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KR-7070 restoration main amp & fm tuner

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by avataar, Apr 30, 2018.

  1. avataar

    avataar New Member

    Messages:
    10
    Location:
    Sofia, Bulgaria
    Hello! AK newbie here looking for advice on restoring a KR-7070. I got it on eBay and the only thing I knew was that it powered up. The unit is in lovely cosmetic condition with an ok-ish plywood case. All switches are snappy and the glass is virtually scratch-free.

    After unboxing I tried it out and I only got sound on the right channel. It sounded lovely. The left one appeared to be completely dead. Firstly I discovered a shorted emitter-collector on the power transistor Q2 (2SC898) and I thought that'd be all but it turned out there was more damage on the main amp board. The obvious was a couple of very crisply fried or cracked resistors (Re33, Re43, Re45, Re47), while the not so obvious was an opened transistor Qe9 (2SA566). All other transistors passed a basic test with a multimeter without unsoldering anything, including Qe7, which is right next to crispy resistors. I marked all failed components with a red dot on the schematics and a couple of never-installed resistors with a blue dot.

    Interestingly, the schematics gives Qe7-8 as 2SC484Y and Qe9-10 as 2SA484Y but the installed transistors are 2SC680 and 2SA566. There's also an extra 120 kohm resistor between the base of Qe12 and the emitter of Qe14 that isn't on the schematics. As far as I can tell it appears to have come like that from the factory. The resistors Re43 and Re44 are also a bit of a puzzle as the schematics says 33 ohm each but colour-decoding Re44 gives me a 68 ohm one. Re43 was probably the same but it it's too discoloured to tell.

    Now, apart from the obvious replacement of resistors and some/all capacitors I'm wondering what to do with the transistors. Both 2SC898 and 2SA566 are hard to obtain. A generally available modern replacement for 2SC898 is MJ21194G and for 2SA566 it's MJE15033G (learnt that from other posts on the AK forum, thanks!). Should I try to get genuine 2SC898 and 2SA566 and replace only the failed transistors or replace the failed transistors with their modern more-or-less equivalents? Or should I replace all of the power transistors Qe1-Qe4 with MJ21194G, Qe9-10 with MJE15033G and Qe7-8 with MJE15032G (the modern equivalent of 2SC680)? Mind you I gave up the practical side of DIY low-level electronics more than 20 years ago and now I'm slowly getting back into it.

    I'm also curious what triggered the failure if anyone can tell from the schematics and the failed components.

    Later work: tuning meter and stereo decoder don't work. I think the stereo issue is due to a failed lightbulb in an opto-isolator (early model of lightbulb + photoresistor).
     

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  2. paul_ral

    paul_ral Active Member

    Messages:
    258
    Check STV-3 diode and all De1-De5 diodes too.
     
  3. avataar

    avataar New Member

    Messages:
    10
    Location:
    Sofia, Bulgaria
    Thanks, I already did but forgot to mention it. All diodes are fine.
     
  4. avataar

    avataar New Member

    Messages:
    10
    Location:
    Sofia, Bulgaria
    It took me some time but I managed to find (hopefully) genuine 2SC898 and 2SA566. It took me even more time to solder in everything and try it out. I used a light bulb in series to make sure nothing too nasty happens. No smoke. Then I tried it without the light bulb and still no smoke. According to the service manual I should have ~50 V on the collector of Q2 but when measured it was ~75 V. The transistor Qe5 heated more than its counterpart in the other channel. I suspect the culprit is Re43, which according to the schematics must be 33 ohm but I used a 68 ohm one as that's what the colour code of Re44 (the same resistor in the other channel) decoded to. However, after measuring Re44 it seems to be 108 ohms. There is no way that colour code (blue, grey, black, silver) is 108 ohms. I tried three different ohmmeters even. I'll try to replace Re43 with a 108 ohm resistor and see what happens. Any thoughts?
     
  5. avataar

    avataar New Member

    Messages:
    10
    Location:
    Sofia, Bulgaria
    Just tried an improvised ~108-ohm resistor made out of three ~330-ohm ones in parallel. Still ~75 V on the collector of Q2 and Qe5 heating up :(. Could a fake 2SA566 transistor be the reason?
     
  6. Watthour

    Watthour Electron Rancher - JS3600

     

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  7. avataar

    avataar New Member

    Messages:
    10
    Location:
    Sofia, Bulgaria
    It turned out another of the 2SC898 transistors died as well and perhaps failed 2SC680s and/or 2SA566s. Or maybe the new parts I got were not that genuine after all. I gave up on the idea of chasing original transistors and replaced all 8 with modern ones:
    • 2SC898 with MJ21194G
    • 2SC680 with MJE15032G
    • 2SA566 with MJE15033G
    Both channels came back to live with lovely sound! The only issue I could notice was a slight static noise in the left channel. It's volume dependent. I think it wasn't there at first (I was using headphones and also listened without any signal on) but developed maybe half an hour after powering up. I don't notice it with signal on and hardly notice it with signal off on speakers. There is no variation in sound quality with time and as far as I can tell the left channel sounds just like the right one. Perhaps one of the remaining old transistors is to blame? At least it's something I could live with if I don't find the culprit.

    Still to fix: no FM stereo and no tuning meter. I noticed the tuning meter somewhat works on AM so I suspect the stereo and the meter might both be related to the failed opto-isolator.
     
  8. rickl

    rickl Member

    Messages:
    97
    Location:
    Twin Cities
    avataar,
    I shouldn't reply but I really enjoyed your post. I don't have a 7070 or even worked on one.

    If it was me, I'd replace Qe1-4, and Qe12-13 with KSC1845FTA.

    I'd also replace Q1-4 with MJ21194G/MJ21193G. I've used these in a 125w Marantz power amp for outputs. I think you did this from your posts above.

    Get it working and update the electrolytics on the amp board.

    This might also help you http://audiokarma.org/forums/index.php?threads/developing-the-bom-for-a-kr-6160-restoration.760451/

    Looks like there are pre out/main in jumpers. you could swap in another power amp to see if the static goes away. if so, you know the problem on the amp board. The low signal Qe1-4 might be the problem.
     
  9. avataar

    avataar New Member

    Messages:
    10
    Location:
    Sofia, Bulgaria
    I enjoy getting comments so thank you for the reply and the tips. I indeed used MJ21194G for Q1-4 (all were 2SC898 before) and I'm considering replacing the small transistors too so the tip on KSC1845FTA is much appreciated. The electrolytic caps are maybe next.
     

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