QuestionQUESTION: Hello, I have a Technics SE-A808 power amp part of the set up with the associated preamp/radio unit, IR unit and tape deck. When I try to play sound through the power amp I get no sound. I have tried pause recording the radio through the tape deck and get sound. i have also bypassed the infra-red to the power amp and still gotten no sound. Have bypassed the power amp and used a different amp from the preamp, and it has played. I have some knowledge of Electronic circuitry.
Are you able to determine what may be the problem, as the power light still illuminates. (I have not checked the speakers, but the wires do not short anywhere)
Russell
ANSWER: Suggestion: Disconnect the amplifier from the system and feed an external audio input directly from a portable cd player or radio. Connect a known good speaker to the output. This will allow you to test the amplifier independently. Then you will know for certain the amp is working on its own or not. If it works well independently but does not when connected back into the system then you know the problem is external to the amplifier. (Speakers with shorted voice coils can cause the amplifier to go into protection mode, for example. You can test the speakers with an ohmmeter and if you do you should read a dc resistance of just less than the impedance rating of the speakers. If they are rated at 8 ohms, for example, you will measure a resistance of about 6 ohms. You can purchase a vom that has an ohmmeter section at Home Depot or Walmart for under $10.)
If you are sure the amp is working (from the independent test above) but not in the system then you must look to the feed lines from the tuner/preamp to the inputs of the amplifier.
WARNING: We have seen this problem many times in old audio equipment where the only thing that was wrong was the tape monitor switch was in the wrong setting! So, check that out.
Good Luck. Let me know if you need more.
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QUESTION: Thanks for your advice.
Because of where this device is located I haven't done quite as much of this as I'd like.
I know that the preamp has sound from its output (I have tested with another amplifier that was close by).
So what I have done, is unplugged all speakers from the unit. bypassed the infra-red Unit and fed that into the power amp.
As I did not have a speaker available. I left the speakers unplugged and plugged in headphones into the headphone port. I know the head phones do work, they were also tested.
I still received no sound. The power light still comes on on the front of the unit. Do you have any other suggestions or know what might be the culprit?
ANSWER: AFTER BYPASSING THE EQ AND GOING DIRECT TO THE AMP AND GETTING AUDIO TO THE HEADPHONES, CONNECT ONE SPEAKER AND SEE.
THE PROBLEM IS ISOLATE THE PROBLEM. IN MANY AMPLIFIERS THE HEADPHONE OUTPUT IS TAPPED OFF THE MAIN OUTPUTS AND IF YOU GET AUDIO IN PHONES IT MEANS THERE IS AUDIO TO THE MAIN SPEAKER OUTPUTS AS WELL. BUT, IN OTHER AMPLIFIER CIRCUITS THE PHONE FEED COMES OFF THE PREDRIVERS IN THE AMP AND GETTING PHONE AUDIO DOES NOT INDICATE AUDIO AT THE OUTPUT; IN OTHER WORDS THE OUTPUTS COULD BE GONE AND YOU WOULD STILL GET PHONE AUDIO.
IN MANY AMPS IF THEY ARE OVERDRIVEN THEY CAN OVERHEAT THE OUTPUT TRANSISTORS. ESPECIALLY THIS IS TRUE WHEN THE UNIT IS OLD. THE THERMAL HEAT GREASE MOUNTED ON THE HEAT SINK TO KEEP THEM COOL GETS DRY AND LETS THE TEMPERATURE OF THE TRANSISTORS GO UP TO BURN OUT. WHEN THIS OCCURS THE OUTPUT TRANSISTORS MUST BE REPLACED (AND REGREASED).
OUTPUT TRANSISTOR REPLACEMENT IS A TECHNICIAN JOB UNLESS YOU ARE EXPERIENCED IN SUCH THINGS.
DO WHAT IS RIGHT!
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QUESTION: sorry, i miss worded my reply, the headphone port produces no sound, but head phones are not the cause.
there is a service manual for the amp in question at users.adam.com.au/rdpeake/TECHNICS_SEA808_service_manual.PDF
it contains a schematic, and circuit diagram as well. sorry for my trouble. i still have no sound from headphones or speakers to clarify.
hopefully a schematic can help you.
AnswerAs previously stated, once it has been determined the problem is internal to the amplifier it will require testing to find the culprit. As in the last reply a famous cause of failure in older audio amplifiers is the drying out of the silicon heat grease causing the output section to go down. But, it still requires testing to determine if that is the cause or if it is one of the upfront amplification stages or elsewhere.
With the schematic you can go into the unit with a voltmeter and check a few of the voltages which may lead to the trouble area. And, with an ac signal tracer or oscilloscope you can check down through the amp to find out where the signal goes astray. That would be the troubleshooting methods used by a technician at the repair shop.
Carry on!
