Marshall Valvestate 2000 ATV 150 - prank on repairperson?

[xtian]

I have an ATV 150 on my bench with the helpful complaint of "no sound." Power lamp illuminates, but no LEDs on the panel light up.

I open it, and after tracing the power and gazing at the PCB boards, I notice two empty spaces where resistors used to be (see photo). With these missing, NO power can get to the circuit. It's as if someone clipped all the spark plug wires out of a car, and sent it to the mechanic, saying, "Won't go."

Is this a prank? What the heck? Someone clipped these resistors, buttoned the combo back up, and sold it. OK, then.

So I tack in a pair of resistors. Now I have panel lights, fans are whirring, and single preamp tube is glowing. But still no sound. Can't access any of the preamp circuitry without removing all the knobs and exposing the panel PCB.

[xtian]

Schematic.

[mr_hankey]

If I had to take a guess based on almost no information, I might say that the owner tried to "mod the amp", messed it up completely, and is too embarassed to admit it.

[roberto]

I've repaired one with same problem. The small PA IC was blown.
IIRC red leds on the board are simply limiters.

[xtian]

I've repaired one with same problem. The small PA IC was blown.
IIRC red leds on the board are simply limiters.

Which component(s), roberto?

[roberto]

Hi xtian,
its name is TDA7293, it's on the last page of the schematic.
Looking the naked amp, it's the one mounted alone on a pcb (plus heatsink) in front of the fan.



I'd take care of connections too.

[roberto]

To see if the preamp works, connect a speaker cab into the input of the amp.
the other end floating. Then set the multimeter on Vac and follow the signal.

This is the cheapy way.

[xtian]

Woah! Lost me. Plug a speaker cab into the input?!?

[Leo_Gnardo]

Woah! Lost me. Plug a speaker cab into the input?!?

roberto's usually on the ball but today there's some sort of distranslations.

I think he's suggesting you run a signal generator into the input, leave the output disconnected, and trace the signal thru the amp with AC voltmeter. You could also use a scope or signal tracer. Regular stuff.

A speaker can be used as a microphone of sorts but in this case I don't see how that would apply.

Good luck!

[roberto]

Eheh! Sometimes I'm dislessic.

Leo you are right about the low cost suggestion.
I suggested a low cost signal generator (an unshielded cable) on the input and a low cost oscilloscope (a multimeter set on AC voltage) to check the circuit.

[xtian]

Should have guessed! No worries--I'm outfitted with a signal generator, a scope, and a dummy load. Problem is, I'll have to remove the front-panel PCB to get to any of the preamp components. I already know signal is not making it to the main power amps, the TDA7293's Roberto mentioned. But no one can reach the owner to ask if he wants to pay for the investigation.

So here it sits, boat-anchoring like a champ.

[roberto]

You certainly have to remove the front-panel pcb to repair it, but you can simply investigater on the signal tapping resistances (their pins are accessible).