Marantz MA700 power issue - Page 1
Forum section: electronics / repair

author: mjrpainless
author_rank: Contributor
timestamp: November 18, 2014, 10:13:36 pm
content: I have a Marantz MA700, that has a power issue. I plug the unit it and it powers on, the led on the front of it continuously flashes, which it is not supposed to do.I have replaced a number of components in the unit that were either out of spec, burnt up, or totally the wrong spec in relation to the service manual.The oddest thing that I have found is that I get 40+/- Vdc in the majority of locations. I find that the unit also ramps the voltage up I'm not to sure how high it goes, but Ive not wanted to let it go very high as i don't want to cook more pieces.I have checked all the main caps both by to separate ESR meters, and by a capacitance metre they are all good.I have checked all of the transistors with a 520B transistor tester (very useful piece of equipment i find)I have a scope if needed as well.Any help would be greatly appreciated!Link for the service manual.https://www.dropbox.com/s/aewb76qvccv2jk6/hfe_marantz_ma700_service.pdf?dl=0Edit: Change link to the service manual (hopefully better quality)

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 18, 2014, 11:05:04 pm
content: I would check capacitor number CN06 (If I read the low-res schem correctly) a 22uF 16 volt capacitor that I suspect is located near the TA7317P protection circuit IC. It is a fairly common fault in this situation, as this capacitor provides a sharp cut-off for the protection relay if power is interrupted or turned off. If there is an AC waveform there instead, the amp is probably in a "waiting" sort of mode. My two cents worth, anyway. Of course, there could be an actual amp failure, such as a DC offset or the like. This can also be pursued if necessary.

author: mjrpainless
author_rank: Contributor
timestamp: November 19, 2014, 12:10:15 am
content: Pulled out CN06, and it was fine, mind you it has been replaced with a 50V and not a 16V, but still .22uf.Dug the scope out and there was a TINY ac ripple of approx 70 mv.But on having the amp on for a few mins, an noticed the smell of burning,Turns out R707, and C707 decided to heat up one heck of a lot. R707 has been replaced with a 1/2watt resistor.

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 19, 2014, 01:29:47 pm
content: Well, the cap I was talking about is 22uF not 0.22uF, the schem is hard to read but it is shown under resistor RN14 near the TA7317P protection IC.Have you checked for DC offset at the output of the amp channel?

author: mjrpainless
author_rank: Contributor
timestamp: November 19, 2014, 04:31:31 pm
content: OH yeah! Sorry, that was the cap i checked on, and it was fine.I had a look for DC offset at the speaker output terminals. and there was non, But when measuring AC.... it would spike at about 1V then drop down... then spike... and so on and so forth.I attached a Dropbox link to the service manual as well.

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 19, 2014, 11:31:47 pm
content: And you've looked at the DC level on that cap with a 'scope, yes?

author: mjrpainless
author_rank: Contributor
timestamp: November 20, 2014, 06:09:27 am
content: Damn, Ill have to check that when i get back from work in a week. Thanks for the help so far! Just out of curiosity, that line goes to the MUTE function if i remember correct, would you think that would be cause the problem of led blinking? If so, that wouldn't explain the over heating R707, and C707

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 20, 2014, 06:52:57 pm
content: Quote from: mjrpainless on November 19, 2014, 12:10:15 amThat cap should never get hot on it's own. Replace it. There might be an oscillation problem.