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bias question

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Othello

Member
Hopefully I managed to get the circuit diagram up...


I am troubleshooting one of these amps and there are a few things I don't understand (I had trouble liek this last year).
My circuit right now contains all passive components and only T1.

I connected a sine input of about 1 V and I get a distorted waveform (clipped positive wave) on the collector of T1.
The base of T1 shows about 40V dc and the input voltage is 70V.

Why is there any clipping?


Uwe
 

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Othello

Member
Well Nigel, people test circuits in stages all the time, so of course it can be done.

But maybe you are right that I do not have enough parts to make the input stage around T1 work, but this is why I posted my question, since the circuit does give me trouble I need to test it in stages.

So I do not want to let your comment discourage me from doing so, I will rather look for a solution which allows me to test just this one stage with it's bias setting components.

And if you know how to do this then I would welcome your comment(s).

Uwe
 

mneary

New Member
To test T1's bias, look where its emitter voltage comes from: the output stage. If you set the right side of R8 to Vcc/2, T1 should behave. If you then install T2 and T3 at the same time (sorry, they are highly interdependent), then you might see how they work.

T6 and T7 then might be checked by replacing their emitter resistors with 100 ohms (50 watts each).

Finally, T4 and T5, which are overload detect, will be difficult to test.
 
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Roff

Well-Known Member
Othello said:
Hopefully I managed to get the circuit diagram up...


I am troubleshooting one of these amps and there are a few things I don't understand (I had trouble liek this last year).
My circuit right now contains all passive components and only T1.

I connected a sine input of about 1 V and I get a distorted waveform (clipped positive wave) on the collector of T1.
The base of T1 shows about 40V dc and the input voltage is 70V.

Why is there any clipping?


Uwe
If your transistor had bias current, the small-signal gain would be approximately (R5+R6)/R7, or about 70. The biggest problem is that you have almost no bias current through the transistor, so it will only conduct on positive half-cycles of the input voltage. Nigel is right.
 

audioguru

Well-Known Member
Most Helpful Member
T1 has no voltage gain so adjusting the P1 trimmer pot won't do anything.
The base of T2 has 3 diodes biasing it so the small change in current from T1 through R6 won't change T2 much.

I think R6 and the 3 diodes should be removed.
 

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kchriste

New Member
Forum Supporter
The base of T2 has 3 diodes biasing it so the small change in current from T1 through R6 won't change T2 much.
I think R6 and the 3 diodes should be removed.
If you do that, then when T4 conducts during current limiting, you can say bye-bye to T2. The diodes cause T2 to become a constant current source when T2's collector current exceeds apx 25ma. At currents less than 25ma T2 behaves like an ordinary common emitter amplifier stage.
Follow Mneary's advice and set the right side of R8 to Vcc/2 if you want to simulate a partial circuit.
 
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Roff

Well-Known Member
I think it would have been better had you not started another thread on the same topic.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Othello said:
Well Nigel, people test circuits in stages all the time, so of course it can be done.

But maybe you are right that I do not have enough parts to make the input stage around T1 work, but this is why I posted my question, since the circuit does give me trouble I need to test it in stages.

Yes, people test in stages all the time (as a service engineer I do it on a daily basis) - but the entire DC coupled amplifier is a 'stage', you can't break it down any further because you're no longer testing the 'stage', but remaking any single part into a completely different circuit.

So I do not want to let your comment discourage me from doing so, I will rather look for a solution which allows me to test just this one stage with it's bias setting components.

And if you know how to do this then I would welcome your comment(s).

Like I said, you can't, you need to test it all in one - you could completely rewire the first stage to make it work so you could test it?, but it wouldn't bear any resemblance to the circuit, and would be of no value at all to you.

What you could do though is simplify it a little, T4/T5 and associated components are for over current protection, you can remove and ignore those for testing. You could also short out T3 (collector to emitter), this sets the bias to zero, so stops thermal runaway - but gives high distortion at low levels. Once the amp is tested and working you can remove the short across T3, ensure the bias is set to minimum, and then gradually increase it while you monitor the current.
 
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Othello

Member
Well first of all I do thank you all for the attempts to help.

I do have half knowledge or less when it comes to electronics.
I can follow the explanations why some things work the way they do but if then they do not work properly it requires an altogether different level of knowledge to know where to tweak it.

The amplifier I mentioned here simply blew the fuse and the two output transistors when I switched it on. I have 5 working ones, this one does not work.

Obviously I am reluctant to simply put two new transistors in and switch it on again. I am trying to figure out why it did this and the safest way would be two build it in stages and 'inch' towards the complete amp. Nigel strongly argues that that is not possible, others are less strict and suggest testing stages here and there.

As I said I am in no position to argue either case, all I hope is to learn something here and in the process -hopefully- also finish this amplifier without having to buy output transistors by the caseload.

And now I wonder which advice I should follow, test the whole thing as one or try some partial solutions?!?!

Uwe
 

kchriste

New Member
Forum Supporter
Since the fuse and both T6 & T7 blew on power up, the most likely problem is that the wiper of P2 is open or P2 is set for maximum resistance. The next likely thing would be that T3 is defective (open).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Othello said:
Well first of all I do thank you all for the attempts to help.

I do have half knowledge or less when it comes to electronics.
I can follow the explanations why some things work the way they do but if then they do not work properly it requires an altogether different level of knowledge to know where to tweak it.

The amplifier I mentioned here simply blew the fuse and the two output transistors when I switched it on. I have 5 working ones, this one does not work.

Obviously I am reluctant to simply put two new transistors in and switch it on again. I am trying to figure out why it did this and the safest way would be two build it in stages and 'inch' towards the complete amp. Nigel strongly argues that that is not possible, others are less strict and suggest testing stages here and there.

As I said I am in no position to argue either case, all I hope is to learn something here and in the process -hopefully- also finish this amplifier without having to buy output transistors by the caseload.

And now I wonder which advice I should follow, test the whole thing as one or try some partial solutions?!?!

I've been repairing amplifers professionally for 36+ years, follow the advice in my last post - never change just the output transistors, it's quite likely that they will blow again, change ALL the transistors in the DC chain, and check resistors and capacitors (and force bias to zero).

Completely changing the circuit to try and check each section separately doesn't achieve anything, what you're checking needs modifying so much it bears no resemblance to the real circuit, and when you restore and connect them all it's the interaction that makes it go BOOM!!!.
 
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