Martin Logan Montis Power Supply Board Help

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#41
Dear Bolrest

highly appreciate of your persistent support and advice , seems that it is beyond my competency, i had dismantle the power board and the cross over section and send it to local electronic repair shop . I will convey all technical information you give to him . Hopefuly he can find which part is malfunction . I will update soon with the result .
 
#42
neil5m said:
Hi, I recently replaced the PS supplies on my Clarity. A very knowledgeable forum member sent me the following circuit chart and explanation to understand the board better. Maybe it could be helpful:

Here is a typical ML high voltage board...the top left section is the music detect circuit, the input resistors and 4 diodes by the connector BB2 clamp the max music input voltage to IC1D to + or - .7 volts. IC1D amplifies the music signal and turns on Q2, discharging C6. Timing capacitors C5 and C6 give a turn-off delay, so it stays on for several minutes after the music stops, to keep the speakers charged between songs and album changes.
The high voltage section starts at the AC mains connector AC1, further down on the left, which feeds a step-up transformer, T1, then on to a voltage multiplier ladder circuit with diodes and capacitors around D15 to D20, on to four 15 meg current limiting resistors, and out to the speaker @ connector BB1.
When it detects music, C6 discharges, and the output of IC1C changes state, this puts voltage on connector BB3 to light the speaker LED, it also goes through an on-board LED D1, and turns on a triac TRIAC1. This enables the high voltage ladder circuit, and gives you the high voltage for the speaker.
Diodes D12, 13, 26, 27 form a bridge rectifier...that in conjunction with Zener diodes Z3 through Z13 clamps the maximum output voltage of the step-up transformer. By adjusting the number and voltages of the Zener diodes, you can adjust the high voltage that goes out to the speaker...(this board uses five 47 volt zeners [1N4756a], and five 12 volt zeners [1N4742a]...add them all up, and you get 295 volts...plus the two .7 volt drops of the bridge rectifier diodes) = 296.4 volts max output from the step-up transformer feeding the voltage multiplier ladder circuit...(this will likely vary depending on which speaker model that the PCB gets used with).
High voltage from the ladder multiplier circuit can be from 2500-5000 volts, and the capacitors (C13 to C22) will retain that voltage for quite some time, even after power is removed. Be VERY careful working on the PCB.
Other components on the board give you the low voltage DC to run the circuitry, etc.
If there was a problem in the high voltage section of the board, the music detect portion could still work, and light the LED OK, but not give the high voltage it should.
Click to expand...

Was reading your post of 29 June 2016 explaining the Martin Logan high voltage power supply . Thanks, very informative.

I am converting couple of 110v 60Hz board (same as the circuit diagram you attached then) to 220v 50Hz. I have replaced the 3W input resistors and the jumper as required on the input to the transformer, but it powers on for a few seconds before it powers off. I guess there might be a mains frequency check going on - apparently ML are famous for it!

I was wondering where you think in the circuit the mains frequency (50Hz/60Hz) takes place?
 
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