Marantz MA500, only 50Hz output?

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Hi,
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
 
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 :)

Cheers,
Chris
 
Chris,

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.
 
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.
 
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.
 
SOLVED

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 ;)

Cheers,
Chris
 
They are advertised to run a 4 ohm speaker per mono block.

When you mono bridge them you use two of them. One drives the positive waveform of the signal and the other the negative.

Just because a amplifier is advertised to drive a 4 ohm load doesn't guarantee it will do it in bridge mode. In all reality I believe that each channel is actually driving a 4 ohm load in bridge mono. Please correct me here if I am mistaken.

According to Marantz the impedance needed when in mono bridge is 8 ohms. My experience with these amplifiers has proven that low impedance loads will result in the magic smoke being let loose.

Typically the impedance of that driver will dip even lower than the 4 ohm nominal impedance and cause the amplifier even more stress.

Don't get caught up in the numbers game. Under ideal conditions if an amplifier (stereo) or two mono blocks are designed correctly one might expect power to increase by a factor of 4 in mono bridge. In all reality it is usually around 3 times. If you have an amplifier that puts out 200 watts per channel and everything was perfect you would get around 800 watts in mono bridge. Now the difference in level between 200 watts and 800 watts would be 6db.

You need to ask yourself if the extra strain on everything is worth this little difference in level. I would suggest selling that JBL driver and purchasing a professional sound model which would more than likely be 8 ohms and of higher sensitivity. It sounds like that JBL subwoofer is of the car vintage.

Drop me an email and I will give you a copy of the Marantz service manual for the MA500.
 
Burnedfingers-

The speaker is the S120p whose specs are that it has "nominal impedance of 8 ohms". But when I put it on a meter, i get a reading of 12 ohms which quickly drops to 5.4 ohms. So I was guessing that it would need an amp that could handle 4 ohms because it will most likely drop down to 4 ohms. What do you think about that?

Robert
 
burnedfingers said:

Just because a amplifier is advertised to drive a 4 ohm load doesn't
guarantee it will do it in bridge mode. In all reality I believe that
each channel is actually driving a 4 ohm load in bridge mono.
Please correct me here if I am mistaken.

Hi,

Bridged each channel driving an 8 ohm load sees a 4 ohm load.

A 4ohm capable amplifier = an 8 ohm bridged capable amplifier.

To bridge into 4 ohms you need 2 ohm capability per channel.

:)/sreten.
 
sreten said:


Hi,

Bridged each channel driving an 8 ohm load sees a 4 ohm load.

A 4ohm capable amplifier = an 8 ohm bridged capable amplifier.

To bridge into 4 ohms you need 2 ohm capability per channel.

:)/sreten.

Okay, I understand that now. So, how does that effect driving a speaker whose nominal impedance is 8 ohms but whose impedeance drops to 5.4 ohms on a meter?

This is the same impedance for the woofers in JBL's L890/L880 towers that I own. The question here is whether the Marantz mono blocks will drive such a speaker when bridged.
 
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