Heathkit AA1640 Power Amplifier

Discussion in 'The Projects Forum' started by Meerkat 67, Jan 29, 2015.

  1. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I recently acquired this beast and got it up and running until the left channel went dead. More importantly, I now have a power (surge) relay problem.. It buzzes all the time, although the amp does power up. The relay is a small open frame type, SPST from the schematic.. The coil on it is not open but the contacts look bad. I could bypass it but I want to keep the amp as stock as possible as I plan on selling it once it is up and running. I am not a certified technician, but can read schematics and check components so if anyone has any ideas let me know.. I know that NTE has replacement relays of the same type or close so a substitute is possible depending on size etc. Cheers Wayne
     
  2. crutschow

    Expert

    Mar 14, 2008
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    The dead left channel may be drawing too much current, causing the relay to chatter.
    Can you disconnect the power to the bad channel to see if that makes a difference?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    The schematic is here.
     
  4. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I have pulled both output boards and checked the chassis wiring according to the assembly manual I have. Even with nothing plugged in , including the power supply board it still chatters.. I pulled the power transformer and checked the primary windings and there is no short so I do not believe it is that. There are a couple of small caps and a resistor in the circuit as well, you can see that from schematic, but they seem okay.. Is there a safe way to bypass the relay and test that way? Thanks
     
  5. crutschow

    Expert

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    The relay function appears to be reducing the startup current surge. You could just jumper across the contacts but that could blow the fuse on startup.
    You could likely replace the relay with a 10A thermistor inrush current limiter.
     
  6. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    Thanks for the advice. I had a look at some of the websites and found it a bit confusing with regard to the type of thermistor to use. The relay has a 5 ohm resistor across the coil so I hopefully can assume a 10 amp 5 ohm unit would be workable. I get most of my parts from MCM or Parts Express as I live in Ontario Canada and there are no real electronic distributors that sell online. I will see if I can get one from there.. I can get a compatible relay and just rewire it but the thermistor sound more up to date. Thanks for your help.. I will get back to you and let you know what happens. If you see smoke North of the border you will know it did not go well Cheers Wayne
     
  7. crutschow

    Expert

    Mar 14, 2008
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    So possibly some Northern Smoke to go with the Northern Lights? :D

    Yes, you want a thermistor that can carry about 10A nominal.
    MCM has these units. The 4Ω, 9.5A unit at the bottom should work for you.
     
  8. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    Thanks again for your help.. I will order the parts tomorrow and will also be changing the AC line cord as it is pretty stiff and old. I did look for a replacement relay and they are available but the Thermistor seems more current. The next step will be the dead channel.. Hopefully I can use the working channel as a guide to finding the problem.. I will also have to check all the output and drivers to make sure there are no bad transistors. Will keep you up to date and if I need more help hopefully I can call on you. I do have the manual with all the initial test and resistance checks so this will help.. Thanks again.. Cheers from Far North.. Wayne
     
  9. crutschow

    Expert

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    Feel free to post if you have any further problems.
    Good luck in getting it all to work. :)
     
  10. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    Just to let you know that the amp is up and running.. I fixed the relay problem, it turned out to a couple of .01 mfd caps that were across the ac line. I replaced them. The one channel was a ground wire off at the input board which took a bit of searching to find. So far it is running cool and plays well.. Almost blew out the test speakers, they only handle 25 watts.. Thanks again for your help Cheers form the Great White and now Snowy North.. Wayne
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Very very cool. Did you find that thread on how to modify this beast for even more power? I saw it and can probably find it again if you have any interest.
     
  12. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    Thanks for the tip. Unfortunately after playing for some time and when I fired the amp up again the line fuse blew. I did not change anything so some more parts must have failed. I removed the two amp sections and the line fuse holds and everything works, speaker relay clicks etc. so the power supply is not the issue.. So now I will have to look at the channels to find out what is wrong.. I am going to temporarily hook up each side to see if there a common problem or just one channel. This beast has given me grief but I will prevail.. As I said before, I do not know the history on this thing and it could have been really pushed causing some parts to fatigue a bit. After all it is old. I will keep you posted. Cheers...Wayne
     
  13. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    I found out what may have caused the issue I have. A small metal clip was found inside the amp, I did see it before but assumed it was secured well. It may have come loose and shorted some parts on one of the output boards. Will continue to look for burn marks etc on both.. \hopefully one one channel was fried by this. Cheers. Wayne
     
  14. wayneh

    Expert

    Sep 9, 2010
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    Good catch, and good luck.
     
  15. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    Found the poblem channel. Two of the output transistors are shot.. I do have replacements, MJ15024G bit I read somewhere it would be better to change all the transistors in case some of the remaining are weak to prevent further problems.. Does anyone know where to get the original Heathkit Transistors RCA 1B05...! Anyone have any thoughts on this full replacement. Cheers Wayne...
     
  16. crutschow

    Expert

    Mar 14, 2008
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    Generally transistor don't get weak. They either work properly or totally fail (short or open), so I wouldn't think you would have to replace all of them.
    But it would be good if you could slowly bring up the power after a fix and monitor the current. Do you have access to a Variac?
     
  17. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    No variac, but can you use light bulb instead ? I have heard of this but do not know what wattage to use. You put the bulb in series with one side of the AC line I believe.. Any suggestions if I cannot get my hands on a variac Thanks Wayne
     
  18. crutschow

    Expert

    Mar 14, 2008
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    A bulb can be used to limit the current although it's not as good as a Variac.
    Start out with a low power bulb, such as 10W and work up from there while you observe the amp voltages..
     
  19. Meerkat 67

    Thread Starter New Member

    Jan 29, 2015
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    Thanks.. Tried the bulb and no problems up to full voltage and power. I am letting the amp cook out while playing see if there any issues. After 24 hours all voltages are stable and the amp just runs warm no overheating.. I do not know what I am going to do with this thing, it is so big and heavy.. The relay still sparks a bit on start up as I did not go the thermistor insurge route.. Maybe I will change this and go for the thermistor. Thanks again for all the advice, as I said before I am not a certified technician although I have electronics as a hobby and enjoy a challenge. Cheers from the Great White North... It is snowing again and minus 15 F Wayne
     
  20. crutschow

    Expert

    Mar 14, 2008
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    Glad you got it all to work.
    Sounds like you'll need to find a cabinet to hide it in.
    If the relay operates properly, I wouldn't change it until/unless it fails. My motto: "If it ain't broke, don't fix it."
    And cheers from LA. I suppose it's mean to mention that we're near record high temperatures here. :cool:
     
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