AB amplifier help please

Original Burnedfingers · 14th October 2006, 11:06 AM

I have a AB Precedent series model 1550 that I need to score a schematic for. I have looked on the AB site in their archives with zero luck. They make no mention of this model. Can anyone help me here?

Its about 10 years old according to the date code on the output transistors. They are showing a late 95 date.

Original Burnedfingers · 14th October 2006, 11:51 AM

Some pictures

Under the model number it says Logic Gated Output

Original Burnedfingers · 14th October 2006, 11:53 AM

and another

Original Burnedfingers · 14th October 2006, 11:55 AM

picture of damage

edl · 14th October 2006, 01:36 PM

Hi,

What's the problem with the amp?
Maybe there is no need to have the schematic to repair it.

Regards,

Original Burnedfingers · 14th October 2006, 02:30 PM

The amp is from a local grade school. Unfortunately they had someone who thought they were knowledgeable in audio that tried to drive a dead short on channel two of the AB amplifier. The result as one can imagine was the downfall of ch two. To make matters worse they blew the fuse which was a 10 Amp and substituted a 15 amp in its place and blew it also causing more damage.

It only gets worse from there. Their audio person not understanding the situation paralled the 1st row of delay speakers which were on channel one of the blown amp to main cluster amp overloading it.

This is a small system having a main cluster up front on one amp. The second and third rows of speakers were delayed and on channels number one and number two of the AB amp.

In addition someone fed the output of the main cluster amp to the line level input of the delays and blew them all to hell.

I guess a schematic would be a good thing considering the amount of burned up parts on the board and me not being familiar with this model of AB amplifier.

I'm open to any suggestions...

Original Burnedfingers · 14th October 2006, 07:56 PM

Damage to the amp so far seems to be the bias transistor(2n4920) , one MJ21194 output and two 22 ohm 1/2 watt resistors.

The output stage is 3 pair of MJ21193's and 3 pair MJ21194's. Since one blew I feel they all should be replaced.

Any thoughts?

anatech · 14th October 2006, 08:10 PM

Hi Joe,
As usual, I think you are right. Replace the drivers too.

You have some work ahead of you. Are you sure you want to warranty this equipment with "that guy" operating it?

-Chris

Original Burnedfingers · 14th October 2006, 08:35 PM

Hi Chris,

I forgot to mention replacing the drivers but you knew exactly how I think. They are on order also.

I would feel better if I just had the schematic for the amplifier. I could go thru it and figure any additional problems that might show up as a result of the damage. I've never seen workmanship like this amp has

tacked on parts that were an after thought for sure.

I have a feeling there will be an additional charge to cover my butt against the uneducated operator. I will warranty my work as usual but like I said ...there will be a little extra in the bill.

anatech · 14th October 2006, 09:17 PM

Hi Joe,

Quote:

I would feel better if I just had the schematic for the amplifier. I could go thru it and figure any additional problems that might show up as a result of the damage. I've never seen workmanship like this amp has tacked on parts that were an after thought for sure.

I know what you mean.

Quote:

I have a feeling there will be an additional charge to cover my butt against the uneducated operator. I will warranty my work as usual but like I said ...there will be a little extra in the bill.

You can't win some days. Extra charges make you feel better, and soften the blow.

-Chris

14th October 2006, 11:06 AM	#1
Original Burnedfingers diyAudio Member Join Date: Nov 2002	AB amplifier help please I have a AB Precedent series model 1550 that I need to score a schematic for. I have looked on the AB site in their archives with zero luck. They make no mention of this model. Can anyone help me here? Its about 10 years old according to the date code on the output transistors. They are showing a late 95 date.

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14th October 2006, 01:36 PM	#5
edl diyAudio Member Join Date: Oct 2004	Hi, What's the problem with the amp? Maybe there is no need to have the schematic to repair it. Regards,

14th October 2006, 02:30 PM	#6
Original Burnedfingers diyAudio Member Join Date: Nov 2002	The amp is from a local grade school. Unfortunately they had someone who thought they were knowledgeable in audio that tried to drive a dead short on channel two of the AB amplifier. The result as one can imagine was the downfall of ch two. To make matters worse they blew the fuse which was a 10 Amp and substituted a 15 amp in its place and blew it also causing more damage. It only gets worse from there. Their audio person not understanding the situation paralled the 1st row of delay speakers which were on channel one of the blown amp to main cluster amp overloading it. This is a small system having a main cluster up front on one amp. The second and third rows of speakers were delayed and on channels number one and number two of the AB amp. In addition someone fed the output of the main cluster amp to the line level input of the delays and blew them all to hell. I guess a schematic would be a good thing considering the amount of burned up parts on the board and me not being familiar with this model of AB amplifier. I'm open to any suggestions...

14th October 2006, 07:56 PM	#7
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Damage to the amp so far seems to be the bias transistor(2n4920) , one MJ21194 output and two 22 ohm 1/2 watt resistors. The output stage is 3 pair of MJ21193's and 3 pair MJ21194's. Since one blew I feel they all should be replaced. Any thoughts?

14th October 2006, 08:10 PM	#8
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Joe, As usual, I think you are right. Replace the drivers too. You have some work ahead of you. Are you sure you want to warranty this equipment with "that guy" operating it? -Chris

14th October 2006, 08:35 PM	#9
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Hi Chris, I forgot to mention replacing the drivers but you knew exactly how I think. They are on order also. I would feel better if I just had the schematic for the amplifier. I could go thru it and figure any additional problems that might show up as a result of the damage. I've never seen workmanship like this amp has tacked on parts that were an after thought for sure. I have a feeling there will be an additional charge to cover my butt against the uneducated operator. I will warranty my work as usual but like I said ...there will be a little extra in the bill.

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