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Old 25th January 2013, 01:06 AM   #121
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Hi,
I do not know if you already did it but I would short the RT1 after the switch. You mentioned that the sound it is like cracking and RT1 in the way it works it decreasing the resistance as the RT1 get hotter. Just short it and turn on the amplifier and see if the problem goes away. Re install it after the test.
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Old 25th January 2013, 01:38 AM   #122
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Nigel

I understand you weren't saying to toss all the transistors. I was just needing clarification and I got my answer. Thank you!

Speaking of mismatched transistors. I asked a question in an earlier post and never got an answer from anyone.
Q18 was replaced with a TIP31C which is an RF transistor.
Q19 was replaced with a TIP32C which is a bipolar transistor.
I know these are the correct NPN and PNP configuration respectively, but one is an RF and the other is a Bipolar. Is that something to be concerned about?

Thanks for your time.

BR
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Old 25th January 2013, 01:43 AM   #123
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Quote:
Originally Posted by badraven View Post
Nigel

Speaking of mismatched transistors. I asked a question in an earlier post and never got an answer from anyone.
Q18 was replaced with a TIP31C which is an RF transistor.
Q19 was replaced with a TIP32C which is a bipolar transistor.
I know these are the correct NPN and PNP configuration respectively, but one is an RF and the other is a Bipolar. Is that something to be concerned about?

Thanks for your time.

BR
This is a problem when replacing obsolete transistors.
The RF one should work ok just look out for oscillation on the output indicating instability.
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Old 25th January 2013, 01:44 AM   #124
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Default tauro0221

Update: Ok, I gotcha! You're talking about the (Cap I think it is) right after the fuse that conditions the power or something. I looked that up a couple weeks ago to see what it does. Ok. I will try that. I hope shorting that isn't going to cause problems. Would it be better to remove it and short the contact points for this test?

Pardon me if this is a "stupid" question but what is the RT1?
Please explain how to do this and I will give it a try.

Thanks!

BR

Quote:
Originally Posted by tauro0221 View Post
Hi,
I do not know if you already did it but I would short the RT1 after the switch. You mentioned that the sound it is like cracking and RT1 in the way it works it decreasing the resistance as the RT1 get hotter. Just short it and turn on the amplifier and see if the problem goes away. Re install it after the test.
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Last edited by badraven; 25th January 2013 at 01:48 AM. Reason: Clarification
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Old 25th January 2013, 01:46 AM   #125
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Quote:
Originally Posted by badraven View Post
Pardon me if this is a "stupid" question but what is the RT1?
Please explain how to do this and I will give it a try.

Thanks!

BR
Rt1 is probably a thermistor.
It has high resistance at power up and slows down inrush of current.
Once warmed up its resistance lowers.
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Old 25th January 2013, 01:50 AM   #126
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Default Nigel

Quote:
Originally Posted by nigelwright7557 View Post
This is a problem when replacing obsolete transistors.
The RF one should work ok just look out for oscillation on the output indicating instability.
Oscillation is something I can hear or something I measure to detect?
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Old 25th January 2013, 01:51 AM   #127
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Originally Posted by badraven View Post
Oscillation is something I can hear or something I measure to detect?
You usually get high frequency oscillation if there is a problem.
You will need a scope to see it.
If it gets bad it can cook your amplifier.
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Old 25th January 2013, 02:00 AM   #128
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Ok I looked it up again. The NTC100-15 is a conditioner or Thermistor. Kind of like a shock absorber for the PCB components as power comes rushing in. As you said, it slows the in rush of current.

I don't have access to an Oscilloscope to I guess I am out of luck on that one.
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Old 25th January 2013, 02:37 AM   #129
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I shorted RT1 and tried it. Still distorted or fuzzy on the clean channel and still overheats after a few minutes.
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Old 25th January 2013, 12:20 PM   #130
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Quote:
Originally Posted by badraven View Post
I understand you weren't saying to toss all the transistors.
As someone who repairs amps professionally I would definitely say 'toss the old transistors', often one of the earlier stages can be intermittent, and blow the rest. It's good practice to replace all transistors in the DC chain, and gives a MUCH better chance of a lasting repair.

Just replacing the outputs (and even drivers) often leads to a swift failure again.

Quote:

Speaking of mismatched transistors. I asked a question in an earlier post and never got an answer from anyone.
Q18 was replaced with a TIP31C which is an RF transistor.
Q19 was replaced with a TIP32C which is a bipolar transistor.
I know these are the correct NPN and PNP configuration respectively, but one is an RF and the other is a Bipolar. Is that something to be concerned about?
A TIP31C isn't an RF transistor, it's a normal NPN medium power audio transistor, and complimentary to the PNP TIP32C. Personally I only stock the TIP41/42C which are higher rated versions of the 31/32, and commonly use them for drivers in power amps (or even outputs in smaller amps).

So TIP31/32 are perfectly fine for your situation.

DC coupled amps are VERY difficult to repair, you need to understand how they work and why they were designed that way, and even then a slight mistake blows everything again
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