor FV-1 and comes preprogrammed with a few stereo reverb and delay effects. With some additional circuitry it should fit nicely with the stereo mixer.

The last effect I am going to add is distortion (or fuzz). I have yet to create this module but i might try out the MFOS fuzz module to begin with.  The fourth module is also from MFOS and is a Stereo Auto Panner. Hopefully, this unit will provide some cool effects. Lastly, since delay and reverb does not fit nicely with low frequencies, I am going to add a variable high pass filter for the effect out part of the mixer. I might give this variable 20-200Hz filter a try.


One of the biggest challenges with the mixer is to create the front panel. Inspired by the latest Soundlab MkII from MFOS I decided to use the BUD-box AC-423. It is a 17x7 inch box in aluminium.
The status now is that I have soldered most of the boards and I have created the first version of the front panel in Inkscape.

First prototype of the layout (some text is missing)

The design is inspired by other MOTM-style synth-panels, like this one.

I also got useful tips about creating front panels in Inkscape here. Schaeffer is a popular choice for manufacturing the front panel. A more DIY-ish solution is to use LazerTran. However, I will probably just laminate an A3 paper and glue it to the AC-423 in the first version.

This project is still work-in-progress, and I will use this blog as my build log and as a place-holder for all the links I collect.