Thursday, December 15, 2016

Pioneer PL-250 turntable restoration

Pioneer PL-250 is direct drive auto-return turntable with s-shaped tonearm. This turntable is manufactured by Pioneer in around 1979 - 1982.

The PL-250 which we got had many electronic and mechanical issues. The main electronic problem in this unit is it's varying speed. The speed controller of this turntable is based on PA2004 and PA2005 ICs. After checking each an every electronic component in speed controller PCB, we figure out that the problem is in the "Pitch Control" potentiometer (VR1) of this unit. After replacing VR1 with 5K (log) potentiometer, the speed problem got resolved and motor starts to spin with correct speed(s).

Servo motor and speed control assembly of Pioneer PL-250 turntable.

The turntable which we got comes with original Pioneer cartridge with "broken" stylus. We test several  cartridges with this player including cheap US $ 3.00 cartridges (which are available in eBay), Audio Technica AT-91 and Audio Technica AT-95E cartridges. Out of all these cartridges, chap Chinese (US $3.00) cartridges produces worse results and the most noticeable thing with that cartridge is it's heavy distortion in 1.0kHz - 6.8 kHz range. Both Audio Technica cartridges produces excellent results with this player.

After fixing all the issues this unit starts to work perfectly and produces excellent results. We mainly test this turntable with Pioneer SA-420 power amplifier system.

Monday, November 21, 2016

HTML5 Mandelbrot Generator

This quick post is to introduce "HTML5 Mandelbrot Generator", which is simple JavaScript application to generate Mandelbrot set and Julia set fractals in HTML5 complaint web browsers. This application is derived from Mandel source code (which is available at and it covers most of the features available in that original application.

Online version of this HTML5 Mandelbrot Generator is available at Few fractals generated with "HTML5 Mandelbrot Generator" are shown in below:

Mandelbrot and Julia set fractals generated from "HTML5 Mandelbrot Generator".

All the source code of this application are available to download at under the terms of MIT Licenses.

Monday, November 7, 2016

100W AF dummy load

This is a quick post to showcase 100W 4 channel AF dummy load which we build to test the output stages of audio power amplifiers. This dummy load use 4, 8Ω 100W wire wound resistors which are available in eBay. In this unit all 4 resistors are mounted on 170mm × 40mm × 60mm high gauge aluminum heat sink.

Prototype version of 100W AF dummy load.

It costs LKR1200 to build this unit and most expensive component in this unit are wire wound resistors. At the time of writing this 100W resistor packs are available in eBay for USD $6.00.

Monday, October 31, 2016

Extending the frequency range of Pioneer TX-7700 tuner

TX-7700 is AM/FM stereo tuner manufactured by Pioneer in around 1975 - 1980. This is basic tuner system which is specifically designed for "rack style" stereo systems. Compare with other tuner systems this tuner is not very popular and very limited information are available in the internet about this model.

This tuner is mainly build around HA1138 AM radio receiver, HA1156 stereo decoder and PA3001 (HA11225) FM IF amplifier IC.

The TX-7700 which I got is manufactured for Japan and because of that it is designed to work with 100V AC and frequency range is limited to 76MHz to 90MHz. In this quick post we demonstrate how to tune this stereo receiver to 88MHz to 108MHz by without modifying the circuit(s) of tuner.

In this tuner the main component which need to adjust is 5 turn inductor located in FM front end circuit (refer A in below photograph). The frequency range of the tuner can be adjusted by changing the spacings of this inductor. If tuning is getting difficult, slightly adjust the trimmer capacitor in that module (marked as B in same photograph).

FM front end components to adjust.

After carefully adjusting the inductor and trimmer capacitor we were able to tune TX-7700 into 88MHz -108MHz frequency range. Once the receiver is desired frequency range the clarity of the reception can adjust by tuning the compression trimmers located in tuning capacitor (refer C and D in below photograph).

