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About VML

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  • Location
    North Carolina
  • RealName
    Vin Lisica
  • Occupation
    Electronics Engineer

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  1. Hello - I'm interested in some help debugging a problem with my M 200-t amplifier.  I find that I am able to play content from a tuner without any issues but find that I have protection circuit trips with records that have strong bass content. The Doobie Brothers are particularly good at causing a "trip" to occur. I have tested the various protection circuits per the service documents where it is indicated that a 2hz signal will cause a trip and a 4 hz signal will not (on the bench with 8 ohm 120W loads). I thought this frequency difference was extremely close, almost insignificant, but the additional 2 hz did in fact matter as was indicated. The infrasonic filter was active and I am using a magnetic cartridge feeding input 1 ( not the moving coil input ). I get protection tripping at about 50% volume playing. This is a louder than normal use for me but it causes tripping reliably in about 1 hour.  BTW: Full power is set at 32 Vrms into 8 ohms with 56.0v at now recapped 3000uf electrolytic caps ).  

    The very low intermittency has made this a pain to trouble shoot. I'm feeding 60w Wharfdale speakers and I have tried other units as well. No difference. I have a decently equipped electronics lab and I'm interested in suggestions in narrowing the problem to a solution while still maintaining the original protection limits. 

    1. Rob


      I am thinking that when you say 3000 uf caps that you have replaced the original 55v rail caps with (4) 3000's? It might not make a difference but have you tried replacing them with (2) 63v 6800uf caps? When the unit goes into protection mode, did you measure voltages at the IC2 pins to see what changed? I am not a big scope user but do use the meter a lot. I would look for voltage changes and then check components in that circuit. 

    2. VML


      Hi Rob and thanks for your comments. The new replacement caps actually measured around 3200uf and yes there were four of them.  The cost differential on the smaller, lower capacitance  caps justified the added work to mount the additional two units.  I think you are correct about measuring the IO of IC2. I was heading in that direction just as you posted your reply. Catching the trip even with a meter might be difficult. As I indicated I have used a generator to provide low frequency signals on the bench and the unit tested OK. I did not have the same run time however as when playing a record so there might be a heating factor involved as well. I'm trying to get a handle on the signal margins for a trip from the various protection circuit elements and I also want to identify cold and hot reading for these areas.  It will be a while before I can get back to the Carver. I am also chasing down a Marantz that took a lightening strike at the moment. Vin 

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