Hi tedsorvino1,
-Chris
Well, then if we are focusing on the weird, you have an open emitter resistor and low current loads operate fine. But the output goes through the headphone jack and tests fine on headphones. So it is either the jack contact or something that will deliver low current but not high current.
-Chris
Lets stick with the right channel.
So the bias does indeed adjust correctly... this is really bizarre you know
First thing is to set the bias back to the correct level which is around 13 millivolts between the test points.
We have to look at this dynamically now. Make sure things like the tone controls are centred (because these are part of the feedback loop for the power amplifier) and set the scope and signal generator up again.
Have NO speakers or headphones connected.
Apply a known amplitude sine to the input, lets say we use 100mv peak to peak again and 1Khz. Measure this on the input coupling cap I circled earlier. Be certain you have 100 millivolts peak to peak.
Now tell me what the peak to peak amplitude is at the output. Measure on that coil at the output.
What peak to peak amplitude do you see?
Is the sine clean or is it distorted looking?
Now plug the headphone in.
Has the amplitude or shape changed?
Hi Mooly.
We are sticking with the RIGHT channel only.
The input amplitude is 100mV, All controls centered, all switches (filters, loudness etc) on the off position.
On the input coupling it's about 1,5mV clean looking
On the output coil it's clean looking and it's amplitude depended to the Volume pot. It gives 100mV (similar to the input) when the pot is at around 2.5 (So there is a lot of headroom).
Same results with headphones in and without them.
Then I tested the switched headphones out (just for the sake of it. On the headphones pin things are the same. On the speakers pin out there is no signal (just two lines, since it's a switch).
Hope it doesn't make things even weirder, although things seem normal to me. Is this right?
What would the behaviour be if the output transistors I 've placed are not an exact match?
Thanks for the input Regenpak. Much appreciated and really really helpful.
We 'll come back to the LEFT channel (hopefully) soon.
For the time being I follow Mooly's advice on the RIGHT channel only.
Thanks for looking in
and it does seem that way at the moment. Weird. Very weird.The amp has some history if you read back, a recap and a little accident with a slip of the meter probe
DC offset is good but audio very distorted on speakers which really points to an output fault...... however
All passives around the outputs and drivers seem good and so as a quick check I suggested taking the bias adjustment way over the recommended value to see if the output stage really conducts correctly... and it seems to.
It could be taken to over 90 millivolts across the two 0.33 ohms. (so 130 milliamps instead of the recommended 20ma). Plenty of range and showing that the outputs conduct correctly.
The distortion is a real mystery and hopeful some scope checks might give some clues. Watch this space
Thanks
Left channel comes next all being well. One channel at a time for this one.
Hi Chris.
Thanks for the input.
No for the right channel it doesn't test right on the headphones with music. Just with sine waves. The music comes out on both speaker and headphones, for this channel (RIGHT) , low and distorted.
What you mention may be correct for the (LEFT) channel. But we will focus on that if we manage to finish with the right first.
Thanks anyways.
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Lets have a look.
This is the bit I don't quite follow. What I was after was an idea of the gain of the power amp, in other words the output voltage vs the input voltage. I just wanted to get a feel for the gain it runs at which is difficult to calculate because of the log pots in the feedback network.
Lets ask it another way. If you set the output (so alter the volume control) to show 8 volts peak to peak at the coil, what peak to peak voltage do you see on that input cap?
You say the output is clean with headphones connected
that's good.Well that does sound like something is open circuit somewhere.
When you pull the headphones out the speakers should switch into circuit.
Connect the scope directly across the speaker terminals (no headphone connected) and see if the audio is present. Have no load connected for this test.
Do you see the audio or not?
If not then switch OFF and do basic resistance checks from each speaker terminal back to the coil for the plus terminal and ground for the negative one.
The speaker terminals should connect directly back to the coil via the headphone socket.
I'll look in later
Yes. This is how it works. And without the headphones the sinewave is good on the speakers terminals. No worries. That's what I meant by mentioning the swiched headphones out
So what happens when you add a load to the speaker terminals.
How does the signal change?
Does it also change the same way when you look at it on the coil?
Lets have a look.
This is the bit I don't quite follow. What I was after was an idea of the gain of the power amp, in other words the output voltage vs the input voltage. I just wanted to get a feel for the gain it runs at which is difficult to calculate because of the log pots in the feedback network.
Lets ask it another way. If you set the output (so alter the volume control) to show 8 volts peak to peak at the coil, what peak to peak voltage do you see on that input cap?
You say the output is clean with headphones connected
Actually I made the amp give me a 3V peak to peak on the output coil and it still gives me around 12mV on the input coupling
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Yes it does.
Actually it doesn't change since I'm using a static sine wave but I have the same wave on both points.
Good news. For no obvious reason the sound came back to the RIGHT channel. I have to admit that I moved the amp to a different place in order to connect it with the speakers. So maybe a contact at the speaker post or something after I replaced the shorted output transistors. I feel lucky.
So before we continue sorry for this long parenthesis. It was an accident that destroyed the output transistors and the fact that it didn't give good signal (obviously for reasons out of the circuit itself) was really confusing.
