ARP 1502 LFO Teardown and Schematic


I recently acquired an ARP MODEL 1502 and 1505 from a friend and was naturally curious as to what was inside. I first opened up the 1502 which is a low-frequency oscillator intended to mix with audio signals to produce electronic effects, like tremolo and vibrato when superimposed with a note. The 15XX modules are a series of modules from ARP that all provide simple effects and analog audio processing meant to work with the accompanying keyboard at the time.

The design is entirely based on discrete transistors. The two IC chips contain 5 transistors, but with the added convenience of being well-matched to form a differential pair. The possible outputs are a square, sawtooth, and triangle wave that can be swept from any value up to 20Hz

Here is an easier view of the traces:

Here are the components you would need to build one yourself. The transistors should all still be accessible but the CA3086 should be able to be replaced with any similar NPN transistor array if desired. Also, the original design used a DPDT switch for the power, supplied by a 9V battery, but with two sets of adjacent pins shorted it’s effectively turning it into a DPST switch. The slide potentiometer has a logarithmic sweep and can be replaced with any audio pot.

Here is a full schematic with my own added reference designators:

Attached at the bottom will also be a pdf schematic which includes smart links to all the components and a list of all the connected nets when downloaded and viewed in Adobe Acrobat. It makes it much easier to find components on the page.

Here are the output waveforms with the slider all the way up at max frequency.

The peak to peak value for all the waveforms is about 4 volts for an open load. The square wave is about 4 volts on high and 0 on low, with a 50% duty cycle. When swept, the duty cycle stayed consistent for all frequencies. The triangle and sawtooth wave both have a 2-volt swing from their ground reference. There seems to be some noticeable crossover distortion on the sawtooth wave.