VML
-
Posts
0 -
Joined
-
Last visited
Content Type
Profiles
Forums
Events
Articles
Downloads
Gallery
Store
Everything posted by VML
-
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.
- Show previous comments 6 more
-
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.