Help with Crossfire BMF 600.2

[kiermee]

Hey All,

I am here to ask your assistance with another repair. The amp is a crossfire BMF 600.2. It's a two channel amp.

Problem I am having , when the amp is powered up it turns on normal and plays normal, but once I turn it off and try to turn it back on it does not come back on. It has one green led and this would give a slight blink as the amp it trying to start. It I disconnect the B+ and leave off for a while 10 mins or more and reconnect and then power back up it turns on normal. If I then turn off and try to turn on it does not come back on.

There was a problem intially where a 5.6 ohm base resistor for a C4467 transistor went open. I changed it along with the the transistor. Everything else seems ok.

Seems as though something has to discharge before the amp can power up again.

Any assistance with this will be greatly appreciated.

Thanks in advance.

 

[Perry Babin]

Post a photo of the board.

 

[kiermee]

Thanks for your quick response. Please see attached.

If you require any other info please let me know .

Thanks again

Please let me know if you got the pics I sent six

 

[kiermee]

Here are three more

 

[kiermee]

I am sending one pic at a time not getting to upload all

 

[Perry Babin]

Is the voltage different on pins 1 and 7 of IC 502 when it powers up normally and when it won't power up?

 

[kiermee]

I measured the voltages to ground, see below:

On normal startup

Pin 1 = 4.84V
Pin 7 = 4.92V

When it won't power up

Pin 1 = 0.112V constant

Pin 7 = pulsing from 0.095V to 4.92V

 

[Perry Babin]

I've been trying to find a similar amp but I can't find one that uses the 494 and an op-amp that has the outputs high normally. I'll keep looking.

 

[kiermee]

Ok thanks I greatly appreciate all you assistance here.

 

[kiermee]

Hey Perry any luck as yet?

 

[Perry Babin]

No and I've looked through a lot of diagrams. I worked on an amp last night that used the 3525 instead of the 494 and had both op-amps high normally. One op-amp was used for thermal protection. Is the thermistor shorted to the case or does it have a broken leg?

 

[kiermee]

Thermistor touches the case. No broken legs

 

[Perry Babin]

When the amp tries to power up, does it produce any significant voltage on the rectifiers (or rail caps)?

Do you see a pulse of DC on the bridging speaker terminals of either channel?

 

[kiermee]

When it tries to start the rectifiers and the caps show 72 volts .

When I place meter across bridge terminals the voltage pulses between 4.21v to 5.2v

 

[Perry Babin]

Are you sure that it doesn't have any leaky/shorted output transistors?

 

[kiermee]

I will pull them out and check. The c4467 that had the open base resistor I changed that one but there are two others in parallel with it I will pull them out and see if the problem still exists. Will do later on and get back to you with my findings.

 

[Perry Babin]

The ones in parallel may have been protected when the first one in the group failed. When one output fails, it's generally one of the opposite polarity that also fails.

 

[kiermee]

True will go back true the two channels as I was running the amp in bridge mode when the problem occurred thanks so far for your time and expertise will get back to you

 

[kiermee]

Took out all outputs, they are all good. Same problem exists. Looks like q119 and q114 mpsa06 were both leaky . Changed them amp starts up normal so far. Tomorrow will put back in output transistors and retest. Will update you. Thanks for all your help much appreciated.

 

[kiermee]

See pic