Yamaha M-70 Thread

Discussion in 'Yamaha' started by Bratwurst7s, Jan 17, 2018.

  1. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    I have an M-70 that I bought last year, it's been sitting patiently waiting to rise to the top of the priority list. It just got there.

    I paid about 1/3 of the going price around here and Knew when I bought it that it had problems. Supposedly it has been sitting in an attic for 20 years and has problems "but not with the output transistors". OK. I have a bag of "yeahright" handy. But for the price it was worth the gamble. I think. Perhaps Hope.

    DSC04806.jpg DSC04807.jpg DSC04808.jpg DSC04811.jpg DSC04812.jpg

    General appearance is quite good, just 2 small scratches on the left upper face corner. Inside, the only thing that stands out is that in the right rear of the main pcb one cap and a zener are non original. Not too dusty, doesn't smell like nicotine, damp, cat piss or smoke.

    The "Power C Board" up front has some DSBG and one small cap that's leaking but no signs of corrosion that I can see at this time. It has the Rifa cap(s) in the power supply that needs to be replaced.

    Cheers,
    James
     
    Last edited: Jan 25, 2018
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  2. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    So after a first visual inspection of the interior the only thing that stands out is that Nichicon FG and replacement zener in back on the right. I assume the right channel has problems.

    I did buy an original service manual last year. Among other things it looks like I'll need to build a 200w 8 ohm test load. And it's probably time to buy a second DMM that is true rms. I do have a signal generator.

    DSC04815.jpg DSC04816.jpg DSC04817.jpg DSC04818.jpg DSC04819.jpg

    It looks like all of the outputs are original, I can't tell yet concerning the emmiter resistors. But all of that is suspect.

    Cheers,
    James
     
  3. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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  4. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    So, now we get to the ugly part. Time to remove the bottom plate and see what's going on down below. First glance didn't look bad at all. No burnt traces. But there are a number of rather shiny solder connections that don't look original. Not bad but not original. Then I took a look at the front pcb that holds the main filter caps and bridge rectifiers. :eek::(

    So that was a big WTF moment. Really? Someone took a drill and severed the traces to the 18000µF caps? REALLY? :thumbsdown::no::crazy:

    DSC04830.jpg DSC04832.jpg DSC04833.jpg

    So that was a bit of a downer. I'm now at the point of developing a plan for going forward. I have to find out if there is a problem in the high power section of the PSU or in the high power section of the outputs or both. At this time I'm assuming that all of the outputs are blown and that electrically this thing resembles a certain radioactive city in the Ukraine.

    That's all at the moment. One thing that I did do today was order some filter caps from Mouser. That's kind of jumping the gun but they only had 5 pieces of the UCC KMH 22000µF/80 that I want and I don't want to wait 16 weeks for a restock. I ordered a pair of KMH 12000µF/80v with them. I won't be ordering any other parts until I start to get a feel for how bad the damage is. Obviously I'll have to jumper those traces, I don't imagine that I'm going to find a spare board anywhere.

    Cheers,
    James
     
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  5. Bert 1100

    Bert 1100 Super Member

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    :lurk: :)
    Let's hope the damage is not extensive :idea: , good luck !
     
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  6. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    Thanks Bert! Working on it...

    2 hours after my order with Mouser I got a shipment notice. Mouser+FedEx is the best. So far the record time from "last click on the order" to "parts in my hand" is 42 hours and 26 minutes. Think about that. Order comes in, parts picked and packaged, shipped from Texas, USA to Munich, Germany in 42 hours and 26 minutes. That's humpin'!

    There are several threads of thoughts going through my head on how to approach this.

    I don't even want to turn it on until the Rifa cap+ has been replaced, as well as the DSBG and leaking cap. With the X power supply I can't use a dim-bulb-tester to start it. So I have to think about a Variac. And normally I want to find and correct all of the problems before going into a general re-capping etc. And at this point while I suspect that the right channel is probably bad I also don't know if I can trust the power supply. Talked that one around in a circle didn't I. o_O

    I think that I will first check out the power supply. Clean it up, deal with the obvious issues and check the components. Next probably de-solder the main output transistors and remove them with the heat-sinks. Check those transistors and then decide what to do from there. I'm not sure if this is the best plan of attack but that's what I'm starting with. I'm also going with the expectation that at least the entire right channel is suspect.

