Marshall Mode 4 (mf-350) IC's blowing up

[Mars Amp Repair]

If you connect your scope from chassis ground to the tip or the ring, you'll still see some representation of the output signal....probably 1/2 of it anyway.

If you have a 2 trace scope, connect as above & then hit the 'invert' switch on one channel and select the 'Add' function. Then as I recall, you just read the resultant as if it were a normally grounded signal.

BTW Conner, thanx for the dissertation on how they have those 4 IC's configured. I'll have to check that out. glen

[Mars Amp Repair]

Sorry I might not have been really clear on how to connect the dual trace scope to the output.

1. Connect both scope probe grounds to the 'CHASSIS'.

2. connect the speaker tip connection to one scope probe tip.

3. Connect the other SCOPE TIP to the speaker jack sleeve.

4. Engage the 'Invert' switch on one channel of the scope.

5. Select the 'Add' function.

As I recall you can then just read the resultant sinewave like any amp output. glen

[Enzo]

Yeah, that will work. Generally, If I find reasonable looking output on either side of the speaker, I am not too concerned if they match up. it is far more likly that part of th waveform would be missing or terribly distorted than that the two sides would disagree on phasing.

[Alex R]

Just to say that the amp powered up ok with the Marshall IC boards. It was good to have Glen's suggestions for monitoring DC offset and the rail supplies to check for faults at very low voltage levels. Thanks Glen!

[Mars Amp Repair]

What else ya gonna do ;-] g

[suha]

parts

where were you able to find replacement IC boards?
i'm afraid I have the same problem. Unable to power up, after replacing the IC's and draining the power first.

[Alex R]

where were you able to find replacement IC boards?
i'm afraid I have the same problem. Unable to power up, after replacing the IC's and draining the power first.

Direct from Marshall.

[pablocarrillo]

I have a mf350 of a friend; aln all 4 ic`s are blowing.

I repplace them; and replace the fuse.

Checked the preamp; it,s ok from imput guitar to send effects.

Voltages semms to be ok on the wires that go to the ic`s pcb`s (rail to rail +47 to - 47vdc and 0v on signal path).

I pluged the wires on all four ic`s pcb´s; and checked without a load and master volume at 0; pluged a male jack on output 1 and i have over 17,7 vdc on the first 16 ohm output sleeve ; and other 17,7vdc on tip of the jack.
I suposs its ok....but not at all sure. Then i pluged a load; a 56 ohm speaker from an old tv; that i have to try amps and other gear; but no sound apears with master volume at aprox. 2.

Recheked sen output with a headphone and its ok; al preamp works fine!!

But i have no output signal from poweramp on speaker..

I have a question about the 7293 standby. When is the ic on standby without output??? The standby is pin 9 of the ic´s and it`s wire number 4 of all the ic`s board; what voltaje should i read here???Because i have over 17vdc to groung on wire number four.

Could anyone post the voltages of all the pin wires of CON 3, CON4 CON 6 and CON 7 with the amp working correctly; with ic`s plugged in and speaker conected????

I messed + 45 and - 45 vdc on pins 14, 15 and 12, 13 and pin 3 of con3 and con4 is -46 too.

What are the other correct voltage readings for the other pins or wires??

I have a signal inyector; but i don´t really know very well how to inyect to these all four ic`s ....perhaps they are muted....
In wich part of the circuit should i conect the ground alligator of the injector(i suposed at ground wire 5 it´s ok)?? And where i have to inject with the live tip??on + input; on - input ???? i don`t know it very well....so I ask all you if you could give me a hand......

PD this solid state amps, are so complicated!!! Valve amplifiers are more easy to service and to make!!!


Sorry about my very bad english i`m galician; and thaks to all.

[teemuk]

But i have no output signal from poweramp on speaker..

Long shot guess but try the amplifier with a lower-impedance test load.

The modern Marshalls are designed so that they provide more voltage gain to higher impedance loads (mimicking the same characteristics of tube amps). However, the amp's gain at a 56-ohm load wil jump through the roof and the amp will clip into rails.

The thing is, though, the MF350 Mode Four has a circuit that detects clipping and this very same circuit - when driven excessively - will also turn on the amplifier's standby circuit hence muting the amp.

If I remember correctly, Marshall amps permanently have the muting feature turned off and they use the standby feature insted for the same purpose. You actually need to measure the standby pin's voltage to see what's going on.

Schematic should verify this and TDA7293's datasheet should provide all neccessary information about the voltage ranges controlling the mute/standby functions. If I remember correctly they were compatible with typical 5V logics.

