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Yippee and the 300dc

Arkay

Arkay

Lunatic Member
Yippee! A place for Marantz! :thmbsp: :D Although I had mixed feelings about moving Marantz to its own forum, now that it has happened, I guess I'm glad to see it. I'd hate to see umpteen forums springing up, one for each brand, but Marantz is probably popular enough to warrant it (probably on a par with Pioneer and Sansui).

Just to keep this thread on topic, I plugged in my new 300dc amp for the first time today [bought it untested/as-is from a wholesale place]. Both meters lit up beautifully! :banana: After browsing here a bit, I'm going to check dc offset and assuming it's ok, I'll hook it up and give it a first listening in an hour or two. Will come back and add comments here afterwards. Enjoying the suspense/anticipation in the meantime. :D
 
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Checked the dc offset...

I just checked the offset on the 300dc. Seems to take it's tiime warming up and semi-stabilizing, probably not too surprising since (1) it has large capacitors and (2) this unit may have sat for a while unused in humid conditions (don't know how long, but at least a couple of weeks). It took 20-30 minutes to stop steadily climbing in mV reading, and settle into a fluctuating/unstable pattern that more-or-less centered around 0,95 to 1.005 volts (yes, that's a volt, not mV) on the left side and about 1.1 V on the right.

So the good news is that both right and left channels are reasonably close in value; the bad news is that they are putting out about a volt on each side, and that value is not too stable. Maybe the instability is OK, as it is within a reasonably narrow range and the thing may be warming up and driving off moisture... and maybe the 1 volt reflects the long-term deterioration of aging caps/components, and can be simply adjusted for, for the time being?

This raises a few questions in my mind:
(1) Is it OK to use this thing with speakers, if it is putting out a full volt dc, or should I wait until that is fixed before connecting anything to the outputs? Seems to me that 1 volt is WAY too much for safety/comfort, but I just want to confirm that.
(2) Is it OK for the time being to simply adjust the offset? If so, where do I make the adjustments on the 300dc; i.e., where are the correct variable pots located? (I don't yet have the service manual).
(3) Other than re-capping it, which I plan to do eventually, anyway, is there anything else with this amp that I should check/pay attention to, relating to this DC output? I will go through it more carefully after I get the manual, but would like to be able to at least give it a listen, first.

Thanks for any advice. :D
 
Arkay: Excellent find, the 300DC is well worth the effort to restore. I wish I could help you with the voltage issue but when you get it corrected, the sound is fabulous. Go for the trifecta and get the 2130 and 3650. Sam.
 
1 volt is way too much offset. If you're handy with electronics, get a schematic and go for it. If not, you should probably find a tech. BTW, when the output section of one of these goes down, it takes a lot of components out, so an ounce of prevention here is definitely worth a pound of cure.
 
SoCal, Thanks. I'm definitely "on the lookout" for the accompanying pre and tuner.

Tarior, I will definitely do whatever is needed to restore this properly. I've already fallen in love with the looks of it, and based on what I've read others who own this amp say about it, plus its apparent rarity, I suspect it is the last one they'll be trying to pry from my cold, dead fingers! :yes: :D

An AKer has already offered to get me scans of the manuals for it, and I think I'll be able to fix it myself, perhaps with some help from the kind experts here if I run into any trouble. While I enjoy working on electronic gear, I don't doubt you could kill yourself getting careless with this particular beast, so I'll definitely proceed with patience and caution, and do it right! :yes:

Any further comments/advice on the best approach to dealing with a 1 volt offset on this, would still be appreciated. Otherwise, I'll start with checking components like resistors and re-capping it (at least the smaller electrolytics), then working through it properly once I get the manuals.
 
Some eye candy... The 3650 has two pre-outs so I can control both 300DC's.
 

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Put the top cover back on after ajusting. Air flow over the heat sink will cause the reading to vary.
After setting R719, my 300DC measures about 10mV per side. That's after 2 hours. After 1 hour, it was about 15mV per side. 1 volt is out of spec. Something is wrong.

Ron
 
1 volt at the output won't harm your spakers, but the amp is out of spec and needs attention. Still, I wouldn't connect speakers and listen to music even with 1 volt. The spec is for 50mV. That's max! Anything over needs looking into.
If you have a DMM, check the bias as well. A service manual is a must.

Ron
 
Thanks, guys. I had the top on, but it was sitting not too far from a fan, which might account for some of the fluctuation. Still, I knew 1 volt was not good. I'll get the manual (and start checking obvious and easily-checked components, in the meantime) and go over it properly.

Have a lead on another Marantz amp with two meters, but I don't know yet which model it is. Keeping my fingers crossed that it's another 300dc (or better...) so I can go from 1/4th to 1/2 of the way towards replicating SoCal Sam's "eye candy". [Great stuff, there! :thmbsp: ]

I'm also thinking about the possibilities for a quad system using two 300dcs to amp the four channels. :scratch2: :D
 
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