Trimmer capacitors of FM tuner stage.

This method can be used to tune any analog (FM) tuner which have identical design to TX-7700.

Saturday, September 24, 2016

μPC1237 based 2 channel speaker protector

When working with expensive speaker system, speaker protector is an essential item to avoid any damages to speakers. In this project we build low cost speaker protector by using NEC's μPC1237 IC. The circuit in this project is based on μPC1237 datasheet and it is specifically modified to work with 24V AC power source and with 24V relay.

Prototype version of μPC1237 - 2 channel speaker protector system.

PCB design of this project is limit to 90mm × 43mm and based on through hole components. Depending on supplied voltage and relay, value of R7 resistor is need to be change. The specified value in schematic is for 24V AC power source and for Omron 24V G2R-2 relay. Start-up delay of this unit can be adjust by changing the values of R5 and C5.

Schematic and PCB design files of this project are available to download at

Tuesday, August 30, 2016

CoCA Emerge + Tech - Musical plants project

CoCA "Emerge + Tech" - Musical plants is an electronics, software and plants base exhibit we developed for Cinnamon Colomboscope 2016 exhibition. This installation will feature a touch sensitive sensor that will generate a soothing sound from each plant. By interacting with individual plants, the audience can create musical compositions.

Sensor unit of this project consists with 14 high sensitive, configurable level detectors and USB interface. In this sensor unit we use Samsung's KA2284 as amplifier, level detector, and PIC18F4550 8-bit MCU as data multiplexer and USB I/O controller. PC817 opto-isolators are used to interface between analog front-end and digital data processor. In this design both analog and digital circuits are physically and electronically isolated to minimize the noises and interferences.

In this design we use KA2284 because it is commonly available in local market (in Sri Lanka) and costs less then 0.1 USD. To reduce the interferences we use shielded cables between plants and sensor ICs. With the special wire we are using we may be able to extend the sensor connectivity up to 3m.

Sensor board of musical plants project.

PC application of this project is developed by using HIDAPI and irrKlang libraries. This application is designed to work with Microsoft Windows operating systems and we tested it on Windows 7 and Windows 10.

This project is an open source hardware project released under terms of MIT and CC BY-SA licenses. All the content of this project are available to download at or Conceptual details of this project is available at CoCA homepage.

Sunday, August 28, 2016

Sony TC-500 reel to reel player restoration

TC-500 is 4 track stereo reel to reel player which is build around 1960 - 1970 by Sony Corporation. This is vacuum tube base reel to reel player with 2 × 3W audio output power. TC-500 is dual speed (3¾ and 7½ ips) tape recorder and it support reels up to 7 inches (18cm). Amplifier stage of this tape recorder is build around 12AX7, 6AU6 and 6AQ5 vacuum tubes and 12BH7 tube is used in biasing oscillator. Other than above tubes, 6CA4 is used in PSU for rectification.

Restored Sony TC-500 reel to reel tape recorder.

Recently we got a chance to restore TC-500 reel to reel player which we got from one of our friend. The player which we got is heavily damaged and we spend nearly 2½ months to restore this player.

In first stage of restoration we disassemble mechanical parts of this player and clean everything. In this player capstan idler wheel and take-up idler wheel is damaged beyond repair and we reconstruct those wheels using rubber sheets.

Saturday, May 28, 2016

Modified Heathkit HD-1410 electronic keyer

HD-1410 is quiet popular iambic keyer which is manufactured around 1970's by Heathkit. The original HD-1410 keyer is designed using 5 commonly available 74LS series TTL ICs and 8 transistors. Other than iambic operation it has an option to adjust sidetone frequency and support for (external) single-paddle key unit.

In this project we redesign original HD-1410 electronic keyer with today's commonly available components. In this new design the key components which we replace are transistors, diodes and power supply section of original HD-1410 circuit. Most of the NPN transistors in original design are replaced with KSP42 and KSP10 transistors, and all the PNP transistors are replaced with KSP92 transistors. To keep everything simple we also use same TTL chip-set with this new design.

