Calibra DIGI 1250

[chrisg4]

Hi all,

this amp doesn't turn on, switchs between protect and power leds.

psu and output section dont have any shorts.

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[Perry Babin]

Is the power supply trying to power up to where you see positive and negative rail voltage try to build?

 

[chrisg4]

Hi Perry,

yes the rails are trying to build, I measured at the rectifiers.

 

[Perry Babin]

Are you seeing pulses of DC approximately the same as the rail voltage on the center legs of the output FETs?

You likely already know but this amp is likely similar to the following:
http://www.bcae1.com/temp/bp1200p1_sm.pdf

 

[chrisg4]

Rails get up to about 70v, center legs pulse from 0v to 22v, 30v and back to 0v.

 

[Perry Babin]

Do you have a scope?

 

[chrisg4]

Yes I do

 

[Perry Babin]

Do you see any oscillation on the center legs of the output FETs as the amp tries to power up?

 

[chrisg4]

Ok, I will get out the scope and test.

I've just used a 7.5v 1.5a psu and the amp powered up.... but doesn't with a 12v supply

 

[chrisg4]

An externally hosted image should be here but it was not working when we last tested it.

not sure if this was the issue, earth was loose. amp is turning on now.

haven't gotten my scope yet to check.

 

[Perry Babin]

In some class D amps, a bad heatsink ground can cause an amp to malfunction.

 

[chrisg4]

The amp worked for 1 day, but its back to flashing between power and protect

 

[Perry Babin]

It likely has an intermittent problem or it has a new failure.

Did you check the outputs again?

 

[chrisg4]

i think intermittent fault,

i was checking the voltage regulators, the 7812 outputs 11.1v, could that maybe cause an issue? also the heatsink ground seems to kick the amp into protect, if i disconnect it, it will power up

 

[chrisg4]

Hi Perry,

these resistors circled are getting very hot, is that normal?

 

[Perry Babin]

I don't know this amp but resistors are typically sized based on the amount of heat that they will have to dissipate. You can generally expect large resistors to get hot.

 

[chrisg4]

Can I use the amp without the heatsink ground? when its connected it engages the protect circuit, pin4 of 494 goes high.

 

[chrisg4]

I'm reading a short across the thermistor in circuit.

 

[Perry Babin]

If this is your own personal amplifier that you don't intend to sell, you can do what you want with it.

If someone is paying you to repair it properly, you have to repair it properly.

 

[chrisg4]

it is my personal equipment...

post #18
Could a faulty thermistor cause this issue