what went wrong?

[eyekode]

I was given a very large amp (6 two channel modules using TDA7294's running off +-35v).
Each of the modules has separate inline fuses for the positive/negative voltage lines.
All but one of these 12 fuses are blown.
I disconnected all the amp modules from the ps and tested the power supply in isolation. It looks sane.
I am scratching my head trying to figure out how this happens... All outputs shorted? But that shouldn't blow the fuses because the TDA7294 has short circuit protection as per the spec sheet.
Water maybe? But the PCB's look fine (and the PS looks good). Someone hooked up 12 low impedance speakers (not low enough to trigger short circuit, but enough to blow the fuses) and played them all maxed out at once? Seems an unlikely use model for this amp (it is meant to be used in a whole house AV distribution network).

Also, the ratings of the fuses confuses me. The fuses are listed as F4AL250V. Which sounds like a fast acting 4amp fuse. With 35v rails and 4 ohm load wouldn't 4 amps be a little low? At 20Hz it would be pulling close to 8 amps for ~1/60th of a second or so?

Thanks!
Salem

[AndrewT]

Hi,
I have tested a pair of discrete amps recently.
F3.1A allowed 311W into 4r0, Ipk=12.5Apk.
F4A allowed 453W into 2r0, Ipk=21.3Apk.

The F4A are too big to be effective for a chipamp running with 8ohm speakers. They will take too long to blow and by then the damage may be irrepairable. F2A in each supply rail, AFTER the main smoothing capacitors, should be sufficient for 60W into 8ohm.

+-35Vdc supplies seems a bit high but if the speakers were all 8ohm or above the chips should be OK.
I wonder how hot it all gets inside when 12 amps are all running?

[Nordic]

Firstly... fuses can age and die......

But look at a datasheet for a fuse.....
If it is rated at 4A... this is the current it can carry without ever breaking.... It will break if the current is in excess of that.... but only after considerable time........

To actualy trip at 4A, you would need a fuse in the hundreds of milliamp region....

If all looks healthy... try another 4A fuse... but only once, and quickly... If it trips... first get it repaired...

EDIT... sorry, this message has been in my que since early in the morning... stupid ADSL no bandwidth available... due to stupid government owned telkoms being money sharks....

7294, can be run very efficiently.. with low voltage supply combined with B+ supply for when power is needed.... my yamaha subwoofer uses that principle... same chip..

[eyekode]

Thanks for the info.

I imagine it produces quite a bit of heat. But it seems to have the heat sinks to handle it. It has heat sinks across the whole width of the chassis that are ~6 inches tall. It also has two temperature controlled fans.

I am planning on replacing one set of fuses and bringing the amp up slowly. Wish me luck