Marshall AVT20 no sound

[bfglp]

So I exhausted the web searches with no solutions

R53 was extra crispy so I replaced it leaving it elevated
C71 got a little diss colored but test ok with ESR meter
Board is grounded
No DC load to speaker
Tube test good even swapped it with no luck
Solder joints all look good

With everything on ten I can here faint guitar through speaker, headphones and extension speaker. If I make reverb rattle that’s louder.
Faint output through CD

i ordered a new TDA2050

Any help greatly appreciated thx

[The Dude]

Schematic please? Those circuit board designation numbers mean nothing without one.

[Jon Snell]

It is unlikely to be the TDA as they either work or not.
Check the HT for the valve, check for supply to the rest of the circuit, check for bad loop through jack connectors.

[Mick Bailey]

The burnt components suggest oscillation, which will quickly damage the output IC. Make sure the components surrounding it are good - check the data sheet for those components listed as "danger of oscillations" - such as the supply caps C47,C48,C45,C49,C50. Make sure these check out as well as their solder joints. Also, there are fake TDA2050s around - buy only from a reputable suplier.


Marshall avt20_60_02.pdf

[bfglp]

The burnt components suggest oscillation, which will quickly damage the output IC. Make sure the components surrounding it are good - check the data sheet for those components listed as "danger of oscillations" - such as the supply caps C47,C48,C45,C49,C50. Make sure these check out as well as their solder joints. Also, there are fake TDA2050s around - buy only from a reputable suplier.


Marshall avt20_60_02.pdf

Thanks for pointing the schematic I couldn’t get one uploaded and
the supply caps checkout
I’ll post an update after I swap out the TDA2050. I was pushing the amp a bit hard with some pedals that may have triggered the oscillation

[Jazz P Bass]

I would replace R53 & C71, just to be sure.
Here is the TDA2050 datasheet.

[Mick Bailey]

Take a really close look at all the tracks/pads. A microscopic cracked solder joint could give rise to oscillation and the TDA2050 is susceptible to this.

[bfglp]

I would replace R53 & C71, just to be sure.
Here is the TDA2050 datasheet.

I see people on other forums raised the value of R53 from 1/2 to 1w what’s your thoughts on this?
i replaced with another 1/2w just elevated 1/4” above the board

[Mick Bailey]

There's no issue with increasing the rating, but if the original resistor is burning up there's something else going on with the amp which is causing the resistor to cook. Changing the value may make the resistor last a little longer under a fault condition, but my thinking is the problem with the amp is not the resistor wattage.

[bfglp]

Take a really close look at all the tracks/pads. A microscopic cracked solder joint could give rise to oscillation and the TDA2050 is susceptible to this.

The solder and circuit board look quite good under my desktop led magnifying glass

[bfglp]

Take a really close look at all the tracks/pads. A microscopic cracked solder joint could give rise to oscillation and the TDA2050 is susceptible to this.

finally getting back to this project and after another inspection I found a piece of wire looks to be a guitar string clipping under the red led lights near the input jack. It was loose and came rite out

[Mick Bailey]

It may be the reason for your problem, or coincidence. Either way, it's not good to have something loose inside the amp that could cause a short.

[bfglp]

It may be the reason for your problem, or coincidence. Either way, it's not good to have something loose inside the amp that could cause a short.

The previous owner said it had an intermittent problem, I had no problems before this

so so I replaced r53, c71, the TDA2050, c47, c48, c49, c50 and still no sound through the guitar input but it’s louder through the CD input
all the shunts are functional on the input jacks and the 12ax7 is good

[Mick Bailey]

It's louder through the CD input, but is it as loud as it should be? If not, then the problem is simplified because the CD input connects to IC4b. Check the DC supply voltages on that IC - should be -15v on pin 4 and +15v on pin 8. While you're at it, check to see if there's any DC on pin 7. If there's anything significant on pin 7 the IC is bad.

[Jon Snell]

The previous owner said it had an intermittent problem, I had no problems before this

so so I replaced r53, c71, the TDA2050, c47, c48, c49, c50 and still no sound through the guitar input but it’s louder through the CD input
all the shunts are functional on the input jacks and the 12ax7 is good

AsI said, TDA 20** either work or they don't.