Marantz 300DC Problem

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I'm new in the forum and maybe someone here could give me a lead on what the problem could be.
One of the channels is developing a DC Offset of +1.5 Volts. I opened it and looked for the DC offset pot but the best that I could do is bring it down to 0.5 Volts.
This behavior happens after about 2 minutes after I turn it on.

Any help would be appreciated.

Ricardo.
 
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Hi All,
Wow, hold on now. This is a great amp, I fix them and am using one in my main system. Don't just start changing stuff yet.

Firstly, the voltage amp is run off a couple regulators + / - 70 VDC. Check that first. Each channel is separate from the other. You may find some parts on those boards to be overheated. Clean the connectors while you are there.

Second, each main amp comes off. There are many parts that run very warm. Again, parts may have overheated. Certainly solder connections. Repair this amp carefully and it will reward you. If you make a mess or use cheap parts you will ruin a really nice amp. When changing transistors, match them (I am not kidding, it makes a difference here). The diff pair is a dual J FET. They are normally fine. The grease on the outputs need replacing by now. Use fresh grease and new mica insulators.

Please PM me if you need more help. I could not attach the schematic, too big.

-Chris
 
Hi Chris,
Thanks for your help.
Given my electronics limitations and scared to death to take the Amp. To a local shop that might end up solving the problem temporarily and blow something else later on for using the incorrect component, I spent a good time yesterday night trying to figure which component was causing this problem.
It seems that temperature is affecting a pair of transistors. I have the schematic and they are Q707 and Q708. On the good channel they are shrink wrapped and heat sink compounded. On the channel that is presenting DC voltage they are not. So I decided to carefully spray contact cleaner to cool one of them down and the DC came down. Doing the same procedure to the other one the effect was opposite, DC came up. So I proceeded to apply heat sink compound between them shrink wrapped and readjusted the offset. It stayed stable for longer than an hour but not as stable as the good channel. After that, DC voltage came up again. I re sprayed and again it came down.
My question is, could those transistors be the only problem? that they are shot and temperature is altering their values more than it should?
Thanks for your help Chris!
Regards,

Ricardo.
 
The problem persist. Any idea?

Besides getting the amp to someone who knows how to really fix it? ;)

I do not have the schematic in front of me, but if this input stage is composed of bipolar devices, then there will be an electrolytic capacitor in the feedback loop, connected to the base of the 'right-hand' transistor through a resistor (of about 1.5K to 6K or thereabouts). If this cap goes leaky, then excessive DC offset is the result.

I've also had problems with the Marantz pots in the past..but these are some pretty decent units, and all I had to do was drench it good with DeOxit while I worked it around a bit, and they about always start working fine.

Lastly, since you say that one side has diff-pairs that are thermally coupled (like they were when they left the factory), and the other doesn't (which strongly suggests that they were replaced), it is possible that the transistors are just horribly mis-matched. Replacing them with a matched (for gain) pair should get the offset close enough to where you can zero it out with the pot.
 
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Hi cayo71,
It looks looks those parts are not matched. It is essential that the proper parts be used and matching of those parts occurs. This type of work is well beyond most repair shops, only because they don't understand the reasons. As such, they normally have not invested in proper equipment (they think they have) nor do they have a proper parts stock.
When replaced, they should be installed as you have seen on the other channel. It is these details that seperate the level of service. It is a wide gulf.
I believe that you have the ability to do this. You may need to buy 10 - 20 pcs of the correct Japanese part or sub to do this. Many inexpensive digital meters have an hfe test that should work well enough. You have used good troubleshooting technics so far. Continue. :magnify:

Echowars, this amp uses a dual matched FET pair for the front end. 2SK109 I believe. Very rare to have failures here.

-Chris
 
Well then, if the 300DC uses a FET diff-pair, then the feedback is DC-coupled...no worries about the cap I described above.

Isn't the 2SK109 a 6-pin DIP package? If so, then what are the devices that cayo71 is describing as thermally coupled in his posts? And what devices are you suggesting he match for hfe?

Ah well...such are the trials and tribulations of troubleshooting an amp over the web with no prints and no unit for comparison.:dead:
 
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Hi EchoWars,
The vas is constructed as a darlington diff pair (PNP) with a current mirror on the neg rail to convert the signal to a push - pull single ended signal. The collector of the current mirror is connected to the other side of the differential in the vas. You may want a schematic to see what I mean. Want it??
You are correct about the fet pair. It's monolithic, so it tracks for temperature really well.

I hold this amp in high regard as an undiscovered treasure. I am sure it can be improved.

-Chris
 
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Ever lift one up? They are cheap on a per pound basis. :tons: They are certainly heavier than they look. If you listen to one you may find the price a bargan. Just my opinion.

It's too bad the matching 3650 has switch contact problems, the phono stage goes into thermal runaway as well. One of the best phono stages I have heard. SC-9 shares the same basic construction.

-Chris
 
Yes, but fixing something that is working is also dangerous.

Because, some human error can create 50 volts of off set!

You are rigth....there's a problems...something that will create bigger future problems.

The one must decide to have those risks or not...depends how skilled he is.

As i am deeply skilled...and hundred times the screwdriver scaped from my hand and BOOOOOOMMMM!

I prefer to tell that to remember that human makes mistakes.

"never change some team that is winning...result can be worst!"

regards,

Carlos
 
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