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Marantz 4230 P700 repair questions

daveyh

daveyh

Super Member
I am having problems with one of my Marantz 4230 amplifier boards. I recapped both boards and recapped the power supply. I have also replaced all the transistors on both boards. I used TIP31CG for the outputs. I have pulled and checked the diodes and resistors. I have not replaced (but have on order) the 0.5 3w resistors R759-R762 and though the seem to check out OK I will be replacing them when my order comes in from Mouser.

My problem is that the right channel keeps burning the R747 resistor. It runs for awhile and when I turn up the volume, POOF. Magic smoke. It runs fine and my DBT does not show a short but when I plug it into current that's when it happens. It also takes out fries the two TIP31CG outputs.

Any one have an idea as to what is going on?
 
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I assume you are running it in 2ch power mode, with the rear panel Power Mode switch in 30Wx2. (BTW, never, ever cycle this switch with the unit powered on)
I do all my quad testing with the power mode switch in 2 Channel mode, ie,12Wx4.
This separates the power amps so you have 4 individual conventional power amplifiers instead of 2 'bridged' power amps.
I don't understand why you would replace all the transistors, I find it unlikely that they were all defective. Were you able to find exact EIAJ replacement parts from a trusted source?

Before it self destructed, with no input signal and no speakers, in 12Wx4 mode, were you able set the bias on each amplifier (12Wx4 mode) properly? Did you check any DC voltages on the 'bad' amp and compare them to one of the 'good' amps?
It seems that the 'front-right' power amp is running away for some reason.
I seriously doubt that the half Ohm resistors will fix anything.

Tom
 
Thanks Tom. I am running it in 2ch power mode. I do know to never cycle this switch with the unit powered on. I have own several of these units before and work on them also. I wanted to replace all the transistors to bring the unit up to date. This unit had the same bad board in it when I acquired it. Since I have fixed several of these before I made a list of replacement parts for the boards I used. I always buy quality parts from a trusted source.

After I thought I fixed the unit I was able to set the voltages and it played for several weeks with no problems in all modes. However was moved and not sitting in the same place all the time.

I am thinking it is a adjustment trimmer or pot going bad and when I turn the volume up loud it is affecting the current flow. I pulled both trimmer pots on the affected channel and the 200K (I think it is R727) seems kinda weird when connected to my DMM and hand adjusting. Am I off base? This would cause run away would it not? If I were to replace these trimmers what might you suggest would be a good replacement.
 
If R727 opens, that would certainly affect the DC balance., I'm not sure what that would do as far as the parts destruction problem.
Many here use Bournes trim pots, I don't have part numbers for you, perhaps someone else reading this does.

Tom
 
Just trying to understand what could be causing this to burn components on this channel. It's like a power surge or something. It has all been rebuilt the same on all channels. Same type of components. All of the components I installed are in correctly. I have even pulled these and double checked their condition again before I reinstall the board. Any other suggestions on what I might look for before power up? I am missing something or it would not be happening.
 
After inspecting everything for shorts,solder bridges, backwards caps, improperly installed semiconductors, etc., I would test it extensively in 12Wx4 with four dummy loads to see which 'amp' is really causing the problem.
I agree, something is being overlooked, probably something simple.

Tom
 
Sometimes its just better to walk away and let it sit for a few weeks and clear your head. Revisit it later. Bad trimmer adjustment pot. I just replaced them all with new Bourns.
 
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