Discussion in 'Marantz Audio' started by Dearslayer, Dec 5, 2016.
I think I have some of those so will check them out. Do you know if the 2sc1000's are troublesome?
No, but any time it sounds good for 20-30 minutes and then goes into distortion, you have to suspect semis. I might even replace them, try it out, and if it does the same thing, put them back in.
I would do exactly that, put the old one back in if the issue isn't cured. Do one part at a time too.
I recently subbed in some KSC1845's for 2SC1000's and incurred that dreaded low frequency oscillation. I had installed the 47 ohm isolation resistors on the output already. Maybe the 1815's won't do that, I'm not familiar with them.
I had some 1845's on hand so changed the 4 2sc1000's out one at a time, but I still have only about 5-10% volume on that left channel so no change. Have to get ready for the dreaded nightshift so will have to research more from work tonight. Any ideas where to look next. Oh I changed out the TT just to eliminate the possibility there was a problem there but that obiviously isn't the issue.
Check every wire in and out of the board. I have had the input wires at the bottom and output wires at the top develop bad connections either at the board or even at the other end or in the middle of the wire -- probably from all of the manipulation of the board.
I did give a quick look over of the wires but will again tomorrow when I get home. I even prodded around the area with a plastic trim probe to see if I could change the signal somehow in case there was a loose connection somewhere. Also checked the wires at the phono inputs at the rear but can't see anything amiss there either.
Time to borrow a scope maybe? First compare dc voltages comparing channel to channel with no audio playing.
I got a old used scope and function generator just last week but I now have to learn to use it ,as I have ZERO experience with it and haven't had much time lately to even research how to use it. And now I have to ask where I should be taking the measurements for the voltages , at each of the transistors on the 700 PCB?
I was poking around the phono and preamp board last night before work trying to get any type of reaction at all that would lead me in the proper direction with this left channel issue. When I touched the transistor at H401 on the preamp ( while using the Aux input ) I got a very loud pop and distortion on the right speaker but no time to investigate it before heading off to work. Came home this morning and checked the back of the board only to find one leg of that transistor not soldered in place. It was on the same solder blob as the Film cap C401 that I replaced so I must have lifted the leg of the transistor when removing the old cap. Soldered it in place and the "pop" disappeared. Back to the phono board because this didn't fix the issue with no left channel on Phono. Removed the 700 board to check it out. I resoldered one sketchy looking pad and also clipped one lead that looked like it was forgotten or just not clipped close enough and so far the left channel is back in business. I should just stay away from working on this when I'm working night shift. Seems like that's when I make the most mistakes. I usually triple check the boards after I'm done working on them but I guess I didn't this time. A lesson to be learned here. Fingers crossed that the issue doesn't return and my rookie mistake was the reason for this grief.
No matter how many restorations you do it's still far too easy to miss a little detail like clipping a lead. I have double checks in place and still happens every so often. Glad you found it.
well done finding the mistake .. we all make them especially if disturbed .. i dont allow being disturbed these days but never get much done . go figure ..
i do have a habit of solder one lead and clip one lead .it takes longer that way for sure but saves missing one . it is whatever works for you at the end of the day .
Thanks Mike but I still think there is a loose wire somewhere. Just came downstairs to listen to an album. Damned if the sound didn't drop again. Removed the 700 board again for a real close inspection for pinched or loose wires and don't see anything. Moved a couple wires to ensure they aren't pinched and now I'm listening again to Robert Cray on Vinyl and it sounds great. Waiting to see if I can manipulate the unit to drop either channel.
Looks like the channel drop hasn't happened for a while now so hopefully all is good in that area however after installing the MJE243 as a replacement for the Bias H760 it seems the Bias is actually higher that it was before. With the clips on J760 and J754 it measured 34.7mV so quite a ways from the 10mV recommended.
can you not dial it in with r763 ?
No it's already dead stop CC. Warmed up to stabilize at 43mV! It was still climbing a little when I shut it down. The other channel still dials in no prob at 10mV. I thought for sure that that new transistor would do the trick. Back to square one again.
best bet would be get a transistor more suited to the job .
you could probably mess about with r773 r774 but i have not really got any idea if it will help or what it might do . best not mess around too much until better advised .
Another one was suggested in a PM so may have to place an order for it and hope it works. Thanks for the advise Pete.
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