moby

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About moby

  1. When the output mosfets die (not uncommon), the gate driver boards often die in sympathy. Follow back the gate lines. M
  2. Can anyone suggest a way of separating the two halves of Xti output device heatsinks to enable replacement of blown transistors. Everything I've tried involves force (hammers, jemmys ect) and leaves blemishes. Cheers Mike
  3. Couple of iTechs showing 2 ohms on both network and front panel monitor menu, when being used with an 8 ohm bench load (resistive). Other iTechs similarly rigged up work fine. Amp is otherwise working correctly and drawing no more current for a given output than good units. Where to look ???? Mike
  4. Common fault - the driver transformer is kaput. Primary should read at least 400uH (out of circuit) Replace transformer and ZTX transistors. M
  5. Network N102 - appears to provide reference for the low side driver cheers M
  6. Troubleshooting the Pulse 1100 power supply : Remove the PCB & front panel from the chassis Do not connect to mains Discharge mains resevoir caps and C75 with a stout resistor (VERY important - these things hold charge for ages) Short out pins 4 and 5 optoisolator OPT1 Hook a 250ma current limited 40 volt bench PSU across C75 and observe current drawn - it should be about 100ma You should be getting non overlapping 20 volt squarewaves across the E/G pins of the IGBTs Excessive current (anything over 120ma) usually means the gate drive transformer TX4 is kaput - they spring apart over time and loose inductance - they should measure around 600uH out of circuit between pins 1 and 2, anything less than 400uH is indicative of a bad transformer. If there is signs of PCB discolouration around Tr40,41,42,43 (caused by heating ie driving bad transformers) replace these transistors as well. With C1100's in protect, if the 50r ceramic plate resistors are broken, it is usually a channel, not the PSU, that is at fault. Many subcatastrophic channel failures sheet back to dodgy BF422s or BF423s, it is also vital to ensure rail commutating FETs are not shorted (amp will run, but overheat) M
  7. Edited - situation resolved, faulty resistor network. New Question - is there any reason why these little CTS networks cannot be emulated with 1% resistors M
  8. I have a CTs 2k which, when you switch it on, only the blue power LED comes on. None of the others ever come on. The +/- 157 volt and +/- 24 volt supples are present, as are the downstream +/= 15 volt and 5 volt supplies. The output FETs are OK. What could be preventing this amp kicking into life ?. Is there any "sequence of startup events" or other troubleshooting document available?. Cheers M
  9. I have 3 Macrotech amps in for repair (2 1200s and a 2400) all exhibiting the same symptoms. Setting the amp for stereo mode and driving channel one input only (leaving ch2 input either shorted or unterminated, it makes no difference) Setting input level such that ch1 output is 20 volts pk/pk, I'm getting the following signal appearing on the Ch2 output terminals. 100Hz - 250mv p/p 1kHz - 50mv p/p 10kHz - 15mv p/p So the crosstalk increases as the frequency goes DOWN - worse than 40dB at 100Hz Observations : Position of ch2 gain control makes no difference Crosstalk is unchanged by an (8 ohm) load on ch2 or no load (only the scope). The reverse situation is not true (ie Ch2 does not leak into Ch1) Note that the customer complaint was only of crosstalk in one of the amps - the other two were merely blown up. I just got curious and tested them after fixing the blow ups to use as bogeys for the one which had crosstalk complaints. Is this endemic to the breed ?. cheers Mike
  10. I have two XTi 2000 amps which are both exhibiting the same symptom - the fans run immediately at turnon, even if the amps are dead cold. There are no shorted transistors in the fan driver circuit. The CPU PWM output to the R389/C75 integrator is about 70% high (17ms on, 7ms off) ie the fans are being told to turn on. The three finger salute diagnostics reports LM35 temperatures below 30C for both channels and power supply so it seems to be a CPU issue. Have updated to firmware 1.1.4.1 but this didn't help. Amps work normally otherwise (after replacing the usual dodgy ribbon cable). M