PCB of modified Heathkit HD-1410 electronic keyer.

Tested PCB layout for this new keyer is available in the project archive and this PCB layout is based on standard though hole type components. Dimensions of this PCB is 110mm × 102mm.

We test this modified HD-1410 design with several transceivers and it performs extremely well. This new design also behaves similar to original HD-1410 unit and the parameters which got change are DC input voltage range and power consumption only.

Schematic, PCB and wiring diagram of this modified HD-1410 keyer are available to download at The original Heathkit HD-1410 schematic and details are available at

Sunday, May 1, 2016

Simple crystal tester

This is simple Colpitts oscillator to test commonly available passive crystals which range between 2MHz to 27MHz. This unit must connect to an oscilloscope and/or frequency counter to get the frequency of the crystal.

This circuit is design to work around 9V to 12V DC power source. Both 2SC930 transistors can replace with any high speed NPN transistor such as 2SC829, 2SC933, etc.

Testing 5030kHz FT-243 crystal using prototype version of crystal tester.

In our lab we use Manhattan style layout to prototype this crystal tester and it produces highly accurate results with above specified frequency range. According to our observations, after 27MHz, amplitude of the waveform starts to decrease steeply.

Schematic and Stripboard wiring layout for this crystal tester is available to download in here.

Monday, April 18, 2016

USB curve tracer for NPN transistors

Curve tracer is an electronic test instrument to analyze the characteristics of transistors and other discrete semiconductors. In this post we construct USB base curve tracer to analyze properties of NPN transistors. This curve tracer is build around Microchip's PIC18F4550 MCU and it use simple Windows based GUI application to plot captured data of a transistor.

In this design PIC18F4550 MCU is used to establish USB connectivity, perform voltage readings and control current/voltage flow into the test subject. To minimize the cost and to make it simple, we use R2R ladder circuit to generate discrete collector-emitter voltage levels for the transistor on test. In each scan session collector-emitter voltage level get increase from 0V to 7.5V in 256 steps. In this design, "tracer" scans the transistor for 7 base current levels which are in between 7µA to 60µA. In viewer application collector-emitter voltage levels are plotted on x-axis and collector current is plotted on y-axis.

Results generated for 2SC945 NPN transistor.

In current firmware, PIC18F4550 communicates with the PC over USB-HID class and because of that this may not need any special device drivers on host terminal. In most of Microsoft Windows operating systems the viewer application may be able to communicate with the device directly. Viewer application bundled in this project package are developed using Microsoft .net framework and it can run immediately on any new PC with minimum amount of prerequisites. Based on our experiences the most recommended operating system for this application is either Windows 7 or Windows 10.

Results generated for 2SD400 NPN transistor.

To get the perfect results we highly recommended to construct this project on PCB. According to our experiences breadboards and veroboards may generate lot of noises and may finally leads to wrong output(s).

This is an open source hardware project and all it's design files, source codes and compiled binaries are available to download at All the content of this project are released under the terms of GNU GPL version 3.0.

Saturday, April 2, 2016

Automatic monitor brightness controller

This is an automatic "monitor brightness controller" based on environmental light conditions. This system use USB port base sensor unit to measure the light level and control monitor brightness accordingly. We design this system to reduce the eye stress by matching the monitor brightness with environmental lighting.

The sensor unit of this system is build around PIC18F2550 8-bit microcontroller. To measure the light level we use LDR with MCU's inbuilt ADC. The control software of this unit is design to work with Microsoft Windows operating systems and it use Windows API's DDC/CI related functions to control the monitors/display devices.

Firmware of USB sensor use 50-point boxcar filter to get the average readout from the ADC and that value is passed to PC over USB HID interface. Thanks to this HID interface this system may not need any special device driver, and it can use in any compatible PC with minimum amount of configuration changes.

