Question about Marshall MG100HDFX 100-Watt Amplifier Head

1 Answer

Blows main fuse. Found burnt place on circuit board where connection 1 is. It's the board that's mounted to the heat sink with fan at the back. Disconnected connector 3 that goes to con 1. Now fuse does not blow and I can play through the head phones. Everything seems to work except the digital effects section with master volume control. Any help?

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  • Anonymous Mar 19, 2014

    with power in and phantom switch , transformer and circuit board assembly removed from console, fuse still blows. do you know how to test

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  • Marshall Master
  • 5,603 Answers

Chances are a power transistor has shorted. Sometimes input rectifier diodes short or electrolytic caps.

Here is a hint to troubleshoot this: Connect a 150 watt lamp in series with the hot side of the power line to the amp. This will limit the current (and voltage) and prevent the fuse from blowing. You can do a lot of troubleshooting at the much reduced voltage.

If board is burnt to carbon, this "cancer" needs to be cut out and removed as the blackened board is slightly conducive instead of an insulator. Often a circuit trace has fried the board. One must install jumpers around ones that have smoked.

Posted on Sep 24, 2009

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1 Answer

It popped and went off and now just blows fuses


Blown fuses indicates a short. A short is when one electrical component is touching another component or touching ground. Visually look at the circuit board to see anything touching or any signs of burnt marks. Always do this while the unit is unplugged. Do not touch live equipment. Good luck.

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I rather suspect IC1 TDA7293 is the cause of your problem. If this is the case you will need to check the speaker/s

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i) I assume the fuse you mentioned was in the mains plug - did you also check the fuse in the TH15A mains input socket? This is accessed by pulling out the tab in the bottom section of the socket (below the centre pin). This is a 3.15Amp antisurge fuse, 20mm type.

ii) If this fuse has blown, then replace it & all may be well ...... but if it blows again, then move to step iii).

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Probably smoked the output IC(it is the chip mounted to the heat sink attached to the fan) which can be purchased from marshall directly or from one of their service centers. Also P-H216 sold at tubesandmore.com should work. No soldering required. Look at that IC(chip) to see if it looks burnt or has a crater on it which would indicate that that is indeed you bad component.

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If the IC is MC1436, Pin one is not connected.Pins 2, 3 and 6 are the signal carriers, 6 is out 2 and 3 are inverting and non inverting inputs. CR13 and CR 14 would be simply something on the order of a 1N60 or a !N914 depending on body size,

If it works fine with heat snk screws removed, check to see if they short something out. Also look for small cracks in the board with the screws in place and snug. It also would not hurt to reflow the solder on all component leads and headers that are in the vicinity of the heat sink. Just because they "look" okay, does not mean they may not be faulty. Reflowing eliminates a cold solder joint as a possibility- which by your description is actually the most probable issue. Followed by cracked board and inadvertant short. Esepcially if the heat sink is supposed to be electrically isolated. If it continues, try using nylon mounting hardware for the heat sink, Also when powered up tap each IC, electrolytic capacitor and transistor with a pencil eraser- those that make noise may be the source of the problem. The applied stresses of the screws might deflect the transistor just enough to make a contact intermittent internally in the device..

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1 Answer

I'm repairing the power supply. I replaced T3,T4,Ic1,D24. but the disipator gets hot when I try to disconect a bulb I conected it in series at the input cord, and then connected back with out the...


Voltage readings are normal. I don't know what you mean by dissapator? There are five separate heat sinks on the board. Which is heating? Is this with the load connected? If no load is connected none of the heat sinks should get hot. In regular operation with the fan running they may get warm. If no load and the big one is heating, then one or more of the three terminal regualtors has shorted. If the one with the big diodes near the 70 volt caps is heating, look for shorted surface mounted caps on the bottom of the board near the 70 volt connectors. If the heat sinks with the IGBT switchers are heating, look for a bad gate speedup surface mounted diode. It is unlikely the input diodee heatsink is heating.

