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BeoMaster 4400 Power Fault/DC on Output

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christianc
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christianc posted on Wed, Jul 22 2020 2:57 AM

Hi there, 

 

I have a BeoMaster 4400 which recently stopped working. I've noticed the Fault Relay activates when I turn the unit on and I am getting no sound to either speaker because of this. I have a bit of electronics knowledge, however not enough to be able to diagnose and fix this issue. 

From browsing these forums in the past, I turned to replacing a few of the Output Transistors. I replaced OIC 200/201/202 and 203. I found that when I did this I would get a small amount of sounding coming from the speakers when I turned the unit off. 

Since then, I have taken it to a Hi-Fi repair place which told me the OutPut Transistors needed to be replaced, however, when they replaced these transistors, one of them was blowing so they gave me the unit back unfixed and I feel like they might have done something else without admitting so.

 

Since then, I have tried replacing the Output Transistors they left out of the BeoMaster 4400 when they gave it back to me. OIC 201/202 and 203 were missing. I replaced these only to find that OIC 202 will blow straight away when I turn the unit on. I'm replacing these with the required TIP141/146.

 

Could it possibly be that one of the MPSA54 transistors is defective? I have read about these doing so online. 

 

Please help, I would love to get this amazing unit working again, and it will make more than me happy!

Thank-you

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Beo_Jean
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Is the unit was serviced before that?

I mean full checkup and capacitors replaced?

It can be so many things but the relay should kick in before blowing transistors IMO

You will need the proper tools to diagnose this problem.

sonavor
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Don't keep changing the transistors. If they are blowing then something is causing them to blow. You need to find the root cause. The Beomaster 4400 is not an easy amplifier for a novice to work on either. It's not impossible either but you need to do a lot of homework and, as Beo_Jean said, you need proper tools.

Keep in mind that this receiver was the top of the line receiver by B&O back in its day. The Beomaster 4400 owner was not intended to open up the inside and work on it. The technicians that worked on them were trained and they had good tools. 

There can be a lot of reasons the Beomaster is failing. If it hasn't been restored with new electrolytic capacitors then that needs to be done before you do anything else. If transistors blew then there could be a short somewhere. The circuit board needs to be checked for good solder joints and for any burned traces. 

I have just started in on another Beomaster 4400 restoration myself.  I also have links to previous Beomaster 4400 restorations. Each one can be its own adventure. They can be fairly smooth going to a basket of trouble. It just depends on the state of the Beomaster when you begin the restoration. There are also a few Beomaster 4400 restoration examples here on the Beoworld Workbench.

There are a bunch of people on this forum that can help but you need to post photos of the problems for people to see what is going on. You can't fix one of these amplifiers blind...at least I can't.

-sonavor

christianc
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@Beo_Jean

Before that, the unit had 8 of the electrolytic caps replaced by an authorized B&O repairer. I'm not 100% which ones were replaced but I should be able to tell by inspections. I did however notice that 2 of the caps were bulging and upon investigation I found these were in backwards. I have since replaced those in the correct orientation without any issues so far.

I definitely agree, I have a bit of electronic experience due to a job I worked at servicing microphones/audio interfaces but this is a lot more advanced for me.

 

@sonavor, I have only put them in once since I got the unit back from the Hi-Fi repair place. I don't plan on changing them again until I can get to the root of the issue! 

I will check your link now :) It's a beautiful amplifier, hence why I want to get it working again. 

I will also take photos of the amp and post back soon with those :)

christianc
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I have attached some photos below:

 

 

EDIT:

 

 

I have also previously replaced the main reservoir capacitors as they were much below their correct values. I have tested the unit to make sure I'm getting 35V from the transformer. 

 

I've noticed that some of the polyester capacitors also need replacing. Any more ideas would be greatly appreciated! :) 

chartz
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chartz replied on Thu, Jul 23 2020 7:10 AM
Hi,

The output transistors seldom blow, my bet would be a small driver ou predriver MPSU transistor.

Leave polyester caps well alone. Those last forever. I have never replaced any in 40 years.

Jacques

christianc
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Thanks chartz! I appreciate it. Also my apologies, I have updated the images.

 

Okay, I won't touch those. I had to bridge one of the connectors from a electrolytic capacitor I replaced as the circuit railing had come away from the board. 

 

That was my thinking. Maybe the MPSH54? I've read on another tread that was causing problems to the output transistor. Thank-you :)

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