Marantz 300DC Problem

[cayo71]

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.

[Netlist]

I bet there are some leaky caps in the signal path/input stage.
Do you have a schematic?

/Hugo

[Giaime]

I am that Italian one with the never ending Marantz 1060 question... all in all these amps are good sounding but way too difficult to repair...

Resistor keeps burning in my Marantz 1060!

[Netlist]

Well, I think Ricardo has the advantage that his amp is not broken
and that it's not an integrated one.
Some good service like replacing all caps will probably fix his problem.

/Hugo

[cayo71]

Netlist,
I do have the service manual which includes the schematic. What should I look for? Or just replce every single cap??
Thanks,

Ricardo.

[Original Burnedfingers]

If it were me I would replace the diff transistors also.

[anatech]

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

[cayo71]

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.

[cayo71]

I replaced all the electrolitycs on the amp board....The problem persist. Any idea?
Thanks in advance,

Ricardo.

[EchoWars]

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.