BLU-32 / 80 power supply problems when no cards fitted.
DeltaXRay
Posts: 8
We found an application for a BLU-32 without any audio cards (linked to another unit) and it ran fine for 2 years before zenner D1802 failed. This pre-regulates the 3.3v supply.
On investigation (and cross-checking with other units from the series), it appears that without the load of some audio cards, several voltages on the multi-pin connector between PSU and main board are substantially higher then when cards are fitted (see table below). One of these voltages, on pin 6, is 4.5volts higher, and it is this rail which is divided by R1810 & D1802 for the 3.3v SMPS.
We've replaced the zenner and all works again, but without audio cards it still gets unacceptably hot. We don't feel confident in returning it to service for that reason.
As a work around, we've loaded the +15 & -15 supplies on pins 5 & 6 with 120R power resistors, to increase the current and therefore reduce the voltages but these get hot too, and clearly this is not a realistic fix.
Can you advise a long-term solution to protect the pre-regulator when no cards are fitted?
Thanks
Voltages on Power connector with audio cards loaded/unloaded:
Pin 1 n/a
Pin 2 2mV / 9mV
Pin 3 5.11V / 5.14V
Pin 4 0V
Pin 5 -14.7V / -18.9V
Pin 6 14.7V / 19.4V
Pin 7 52.3V / 72V
Pin 8 15.5V / 21V
Pin 9 9.2V / 13.7V
Pin 10 -15.4V / -20.5
Measurements taken from a BLU-32. BLU-80 within 3% of the above.
On investigation (and cross-checking with other units from the series), it appears that without the load of some audio cards, several voltages on the multi-pin connector between PSU and main board are substantially higher then when cards are fitted (see table below). One of these voltages, on pin 6, is 4.5volts higher, and it is this rail which is divided by R1810 & D1802 for the 3.3v SMPS.
We've replaced the zenner and all works again, but without audio cards it still gets unacceptably hot. We don't feel confident in returning it to service for that reason.
As a work around, we've loaded the +15 & -15 supplies on pins 5 & 6 with 120R power resistors, to increase the current and therefore reduce the voltages but these get hot too, and clearly this is not a realistic fix.
Can you advise a long-term solution to protect the pre-regulator when no cards are fitted?
Thanks
Voltages on Power connector with audio cards loaded/unloaded:
Pin 1 n/a
Pin 2 2mV / 9mV
Pin 3 5.11V / 5.14V
Pin 4 0V
Pin 5 -14.7V / -18.9V
Pin 6 14.7V / 19.4V
Pin 7 52.3V / 72V
Pin 8 15.5V / 21V
Pin 9 9.2V / 13.7V
Pin 10 -15.4V / -20.5
Measurements taken from a BLU-32. BLU-80 within 3% of the above.
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