Prototype version of Automatic monitor brightness controller sensor unit.

We successfully test this setup on several PC configurations. At the designing stages we mainly use Viewsonic VA1901wm LCD monitor to test this setup.

This is an open source hardware project and all it's source codes, design files and compiled binaries are available to download at All the content of this project are distribute under the terms of MIT Licenses.

Monday, March 21, 2016

Low voltage power supply unit

In this post we introduce simple and flexible, regulated low voltage power supply unit. This power supply has provision for 4 outputs such as 1.5V, 1.8V, 2.5V and 3.3V. We mainly build this low voltage power supply unit to test (and power-up) low voltage MCUs, CPLDs and radio receivers. For this power supply we choose 1.8V, 2.5V and 3.3V to get it compatible with most of the LVTTL/LVCMOS devices. Other than that, we include 1.5V because there are several analog ICs are available for that voltage level.

This power supply unit is based on LM1117/AMS1117 voltage regulator series and for this design we use AMS1117-1.5, AMS111-1.8, AMS1117-2.5 and AMS1117-3.3 fixed voltage regulators. Except to above regulators this board can be use with AMS1117-2.85 and AMS1117-5.0 regulators.

Internal view of low voltage power supply unit.

In our prototype we use center-tapped 6V (1A) step down transformer with this power supply unit. All the wiring diagrams to transformer and rotary switch are provided with the schematic.

The given PCB design for this power supply unit is based on SOT-223 (SMD) version of voltage regulators. Even though it use SMD components this PCB can build with single side PCB. Dimension of the PCB is set to 89mm × 69mm. All PCB designs and schematics of this power supply unit are available to download at

Friday, March 11, 2016

Mullard 3-3 Amplifier Project (Part 3)

This is quick (and final) post regarding our Mullard 3-3 audio amplifier project.

In last few months we got several e-mails regarding this project and most of the people are asking for modified schematic related to this project. The schematic of our amplifier is available to download in here. Please note that this schematic is heavily based on original Mullard 3-3 amplifier design and main component which we replace in that original schematic is it's tube base power supply unit.

As a final step of this project we construct wooden enclosure for that amplifier and attached it to 10 inch (8Ω) full range speaker. High resolution photographs of our amplifier is available in here.

Final view of Mullard 3-3 audio power amplifier

As a conclusion this Mullard 3-3 audio power amplifier is high quality low power tube amplifier with minimum complexity. We really impressed with it's audio quality and overall performance.

Tuesday, March 1, 2016

Artificial star unit

Star test is the most recommended method to test and collimate telescopes. To do a "star test" we need to aim and track telescope with real star and most of the times this is quite difficult to do because of fast movement of stars and bad sky conditions. Another problem with real star is air turbulence and some times this effect may leads to false conclusions.

However artificial star is an excellent alternative for "real star test" and in this post we introduce easy to build "artificial star unit" to test and collimate telescopes. This unit is based on commonly available electronic components and it can operate with 9V battery. At the time of writing this unit can build around LKR 1000.00 (Approximately 7.00 - 8.00 USD).

In this given design we use NE555 timer IC to generate PWM output to LED and this allow us to control intensity of the artificial star. In our prototype we use high-bright 3mm white LED as our light source.

Apart from circuit the next most important thing in this unit is it's LED mounting. Since real star appears like "point of light" this artificial star must appear as a point of light. To archive this same effect we dill 300 micron (0.3mm) pin-hole in our casing as shown in below.

Prototype version of artificial star unit. Output pin-hole is located in left side of this box.

We test and collimate several telescopes with this unit, which including 8'' and 10'' Newtonians, 6'' and 11'' Schmidt-Cassegrain telescopes. Now we use this unit periodically to test our scopes for collimation and related issues.

