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OK to use Variac? DBT?

Discussion in 'Yamaha' started by FlaCharlie, Dec 6, 2018 at 12:46 PM.

  1. FlaCharlie

    FlaCharlie Well-Known Member

    Messages:
    615
    Location:
    Gainesville, FL
    I've got a couple of pieces of Yamaha gear that haven't been powered up in a while - a CR620 receiver and an M-60 power amp. I powered both up when I first got them but they've been unused for several years.

    I'm used to working on tube gear and using a variac and/or DBT on startups. I did some searching on the SS forum and here and read some posts that suggest that Yamaha gear, or at least some of it, should not be powered up with variacs or DBTs and that doing so might actually cause damage.

    So, what about the models listed above? Safe or not?
     

     

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  2. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    The models in question are actually right in the middle of your 2 examples.

    Problematic are the M-50 & M-70 power amps and A-760, A-960 integrated amps. Basically anything with the "X-Power" power supply, ca 1982-1983. They don't like to be brought up on a dim-bulb-tester but have no problems with being powered up with a variac.

    Cheers,
    James
     
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  3. mbz

    mbz Super Member

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    Thanks for the tip. I have a dead M-70 and was planning to build a DBT for it, but will settle with the trusty variac.
     
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  4. FlaCharlie

    FlaCharlie Well-Known Member

    Messages:
    615
    Location:
    Gainesville, FL
    Thanks for the response but I'm a bit confused by your answer.

    I can see that the M-60 is obviously part of the series that includes the M-50 and M-70. Not sure how the CR-620 is connected to the A-760 and A-960. I thought the other models in that series were the CR-420, CR-820 and CR-1020.

    And when you say "they" don't like to be brought up on a DBT but that using a variac is OK, I'm confused about which models you're referring to? The M-60? . . . or the M-50 and M-70? All three of them? The CR-620? . . . or the A-760 and A-960? or all of them?

    From what I've read the issue seems to be with the "X-Power" power supply and the use of VFET transistors.

    Do the models I have use this type of power supply and/or the VFETs?
     
  5. savatage1973

    savatage1973 Addicted Member

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    You have to realize that although the DBT and variac perform a "similar" function, they do not function the same. Most variacs limit/control voltage only, and have no provision for limiting current in-rush. A DBT limits current, and correspondingly voltage. As the filament of the bulb heats up, the resistance increases, which in turn limits current, and also drops the voltage due to the resistance increase. In many cases, it is prudent to use both at the same time--wall source to variac to DBT.
     
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  6. FlaCharlie

    FlaCharlie Well-Known Member

    Messages:
    615
    Location:
    Gainesville, FL
    I understand the difference between the variac and the DBT and how they work. I've been using them for many years working on tube gear.

    My question involves using them on these specific models of Yamaha SS gear. I've seen some posts that indicate that certain models of Yamaha can be damaged if a variac, DBT or some combination is used. I'm trying to find out the correct method of safely powering up these particular models.
     

     

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  7. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    I'm sorry for not being clear. I'll try to correct that...

    The M-50 and M-70 are unique power amps in the Yamaha M-xx lineup and have the x-power supply. They along with the A-760 & A-960 (which also have the x-power supply) are not recommended to be powered up with a dbt. I don't know if the A-760II and A-960II still have the x-power supply. The M-50/70 do not have vfet transistors. The B-6 power amp also uses the x-power supply, I don't recall if it has vfets.

    The M-70 service manual specifically calls for using a variac when troubleshooting the power supply (page 8).

    The M-40/45, 60/65 and 80/85 have a completely different psu than the M-50/70 etc and using a dbt with them won't cause any problems.
    The CR and CA amps pre-date all of the above mentioned amps and have a conventional psu to the best of my knowledge and using a dbt with them wont afaik cause any problems.

    Cheers,
    James
     
  8. FlaCharlie

    FlaCharlie Well-Known Member

    Messages:
    615
    Location:
    Gainesville, FL
    So using a DBT should be OK with the M-60 and CR-620. What about using a variac on them?
     
  9. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    I've never heard of any problems from powering up with a variac. Double check that by asking Avionic.

    Cheers,
    James
     
  10. avionic

    avionic " Black Knights " Subscriber

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    According to the schematic they do.
     
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  11. avionic

    avionic " Black Knights " Subscriber

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    None of these have VFET
    No
     
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  12. avionic

    avionic " Black Knights " Subscriber

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    DBT or Variac won't hurt anything.
     
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  13. FlaCharlie

    FlaCharlie Well-Known Member

    Messages:
    615
    Location:
    Gainesville, FL
    Thanks for the definitive answer.
     

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