|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting
VHF channels will not tune Magnavox T809 (Photofact 2026-1)
Channels of this Magnavox T809 receiver would not tune up or down the VHF band. Although the mixer B + , RF-oscillator B + and band-switching voltages were all normal at the tuner, the tuning voltage remained a steady + 39V when attempts were made to tune the channels of the VHF low and high bands.
As I looked over the tuner-control schematic, I decided to begin by checking the binary logic outputs at port 1 of the microprocessor. This is where the tuning-voltage decision originates, so I thought it would be a logical place to begin the tests. After all, these circuits are very complex and seldom are checked completely during repairs. As I punched in different channel numbers, the port 1 outputs changed binary states. This is normal, and I concluded that the microprocessor was not defective.
From the microprocessor, the digital information passes through buffers and enters a digital-to-analog converter made of bilateral switches and op-amps. Rather than trying to trace through this jungle of circuitry, I merely went to the output, where the analog voltage should appear. This point was the filtered do voltage from R10 and C22 that is connected to pin 3, the non-inverting input of IC13, the final tuning-voltage amplifier. Notice pin 2, which brings in any AFC tuning corrections. Pin 6 should have the VHF- and UHF-tuning do voltage (notice on the full schematic the series 10k2 resistor and the 4.47µF bypass for the tuning voltage at the UHF-tuning end). Of course, this voltage should be different for each channel.
As stated before, the pin-6 voltage remained at + 39V although an attempt was made to tune in several channels. Finally, I decided IC13 was the most likely suspect. After 1C13 was replaced, all channels tuned properly.
David Luckner Corning, NY
No picture, very little sound RCA CTC120A (Photofact 2296-2)
After I removed the back and moved the machine to a serviceable position, I verified the complaints against this CTC120A-chassis RCA television. Volume could be turned loud or soft, but the sound was just noise except for weak, noisy sound from our local Channel 2. None of the channel-selector but tons operated; LED channel-readouts did not change.
Using my digital multimeter, I checked voltages around the tuner and the tuner-control module looking for any incorrect voltages. On tuner module MSC012A at pin J1, I found the + 5.1V missing.
Next, I moved to the source of the + 5.1V source at TP2509 (regulated by zener CR2504), but there was no voltage. The + 5.1V source is produced from the + 8.7V supply at TP2503, but there was no voltage there either. Diode CR2503 rectifies pulses to pro duce the + 8.7V supply. Therefore, I checked CR2503 for shorts. When it was found to be OK, I tested the associated filter capacitor C2509, but it, too, was not defective.
Finally, I brought out the heavy artillery, my trusty oscilloscope, and began looking for negative horizontal pulses at J2/pin 6 on module MCS012A, but they were missing. At the source, pin 7 of flyback T402, the negative pulses were normal. Something between those two points was stopping the pulses. I traced wiring back to the main chassis and J302 pin 2, but no pulses. I suspected that L104 might be open. When I checked on the CircuiTrace 125 side, the pulses were normal, but not at CircuiTrace 126.
This pointed precisely to an open in L104, but as I was examining L104, I noticed a bad soldering connection on the 126 side of the coil. I cleaned off all the old solder and re-soldered the lead correctly.
Pulses now were reaching the tuner-control module, allowing the + 8.7V and + 5.1V voltage sources to operate as intended. The tuning buttons and the channel-readout LEDs were bringing in the correct stations. The job was over except for minor adjustments and replacing the back.
Since the problem of the open coil, I have found another in the same model and heard of several in different brands and models that . had the same symptoms because of a loss of pulses to the tuner control circuit. Keep this irk mind.
Many defects produce symptoms that point to ward the cause, making our diagnosis easier, but that was not my good fortune with this open-coil symptom.
Ivan L. Wood
Prev. | Next | Index