Marshall MG100HDFX

[repair2k9]

I've been working on a Marshall MG100HDFX where theTDA7293 blew out. This is my son's head and he plugged the output into the output of another amp - oops and ouch! So I replaced the 7293 but still not working.

Enzo suggested (by the way Enzo - a big thank you for your advice and schematic) to check the standby control (TR2, C98, D14) but found nothing wrong there. I also checked power supply including +/- 15V and +5V and all fine. There is output thru the headphone jack (another Enzo tip). I was looking at the internal LED pairs (LED 1 & 4 and LED 2 & 3) and noted when the CLEAN channel is selected (CLEAN/CRUNCH button OUT, CH SELECT button OUT) that none of the LEDs come on when guitar is played. I actually went thru the various states of CLEAN/CRUNCH, Overdive, OD1/OD2 button states and can post that if it helps. There are states where all LED's respond to the guitar for what it's worth.

Anyway, at this point I am suspecting the IC7B 4558 driver but am looking for additional feed back from some of you more experienced guys. This is new stuff for me and any advice would be greatly appreciated.

[Enzo]

If the headphones work, then everything before the headphones works.


The LEDs are just part of the clipping circuit and wouldn;t be on during clean.

SO the high voltage rails are also OK?

Follow the signal. If it is in the phones, the phones are before the master volume aren't they? (It doesn;t turn them down does it?) ANyway, the signal is there or not, and follow up to the power amp modules, where the signal either gets to the input pin or not.

[Mars Amp Repair]

It also might behoove you to actually check the standby voltage at pin 4 of the power amp board. Acording to my notes, the level needs to be 6.8V to function (according to the data sheet, min 3.5V to come out of stby).

I've had issues with connections at C76 (near D14 & Tr2) which feed the AC to become the negative DC bias on the base of TR2 to keep it turned off & unmute the power IC.

Just be certain the transistor is turned off & the voltage at the cathode of the zener D-14 is 6.8V.

Also as Enzo mentioned, the headphones are fed before the master volume control & also have their own power amp ic. glen

[Mars Amp Repair]

ooops,
I meant that until C76 charges it holds the voltage at the base of tr2 positive via R46 to keep the amp muted at power up until the + & - 16V supplies stablize (timed by C76 & the other resistive components.

D17 probably protects the base emitter junction of TR2 from exceeding the reverse voltage rating. glen

[repair2k9]

Got it working!

Just wanted to let everyone know I got this thing working.

After checking for the standby signal (thanks MARS) and finding nothing there, I poked around and found the 6.8V zener (D14) shorted forcing the standby low. Replaced the zener and my days of a peaceful household are over (my son is thrilled, it's his amp although his sister is less than happy)!

Anyways, a great thank you to both Enzo and Mars for your advice and schematics. This was my first time out into a forum like this and it proved to be very helpful with really good people involved. I may return sometime but hopefully not too soon ( I'm an engineer and don't get into repair unless necessary).

THANKS AGAIN!

[Enzo]

Well, happy all around.

But most of us technicians would wish more engineers involved themselves in repairs a bit more often. Then there would be fewer unservicable products out there.

I can;t wait until the guys at Fender et al thinkto leave us just a mere 1/8" clearance between chassis and cab, so I no longer have to tear through foil and tolex to get the chassis out.