Mackie SDR 24/96 Power Supply Problems(DC to DC converter)

[inkster]

I have a Mackie SDR 24/96 on my bench with power supply problems, the issue lies with the DC to DC converter supplying analog board with +/-16V using a 150 khz oscillator.
The oscillator is not oscillating so it's not producing the associated voltages of course.
I replaced the 2 dual mosfet packages and that did nothing, I diode tested all the transistors in the circuit and all of those test ok as do the diodes.
Transformer windings seem ok too.
Never made much of an effort to wrap my head around this type of circuit so I'm encouraging anyone with a good knowledge of this stuff to help me understand it better and perhaps point me in the right direction.
The 12V, 5V and 3.3V supplies on the board are all present.

 

[Khron]

I replaced the 2 dual mosfet packages and that did nothing

As in, those dual 4401 / 4403 non-MOSFET devices (dual NPN and dual PNP transistors, respectively)?

Is R5 still in one piece? Seeing 12V across C3/C4?

 

[inkster]

As in, those dual 4401 / 4403 non-MOSFET devices (dual NPN and dual PNP transistors, respectively)?

Is R5 still in one piece? Seeing 12V across C3/C4?

R5 is intact, I'm getting 9.97V at C3&C4, the mosfets are Q5a&b, Q8a&b

 

[ccaudle]

Look at the voltages around the MOSFETs. When the 10V is applied to the pin3-pin2 center-tap of the transformer, that should send 10V to the drains of Q5A and Q5B, which then sends via R1 and R2 10V to the bases of Q1 and Q2. That turns on Q1 and Q2, which sends positive voltage to the gates of Q5A and Q5B, which causes the voltage at the drains to go low.
That would cause both MOSFETs to turn off, which would cause the drain voltages to go high again.
I think that R14/C11 is supposed to form a resonant tank with the inductance of the transformer primary so that you get the two ends of the transformer primary alternating, rather than moving together.

Do you have a digital 'scope which would let you capture the waveforms at TP1 and TP2 as you power up?

 

[Ike Zimbel]

I have a Mackie SDR 24/96 on my bench with power supply problems, the issue lies with the DC to DC converter supplying analog board with +/-16V using a 150 khz oscillator.
The oscillator is not oscillating so it's not producing the associated voltages of course.
I replaced the 2 dual mosfet packages and that did nothing, I diode tested all the transistors in the circuit and all of those test ok as do the diodes.
Transformer windings seem ok too.
Never made much of an effort to wrap my head around this type of circuit so I'm encouraging anyone with a good knowledge of this stuff to help me understand it better and perhaps point me in the right direction.
The 12V, 5V and 3.3V supplies on the board are all present.

You haven't said if you've dealt with the electrolytic capacitors or not. If not, you most likely need to replace them, with 105* low ESR types like Panasonic FM or FR. I couldn't see the schematics well enough to make out part values, but SMPS have a small e-cap at the front end of the circuit that is part of the start-up circuit. If they go high ESR, they do not have the HF response to get that oscillation going.

 

[inkster]

I neglected to mention that I replaced the STS5DNE30L Mosfet packages with IRF7313TRPBFXTMA1 which is waht I had quick access to.
I'll check out the caps too when I get time.
Thanks for all the great info!

 

[MidnightArrakis]

I have a Mackie SDR 24/96 on my bench with power supply problems

I also have a MACKIE SDR24/96 that I used for doing "LIVE" recording sessions for years and years. However, I haven't been able to use it at all ever since the RAM battery died and I have never gotten around to taking it all apart to try and locate what type of battery this thing uses!!! Two questions:

1) Do you know what type of battery the SDR24/96 uses? (I'm simply guessing it might be a standard "2032" type).

2) Do you know where it's located once the cover is removed?

THANKS!!!



/

 

[Mihobro]

If I remember correctly, the CMOS may need to be reconfigured, as well as firmware may need to be updated if the CMOS battery died, Some boards had 3.6V 2/3AA batteries but I think you will need to take the cover off to be sure. This it is pretty much a PC so you need a keyboard, mouse and display, If there is no display card or port on the main PCB you have to find a PCI display card and monitor and keyboard with a large round circular connector and MINI din connector mouse. Anyway, further info is available in the discontinued products pages on the Mackie site.

 

[Hubbub]

For the supply to start the threshold voltages of the STS5 parts needs to be low enough that one of Q5A/B turns on some. If your IRF devices have a higher threshold specification the supply may never start.

 

[Mihobro]

Also a short in the analog board often will load down the power supply. try removing the analog board and see if the rails come back.