Marshall AVT100

[Carlosraj]

Dear Everyone,

A friend of mind gave me a Marshall AVT100 guitar amp and asked me to help him fix it. The power supply was OK, but no sound. I connected the "line out" to another amp and it works fine, so I replaced the 15 pin power output IC (TDA7293) and it works. But there were 2 problems.

1) When I plugged in the cooling fan connector, the fan works but the sound becomes distorted and slowly fades away (no sound).

2) On the "Clean" channel, when I turn on the "gain' and "volume" with the bass, mid and treble completely turned off, there is no sound. Only when I turn on the tone controls (Bass, treble, mid) then there is sound. Normally I guess when you turn on just the volume and gain control, there should be sound totally independent of the tone controls.

I replaced both the +15 and -15 voltage regulators, but no change. As I am new to this amp, can someone kindly advice or provide me some hints on what could have gone wrong.

Thank you

Carlosraj

[Ronsonic]

First things first, assumption number two is completely unfounded.

Now the actual problem. You're saying that the amp fades out when the fan is plugged in? I don't have a schematic to hand, but you've got the amp there. Which voltage supply is going away with the fan plugged in?

[Carlosraj]

Marshall AVT199

Dear Ronsonic,

What I mean is, with the bass, mid and treble off and I turn on the gain and volume on the clean channe, there is no sound. With the gain and volume turned on, then when I turn the bass or treble or mid, then there is sound. This is only on the clean channel. I would not know which voltage supply is going off because I do not know where to take the readings, but the sound distorts and then fades away. At this point, when I take the line output and plug into another amp, it works well, which means only the supply or signal to the power amp is effected. This happened only after I replaced the power output IC (TDA7293). Could there be any component shorted, where should I check the voltages.

Thanks

Carlosraj

First things first, assumption number two is completely unfounded.

Now the actual problem. You're saying that the amp fades out when the fan is plugged in? I don't have a schematic to hand, but you've got the amp there. Which voltage supply is going away with the fan plugged in?

[Jazz P Bass]

Marshall AVT100 fan circuit

Pin #1 of the fan connector ties directly to the input pin of the -15 volt regulator. The input pin is the pin in the middle of the package.
It is marked as REG200.
Pin #2 of the fan is ground.
If the fan is pulling that point down, you may have a bad fan.
I would go back & check the work done on & around the -15 regulator.

As to the tone controls killing the signal when turned full left, I would recheck that all the connectors are properly installed.

[Ronsonic]

Dear Ronsonic,

What I mean is, with the bass, mid and treble off and I turn on the gain and volume on the clean channel, there is no sound. With the gain and volume turned on, then when I turn the bass or treble or mid, then there is sound. This is only on the clean channel. I would not know which voltage supply is going off because I do not know where to take the readings, but the sound distorts and then fades away. At this point, when I take the line output and plug into another amp, it works well, which means only the supply or signal to the power amp is effected. This happened only after I replaced the power output IC (TDA7293). Could there be any component shorted, where should I check the voltages.

Thanks

Carlosraj

I don't have one of these amps or the schematic to hand. BUT, it is normal for the standard Fender / Marshall tone stack that turning all the tone controls down kills the signal.

Or are you saying that turning any ONE of them down kills the signal?

The problem with the fan is a bit odd. Since it happened after you did your work, I would check for solder bridges and things left unsoldered first.

[Jazz P Bass]

Marshall tone control

When you use the line out does the Clean channel tone control now function normally?
That would be odd.
Here are the schematics.

[Carlosraj]

Marshall AVT100

I don't have one of these amps or the schematic to hand. BUT, it is normal for the standard Fender / Marshall tone stack that turning all the tone controls down kills the signal.

Or are you saying that turning any ONE of them down kills the signal?

The problem with the fan is a bit odd. Since it happened after you did your work, I would check for solder bridges and things left unsoldered first.

Dear Ronsonic,

The signal gets killed ONLY when ALL the tone control are down. Thanks for your advice on the Fender/Marshall tone stacks. As adviced I will check on the soldering.

[Carlosraj]

Marshall AVT100

When you use the line out does the Clean channel tone control now function normally?
That would be odd.
Here are the schematics.

Dear Jazz P Bass,

When I use the line out, it is the same. the signal gets killed when all the tone controls are down. As adviced by Ronsonic, I assume it is normal with Fender/Marshall amps. Thanks alot man for the schematics. They will be of a great help.

Carlosraj

[dobrota]

Dear Everyone,

A friend of mind gave me a Marshall AVT100 guitar amp and asked me to help him fix it. The power supply was OK, but no sound. I connected the "line out" to another amp and it works fine, so I replaced the 15 pin power output IC (TDA7293) and it works. But there were 2 problems.

1) When I plugged in the cooling fan connector, the fan works but the sound becomes distorted and slowly fades away (no sound).

2) On the "Clean" channel, when I turn on the "gain' and "volume" with the bass, mid and treble completely turned off, there is no sound. Only when I turn on the tone controls (Bass, treble, mid) then there is sound. Normally I guess when you turn on just the volume and gain control, there should be sound totally independent of the tone controls.

I replaced both the +15 and -15 voltage regulators, but no change. As I am new to this amp, can someone kindly advice or provide me some hints on what could have gone wrong.

Thank you

Carlosraj

Did You mesare +-15V voltage supply after connecing fan ? Maybe it has short circuit, it decrease minus voltage for power amp / preamp and than sound is distorted.

[Mars Amp Repair]

Also notice that the source for the filament of the 12AX7 preamp tube comes from the same point that the fan does.

If there is something wrong with the - supply from the bridge rectifier diode, loading it down with the fan could be the reason the sound 'fades' out as the filament slowly extinguishes.

I'd monitor the - voltage out of the bridge & see if connecting the fan causes it to diminish. I'm guessing that voltage will be about 20-24Vdc. Also if C48 2200uf filter cap were weak, that could also create less minus voltage available for the fan & filament loads.

Also check connections on the larger components ie: bridge rectifier, filter caps & cement block resistors. Also an intermittent bridge rectifier diode (B201) has also been known to cause similar issues in some models.

glen