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VML

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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. Show previous comments  6 more
    2. Rob

      Rob

      I always use these caps. Make a jumper and they fit where the old ones came from.

      IMG_5082 2.JPG

    3. VML

      VML

      Hello Rob - Thank you for forwarding your values for the pinout on IC2. After reviewing them I concluded that the most significant differences could be driven by the 12v supply. The differences at pins 7 and 11 concerned me the most. 
      Other differences were caused by my measurement errors as follows:
      Pin 3 was poorly labeled by me. The -12.238 vs your + 11.6 is not a true difference. I meant the apparent "-" as a hyphen not a negative value. The actual difference is 0.638v. 
      Pin 7 indicates a difference of 0.726v. This is a supply rail. This should be closer to -12 as your value indicates. This supply is Zener controlled at D141. I will recheck the reading I took. 
      Pin 8 is almost directly driven by the 12v rail which is controlled by D140 Zener and D1, D51 downstream circuitry. Although I saw no contribution from the Over Current sensor in the trip event I think my reading is definitely too high. Again I will revisit it. 
      Pin 11 looks to be grossly incorrect. This is the +12v rail. Might be flux on the pin ? Will recheck.     
      ******************************************************************************
      Following the 12v supply issue:
      I measured the T1 transformer output at +/- 16.1 VDC after the rectifiers. The Carversite schematics that I downloaded were marked at +/- 15.5VDC. So T1 is a bit hot, but the Zener's should address that. 
      Based on this I decided to replace the 12 volt Zener's at D140 and D141. Given that at almost 0.9 v delta, I thought this voltage should be better behaved. This did not make sense however given that a higher magnitude 12v supply would tend the to reduce the sensitivity of the Subsonic Offset circuit that I caught miss behaving rather than making it more prone to false triggering. I went forward anyway thinking that these changes would bring my numbers in line with yours. 
      After replacing D140 and D141 the 12v supplies were indeed closer to ideal at +/- 12.2v. Checking the specs on the original Zener's I found they were only 1 watt components. This would be very close to the operating limits given the high transformer output. I installed a 3 watt part. This change caused a difference in the +55v supply as well as the final drive bias. After resetting them to 56.0v and 3ma respectively I remeasured IC2.
      My new numbers:
      1) + 0.021   8) +12.21
      2) + 1.50    9) + 8.37
      3) + 11.76    10) - 11.71
      4) - 4.33    11) + 12.3
      5) - 1.42    12) -11.73
      6) - 1.42    13) + 0.53
      7) -12.21    14) + 0.61
      To test, I drove the amplifier with a swept sine wave from 100 hz to 10 Khz with the input at 1 volt pk-pk into both channels simultaneously. I could let the amp finals cook up to 150 F whereas It would trip at about 130 F earlier. I'm not going to say I understand this new better behavior, but I will take it.
      I followed up with the ultimate test - the Doobie Brothers album at 2/3 volume into resistive loads. The amp responded by never exceeding 120 F during the playback. I repeated the test 4 times and no tripping occurred. I connected the speakers and again no protective tripping. 
      I would be happier if I could explain why this change mattered to this degree ( I never caught any events associated with the the 12v supplies ).
      Thanks for your assistance. Your values drove me in the correct direction and we will never know why. 
       

    4. Rob

      Rob

      I am glad you were able to get somewhere with the help I gave ya. I try to take diagrams and write in good voltages so it can help me down the road when diagnosing issues. 

      ~^

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