If you want to offer any input regarding the left channel it would be more than welcome.
So before we continue sorry for this long parenthesis. It was an accident that destroyed the output transistors and the fact that it didn't give good signal (obviously for reasons out of the circuit itself) was really confusing.
If you want to offer any input regarding the left channel it would be more than welcome.
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Good news
OK, so the right channel has started working for no obvious reason... all you can do here is go over it all very carefully and look for any connectivity problems. Anatech mentioned the headphone socket as a possible problem area.
It's worth looking at all possibilities here.
Earlier in the thread you mentioned this:
So is the right channel all 100% OK now?
Let me look where the left channel was up to...
OK, so the right channel has started working for no obvious reason... all you can do here is go over it all very carefully and look for any connectivity problems. Anatech mentioned the headphone socket as a possible problem area.
It's worth looking at all possibilities here.
Earlier in the thread you mentioned this:
So is the right channel all 100% OK now?
Let me look where the left channel was up to...
Good news
OK, so the right channel has started working for no obvious reason... all you can do here is go over it all very carefully and look for any connectivity problems. Anatech mentioned the headphone socket as a possible problem area.
It's worth looking at all possibilities here.
Earlier in the thread you mentioned this:
So is the right channel all 100% OK now?
Let me look where the left channel was up to...
I've cleaned up all ins and outs, pots and switches thoroughly.
I don't know if different output transistors have different sound characteristics, but the new TIP 41C-42C sound exactly similar to my speaker (Right channel) and headphones like the original 2SD313 - 2SB507
I think we need to go back to basics for the left channel and just recheck a couple of things.
TR05 collector you say was unstable. That should reflect as an unstable output voltage and yet it appears not to.
Lets try something different here.
Connect your black meter lead to amplifier output (the coil)
Now measure and record the following voltages.
TR13 base=
TR09 base=
and:
TR15 base=
TR11 base=
Measure all those. Lets see what you have. You should see approx 0.6, 1.2 and -0.6 and -1.2 in order.
If you compare with the 'good' channel you will see what they are... but no slips with the leads.
TR05 collector you say was unstable. That should reflect as an unstable output voltage and yet it appears not to.
Lets try something different here.
Connect your black meter lead to amplifier output (the coil)
Now measure and record the following voltages.
TR13 base=
TR09 base=
and:
TR15 base=
TR11 base=
Measure all those. Lets see what you have. You should see approx 0.6, 1.2 and -0.6 and -1.2 in order.
If you compare with the 'good'
channel you will see what they are... but no slips with the leads.
The transistor type shouldn't really alter how it sounds at all. The only time they do is if they are unsuitable and cause instability and oscillation... which you would see on the scope.
Matched transistors... not for an amp like this. It's more important matching multiple pairs when they are used in parallel but here they are just notionally complementary types. So TIP41C and 42C are fine.
I think we need to go back to basics for the left channel and just recheck a couple of things.
TR05 collector you say was unstable. That should reflect as an unstable output voltage and yet it appears not to.
Lets try something different here.
Connect your black meter lead to amplifier output (the coil)
Now measure and record the following voltages.
TR13 base=
TR09 base=
and:
TR15 base=
TR11 base=
Measure all those. Lets see what you have. You should see approx 0.6, 1.2 and -0.6 and -1.2 in order.
If you compare with the 'good'
TR13 base= 70mV
TR09 base= unstable -0.8v - -0.750V
and:
TR15 base=0 - -0.1mV
TR11 base = unstable -2.5V - -2.3V
No load connections on the output
Extra care with the slips (even if not so good probes take extra touch for readings)
So those results tell us a lot.
TR15 base is near to zero. That means TR11 emitter is also at the same voltage.
TR11 base is at negative 2.5v (ish) and that is the big clue. It means base/emitter junction of that transistor is faulty as it should only be able to drop a maximum of around 0.7 volts (just like a diode).
Polarity is all important on tests like these. The emitter is the more positive (at zero) and the base negative. So no doubts at all that this particular transistor is faulty.
I would probably look at replacing both drivers, the NPN and the PNP.
You must also now turn the bias control to give minimum bias and that is when the pre-set is on maximum resistance. That is very important.
Are you using a bulb tester... you should be really
What transistors are you using? They really need to be able to withstand the total rail voltage plus a bit more for safety and so should be 65 volt or higher.
TR15 base is near to zero. That means TR11 emitter is also at the same voltage.
TR11 base is at negative 2.5v (ish) and that is the big clue. It means base/emitter junction of that transistor is faulty as it should only be able to drop a maximum of around 0.7 volts (just like a diode).
Polarity is all important on tests like these. The emitter is the more positive (at zero) and the base negative. So no doubts at all that this particular transistor is faulty.
I would probably look at replacing both drivers, the NPN and the PNP.
You must also now turn the bias control to give minimum bias and that is when the pre-set is on maximum resistance. That is very important.
Are you using a bulb tester... you should be really
What transistors are you using? They really need to be able to withstand the total rail voltage plus a bit more for safety and so should be 65 volt or higher.
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