PCB, schematic and wiring diagrams of this "Artificial Star Unit" is available to download at

Monday, February 22, 2016

Mullard 3-3 Amplifier Project (Part 2)

This is a second article related to Mullard 3-3 Amplifier Project and in this article we introduce HT transformer and HT power supply related to this amplifier. As mentioned in previous post, power supply unit of this tube amplifier is constructed using 400V 5A bridge rectifier, 220µF (400V) and 82µF (400V) electrolytic capacitors.

Homebrew HT transformer and PSU

The most vital component of this power supply is HT transformer and due to limited availability we construct this transformer by ourselves. We construct this transformer to obtain following voltages and primary winding is design for 230V AC mains lines.

Monday, February 15, 2016

Simple door bell for access control systems

This is very simple electronic doorbell circuit which is specifically design to interface with commonly available access control systems. This circuit is designed around NE555 timer IC and LM386 audio power amplifier IC. Because most of the access control systems use 12V power supplies this doorbell is also design to work with 9V to 15V DC power source.

Final view of our doorbell project prototype.

We test this doorbell with several YOUHE access control systems and it works well with all the tested models.

In this design NE555 is setup to obtain 840Hz - 860Hz output with above 80% duty cycle. To get more effective output we use small 3.5 inch, 8Ω tweeter with our prototype design.

The PCB design and schematic of this design is available to download at

Sunday, February 7, 2016

Musical trees

"Musical trees" is a part of creative arts installation and this project is capable to produce different audio tones by detecting human touch to its attached plants. Existing version of this driver is capable to monitor 8 plants and produce different sounds for each plant.

Prototype version of "Magic Tree" project with 2 plant samples.

This project is build around PIC16F628A 8-bit microcontroller and PT2399 echo processor IC. To drive the sensor electrodes we use pair of CD4011 quad 2-input NAND gate ICs. This prototype use TDA7052 1W audio amplifier IC to drive the speaker(s).

The existing firmware of PIC16F628A is designed to produce tones in between 155Hz to 1244Hz and this range can be change by modifying the supplied firmware source code. In this design PT2399 echo processor IC is used to get more natural (/ deep) sound effect by adding some reverberation to the output.

Schematic and firmware of this project is available to download at

Saturday, January 16, 2016

Victor SEA-50 graphic equalizer restoration

Few weeks back I got Victor SEA-50 stereo graphic equalizer from one of my friend. This is 10 band stereo graphic equalizer manufactured by Victor Company of Japan (now JVC Kenwood) around 1970 - 1980. SEA-50 consist with 63 transistors and it provides ±12dB control range for 32 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz, 12 kHz / 16 kHz / 20 kHz center frequencies.

The SEA-50 which we got is not working properly and most of its controls may generate distortion above +8dB and below -8dB. Apart from that left channel of this receiver has low volume and distortion.

Defective transistors in control board (next to 2SA493).

After few hours of checkup we identify few defective transistors (2SC982) in control board and we replace those with 2N3904 NPN transistors. Subsequent to this fix, all controls of SEA-50 start to work again on its full range (±12dB) by without producing any distortion or clipping.

Sunday, January 10, 2016

Mullard 3-3 Amplifier Project (Part 1)

Mullard 3-3 is quiet popular 3W tube amplifier introduced by Mullard Ltd in 1956. A schematic and design detail of this amplifier is available in "Mullard Circuits for Audio Amplifiers" book and in National Valve Museum article. This amplifier is based on EF86, EL84 vacuum tubes and EZ80 full wave rectifier tube. In this project we decided to construct this original Mullard 3-3 Amplifier with some slight changes and commonly available electronic components.

Top view of Mullard 3-3 amplifier with 6P14P and 6J8 tubes.

In our prototype we replace EZ80 tube with 400V 5A bridge rectifier which is commonly available in many electronic spare parts shops. Also we replace EL84 with 6P14P pentode and EF86 with 6J8 pentode. Both of these valves can directly use with this circuit and those values are available for lesser price than EL84, EF86 tubes.