LED Diagnosis/Troubleshooting
Board 2

LED Diagnosis || Board 1 || Reverse Polarity Damage

Second Mixer || Crystal Oscillator || Oscillator Amplifier Diagnosis || 455 IF || AGC
S-Meter || BFO || BFO Amplifier || Product Detector || Audio Amplifiers

Second Mixer

>If the LEDs do not light, use a Frequency Counter and check for a reading at the .01 capacitor located at the upper right corner of the mixer to the left of the 470 ohm resistor. This is the coupling capacitor between the Crystal Oscillator Amplifier and the Second Mixer.

>If you get a frequency readout, then the Crystal Oscillator is OK and the problem is at the Second Mixer.

>Lift one end of the 470 ohm resistor connected to Gate 2 (MOSFET on the right hand side of the mixer), between two .01 capacitors in the upper right hand corner of the mixer, and the LEDs should light.

If the LEDs do not light, replace the 470 ohm resistor with a 100K resistor, then do the procedure above under the section "MOSFET Amplifiers".

Put the 470 ohm resistor back in the board after running the MOSFET Amplifiers diagnosis.

>Make sure that the LEDs installed in the mixer are either the smoked red LEDs provided in the kit or the special ordered Bright Reds. Do not use the Super Brights (will not be red colored when powered up) or the mixer gain will be lowered considerably.

Back to Top

Crystal Oscillator

Note: If the LEDs are modified/changed from the Bright Reds supplied with the kit, the circuit may not switch properly. The fix is to add a 100K resistor between the gate and 12 volts of the IRFU220 closest to the 12 Volt box near the row of holes at the bottom of the circuit. See Crystal Oscillator/Amplifier/LED Switching of the Crystals and the schematic for more details.

This frequency of this circuit is controlled by the pair of IRFU220's, which are switched by the Phototransistor. The IRFU220 switching circuit switches the crystals in the oscillator circuit.

>If the LEDs at the crystals are switching at random times, check the switching circuit at the Crystal Filter. One or both of the VN0106N3s could have a bad solder joint. Place a finger on each one of the VN0106N3s and see if the LEDs switch with movement of the VN0106N3. You might see the dark LED at the Crystal Filter show a little light.

>If both LEDs come on at the same time, there is just enough light on the Phototransistor to barely turn on the 30/17 LED. Make sure the IR path between the Crystal Filter and the Phototransistor is covered by the black tubing supplied with the kit. Do not use colors others than black, they do not block stray IR light from shop lights or the sun.

>If the phototransistor is covered (try a piece of tape over the lens), and both LEDs are on, one of the IRFU220's could be bad, or a missed/bad solder joint at the 47K resistor (to the right of the top IRFU220), between a .01 capacitor and a 1K resistor.

This will also cause a loud howl out of the speaker, because 3.547 MHz oscillator RF energy is getting through the Crystal Filter. Blocking ambient light getting to the Phototransistor will cure the problem.

>Covering the IR path between the 30/17 Crystal Filter IR LED and the Phototransistor must be done, for the Phototransistor is sensitive to ambient visible light, and can easily turn on, causing a loud howl in the speaker.

>When no light/IR energy is on the Phototransistor, the 40/20 LED should be on. When light/IR energy is hitting the Phototransistor, the 30/17 LED should be on.

Do not put a wire jumper across the 200pf Mica capacitors, as they block the DC current/voltage from going to the crystals. These capacitors can be changed to any value to fine tune the BFO tones, but do not eliminate them and replace with a wire.

Checking for a Bad Crystal

If the LEDs at the Second Mixer turn off when the Crystal Oscillator is switched, one of the crystals are bad. An LED indicates which crystal is on, so if an LED is on at a crystal, and the Second Mixer LEDs are off, that is the bad crystal.

If the LEDs at the mixer turn on when the other crystal LED is on, this shows that all the circuitry is working except for activity in the bad crystal.

Another indication is that the LED at the Oscillator Amplifier will not flash when the crystals are switched. The LED will dim very slightly when the bad crystal is on.

Back to Top

Oscillator Amplifier Diagnosis

The LED in this amplifier is brighter than the other MOSFET amplifiers in the receiver. The amplifier LED will flash when changing crystal frequencies.

If the MOSFET amplifier LED does not flash when changing frequencies, there is a problem with the MOSFET circuit or the oscillator.

One problem is that one of the crystals in the oscillator could be bad (see above).

If all the components are installed properly, and the LED does not flash, then the MOSFET is probably bad (short between drain and source). See LED Diagnosis for checking the MOSFET amplifiers and MOSFETs.

455 IF

>If one of the IF amplifier LEDs are off, check that individual amplifier. Go to LED Diagnosis and "MOSFET Amplifiers".

>If both of the IF amplifier LEDs are off, check the AGC circuit. The IF amplifiers are the first two amps, the third IF amplifier is an AGC amplifier and that LED should be on all the time.

AGC Circuit

Almost all problems in this circuit have been missed solder joints. Look over this area of the board carefully.

>If the AGC voltage is near zero, or low enough to dim the IF amplifier LEDs, check for a bad/missed solder joint at the 10mfd tantalum capacitor next to the 2N3904.

>It is unlikely there will be trouble with the 2N3904, but check for a solder bridge between the transistor pins.

>The S-Meter works off the AGC line, so if there is a solder bridge at the 2N5486, the AGC voltage will be low.

>Double check the diode polarities - make sure the band on the diodes (4 - 1N914s and 1 - 1N270) are the same as the footprint on the PCB.

>No AGC action on live signals, receiver is playing OK, look at the third IF amplifier and the secondary on the 50A02 transformer. Also check for cracked glass on the 1N270 and missed solder joints.


