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MA5000VZ soft start problem


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#1 kevvywevvywoo

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Posted 05 February 2013 - 03:49 PM

I have another odd one here, a repair that came in for one fault but is displaying another.

While testing, I found the amplifier was not soft-starting on ChB (I found out by lifting each main transformer fuse).

I found the 5R thermistor blown and scorched. I replaced it and checked the pwm pulses on the opto-triac were ok. When I switched on, the amp soft-started properly and didn't trip any breakers.

However, occasionally, maybe one in 10 starts, the soft start triac operates in half-wave. This causes excessive current at startup until the relay closes, a loud buzzing from the transformer, and a strobing desk lamp. The amp works fine apart from that.

I have changed the triac, snubber capacitor, checked the snubber resistor and scoped the pwm waveform, all ok.

Any ideas?

<_<

#2 joust

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Posted 08 February 2013 - 02:42 PM

Triac displaying half-wave? That is quite odd... Unless it receives a half-wave command.
I doubt very much that the ss Triac, or snubber and/or al. could be the culprit since they work with full wave. So, since the command must come from the gate side, the MOC (opto) could be at fault. The only way the command could be half-wave is if the MOC's triac operates in half-wave! Since the MOC'S triac gate should be receiving the full intensity of the driving LED, then the only culprit should be the MOC!

Alain

#3 kevvywevvywoo

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Posted 19 February 2013 - 03:25 PM

I already changed the MOC opto. Scoping the drive waveform, its fine. When the fault occurs, the amp takes a steady 2A mains current until the bypass relays operate, then the current falls to normal idle level.

#4 joust

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Posted 22 February 2013 - 10:16 AM

When the ss (soft start) Triac is in half wave, the command at the gate appears normal? I have an MA5K and I'll report soon on the gate's signal.
Stay loose!

Alain