    Cheers,
    James
     
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  7. avionic

    avionic " Black Knights " Subscriber

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    Replace that largest Rifa PIO cap. Its a fire hazard in the 220v model.
    [​IMG]
     
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  8. avionic

    avionic " Black Knights " Subscriber

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    Needs to be and XY rated "across the line" safety cap.
     
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  9. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    Is a dim bulb always a no-no on X power gear? I've used one on several M-60s, an A-1020 and an A-1000 with no problems.

    The damage to the traces is insane. I wonder what in the world they were thinking. At least there is a lot of width to work with to make repairs.
     
  10. avionic

    avionic " Black Knights " Subscriber

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    At this point I'm "on the fence". I know I've said " no " in the past. Its pretty nerve wracking not using one. Did I mention B-6's have the x-power supply.I had to rebuild 1 channel in each of my B-6's.
     
    Last edited: Jan 17, 2018
  11. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    Initial power up after major repairs is my least favorite part of the whole process by far. I think I'd probably have to pay someone to plug in a B6 I repaired without a DBT because doing it myself would give me a heart attack.

    The DBT definitely saved me some grief when I was repairing that M-60 where someone had replaced resistors with whatever they had handy. Talk about weird voltage measurements when you have something like 4.7ohm subbed in for a 33k or other higher values in a few random places.
    With the "repairs" that this M-70 has done to it I have a feeling there could be surprises like that hiding around.
     
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  12. mbz

    mbz AK Subscriber Subscriber

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    A little off post but, best time to Melbourne is about 60hours, order monday 10pm, arrive Thursday 10am (normally thursday 2pm) with UPS.
    It takes 4 days to get a parcel within australia, Unbelievable.
     
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  13. avionic

    avionic " Black Knights " Subscriber

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    I think I have like 350 tied up in three B-6's. So I still have 1 more to finish up.
     
  14. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    Thanks Avionic. Understood. That cap in my M-50 blew up the first time that I turned it on when I moved over here.
    I did a general search concerning this amp and came across this thread...
    http://www.audiokarma.org/forums/index.php?threads/smoke-from-m-70-what-to-do.376536/#post-4747576

    The cap that you and Brutal recommended was this one...
    https://www.mouser.de/ProductDetail/Panasonic-Electronic-Components/ECQ-U3A474MG/?qs=sGAEpiMZZMvQAn3cSZoTr1%2b0Ye1BktuzFq%2bm14xyTdw=

    And for the 3 small 6800pF Rifa caps I'm looking at this one...
    https://www.mouser.de/productdetail/panasonic/ecq-u2a682kl?qs=sGAEpiMZZMv1cc3ydrPrF0%2bjlB8SXIRuFjuQdipUHOg=

    Question:
    On the bottom sub-extention of that board there is a huge 0.1µF/450vac axial lead electrolytic, C611.
    I was thinking of using one these to replace it...
    https://www.mouser.de/productdetail/arcotronics-kemet/r73tw3100se00k?qs=sGAEpiMZZMv1cc3ydrPrF42R8ZpcOXPb/VGN6n3zWPo=
    https://www.mouser.de/productdetail/epcos-tdk/b32686a7104k000?qs=sGAEpiMZZMv1cc3ydrPrF1u7m5EY228Lht/U8VTV1W8=
    Do you think that they will serve?

    Thanks,
    James

    edit: the parts list shows C611 as an oil cap.

    DSC04840.jpg
     
    Last edited: Jan 27, 2018
  15. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    That's pretty darn good considering the distance.

    Cheers,
    James
     
  16. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    We have a huge test bench at work with a variac built in (Regeltrafo). But it would be a pita to drag the amp in there to use it. Next month I would be able to afford to buy a variac (ca €150). I just have to decide if it will get used enough to make the purchase worthwhile.

    Cheers,
    James
     

     

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  17. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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  18. Beau Geste

    Beau Geste Rione Traiano Subscriber

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    Thanks, James, your works are always nice :)
     
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  19. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    Thanks BG. In this case I haven't even really gotten started yet and I'm already getting nervous. :eek:

    Cheers,
    James
     
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  20. Beau Geste

    Beau Geste Rione Traiano Subscriber

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    It always happens to the best. You just have to find the key to follow the right path. It will not be difficult because you are really "Bravo" and you have with you the most powerful weapon: patience :)
     
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