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Old 19th February 2013, 12:49 AM   #231
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Well this is depressing.
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Old 19th February 2013, 12:55 AM   #232
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JMFahey actually what you say about removing everything to the right of C40 makes sense since the drive and such is all created in the preamp on this amp. So are you suggesting I build a power amp like you describe or are you suggesting I purchase a power amp? Do you have something particular in mind? Thanks!
BR
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Old 19th February 2013, 12:56 AM   #233
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I'm not an electronic pro here, so I can only sit back and watch and learn like you. I have knowledge about certain amps as I've dealt with them before, but this isn't one of them.

I'm sure you will get to the bottom of this. You started with a busted amp, broke it further blowing fuses and all, then over the course of a hundred posts were guided into getting the thing (almost) working. You are close to finishing.



if you go the route and get a seperate power amp, may I suggest a Class D board?

Something like this.

http://www.parts-express.com/pe/show...number=320-303

You can just use one channel.

Last edited by ouimetnick; 19th February 2013 at 12:59 AM.
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Old 19th February 2013, 01:05 AM   #234
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If the heat-sink is getting to hot then the output transistor are passing too much current.
The first few stages of the amp are about voltage amplification so shouldn't affect it.
Then the bias stage gives the correct bias voltage to the output transistors.

You could try changing r95 and r96 to 3k3 and see if the problem goes away.
This would decrease the bias voltage so you might get some crossover distortion but it would prove where the problem is or isn't.
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Old 19th February 2013, 01:10 AM   #235
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Default Nigel

I have been sitting here for the last little bit thinking the same thing. I thought, if I change the right reisistor(s) I could cool the output transistors off and eve if it were to drop my power who cares, this amp has plenty of power. I will give this a try. Thanks!



Quote:
Originally Posted by nigelwright7557 View Post
If the heat-sink is getting to hot then the output transistor are passing too much current.
The first few stages of the amp are about voltage amplification so shouldn't affect it.
Then the bias stage gives the correct bias voltage to the output transistors.

You could try changing r95 and r96 to 3k3 and see if the problem goes away.
This would decrease the bias voltage so you might get some crossover distortion but it would prove where the problem is or isn't.
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Last edited by badraven; 19th February 2013 at 01:12 AM.
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Old 19th February 2013, 01:14 AM   #236
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Default ouimetnick

Thanks for the advice. If I can't correct this heating problem soon I think I will try this. Thanks!

BR

Quote:
Originally Posted by ouimetnick View Post
I'm not an electronic pro here, so I can only sit back and watch and learn like you. I have knowledge about certain amps as I've dealt with them before, but this isn't one of them.

I'm sure you will get to the bottom of this. You started with a busted amp, broke it further blowing fuses and all, then over the course of a hundred posts were guided into getting the thing (almost) working. You are close to finishing.



if you go the route and get a seperate power amp, may I suggest a Class D board?

Something like this.

2x100W TDA7498 Class-D Amplifier Board 320-303

You can just use one channel.
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Old 19th February 2013, 02:34 AM   #237
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Just for kicks, connect the Power amp out to a (borrowed) amp similar to the one suggested by Chris in post #229 and hook it to your speakers.
If you like what you hear ...... now you know.

As of adding a new Power Amp PCB, for practical reasons please avoid Class D, ChipAmps, Class A, Zen, etc.
We are not "upgrading" or "modernizing" anything here, we are backtracking to a classic, simple, *well* proven design to be able to go onstage with this amp.
The least "surprises", the best.
Said new PCB should be able to supply around 100W into 4 ohms, when fed +/- 42V , to be able to use the current PSU.
Here simplest is best.
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Old 19th February 2013, 04:04 AM   #238
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Have u checked C40 for leakage or short.

Sometimes, a diode shows good when cold tested but does not work/opens in the circuit.
Check voltage across inputs of each op amp.
You can also check for forward voltages of diodes including BE junctions with power ON.

Gajanan Phadte

Last edited by gmphadte; 19th February 2013 at 04:15 AM. Reason: added content
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Old 19th February 2013, 04:39 AM   #239
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Q16 and Q17 will not show the forward BE voltage as they r protection devices.

Gajanan Phadte
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Old 19th February 2013, 09:16 AM   #240
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Quote:
Originally Posted by JMFahey View Post
Just for kicks, connect the Power amp out to a (borrowed) amp similar to the one suggested by Chris in post #229 and hook it to your speakers.
If you like what you hear ...... now you know.
PREamp out!!

Using the power amp output in its current faulty state could destroy any subsequent equipment.

I think that's what you meant, but I feel we should be clear on this point.

Chris

PS - to the OP, have a look on eBay for Samson Servo. There's a few bargains to be had, if you can get one to fit nicely in the cabinet.

Last edited by chris661; 19th February 2013 at 09:18 AM.
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