HELP! Marshall MG100HDFX

[dan8686]

First of all, hello everyone!
OK .. my 100hdfx blew a fuse a few weeks back and I have had no luck in reparing it! Here's a quick timeline of things i have tried so far...

*replaced fuse, unit turned on, LED's and fan turn on but no output to speakers, tried another set of speakers, same thing.

*ordered new power amp board from marshall, now very low output and very high distortion (can hardly hear it, maybe 0.5 to 1 watt output), this is the situation I am still stick at, but have also tried the following:

*measured voltages everywhere, +/-15V is good, +5V is good, xformer outputs are good, +/-42V rails for power amp chip good.

*replaced caps C22, C24 and C13.

*tried the effects loop issue that ios addressed in a few other posts, to no avail.

*searched and searched for any solder issues.

Some other things I have been able to determine is that everything seems to work except the high power output. When I use the headphone jack, all effects, channels, and controls work well and can play my ipod through the CD input (but only to headphones).

The mute pin on the IC (TDA7293) is connected to the supply for the chip and the standby pin is at about 1.6V.


This is where my knowledge takes me to and I hope someone will be able to help! Can't wait to play on it again..

Thanks in advance

Daniel

[Mars Amp Repair]

hello daniel,
My notes indicate that you need 6.8V on pin 4 of the power IC board (pin 9 of the ic itself) to unmute the IC.
Most likely cause that I have experienced quite a few times is a shorted zener diode in the stby ckt.
D-14 (6.8V) & or TR2 (stby transistor).

glen

[dan8686]

Thanks for the quick response Glen, I have had a quick look at the schematic and cannot find any reference to what transistor TR2 actually is, am I able to replace it with any standard NPN? e.g BC548?

[Mars Amp Repair]

Well,
It is on center bottom of the power supply schematic page. I'm not certain what the number is, but I'd suppose any general purpose NPN would do the trick. the BC458 seems a good choice.

My recollection is that the diode is the most likely cause. I'm not certain I've had to replace the transistor. a quick check with your diode checker setting on the DVM should tell you that.

glen

[dan8686]

sorry about so many questions! on the schem, it has the exact zener that is used (a vishay bzx55b6v8) do I need this exact one? or will any 6.8v zener do the job? also, is the low volume and high distortion typical of this ic being muted? or would it usually be fully off?

Thanks again for your help!

[ParthaD]

TR2 info

On the schematic it's a C3198. The Toshiba 2SC1815 (from mouser or anywhere) works perfectly, the pinouts matching the original perfectly when you orient it the same way as the original (but double-check with the datasheet for both C3198 and the 2SC1815 available on the net). In my situation, a new power module + TR2 + D14 + D17 brought it back to life. The D14 is a zener but I just used a 1N914 (not a zener) since I had that handy. Seems to be working just fine.

[Enzo]

A 1N914 will not limit voltage in the circuit to 6.8V. Might as well not even install a 914.

2SC3198 is not critical, any transistor that meets its basic specs will work.

[ParthaD]

Zener question

A 1N914 will not limit voltage in the circuit to 6.8V. Might as well not even install a 914.

Enzo, I'd like to understand this better... my understanding was that a zener limits the REVERSE direction voltage to whatever it is rated for. Hence a zener will limit the reverse voltage to 6.8 volts not the forward voltage.

However if you use a 1N914 there will be NO reverse voltage at all, (i.e. until 100V which is more than the max volts in this amp, right?) , since it allows current flow in only 1 direction. Hence, my logic is that for this application it would be SAFER since this part of the circuit does not NEED the flow of current in the reverse direction or else the amp would not even work when I replaced the zener with a 1N914.

Thanks!

[Enzo]

A zener conducts forwards like any diode, but in reverse, it blocks voltage up to its zener point. Look at a typical zener regulated power supply. SOme voltage source has a zener across it, with the addition of a series resistor to limit current. As long as the voltage is greater than the zener's rated voltage, the zener will maintain its voltage across itself.

