2285B one channel very weak

Discussion in 'Marantz Audio' started by Ziricote1, Mar 10, 2018.

  1. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    I've got a 2285B that I'm trying to revive, it's turning out to be a real basket case! It had some bad corrosion on the chassis and some of the wire connections, which I have taken care of for the most part. I also rebuilt the power supply and recapped the main amp, pre, and tone boards. Also all transistors replaced in the preamp and tone boards, and triple checked for correct orientation. The amp section is good, bias and offset adjusted properly. Problem now is that the left channel works fine but right side is very weak, and I suspect the problem is somewhere in the preamp. Here is what I've done so far:

    -All switches, pots, and jacks on front and back panel cleaned. No scratchiness present when operating any of these. Volume control on left channel behaves normally, volume on right channel is variable but much lower than left channel.
    -Tried criss-crossing preamp and main-in jacks with patch cables. Problem moves to other channel, which rules out main amp as the problem.
    -Tried both sets of speaker jacks. Tried using Aux and tape 1 input jacks. Same behavior.
    -Checked for broken wires between pre and tone boards, everything is tight.
    -Checked CE40 for proper orientation, I have it installed opposite of the board silkscreen which I believe is correct.
    -at JH09 and JH10 I have around -18v and 18v where the schematic shows -20v and 20v. Voltages on most of the transistors on this board are similar from left channel to right channel but slightly lower than what the schematic lists. The only real difference I could find was QH04 emitter voltage is -4.4v while QH03 is -4.1v. Something is not right there but I'm not sure where to look. These are all new replacements - 2 x ksa992 for the 2sa798 and ksc1845 for the 2sc1345's.

    I'm kind of stumped, do you guys have any suggestions for where to look next?
     

     

    Please register to disable this ad.

  2. Jailtime

    Jailtime Standin' on a corner Subscriber

    Messages:
    11,371
    Location:
    Velva ND
    Maybe something silly like a broken wire? Worked on a 4400 that had a missing channel, maybe a hint of bleed through was present. Is it that weak? The 4400 had a broken wire up on the filter switch board.

    I was thinking relay, but your patch cable experiment rules that out I think.
     
  3. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    It's not quite that weak. If I turn it up to full volume with the balance set full right, it does reach a moderate volume level. I did a once over for broken wires already but I will check again.
     
  4. catrafter

    catrafter Marantz Specialist Subscriber

    Messages:
    6,063
    Location:
    Montana
    Sometimes the switches built into the front panel Dubbing In jacks can cause problems. A shot cleaner in the center hole and repeatedly inserting/removing a 1/4 inch plug might fix it, power off, of course.

    Tom
     
  5. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,487
    Location:
    Cleveland, Ohio
    If that doesn't do it you need a scope or signal injector so you can see where the signal is being lost. Otherwise you're just guessing.
     
    spinroch likes this.
  6. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    Thanks, yes I did clean out the dubbing jacks but no change unfortunately. I've been looking into getting a scope lately, this project may just force my hand!
     

     

    Please register to disable this ad.

  7. KutzlerTrans

    KutzlerTrans Jennie & I with Buddy and Barney. Subscriber

    Messages:
    2,998
    Location:
    Wisconsin
    You could un-solder and swap input channels at the pre-amp itself. That can narrow it down. Or you could pull the ce01 and ce02 and cross them. I believe they are your inputs.
     
  8. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    I hooked up a signal tracer and made some headway with it tonight. I'm losing signal on the preamp board but can't figure out where.
     
  9. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

    Messages:
    12,646
    Location:
    Duvall, Washington
    I ran into a bastard losing signal on the tone board. There was a bad control. The rivet on one post was rotten and I lost connection to the wafer. It was a hard find.

    It was a balance control. It could have been jumped but I had one close enough to make it work.
     
    Scott Hughey likes this.
  10. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    Thanks for the suggestions.. Im going to post my results here in a little while. Will check out the balance control also.
     
  11. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    Here's where I ended up last night. Green circles indicate good signal and red indicate where it goes faint. The tone selector is set to "out". I have good signal coming in at JE06 and JE07, and it's coming out good on JE09 but faint on JE10.
     

    Attached Files:

     

    Please register to disable this ad.

  12. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,487
    Location:
    Cleveland, Ohio
    This signal tracer you are using, is it injecting a signal at those points and you are listening through the speakers or does it have it's own speaker and you are probing places in the circuit?
     
  13. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    I'm injecting a 1khz test tone through the aux port and have my probe connected through a 1uf cap and into a small guitar amp.
     
  14. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,487
    Location:
    Cleveland, Ohio
    Ok, great, then it makes sense. The problem is either in the switch or the second stage. Probe CE32 and QE06 base and collector.
     
  15. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    Ok, at CE32 I'm getting no signal. At QE06, I get a loud "boop" at base and collector.
     
  16. Ziricote1

    Ziricote1 AK Subscriber Subscriber

    Messages:
    61
    Location:
    N. Virginia
    Ok, so I just pulled CE32 which was a new 1uf Wima film. The traces there lifted up very easily, so I'm thinking one was damaged already but hidden under solder. I put in a new cap and extended the lead out directly to the pin on the selector switch. Result is I now have good signal on both channels coming out of the preamp! Thank you guys so much for the help!
     
    quadmatrix82 and Jailtime like this.

     

    Please register to disable this ad.

  17. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,487
    Location:
    Cleveland, Ohio
    Glad I could help.
     

Share This Page