Marantz SR 3100 distortion in one channel

Discussion in 'Solid State' started by chriscats, Mar 13, 2018.

  1. chriscats

    chriscats New Member

    Messages:
    6
    Thought I'd crosspost this to see if I could get any help for this issue.

    Hey all. Quite new to posting here, and still learning about older audio equipment, with my first and current stereo setup being comprised of (except for a Dayton subwoofer) vintage gear.

    I recently came across a Marantz SR 3100 receiver which seems to be a lesser known model and not all that highly regarded among others that preceded it in the marantz line.. however for $40 in its decent cosmetic condition I was okay with buying it to see if it would work out for my secondary PC/Headphone-use setup. The first step I took was a thorough cleaning of all contacts, pots, switches, etc as per the usual with getting an unserviced, uncleaned unit secondhand. Main tuner lamp is blown, but that's actually not a problem at all at the moment, with it being left on constantly, and with it being at my desk, I quite like not having the light glaring in my face :cool:

    My first impression after getting it was quite displeasing, and after playing a familiar record through the phono input, I was faced with very, very, very heavy distortion in the left channel, with deeper bass totally decimating any semblance of sound quality coming out of my left speaker. Right channel was quite nice actually. And the weird part (to me anyways) is that the headphone section seems absolutely perfect. No issues whatsoever with the sound and it is quite a respectable upgrade over what I had my PC running through previously.

    So off goes the case and covers again, and went at a second cleaning with everything I could think of throwing at it, including a second go at the switches, with special care to be very thorough in working the Deoxit in adequately. I replaced the fuse which was just a crudely cut copper tube crammed into the fuse cap.. :eek: And I worked each pot, switch, input, speaker terminal, to see if I could fix the issue. This helped a little bit. One note is that enabling the loudness switch exacerbates this issue quite a bit, which makes sense I suppose.

    So where I'm left now is with a left channel that is still underperforming and sounding a bit off at certain frequencies with a static-y sort of sound; as well as that bass distortion, only at higher volumes now. The issue only comes through both sets of my speakers, which are all 8Ω. A service manual hasn't been easily available on the usual sources, but I did find one for the SR5100 which is very similar in look as well as internally. I got some info from the 5100 manual but not much that I could use in my troubleshooting. Worth repeating, the sound through headphones is very nice and smooth, and might even sound better than my current primary stereo receiver, the Onkyo TX-2500 MK II.

    I have never recapped a receiver, or done any sort of internal electronic part replacement on one, but I reckon I'd be up to it if I could narrow the problem down to a certain area in which to replace parts.. One could hope it'd be that easy..
     

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  2. elnaldo

    elnaldo Addicted Member

    Messages:
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    Location:
    Buenos Aires GMT-3
    Is the headphone OK, and the speakers at the same time sounding wrong? please check both at the same time (headphones and speakers sounding at the same time if possible)
     
  3. chriscats

    chriscats New Member

    Messages:
    6
    Yeah I tried in every configuration I could think of. The headphones just sound totally fine while the speaker badly distorts at moderate volume.
     
  4. elnaldo

    elnaldo Addicted Member

    Messages:
    7,193
    Location:
    Buenos Aires GMT-3
    If the headphones sounds OK WHILE the speakers at the same time are sounding bad, the amplifier stages are OK ( I assume this amp, as most of the amps I see inside take the headphones output from the power amp, right before the speakers, we ), and your problem is probably some relay, speaker selector, speaker connector, broken solder at some point between the power amp output (emitter resistors) and the speaker connector at the back of th amplifier.

    It would be good to check with the schematic, to see what is between the headphone and the speakers, and check those points.
    But is important what I mentioned, headphones sounding OK at the same time that the speakers are sounding bad. It's not the same to drive a pair of headphones (very light load to the amplifier) than a pair of speakers (heavy load to the amp, higher current).

    I've seen amplifiers that could drive the headphones just fine, but attaching speakers caused distortion, since the output and driver transistors couldn't handle that larger current.
     

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