[pablocarrillo]

asuming datasheets characteristics; the ic is muted with 0 vdc on pin 9 and turned on with only 5 vdc at pin 9; and i have over 17vdc on pin 9 but i don´t know (because i´m dumb and not a tecnician)if the mute is on or off!!

What happen if i disconect the wires that go for the mute pin??The tda`s 7293 are going to be on???

I`m not very experienced at solid state amps....sorry....

[J M Fahey]

Check the datasheets but i think the mute and standby pins need *at least* 5 V to turn on, probably those +17V are OK.
What I can tell you for sure is that disconnecting standby/mute wires will leave the amplifiers off.
What worries me is those 17.7V you measure on speaker pins, are you sure your negative multimeter probe is touching actual ground?.
As a verification: touch the negative probe to chassis and read +B, -B, (maybe on the IC power pins); +15, -15, on preamp IC pins 4 & 8, and voltage on tip and ring of the speaker out.
If I remember correctly the only difference between 16r and other speaker outputs lies in the feedback, there is no *real* impedance change.
Nota: quítate ese clavo de encima, te va a enloquecer, y lo peor es que en un par de meses volverá a quemarse y "tú serás el responsable" a los ojos de tu amigo.
En todo caso, que él compre los 4 repuestos originales, instálalos cuidadosamente como explica Enzo, descargando bien los capacitores de la fuente, re-conecta todo usando una lámpara serie (busca "bulb lamp current limiter" aquí en el foro) y si arranca, se lo entregas y "buenas noches". Y si consigues que lo envíe a un centro autorizado Marshall, mejor. Consejo de amigo.
^^^^^^^says: "get away from this lemon"
PS: please post some pictures of the interior, specially the power amp section.
*Maybe*you can junk the power board and install something that works.
Aquí ninguno de los grandes técnicos lo toma en reparación, si puede evitarlo.

[Steve Conner]

Oh no, not the "Splode 4" again. They blow up all the time, and if you make one mistake replacing the chips, all four can blow again the next time you apply power.

If the amp doesn't explode, but makes no noise: Sometimes the TDA chips will shoot rail voltage out of the standby pin when they blow. This can damage the amp's standby circuit, leaving it stuck in standby. Look for a blown component there.

[pablocarrillo]

thanks to all!

I will recheck all you figured out; and yes, use the trick of the limiter current bulb; it`s an old tool that i build when i make my first amp; a jcm-800; for try the firs time on.

This power amp it´s very strange to me; and yes, I think too,that make some power amp arround 100 wats with other or the same ic´s perhaps will be a better way to fix this fucking amp!!! Today , there are kits arround 100wats of preamps at a very low price and more reliable than this power amp.

I don´t think that ic`s are blowing again; but on the other hand; i have no sound on speaker outputs!

I will post some pics of the inside; and the power amp pcb`s.

Maldita sea la hora en que le coji este ampli al colega!!! bueno; aun tengo 4 7293v nuevos; habia comprado 8 por si las moscas.... pero esto ya me da mala espina.Muchisimas gracias J M Fahey por tus consejos. Posteare fotos, y os mantendre al dia de lo que consigo hacer; pero me da a mi que cuando se quemaron los tda`s la primera vez tuvo que quemarse algo mas en la pcb grande! Cuando medi los 17,7 vdc en el jack de salida; los medi respecto a tierra general, no respecto al chasis; pero probare a hacerlo contra el chasis.


Thanks to all ; and i will tell all wath I earn!

[clydey]

great thread thanks
i got lost at the end re hooking up the scope.
i understand no earth in the main scope lead to float the earth.
i used to do this for tv chassis where the chassis was at half mains potential
like enzo i use an 1 to 1 isolation transformer on the amp under test but i got confused as to the situation re the scope mains earth lead i believe it is connected in the plug in this situation as the isolation tx earth is connected to earth so the question is ?
when using an isolating tx for amp under test only and
having the main earth in the scope connected to terra firma (true earth) and the scope leads earths connected to chassis and
checking the output signals of the chips and speakers
will this damage the output chips of the mode 4 mf 350
cheers

[Steve Conner]

I know that on the Mode 4 you mustn't connect the negative terminal of the speaker output to ground. The negative terminal is driven in a bridge arrangement, and carries an equal and opposite voltage to the positive one. So grounding it shorts out two of the amp chips, which can blow them. Like they need an excuse to blow up :-(

In other words, don't hook a scope ground clip to the negative terminal on your dummy load!