A-700 Restore

Discussion in 'Yamaha' started by Scottfer55, Mar 6, 2018.

  1. Scottfer55

    Scottfer55 New Member

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    Martinez, GA
    Hello, Im posting this thread in hopes of guidance through a restoration of my A-700 amp I purchased new while in the navy back in 1983. The amp has been sitting untouched since 2003, it was working but had sound issues, dim and some not working lights. I brought it to a tech and was told then that it would cost more to repair than what it was worth. Ive recently decided to try and restore it myself. Progress will be slow and I will be learning as I go along here. Ive sorta taken on this project for a few reasons, one is to be able to listen to this fine amp once again and another is to learn about repairing electronics gear. As of today I have disassembled the amp, found that old nasty glue and associated corrosion, removed the 2 caps with glue on the main circuit board and found some questionable resistors due to the corrosion. I left the transistors attached to the main circuit board and salvaged the mica ? plates that are between them and the heat sink, however one set hasn't come off and i don't want to pry it risking breakage any suggestions on separating plates and cleaning the old paste? I started to reflow a few of the solder joints and i think i'm doing ok with that, see pictures. Im also in the works of getting a dim bulb tester made up. My plan is to reflow all solder joints, reassemble and test.
    If successful Id like to replace the caps that may improve sound quality.
    Here are a few pictures


    C9836489-074E-4BFA-BB99-49DF6FA5BEE9.jpeg How do these re solders look?

    09418EDC-3312-444A-8E32-E73E94B334F8.jpeg Should I replace any of these resistors with corrosion before testing?

    6785C25C-3B08-402A-AB91-AE90B96AECC6.jpeg How do I remove the mica plate without damaging it and how to clean old paste?

    Thanks, Scott
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

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    Use a single edge razor blade to slide up under the mica. I just use paper towel and elbow grease.
     
  3. avionic

    avionic " Black Knights " Subscriber

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    Resistors are cheap. Replace them.
     
  4. avionic

    avionic " Black Knights " Subscriber

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    They look fine.
     
  5. avionic

    avionic " Black Knights " Subscriber

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  6. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    Reflowing the solder joints is a great idea.
    My A-700 would go in and out of protection and the bias was all over the place until I fixed the bad joints. You can't do much troubleshooting while some key transistors are trying to fall out.

    Definitely replace resistors that got glued. I think there is a Zener that usually gets some corrosion as well.

    One thing to be super careful of is the bias transistors on top of the heat sinks. Make sure you don't smush them while working on it or you may end up with a short.

    Capacitors are a good idea as well. There are definitely some that would check low.
    I'd make a big parts order but fix what you know is wrong before going in for a wholesale recap.
     
  7. Scottfer55

    Scottfer55 New Member

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    @avionic thank you for the reply, I will try the razor blade haven’t thought of that and are those plates one piece or several plates stacked on each other?
    As far as replacement resistors what do I need to know besides the resistance value? Would these be 1/4 watt? Some are fused but how do I determine that?
    That’s a great thread you linked, i will continue to refer to it throughout this process.
    Your help is greatly appreciated
     
  8. Scottfer55

    Scottfer55 New Member

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  9. Scottfer55

    Scottfer55 New Member

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    @OMGCat! starting this thread was suggested by @zaibatsu and the idea to get the amp up and running first was his thoughts also. I’m a beginner at this however I’ve learned through years of being dumb and tough it’s probably best to listen and learn so I’m trying to do just that and probably ask some real elementary questions to some of you.
    The bias transistors you mention are unscrewed from the heat sink and kinda floating around, I’ll pay more attention as I’m handling the amp. Thank you
    I’ll dig deeper and try to determine which zener may be corroded and get a replacement coming with the resistors.
     

     

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  10. avionic

    avionic " Black Knights " Subscriber

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    Single piece per transistor.. Can you read a schematic diagram ?
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Thats going to depend on circuit application. Fusibles are usually documented on yamaha schematics with a resistance value and a current value. example ---- 220Ω 85ma --- Using " ohms law" you can calculate power ie 250 mw or 1/4 watt , 500mw or 1/2 watt..Etc. There are some actual fusible resistors at Mouser or Digikey. But there real slim pickings usually.
     
