Trouble with preamp on 2250.

Discussion in 'Marantz Audio' started by PoDuck, Aug 9, 2018.

  1. PoDuck

    PoDuck New Member

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    I recapped this 2250, and all was well except for the phono section. I didn't have a signal generator with low enough output to not overdrive it when I first went through it. I left it as it was, using everything else just fine for a couple months, then I got a proper signal generator that would do a low enough output. I went to adjust the phono stage, and I had nothing but trouble. Nothing I did seemed to show what I thought it should show on the oscilloscope, and as I was working with it, I noticed the left channel getting weaker and weaker. Then I went to the right channel and it did the same thing. I tested the aux and tape inputs and was getting nothing there either except when the input is turned up pretty loud and the volume is turned up all the way on the Marantz. When I put a signal on the main input, I get great sound, so it isn't the amplifier, and I get sound on AM and FM, so it would seem to be the preamp.

    I assumed it must be an active component on the preamp board, so I started there. I tested the transistors, and one of them tested bad, so I matched up a pair of KSC1845s, and a pair of KSA992s and replaced the transistors, and I started hearing sound from the phono board, but no change in the audio output from other inputs. I then decided that since the only other active components were the BA312s, and I happen to have some of those sitting around, that I would replace them. Still no bueno. Nothing changed. Now I went back to test the transistors again, and another one is testing bad, so I replaced them all again, just for good measure. After all, I have about a thousand of each.

    Anyway, I am quite perplexed here. What would allow me to hear audio from the phono input at 2 mV, but not from any other source, no matter the input level?

    It may help knowing that I am not getting anything like what I expect to see when I test the tape output ports, but I do see a distorted 1khz signal.

    What would be common between both channels, not be in the amplifier stage, not affect AM and FM, and cause distortion on Phono?

    Thanks
     

     

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  2. PoDuck

    PoDuck New Member

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    Sorry this got posted twice. I don't see where to delete the duplicate.
     
  3. dr*audio

    dr*audio Fish fingers and custard!

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    Check the power supplies to the preamp.
     
  4. PoDuck

    PoDuck New Member

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    Pin 3 on on JE01 is getting 34.95 VDC. I believe that is correct.
     
  5. petehall347

    petehall347 the brandy coffee man Subscriber

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    does am and fm not go through the preamp ?
     
  6. PoDuck

    PoDuck New Member

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    From what I understand, they don't. But, from what I understand, the phono amp does go through the preamp. I could be wrong on both counts.
     

     

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  7. PoDuck

    PoDuck New Member

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    From what I understand, they don't. But, from what I understand, the phono amp does go through the preamp. I could be wrong on both counts.
     
  8. PoDuck

    PoDuck New Member

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    I need to get used to this forum. I keep double posting.
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

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    i looked for a block diagram but couldn't find one with signal path without looking at the schematics .. i get lost with the switches .
     
  10. PoDuck

    PoDuck New Member

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    Okay, something is extremely strange. I was probing with my signal generator on the preamp, and I was able to get the right channel at first, but the left channel was sounding funny. As I was probing though, I slipped and shorted pin 3 and 4 of the jumper on the preamp. It tripped the protection relay, and when it opened back up, everything was working fine. I'm still not seeing what I expect to see on the oscilloscope with the phono board, but everything seems to be working.

    I have absolutely no idea what caused that problem though, so I don't know how to reproduce it so I can diagnose it if it happens again.
     
  11. bryans12v

    bryans12v Marantz Junkie Subscriber

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    Hook up a turntable. If it works, leave it alone.
     
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  12. PoDuck

    PoDuck New Member

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    Yeah, I'm working on that now. When I hook up the turntable, it sounds like ass. The volume levels jump all over the place. I then decided to hook up to the aux port, and noticed that when I turn the bass up, it cuts out more and more the higher it goes. Then I fiddled with it a while, and now it is doing what it was doing before.

    It appears there is something wrong with the bass tone control. None of the other tone control knobs seem to do anything.
     
  13. wlhd1610

    wlhd1610 Penny and her new friend Sponsor Subscriber

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    Did you super clean/deox all the controls, switches, and pots already?


    Bob
     
  14. PoDuck

    PoDuck New Member

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    Well, considering that it was acting like a dirty potentiometer, I cleaned the bass pot again, and no change. I have played with things a bit, and I can consistently reproduce the problem, and it seems to have changed since I started messing with this.

    Right now, if I shut things down for a while, when I turn it back on, I get good sounding audio out to the amplifier, as long as I have the bass all the way down. As I turn the bass up, at about the half way point, it starts cutting out, and at fully clockwise, it basically cuts out all sound unless I turn the volume all the way up, and then it isn't all the sound, just the loudest portions of it. Now, as long as I don't turn the sound all the way up, the cutting out goes away as soon as I turn the bass down again, but if I turn the volume up all the way, while the bass knob is all the way clockwise, after a few seconds I no longer even hear the loudest sounds through the speakers, and when I turn it back down, the sound is very distorted and stays that way until I shut things down and wait a bit.

    To me, this suggests one of two things. Either something is getting hot, or a capacitor is causing trouble. Since it goes away immediately when I turn down the bass, I'm thinking that heat is not the issue. That leaves me with a capacitor issue, unless someone else has any ideas.

    So, does anyone know what capacitors might be causing this problem? I already replaced all the electrolytics, but I took them each out and tested them, and they all test out fine. So, it's probably some other cap.
     
  15. morgan

    morgan morgan

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    Speaker relay?
     
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  16. PoDuck

    PoDuck New Member

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    I thought about that. It could be a possibility I guess. It just seems like it would be a bit more consistent regardless of tone. Unfortunately I don't have a spare relay. I guess I could try cleaning it.
     

     

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  17. dr*audio

    dr*audio Fish fingers and custard!

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    I think you may have an open ground connection to the preamp. With the power off, meter on Ohms, connect one lead to the chassis. Then measure resistance to the power supply ground at the main caps, then to every point in the circuit that shows ground in the schematic. Do this for the preamp, phono preamp and power amp. You may have a broken wire going to one of the boards.
     
  18. PoDuck

    PoDuck New Member

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    Well, I just cleaned the relay multiple times with a strip of paper sprayed with deoxit and while I did get some dirt off, there is absolutely no change. This almost always works for cleaning relays, so I am going to assume for the moment that the relay is not the problem.

    I checked all those spots, and resistance is 0.1 - 0.2 ohms, and 0.1 ohms is what I get when I simply touch my probes together.

    Thanks for the suggestions, but I'm still in the same spot.
     
  19. bryans12v

    bryans12v Marantz Junkie Subscriber

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    Was this problem present before the recap?

    Are you having this issue with the aux input or just the phono stage?

    At this point, I would halt replacing parts until you hail down the culprit. I would suggest putting the original ba312s back in though.
     
  20. PoDuck

    PoDuck New Member

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    No, but it wasn't present after the recap either until I went to adjust the phono board. Everything had been working perfectly for months except the phono amp. Now all inputs have trouble except when I connect directly to main.
     
    Last edited: Aug 10, 2018 at 10:43 AM

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