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Marantz 300DC Popping

J

jeff105

Active Member
I just got my 300DC amp out of the closet, hooked it up, played for a minute or two and then it popped through the speakers(cheapies for testing) .

Silence for a few seconds, then I hear the relay click, then I have sound again .
It does this repeatedly,plays,kicks out, plays,kicks out,etc,etc.

I just turned the volume down fairly low and let it go.

Now, it has been playing for about 2.5 hours without missing a beat and sounds great but the first 30 minutes I thought it was going to just quit.

The amp has not been hooked up in a year or more until today.

Is it possible the relay contacts were the culprit or is this likely to be a much deeper problem ?

It sure sounds good right now but this has me a little worried.

If I had a local tech to give it a good going over, I would happy to do that ASAP but my only option would be to ship this heavy little bast@%d off to someone.

It's a nice amp and I sure would hate to see it damaged.
 
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I have a 300DC as well. In fact I just had my restored, along with my 3650 and my 115 tuner. The 300DC is very very heavy, and does kick out a lot of heat. But boy does it ever sound incredible! It cost me a fortune to have it shipped across the continent.

As for your issues, I am not a tech and have no idea. Whatever you do however you must save her. I think the 300DC is the best marantz amp in the solid state arena.
 
The 300dcs are very nice, worth preserving. :yes:
[That's why I keep at least four of them around. :D]

There aren't many caps in that amp; about 18, if I remember right, and it isn't a terribly difficult design to work on, so if you haven't recapped it yet, that would be worth doing. IT might eliminate your pop-pop noise, especially if it sounded a bit like a helicopter or motorboat (definite signs of failing caps, which might have reformed enough while playing after being idle so long to then stop doing it, as you experienced).

Was it having trouble staying out of protection; i.e., was the sound cutting out momentarily, or was it just making noises, on top of the music?
 
I agree that it's worth fixing.:yes:

As far as the speaker pops goes, it would pop through the speakers then go into protection for just a couple of seconds an then come out and play fine.

It would play for a short while (several seconds or maybe a couple of minutes) and then pop, go in protection, come out, etc.

I just let it play and after a while it played for a few hours non stop without a problem.
Before this, it would only play for a few minutes at most before going into protection.

I will have to have a tech fix whatever it is.
I just worry about shipping the thing.

This is one of the few Marantz pieces that I didn't buy new but I have had it a good while.
 
Just an update here.

I thought I would power the amp up today and see how things go.

It plays perfect for roughly one hour, then goes into protection for just a bit but comes right back.

It's now playing and has been for a hour and a half or longer without a problem.

I wish I were a tech so I could understand how something with a problem can sound this good. If the caps are bad, why will it play this long without a hitch.

I would think the problem would get worse as things heat up but it seems to make it better.

Does anybody know if the plastic cover on the relay comes off ? I would really like to see what the contacts look like.

That's probably not my problem but it would be something I could do myself, just in case.
 
Your first action would be to take a measure of the DC voltage at the output of each channel.

Set your voltmeter in DC volt scale, with no input signal. I think you may have some DC voltage at the output.
 
Since you hear the relay click, this is not a problem in the contact area of the relay.

The low offset you measured is not large enough to trig-off the relay.

Besides a DVM, what test equipment do you have?
 
Thanks for the reply.

I don't have anything other than the DVM.

I just now turned the amp off. It has been on since before lunch(about 7 hours as of know) and it only acted up one time and that was about 40 minutes to a hour after powering on.
After that, it was smooth sailing.

Once I get the nerve, maybe I'll pack it up and ship it to a tech for a recap and a checkup.
 
secondslc, Thanks for the link.

I do need to let a good Marantz tech give it it a good going over, hopefully they will check or re-solder all the joints whenever they are recapping it.

These are very good sounding amps worthy of a restoration.:thmbsp:
 
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Thanks Ron,

I sent you a PM.

One thing I haven't mentioned is this. Once while standing next to the unit whenever it went into protection, I did noticed the right channel VU meter jump all the way to the right. (maxed out)

Another thing, the right gain control (right channel pot or variable resistor) has noticeably more wear the left.

Is it possible the pot could be wore to a degree that it would possibly arc :scratch2: or something ?

Not being a tech, please ignore the question if it is totally ludicrous.

By the way, I will hook up just one speaker to see if it is indeed just the right channel giving problems, as another AKer recommended.
 
Don't peg the meters:yikes: The coil is made of very fine wire and they can burn out.
The Gain control will lower not raise the amplifier gain. The amp is always on. If they lower the sound, they're working fine.
 
jblmar said:
Don't peg the meters:yikes: The coil is made of very fine wire and they can burn out.
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Click to expand...

I didn't peg the meter.

I noticed that the needle jumped quickly to the right just before it went into protect.

If I had I had blinked an eye, I would not have even known it.
 
There is a DC offset trim pot (R719) in this amp. I recommand that you clean this trim pot, and try to adjust the offset.

A frequent cause of the symptoms you reported is mismatched transistors in the differential stage.
 
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