Repairing a modern 2-channel audio amplifier

[AB2EZ]

Hi!

It must be my lucky day! I was actually able to repair a Samson 120a 2-channel audio amplifier that I use to plate modulate some of my tube transmitters. Usually, I assume that modern equipment is unrepairable (no schematic, ASICs, etc.)

The key was all of the experience I gained from my Class E transmitter / Class H modulator.

Symptom... large input audio transient causes amplifier to turn off. Fuse is blown. Replace fuse and turn on amplifier... fuse blows again. Ugh! 

Glass of wine, thinking about what would cause the Samson 120a to draw a lot of current as a result of a large transient applied to audio input. Where have I seen this happen before? On occasion I have abused the Class H modulator in my Class E transmitter by applying too large an input signal. Result... shorted FET's in the class H modulator... causing the over-current protection circuitry to trip.

Putting the above together (lubricated by aforementioned wine): One of more of the power transistors in the Samson 120a must be shorted! Power transistors are easy to get to, and easy to test.

Ohmmeter check: Right channel power transistors both okay. Left channel power transistors both toast (0 ohms between any two leads).

Remove left channel power transistors (TIP35C and TIP36C)... replace fuse again, turn on Samson 120a:  right channel works fine.

Order replacement TIP35C and TIP36C transistors from Mouser: $1.99 each. 

Best regards
Stu

[WD5JKO]

Stu,

   Good deal with that amp. Consider that those two blown transistors might not be all of the issue. There could still be a gremlin or two of peripheral damage. A direct coupled SS amp can be tricky for sure. Proceed with caution.

   On another repair my son hands me his Nintendo DS Lite. It had gotten wet and was inoperative. Having never been into one before, I jumped in both feet. Taking one of these apart is not easy, and I soon found out that putting it back together is much harder. Using IPA and a Q - Tip I cleaned up the spill (apple juice?). So now what, still don't work. With no schematic, and 52 year old eyes, I was struggling. That circuitry sure is tiny. Get my magnifier, and magnifying lite, and bifocals, and I could almost see the details. I followed the power from the battery holder, and the + voltage soon disappeared and I traced the problem to a bad plated through hole. Apparently the juice on the board with voltage present resulted in some form of electrolysis that disolved that plated through hole (via). A strand of 30 awg wire wrap wire soon repaired the problem.

I guess my experience with wiring tube sockets, and running Romex helped me with the DS Lite!

Regards, and HNY,
Jim
WD5JKO

[flintstone mop]

You might be  lucky, Stu
That it was just an overload that took out the final transistors. I have had issues with the pre-driver transistors that will eventually strike again and knock out the replacements.
Is this stereo amp bridged to be a big audio source for your transmitters?? Or was this blown channel sitting out there un-terminated?
Usually solid state final audio stages do not mind being un-termninated.
There may have been a nasty AC transient that got it, if you leave this unit on 24/7.
I have a University Sound public address amp that blows finals every two years after steady 24/7 operation.
I gave up on that and have a nice old, ugly, green tube Bogen pa amp giving me beautiful monitor audio from my receiver system.

Fred

[W1RKW]

Stu,
Check the driver transistors as well. You  may want to pull them and check for leakage. If they are OK that is good.  You're not done yet, however.  Check the transistors at the input of the power amp stage.  Typically there are  2 making a differential arrangement.  You may want to check the coupling cap at the input of the power amp stage, just before the differential amp and ensure that is doing what it is supposed to do.  Check the emitter resistors on the output transistors to ensure they're not open. After replacing the transistors and you're ready to power the amp up, power it up it on a light bulb, 75 or 100 watt bulb should do the trick. If all is well the bulb will barely light up at steady state.  While the amp is on the light bulb, check to see if there is DC on the output terminals, no load attached.  Make sure the output is at 0 VDC (give or take 10 or so mV).  If its right around 0VDC connect an 8ohm load, preferably a resistor, to the offending channel. If you use a speaker ensure the cone does not move or make a pop sound when connecting. Observe the light bulb.  It should not change intensity if all is balanced properly in that channel. If it does change intensity check other transistors in that channel  as well as any transistor or diode clamped to the heatsink.  If all is OK, at this point you can remove the light bulb and connect directly to AC mains. Recheck DC balance at the amplifier terminals.  Also do a bias or idle current comparison between the left and right channels.  Simply measure the voltage drop across the output transistor emitter resistors of each channel. If emitter current is drastically off in the bad channel compared to the other channel you still have a problem. Chances are you should be OK. If it is slightly off you maybe able to adjust this if the amplifier has adjustment pots. Simply tweak it so it is somewhat equal to the other channel.  The right way would be to get the manufacturers spec but this should be OK, usually 5 to 10mV across the emitter resistors should do the trick.  There may also be adjustment pots for setting the DC balance on the output.  Simply adjust for 0VDC at the output of each channel.  A couple of final checks, let the amp idle on an 8 ohm load both channels and monitor the heatsink temp for about 20 minutes.  If the heatsink temp is equal for both channels that's a good sign. Also, to ensure if the amp is working correctly would be to put a 1KHz sinewave through the amp (both channels under load) and run it up to full power on a dual trace scope. Observe the waveform for non-linearities or discontinuities. 

[AB2EZ]

All

Thanks for the heads up on possible other problems (beyond the output transistors).

I will keep all of this excellent input in mind as I bring the left channel back up. I ordered 4 each of the output transistors... so I will have a few chances to get this thing up and running.

Fortunately, all of the components are large, discrete, through hole types... so I should be able to further troubleshoot it if necessary. I will also check the DC offsets after I get the new transistors soldered in.

Best regards
Stu

[AB2EZ]

I lucked out...

After I (carefully) replaced the blown output transistors on the left channel (one TIP35C and one TIP36C), the amplifier is working fine again on both channels. The dc offset measured at the output is just a few millivolts... so no adjustment was necessary.

Thanks for the tips!

HNY

Stu