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kaspro 5th May 2007 10:52 PM

Marantz MA500, only 50Hz output?
first of all congratulations for this amazing forum- what a great resource! I have been here a couple of times but without a real project... now I thought to give it a go :)
I have two MA500's, and since finally my speakers are built they get some use...
Here's my problem:
one of the amps is turns on fine (relay clicks), but I only hear a 50Hz on the output when connecting to the speakers. The speakers itself are fine, as tested with the other amp. I did test the speakers a couple of times with the amp, and had 1-2 times wrong connected crossovers... but if so the protection always jumped in(at least by the sound of it :) ).
Does anyone know this amp and can help me out with some tests/hints? I opened the amp, but nothing looks/smells fatal. I don't have an oscilloscope just a multimeter, and I'm not an electronic geek- know the basics, can solder... thats it :)

Thanks a lot in advance,
Cheers Chris

Burnedfingers 6th May 2007 03:39 PM

Both amplifiers don't work?

kaspro 7th May 2007 06:50 AM

No, only one doesn't. After looking again into the whole configuration, it seemed like the input was short-circuited... at least I get a signal now. But once the speakers use up a bit more power, the amp goes into protection mode now :(
I didn't test the impedance yet, but double- checked the crossovers which looked fine... I'll keep you posted- thanks for the reply :)


Burnedfingers 7th May 2007 10:47 AM


I have repaired a number of the Marantz MA500 mono block amplifiers. I have it to a point where I just go in and replace a number of capacitors and this keeps from having to open the amplifier up every 6 months to replace a capacitor in the protection circuit or in the signal path. The Marantz has a crappy circuit board made overseas so extreme care must be taken when working on it. When made the leads are bent over and hard to get out without damaging the foil on the PC board. If you are not capable of doing this I would suggest taking it to a repair facility and having them do it.

Replace C705,C701, C710, CN02, CN03, CN05 and that should take care of your problem. These caps effect gain and or the protection circuit. These are problems the MA500 is famous for.

kaspro 8th May 2007 03:28 AM

would this explain the dropout (protection switching) at higher output?
I'm going to measure my speakers tonight, if they're ok, I'll go for it... Thanks heaps for that quick solution!!! (hopefully :) )

Burnedfingers 8th May 2007 10:36 AM

Replacing these caps has taken care of the problems I have seen with respect to gain and to triggering the protection. Your looking at less than $10 worth of parts here and if you are capable of replacing them you will save yourself several trips to the repair shop in the future. The CN05 part is a 2mfd/50 volt cap. Replace this with a 2/100volt cap.

There is one that I forgot and that is CN01 and that is 22/16v.
Replace that one also.

Burnedfingers 8th May 2007 10:45 AM


would this explain the dropout (protection switching) at higher output?
Yes, I believe it will.

To be on the safe side switch the speakers and see if the amp that is acting up now continues to act up and I'm sure it will. By doing this you will know if there is a problem with your speaker load. I don't think there is a load problem.

The caps with the CN in front of them are in the protection stage of the amplifier and their starting to have problems and need replaced. Thus the drop out as the volume increases.

The caps with the C in front of them can affect gain of the amplifier and as they go bad the gain starts to drop or the amplifier doesn't amplifly hardly anything.

These mono block MA500 amplifiers have problems in these two areas so I would suggest that you change the caps I mentioned so there aren't any problems in the future.

kaspro 9th May 2007 06:41 AM

Yes!! You're my Hero! :D
It works perfectly- it even seems it's 2db louder than the other one... so will do the other, too...
Finally, I can enjoy my just- built speakers... which sound amazing ! :cool:
Thanks so much burnedfingers, you saved my month ;)


Burnedfingers 9th May 2007 10:32 AM


Yes, do the other one so that the gain will be the same and so that you don't have a similar problem with it. Did you change all the caps I listed?

kaspro 9th May 2007 11:06 AM

didn't do the CN01 yet, and the 2.2uF is a 63V... and the 33uF is still the same due to unavailability... didn't get the message early enough :)
Will get the rest of the parts tomorrow, and give the other amp a freshup, too!

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