Marantz 1070 DC-Offset Problem

transformer

transformer

Active Member
I've recently bought a Marantz 1070 integrated amp. Sounds great! The power amplifier idling current reads a healthy 10mV, the power supply adjustment is set to 40V (perfect), but I have a problem with the DC-Offset. Left channel is stable at 0mV but I cannot get the right channel lower than about 120mV. According to the service manual, the right channel should read 0mV (+/-5mV). Way off!

I suspect it could be a faulty resistor or transistor, or both - but which ones? If you are in the know and can take an educated guess at which components I need to replace in order to correct the DC-Offset, would you mind posting the parts list here?

For your reference, and for anyone else that requires it, I have uploaded a copy of the service manual and schematic at this link .... http://www.easy-share.com/1905801642/MARANTZ 1070.zip

Thanks in advance.
 
Last edited:
Register to hide this ad
replace the differential amp!
two transistors stickt to each other with a rubber band.(usually)
these are made bij NEC. you need to replace one set of these transistors.
look at the red markings.

1070differential.jpg



i got a set ones for cheap, and i refurbished everything.
see the link (it's Dutch)
http://marantzforum.nl/forum/index.php/topic,496.0.html
 
Yes, you should replace the differential transistors H701, H703 (H702, H704 in the other channel) by transistors with close hFE.

One easy to find transistor that will work fine is the 2SA733. The P or K are preferable in your amp.
 
Your suggestions are much appreciated. Yes, I think the 2SA763 transistors are the culprits. When I apply freeze spray to them (the pair on the faulty channel side), the DC voltage decreases dramatically. The originals are hard to find but I'll keep searching, otherwise, I will go for 2SA733's. Cheers!
 
The 2SA733 are not the best transistor on earth, but they are better than the original in every respects like noise, gain linearity, breakdown voltage... and so easy to find. Open an old computer monitor and you have 50% chances to find many 2SA733
 
Just a short note in closing to confirm that I have replaced the two pairs of 2SA763 transistors. The DC-offset is now steady as a rock and near enough to being zero volts, just as it should be! All is sweet. I really appreciate the help of all members that have contributed to this post - many thanks!
 
You must log in or register to reply here.
Back
Top Bottom