Marshall AVT150H

[Carlosraj]

Hi


This amp came to me with no power. Upon checking, the fuse had blown. On further checking, I realised that both the power ICs (TDA7293) have cracked, blown. I checked the power supply circuit, all the caps, diodes, bridge rectifiers and resistors are OK. I took the signal from the 'emulated line-out' to another amp and it works fine.After replacing both the power ICs, I brought down the mains voltage from 240VAC to 100VAC plugged in the speaker and without any input, switched the amp ON. There was some buzzing sound and one of the power IC started to get very hot. Before I can turn off the amp, the IC smoked and cracked, the other IC has no cracks but not sure whether it is OK or not. I am wondering what could have caused the IC to blown, even at a much lower voltage (100VAC). Please assist.


Thanks

 

[Osvaldo de Banfield]

Check external components to the IC: diodes, resistances, capacitors, etc. In the next turn on, use a filament lamp in series with the mains, for example, a lamp wired with a pair of short piece of wire an a couple of crocodiles, placed in the fuse holder. So, if anything goes wrong the lamp will bright protecting new components, or reducing probability of severe damage to them. Test any component until light brights very dimmed.

Lamp must be ONLY filament type (Not led nor fluorescent), and more or less, the same power that the DUT's expected power consumption.

 

[Mark Tillotson]

Sounds like the power is wrong, backwards, completely over-voltage, or perhaps a hard short in the output (protection circuitry failed?).


Did you measure the power supply outputs before replacing the amps?


Another possibility is large scale oscillation. For testing that I would place current limiting resistors in series with each rail from the PSU to prevent damage (except to those resistors!)
Perhaps 100 ohms kind of value, assuming no load on the amp.


Any means of limiting current is useful for commisioning/troubleshooting semiconductor circuits.

 

[JonSnell Electronic]

Make sure the ICs are isolated from the chassis. The ic metalwork is at a DC potential!

 

[hex69]

Check external components to the IC: diodes, resistances, capacitors, etc..

If you made sure the ICs are well isolated from the chassis (as suggested by JonSnell), I agree that capacitors may seem good but doesn't mean they will perform properly when power is applied.

Seems that filter caps, the two decoupling caps on both boards (47uF) and (possibly) the 22uF 63v caps (bootstrap) may cause the ICs to blow.

Have a look at the last post here

 

[Carlosraj]

If you made sure the ICs are well isolated from the chassis (as suggested by JonSnell), I agree that capacitors may seem good but doesn't mean they will perform properly when power is applied.

Seems that filter caps, the two decoupling caps on both boards (47uF) and (possibly) the 22uF 63v caps (bootstrap) may cause the ICs to blow.

Have a look at the last post here

OK, I will replace the decoupling and bootstrap caps. Will make sure of the isolation from the chasis and to totally discharge the caps before fitting in the new ICs. Just wondering whether the 2 (CD4011) and one (CD4066) on the main board OR any problem on the front board can cause this problem.


Thanks

 

[hex69]

OK, I will replace the decoupling and bootstrap caps.

Being there, you could also make sure pin 12 on the TDA7293 (Bootstrap Loader) is making good connection to pin 8 on the power amp board connector, pads can be damaged because of heat from soldering/desoldering.

 

[Carlosraj]

I did replace the bootstrap and decoupling caps, discharge the mains caps before connecting the power PCB connectors. Made sure of isolation between the IC tab and heat sink. Now it works fine.

Thanks, all for your assistance. You guys are great!