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Hi pcruzx,
Sometimes replacing the fuse alone with exact voltage and ampere ratings will cure the problem(you're lucky) but not all. Sometimes further damaged gone to other parts most especially the voltage regulator transistor/IC (mounted in heat sink) and the opened the fusible resistor in line with the supply(B+).
First check the fuse, for sure it was already blown. If you've replaced the fuse and immediately blew again after powering on the unit, this was an indication of shorted parts near the AC line. Click this LINK for more info.
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Thanks for using Fixya.

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1 Answer

I have the Marshall MG100HDFX 100-Watt Amplifier Head, the fuse was blown and there is no audio coming out of the the spkr jacks. this is how i received the amp and have no further info to diagnose. im...


If the output transistors have shorted, it is LIKELY that several others in the drivers and even farther back have also gone. HOWEVER some Marshall amps use one or more TDA7293 chips. These require installation using heat sink compound. The TDA7293 can be found at:

http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&WT.z_homepage_link=hp_go_button&KeyWords=tda7293

They come in two sexes depending on which way the leads are bent for installation. You have to see how yours are mounted relative to the circuit board. Cost about $6.

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1 Answer

Loud crack when turning unit on, blows main fuse no lights come on.. bottom of power supply black and signs of burn...


DO NOT REPLACE THE FUSE !!! You will do more damage.

I just repaired one of these. A small ceramic surface mounted cap very near where the +/- 85 volt connectors plug onto the board blew up.

This left a sputtered mess on the insulating material at the right rear corner of the power supply.

The net parts list of parts I had to replace:

1. Main IGBT transistors on heat sinks. You have to remove the heat sinks to get at the mounting screws. 1MBK50D or equivalent. I used DigKey part APT28GA60BD15

2. One 1N4148 surface mount diode that is for gate drive speedup. D24 and D25. (don't remember which one was shorted)

3. All four driver transistors for the gate of the IGBT's. @ each 2SA1020 (PNP) and 2SC2655 (NPN). These are tall TO92-like transistors and I believe that at least one of these I had to get from Mouser.

4. Transistor T9, suface mount BC856B (PNP)

5. Replaced all four surface mount caps, one had exploded.
C50, C51,C54,C56. Replace with 0.01mfd/200 Volt 1206 style surface mount caps Digikey 478-1511-6

6.Replaced ceramic wire wound 10 Watt stand up resistors that were blown open. Replaced with 20 ohm design change. These slowly charge the main input filter caps and when supply is stable, a relay shorts them out. If you have replaced the fuse, you LIKELY blew them out. Original value was 10 ohms on some units. This was a Mouser part. 280-PRM10-20-RC

7. The 2153 switching reg chip IC1. This chip shorts. It is made by International Rectifier and I believe the full number is something like IRF2153. I had these in stock and not sure of which source I had procured them from... It was from one or the other.

Lots of destruction and one really needs a desoldering station to work this. Removal of the blown IGBT's is especially tough. All solder must be removed from the transistor leads AND the thermistor on one of the heat sinks. Once you have verified the leads are loose, one has to rock the heat sinks to get the tabs free. The tabs are NOT soldered! Be careful when restoring to get the thermistor leads started in their holes.

Note: ALL these repair parts were procured from DigiKey.com and Mouser.com. Neither of these places had all, I had to order from both. Mouser had the ceramic resistors.

When restarting this, leave outputs disconnected until it is running again. USE a series 75 watt light bulb in the 120 volt input. ONLY work on the unit using an isolation transformer as a large portion of this is connected right to the power line.

There is a small 15 volt switching supply that comes on first that runs the relay to short out the inrush limiting resistors. With the 2153 shorted as usual, this auxilary supply can't reach voltage enough to turn on the relay. The power for this is supplied through the ceramic resistors.

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1 Answer

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There should be a fuse holder near the place in the back of the amp where the power cord plugs in. Also some Marshalls have fuses in fuse holders on the circuit board. Always replace with the same value and type as the original fuse. Never open up the amp when it is plugged in to the outlet. When replacing the internal fuses do not touch the capacitors as they retain a lethal charge. If the fuse continually blows it is an indication of a more serious problem which will require servicing by a qualified repair person.

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