amplifier biasing help

[muddasirwaheedmalik]

hi
i have replaced TIP35/36C blow out put stage trasistors from Focal amp and replaced with 2SC5200 pairs.

Now there is a REFERENCE POINT and BIAS CHECK point. The voltage across all four channels is 2mV to 80 mv.

VCE= 30.6 volt for all

VBE= 0.35V to 0.55 Volt ?

Any guidance how to adjust bias before i blow them out again.

 

[AndrewT]

The question is back to front with the action.

You ask before you power it up !!!

 

[muddasirwaheedmalik]

No. Actually its second replacement, the yesterday replacement were all well before i put a bridged load on it.

so yes i am asking before powering it up before second time. But i am using series bulbs with the power supply just in case.

 

[Chris Daly]

hi
i have replaced TIP35/36C blow out put stage trasistors from Focal amp and replaced with 2SC5200 pairs.

Now there is a REFERENCE POINT and BIAS CHECK point. The voltage across all four channels is 2mV to 80 mv.

VCE= 30.6 volt for all

VBE= 0.35V to 0.55 Volt ?

Any guidance how to adjust bias before i blow them out again.

TIP 35 is an NPN whereas TIP36 is PNP inferring a complementary pair. 2SC5200 is NPN, It would suggest to me that you need a PNP to match the replacement of the TIP36.The correct complement for the 2SC5200 is a 2SA1943
Cheers / Chris

 

[muddasirwaheedmalik]

just forgot to mention though in my earlier post. yes indeed it 2SA5200 was matched with 2SA1943, and tried to match within 5% of VBe, not sure of hFE though.

Biasing ? VBE ? HELP?

 

[Chris Daly]

just forgot to mention though in my earlier post. yes indeed it 2SA5200 was matched with 2SA1943, and tried to match within 5% of VBe, not sure of hFE though.

Biasing ? VBE ? HELP?

Data sheet here: http://www.fairchildsemi.com/ds/FJ/FJL4315.pdf
Cheers / Chris

 

[DF96]

Have you tried circuit fault-finding to discover why they blew?

 

[muddasirwaheedmalik]

accidental short circuit at output while trying to put an OP transformer. :-(

 

[DF96]

OK, so there is not a fault to trace unless the output transistors took something else with them when they died.

 

[nigelwright7557]

No. Actually its second replacement, the yesterday replacement were all well before i put a bridged load on it.

A bridged load needs to be twice the none bridged load.

 

[AndrewT]

Hi, Nigel,
nice to see confirmation.
Any chance you could go over and explain that to the other Members that cannot or will not grasp that.

I gave up trying different explanations of why their closed in thinking was wrong.

 

[muddasirwaheedmalik]

nigel my question is not about the load in bridge load its about biasing after changing the out put transistors.

I dont think anything else is destroyed as i powered up the amp through a series bulb resistor, it has turned on and with small signal input all the channels are working fine without much current draw to indicate a shortage or overheating,

But how do i check whats the ideal bias current ? at what value should i adjust the bias pots. ?