My 300DC

thefragger

thefragger

Certified Crazy.
Okay, I took this beast apart and started to clean it; cleaned the controls and fired it up.

Dangnabbit. Only one channel puts out music (and neither of the meters work, but I suspect they're clogged with nicotine). Come to think of it, it was demoed to me on one channel-- thank God I only paid $100 for it.

So this beast looks kinda complex and I haven't seen this biasing design in the past. I do have the S/M but I just need the guidance so I don't mess it up any more.

There is no noise out of the dead channel, just silence. First thing I should do is test the output transistors? :scratch2:
 
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The outputs on the high power Marantz amps were fairly unreliable, so that would be a good place to start. You want to get all the outputs matched if you have to replace them. ONSemi makes good replacements.
 
Jailtime said:
The outputs on the high power Marantz amps were fairly unreliable, so that would be a good place to start. You want to get all the outputs matched if you have to replace them. ONSemi makes good replacements.
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Aw shoot. I'll pull the power sections again and test the transistors.

Thanks JT.
 
If it has a relay, you might want to take a look there too. Give it a few little taps with something non-conductive.
 
jblmar said:
That's a classic amp. Get it to a good tech for repair.
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If I had a hand full of thumbs and the cash to do so, I might consider this, but I don't find this advice helpful.

I don't mean to lash out, and I'm aware of what this amp is (I haven't touched it until recently after I bought it), but I feel that I'm more than qualified to track down any problem that amp might have, even though I don't offer my services on AK.

I think it's quicker to ask for any tips and pointers from someone who's worked on one before instead of methodically going through everything one system at a time (which it looks like I might end up doing when I find the time between my studies).


Philip.
 
Jailtime said:
The outputs on the high power Marantz amps were fairly unreliable, so that would be a good place to start. You want to get all the outputs matched if you have to replace them. ONSemi makes good replacements.
Click to expand...

The only output's that were unreliable were the ones in the model 500.
Once replaced like in mine, no problem's whasoever.

The later model's like the 170 and 300 used Toshiba output's and they were very good.

Comming to the point that one channel is dead:
Replacing the original ones with on-semi's wil give you a bias that is to high.
Adjusting with a potentiometer is not possible so you wil have to change out 2 resistor values to come to the recommended bias setting.

This works the same way for the 170DC.

The channel could be fried but'for the same it is not!

In these case's start at the beginning: the power supply and work your way up from there.

Cheers
 
SB Marantz said:
The only output's that were unreliable were the ones in the model 500.
Once replaced like in mine, no problem's whasoever.

The later model's like the 170 and 300 used Toshiba output's and they were very good.

Comming to the point that one channel is dead:
Replacing the original ones with on-semi's wil give you a bias that is to high.
Adjusting with a potentiometer is not possible so you wil have to change out 2 resistor values to come to the recommended bias setting.

This works the same way for the 170DC.

The channel could be fried but'for the same it is not!
Click to expand...

Very informative, thanks! Didn't know that the transistors are Toshiba made- they're got the Marantz name on 'em.


In these case's start at the beginning: the power supply and work your way up from there.

Cheers
Click to expand...

That's the plan! I just need to find some time :)


Philip.
 
thefragger said:
If I had a hand full of thumbs and the cash to do so, I might consider this, but I don't find this advice helpful.

I don't mean to lash out, and I'm aware of what this amp is (I haven't touched it until recently after I bought it), but I feel that I'm more than qualified to track down any problem that amp might have, even though I don't offer my services on AK.[QUOTE

I think it's quicker to ask for any tips and pointers from someone who's worked on one before instead of methodically going through everything one system at a time (which it looks like I might end up doing when I find the time between my studies).





That's the problem with email type responses. Misunderstandings may result.
I wasn't suggesting or commenting on your ability as I have no way of doing so nor do I make judgements on people in general. I was advising that this is a classic amp and not one to learn on. Repair this one the right way or have it repaired correctly and you'll have years of enjoyment.

Ron
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Philip,

You wrote; ''This amp was fine before putting it under the knife?''

You dit nothing more then cleaning the inside?
Look at the input wires!
Big chance the problem is right there!
Or other broken wiring....

Allso messure DC-offset on the speaker terminal on the side that does not work.
If you messure VDC there then my way of thinking is correct.
DC-offset must be within +- 50 mV's but with the blue pot on the driver board you can set it with a little effort to +- 5 mV's.

Cheers
 
SB Marantz said:
Philip,

You wrote; ''This amp was fine before putting it under the knife?''

...

Cheers
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LOL SB, that's in reference to a Sony amp I was working on once- the power supply lived in an aluminum box that the box was screwed to the chassis and that would be the ground connection, so when I tried to run some tests on the amp with the box out on my bench nothing worked because the ground was floating. :nono:

I will check the DC offset and the continuity everywhere. Did Marantz use solid core wires or stranded?


Philip.
 
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