>The pins on the 10K trimmers should be checked for a bad/missed solder joint. It is easy to miss one (Sensitivity pot) or both (Zero Adjust pot) of the grounded pins.

>Double check MTR1 and MTR2 for the correct resistor values for the meter you are using. Check S-Meter page for values.

>Check for solder bridges across the pins of the 2N5486.

The circuit is very simple, not much can go wrong.


>If a tunable (high to low frequency) noise is not present in the output of the speaker when adjusting the BFO, then the BFO is not oscillating.

Make sure that the Audio Amplifiers are working to make sure you are not at the wrong place with the problem. A quick test is to place your finger at the output of the Product Detector (junction of 1.5K and .05 cap next to a 10mfd tantalum at the input to the audio pre-amplifier, audio volume turned up) and check for 50/60 cycle hum or a strong local AM station.

>The most common mistake is accidentally grounding the stator of the BFO tuning capacitor. Use a DVM and check that the stator is not grounded.

>If the BFO frequency changes when the rotor is touched, make sure that the wire connected to the rotor is going to ground. Make sure the lettering "193-10" is showing at the top of the capacitor. Of course, the better test is to use a DVM and make sure the rotor is grounded.

>Check for 5 volts on the output side of the 100 ohm resistor connected to the 7805 regulator. If a 1K or a 100K resistor was placed here, the voltage will be too low for the BFO to oscillate.

>The 2N5486 rarely blows in this circuit. Disconnect the BFO capacitor connection to the BFO TUNE hole and adjust the slug in the coil. If no noise is found in the output of the speaker, then the 2N5486 is bad.

BFO Amplifier

>If the LED is off, first adjust the "Gain Adjust" trimmer to make sure the amplifier is not turned off.

>Next check solder joints at the 50A01 455 kHz transformer. Use a DVM to make sure there is continuity between the primary windings.

>If the LED is dim (about half brightness), take a close look at the transformer primary solder joints and continuity through the primary coil.

>Go to LED Diagnosis, "MOSFET Amplifier and Checking MOSFETS" to check the circuit.

Product Detector

>Check the diodes with a magnifying glass and look for cracks in the diode, especially at the ends. If the diodes were stressed when inserted/soldered into the board, the glass could be broken.

>Check for a cracked transformer. Gripping the transformer with a pair of pliers and twisting slightly will test for a cracked transformer. Do this with light pressure to keep from making a crack!

>The pins on the 455 kHz filter (L40) should have been tinned before soldering on the board. Check the soldering on the pins to make sure the solder flows onto the pin and there are good connections.

If a bad solder joint is found, use a solder sucker or wick and remove the solder from the joint. Use a sharp razor blade or utility knife and scrape the pin clean, then resolder.

Audio Amplifiers

Low Audio

Check the ground connections for the diodes at the product detector. They can be easily missed while soldering the board.

Output will be less the higher the impedance of the speaker (over 4 ohms). An 8 ohm speaker will make a little difference, but 16 ohms and above will be noticeable.

Audio Pre-amplifier

>Touching a finger to the pins of the Audio Control pot should result in a 50/60 cycle hum or a strong local AM station coming from the speaker. If no hum or station, read the diagnosis for the "TDA2002/LM383 Final Audio Amplifier" first.

>Make sure the Audio Control is at maximum volume or adjust the volume control while doing the following step.

>Second, touch a finger to the junction of the 1.5K resistor and the .05 capacitor next to a 10mfd tantalum at the input of the pre-amp. If you do not have the same 50/60 cycle hum or a strong local AM station coming out of the speaker, the problem is in the pre-amp.

>The 2N3904 is rarely bad, so check carefully for correct resistor values, bad solder joints, missed solder joints, or solder bridges.

>If you suspect the 2N3904 is bad, it can be replaced with any general purpose NPN transistor (2N2222, 2N4401, etc.).

TDA2002/LM383 Final Audio Amplifier

The TDA2002 and LM383 for electrical purposes are the same part. Some kits will have the TDA2002 and others the LM383. Kits purchased after October 2004 will probably have the LM383.

Two problems can occur with the Final Amplifier: no output and motorboating. Motorboating is a recurring thumping sound at regular intervals.

No Output

>To check for output, put a finger on the terminal of the Audio Control that leads to the input of the Final Amplifier and listen for 50/60 cycle hum or a strong local AM station.

>If no output, the most likely problem is solder bridges around the TDA2002/LM383.

>Also check for missed solder joints or joints where the solder did not flow around the pin of the parts. These are found by holding the board up to a light and looking on the other side for any holes that light shines through.


>Motorboating is caused by driving the Final Amplifier too hard. Check the "Gain Adj" box in the Audio Pre-amp. If the value is above 2.2 mfd, the pre-amplifier may be driving the final amplifier too hard.


>If you feel like the Audio Amplifier IC is running too hot, replace the 10 ohm, 1 Watt resistor with a 22 ohm, 1 Watt resistor. The IC normally runs warm to hot and is not a problem.

>Other changes that have been recommended are changing the value of the 2.2mfd electrolytic at the bottom right of the IC (12 Volts bypass) to 10 or 20mfd and raising the value of the 1 ohm resistor (going to ground at the bottom side of the 1mfd capacitor right before the speaker connection) to 10 ohm, 1/4 watt. These modifications are to help stabilize the IC, but have not been found necessary during normal operating conditions.

MOSFET Amplifier/MOSFET Device Diagnosis

Board 1 LED Diagnosis/Troubleshooting

Send E-Mail || Amateur Radio Receivers || Electroluminescent Receiver
Back to Ham Radio Instructions || Back to SWL Instructions || Modifying the LEDs

Last Update: 8/17/07
Web Author: David White