    Last edited: Mar 6, 2018
  12. avionic

    avionic " Black Knights " Subscriber

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  13. zaibatsu

    zaibatsu Well-Known Member

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    • You can use a razor blade like avionic said to get the micas off, just make sure you minimise bending as much as possible (as that's what cracks them). Household olive oil is great for dissolving/loosening that white grease, spray some on a paper towel and wipe over the micas while they're on a flat hard surface. When picking them up, sliding a sheet of paper under them works pretty well. The oil then cleans off any parts with household detergent, and a wipe with isopropyl is good on any surfaces that need fresh thermal compound applied.
    • Make sure you use a non-conductive + non-capacitive thermal grease. I like "Arctic Cooling MX-4", but this works too.
    • It's good to resolder the whole power amp board on these amps, but be very careful not to make any solder bridges. At minimum I'd resolder all the transistors.
    • The things you mention about the flickering lamps is pretty abnormal, I'd have some BAV-21 diodes handy in case the single one on the rectifier/fuse board has gone bad. I had a similar issue with an A-720 in this thread, but your issue sounds different again so may not be the same thing.
    • The flameproof resistors are the vertical standing turquoise ones, but the flat ones are just normal carbon films. You can read the value by plugging in the stripe color code here. The 4x that are corroded (behind the big 680uF/80V cap) will be 27kΩ, I used Vishay PR01 metal films for the same ones on my amp:
      resistor.png
      There might be some others you have to match codes for (or measure) yourself.
    • You can desolder the bus bar and give it a light sand / polish to get the corrosion off it if you want, helps to have a desoldering pump (handheld manual one is fine).
    Once you're done with the housekeeping tasks, reassemble the whole thing entirely and bring it up on the DBT (with no speakers or load connected).
     
  14. Scottfer55

    Scottfer55 New Member

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    I’d have to say no because I’ve never really tried, my background is in medium/high voltage electrical. I’m capable of troubleshooting AC control circuits reading one line diagrams. I think now is a good time to start learning as some of the symbology I’m familiar with but functionality of the components is still Chinese.
     
  15. Scottfer55

    Scottfer55 New Member

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  16. Scottfer55

    Scottfer55 New Member

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    Disregard my last post, somehow managed to post a quote without a reply, ugh! How to delete???
     
  17. zaibatsu

    zaibatsu Well-Known Member

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    Don't worry about it.

    The other thing you should replace is any badly corroded jumpers, you can cut some new resistor leads to make them:
    jumpers.png

    The two zeners either side of that 4x 27kΩ resistor bank (D149, D150) are HZ16-3L:
    screen 2018-03-08 at 6.43.18 AM.png
    screen 2018-03-08 at 6.43.01 AM.png

    Modern sub for those:
    HZ16-3L --> 78-TZX16C-TAP

    Also the diode on the PSU/fuse board in case you need it (D511):
    1SS82 --> BAV21

    I wouldn't worry about anything with light surface corrosion for now, just fix the obvious stuff and get it back together so you can find the actual problem.
     

     

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  18. Scottfer55

    Scottfer55 New Member

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    @zaibatsu the olive oil works! Managed to clean up the heat sinks and mica plates. The micas seemed to have thin layers in addition to the thick one, they are so thin and didn’t survive but I have the 4 thick ones ready and will assemble the heat sinks with the transistors attached then solder onto the board. The paste I’m using is Dow 340 heat sink paste.
    I’m still reflowing Solder joints just a few per day as my time during the week is limited. I will clean up that bus bar, those solder joints are visibly cracked.

    Working on getting an order together for the corroded zener diodes and resistors.
     
  19. Scottfer55

    Scottfer55 New Member

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    Trying to determine what wattage resistor to get to replace R271, C4ADF313-E5D1-4762-AB15-3096E71D14F0.png
    4.7k ohm
    70.7v
    .1 watt

    I used the voltage and resistance indicated on the schematic did the math using the calc and cane up with .1
    Do I order a .1 watt or is there a rule to follow?
     
  20. avionic

    avionic " Black Knights " Subscriber

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    No.... 4.7KΩ 1/4 watt probably flameproof carbon film. Not a fusible per say.
    Post a photo of it..
     
    Last edited: Mar 7, 2018
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