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Old 29th August 2011, 08:23 PM   #1
ram88 is offline ram88  United States
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Default marshall mg100 hdfx

Ok after everything I have found it looks like here is the place to try and solve this problem I have a like new marshall mg100 hdfx amp. I had the same problems as others have had but I am at a loss to what to check next. Here is what I know. fuse blew put in a new one it to blew replaced the whole daughter pcb board the one with the tda chip mounted to it fuses stopped blowing I have sound in the headphone out when I play the guitar. I have no sound out to the speakers its an 8 ohm stack, They do work. I have replaced the zener 6.8 diode glass with a zener 6.8 1w glass diode like as was stated in other forums still no sound out headphones still work. their is no smoking fuse blowing and the main board is perfect I did understand how to check the zener 6.8 but the other stuff on how to check that I have no idea thats why I am stuck as I have read it is stuck in standby mute mode what do I do now? Thanks in advance to any advice given and please be simple as possible because I dont know how to check the other stuff in the standby circuit I can take pictures of it for reference. Tools I have are solder iron, digital multimeter and some electronics experience but apparently not enough. oh and zener checked out to be 0.048 before and now the new one attached to the board is 0.050 the others test at .500-.670 but the new one tested like the others on the board before I out it in.
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Old 29th August 2011, 09:51 PM   #2
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I remember when my firend's blew exactly that way. There was a thin trace along
the edge of the board, on the back side where you can't easily see it, burned up...
I had to patch damaged trace with a piece of wire... Yeah, it snakes to the zener
through another jumper wire on the top side... Thats the trace I'm talkin' about...
I vaguely recall being able to change the zener from the topside, so I didn't see
the damage on the back until later. Right at the very edge, might not be obvious.

Your zener still measures shorted out. If new, probably somewhere else though.
Whats left of the burned trace might be curled up against something else...
Might use superglue to tack it back down or cut the damage away with a knife.
You trimmed the leads on that new zener, right? They aren't hitting anything?

Gotta make sure you correctly use the plastic washers to reassemble the TDA
board and fan assembly. Very easy for those screws to short to the chassis.
Blow something all over again.... Maybe that's how the trace gets burned???
I can't say for sure when my trace burned, but I did for sure short a new TDA
assemby to the chassis by not paying attention those plastic washers... It
did not ruin the new TDA, only burn the trace if it wasn't already. But was
only abused in short circuit that way a few seconds before I got wise to it.
Power supply voltages did not come up right, I had a meter on them.

Last edited by kenpeter; 29th August 2011 at 10:17 PM.
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Old 29th August 2011, 10:22 PM   #3
Enzo is offline Enzo  United States
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First, that mute circuit. You replaced a zener, but there is a zener, a transistor, and a cap all in parallel. ANy one of them can be shorted, or even all three. They are sometimes subjected to the main V+/V- voltage when the TDA7293s fail. Powr off and measure resistance across that new zener. If it looks to be shorted, then I'd wager either the transistor or the cap had failed.

Ah, looks like it already has been tested as shorted. Considering how inexpensive those parts are, and your professed lack of experience here, I'd just replace both transistor and cap.
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Old 30th August 2011, 12:32 AM   #4
ram88 is offline ram88  United States
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ok power is off started with the stuff kenpeter said then enzo's the strip that you talked about on the back of the board is ok and I have continuity to the other solder points The tda board is brand new I bought the pack of tdas at first and actually divorced the chip from the first board. using the old pins left from the old tda i soldered on bread board wires to all the little pins and perfectly to the new tda and remounted the tda to the fan sink then i made a new spot for the board itself to keep it from future harm and a quick change if need to. it tested perfect untill i powered it up and still got no output to the cab so I figured my ingenious little mod was bad somwhere thats why I bought a whole new board. Ok Enzo Thanks for getting on this with me.The zener tests on my meter setting arrow to the plus symbol .049 both ways like when i reverse the meter leads the other zeners on the board i checked only measure one way and they get .546 which is what I want but don't have. the transistor you are talking about is it the tda or somthing else? and how exactly do I test the cap.
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Old 30th August 2011, 12:35 AM   #5
ram88 is offline ram88  United States
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Just to let everyone reading this no now when this is fixed I am going to do a video on how to fix these things since thier is nothing on it picture slid show I think.
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Old 30th August 2011, 01:10 PM   #6
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Best way to change the TDA is cut all the pins, and pull them one at a time.
You may have to add fresh 60/40 tin/lead solder to make the joint flow well.
If it don't flow, add flux, then add more flux, and did I mention adding flux???
If you didn't buy extra flux, go get you some, right now!

