TEAC AG-H300 with protection problem

slavslav

Member
2019-11-01 5:56 am
Hi everyone. I have bought a second hand TEAC AG-H300 about one year ago.
The sound of this little unit is quite good for my taste ( I consider my self as a HI-FI enthusiast, with little experience in the field), and it was working alright for some time.

But then I started hearing some crackling in one of the channels , not sure left or right because I don't mind plugging left to right and vise-versa .

After some time the amp started to make a nasty POP for a second , then go to normal if it was just some random electrical interference with lets say a powerful motor , that was switched on in the next room.

And now for the real problem that brought me here: the nasty pop had grown to a repeated protection activation lasting for quite long time.
I had disassembled the amp, cleaning the components and stuff, looking for some extra hot spots with a infrared thermometer , reconnecting all the connectors - with some result. The amp started to work for a day (real test on my work bench) and then after 8 hours - protection was activated again.

It was maybe 3 or 4 month since the amp is disassembled again collecting dust in the drawer . Now I have some time to deal with it again.
So any advice on this old but gold TEAC will be really appreciated.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
You will need the service manual or at least the schematic to trace any faults. Free download after registration here: TEAC AG-H300 - Manual - AM/FM Stereo Receiver - HiFi Engine[url

A fault like that is possibly due to one or more bad solder joints and these aren't easy to see, as they can be fine cracks that may not develop for many years. A remedy may not be difficult as it is often no more than reflowing the solder of suspicious looking joints, adding some fresh flux or maybe just a touch more resin cored solder to leave a nicely filled, tidy joint. Make sure you looked at everything in the area you suspect with a loupe or magnifying glass and strong lighting. You may also need to prod and tap the board gently at various points when it is operating, to check if that noise isn't still there for next time you turn it on. There are videos for reworking solder joints on PCBs, such as this one: https://www.youtube.com/watch?v=h5_dBMuJMNY

I suggest you begin by identifying the various sections in the receiver, such as the AM & FM tuners, the preamplifier/tone and volume controls etc. and the 2 power amplifiers. Identify whether it is the preamp or one or both power amplifiers has the problem by a process of elimination and go straight to it, after checking voltages and noise sensitivity by comparison with its the counterpart in the other channel. Just bear in mind that professional repairers get specialist training or may even start out as engineers. Your first experience may not go so easily but take care, and work systematically.

Before resoldering, you might want to verify that the voltages shown on the schematic are approximately correct and you'll need a good, basic DMM for that and print out the schematic and PCB parts layout so this is easier to follow.

Other suspects are electrolytic capacitors. These can be expensive if good quality and the voltage rating is correct but these can usually be tracked down without difficulty with the help of a little prodding to find if they are sensitive to a little mechanical shock. If the troubles only begin after it has been operating for some time, the rising temperature inside the case is another clue to locating possible faults.

If you are new to repairs, beware that mains voltages are fatal, even if just for a split-second and even if most of the receiver's electronics operate at relatively low voltages. Always keep one hand away from the device and use clip leads, hook probes etc for secure test connections and personal safety - Avoid becoming a part the circuit yourself and another statistic.
 
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