Then cut solder wick braid into 1 inch strips (don't use on the roll), and flux
these strips heavily. Apply 1 strip per hole, and I garauntee they will clear.
Don't try to clear a hole while the component lead is still inside. That won't
help get the component lead to come loose, only make it more difficult.
The component lead must come out first, with a big blob of solder (and flux).
Then you can attack the hole...

Our best soldering girl "Cricket" showed me the trick with cutting the braid.
And explained how it prevented heat from being drawn away by the roll.
The small strips soak up more solder too. The long roll tends to spread the
solder thin, and waste braid... She desolders with 2 irons, one in each hand.
One heats the joint, the other pushed the small strip of braid toward it...

Always clean your flux mess afterward. Never use "no-clean" fluxes...
Pros use water soluble flux, which is both conductive and corrosive!
But if you use that kind of flux, you have to wash your work very
thoroughly in hot water, blow dry, and set in a low oven (65C)....
Obviously if board is unremovable from its place, or unwashabe due
to some kinds of parts, you shouldn't attempt to repair it any type
of corrosive flux.

The sticky fluxes (rosin) clean with strong alcohol, and this will pretty
much dry itself without you having to do anything....

Never mix fluxes, as the sticky kind can make the corrosive kind harder
to wash completely. and nothing but a chisel removes the no-clean BS...

Last edited by kenpeter; 30th August 2011 at 01:40 PM.
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Old 30th August 2011, 02:16 PM   #7
ram88 is offline ram88  United States
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@kenpeter The first board that the tda was mounted to is what I experimented with after I cut the tda off it i basically made a ribbon cable to the tda and the board for better cooling and if this whole event was to happen again I tested each pin to check to continuity before I tested the amp.n it was just an idea. But in the end I still bought a whole new con 1 tda board and heat compound is it is still stock just new parts
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Old 30th August 2011, 04:08 PM   #8
ram88 is offline ram88  United States
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ok c98 and c76 red meter lead on + and ground on chassis the caps test the same 0.050 as the zener i checked them on the diodes setting do I have to remove the caps on c98 and c76 to check them the meter goes crazy just climbs to a point and starts over. also I rechecked everything on the board itself and I see nothing burnt and all the wiring is doing great. I have the board out of the chassis and still everything connected to check from the underside alwell.
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Old 30th August 2011, 11:41 PM   #9
Enzo is offline Enzo  United States
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The parts rarely look any different, good or bad.

You already replaced the zener D14, right? And we made sure it was in right, the line end NOT to ground. So we assume it is OK. Now C98 measures shorted. So either it is indeed shorted, or something around it is.

Using a hand meter to check caps is OK up to a point. If the cap checks as shorted while it is soldered in, then if it no longer checks shorted when it is out by itself, then it is not what was shorted, something else was. A cap naturally wants to charge up whenever current is presented to it. Your meter sends a low current out its probes when on the ohms or diode scales. This will try to charge any cap connected to the probes. To the meter, a cap will look like a resistance that rises and rises...

SOunds like your cap is OK.

That leaves the transistor TR2, a 2SC3198. If you have no 2SC3198, most general purpose NPN types would work. So, look at TR2, is it shorted emitter to collector?

If that point in the circuit where the + side of C98, the cathode of D14, and the collector of TR2 all meet - labelled STBY on the drawing - is down near zero volts, then the power amp is muted. I forget the spec, I think it is anything over about 4v, but to get the power amp ICs to run, that positive voltage needs to be there.
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Old 31st August 2011, 12:26 AM   #10
ram88 is offline ram88  United States
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ok checked tr2 no continuity in any order from e c or b I do have continuity from + side zener to e on tr2 ,and + side c76 to e side tr2 ,and + side c98 ,,,so E on tr2 is linked to all + in circuit. C has continuity to negative side on zener, negative to c98 but nothing to c76 negative. Im thinking that the circuit is fine? or should their be a connection.c76 to c on tr2
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