In this application, the standby circuit runs off I don;t know what - 12-15v or thereabout. There is a cap and the parallel zener, with a resistor from the supply rail feeding them. The 6.8v zener there will establish a 6.8v charge across the cap. And whatever is on that cap is what goes up the STBY line to the power amps. Note the zener is wired anode to ground, so the voltage it faces from the power rail IS reverse to it.

We are not concerned with reverse voltage appearing there, though I supose a shorted rectifier could put AC there. But then you'd have larger problems than the zener. The zener is wired in reverse because that is how zeners are used. But the diodes are not there to protect the little circuit from backwards power.

Your 1N914 will never be forward biased, so it will never turn on. Meanwhile, without the zener, the voltage across the cap will be free to climb to whatever it wants to. Well, to whatever the voltage, circuit leakages,a and so one will cause. The 1N914 will simply have no effect.

Does you amp work fine? Sure, I am sure it does. But the circuit is no longer in the designed configuration, and the resulting STBY voltage will be unregulated. The potential for a problem is there, whether or not one ever actually arises.

The also parallel transistor shunts the STBY voltage to ground to mute the amp. Once it releases, the cap is free to charge back up and turn on the speakers.

[ParthaD]

Got it...

Thanks Enzo, I understand the difference now!

[dan8686]

Thanks for that great explaination Enzo!
while you're here, are you able to shed some light on what function D17 (1N4148) has in this standby circuit? From what I can see (don't have the amp here with me so can't do any physical tests) the voltage divider (R45 & R46) supplies about 5-10V to the base of TR2, in my mind this switches on TR2 and would always be shunting STBY to GND. This is obviously NOT what is happening so my guess is that D17 is doing something here, but I am not quite sure what.

[Enzo]

Look again at what the voltage divider is strung between.

The zener, transistor and cap are all referenced to ground. The bottom line from D17 on to the right is ground. But to the left of C76, that is not ground, that is negative voltage. Note the bridge formed by D10-13 to the left? The ceneter tap is grounded at the transformer, so they form a split supply.

I don;t know what it is, so let's call it +/-15v. After everything is warm, the 6.8k and 4.7k put the transistor base at about -2.8v, turning it off. or it would, but D17 will clamp it to one junction drop. Not sure if the transistor needs the protection or not.

The +15 is filtered by 2200 mikes of C99, while the -15 looks to be filtered by 4.7uf C76.

Note these are not the +/-15v rails for the preamp.

wWhen power is removed, I think D17 helps discharge C76 which allows the remaining charge in C99 to pull up the base of the transistor to mute the power amp as things wind down.


At least that is what comes to mind looking at it.

I am open to alternative interpretations.

[dan8686]

THANKS

ITS WORKING!
Just a final note to say thanks to everyone who has added to this thread and helped with their knowledge and ideas! Enzo, your input to this thread and every other one is invaluable!

I replaced all components within the standby circuit, TR2, D14, D17, C98 and this has fixed the problem, the standby voltage is now around 6.8 volts (thanks zener diode).

Thanks again!

[dan8686]

is there ever only one problem???

ok, the amp works great now, but i thought, while it is open and everything is ready to go i might as well attempt to remedy other issues.
The problem i have now is that when i turn the master volume pot (VR15) it "scratches" quite loudly, i have read that this can be due to dust or DC across the pot, i have cleaned it well and don't suspect that dust is the issue, so i will replace some suspect caps and see what happens.
The caps i will first try will be C89, C90 and C91. Am i anywhere close with these caps or are these unrelated to the issue?

[Enzo]

Get out your meter. Is there DC across the pot or not? No sense trying to cure a problem if you don;t have it.

Dirty pots doesn;t always mean cat hair, lint and garden soil. There could be fibrous material from something, or flecks of dried lube, or who knows what. And it also refers to a clean clear path for the pot wiper. if there are rough spots in the track, they can't be cleaned away.

SOmetimes you can;t detect problems simply. If I feel real anal, sometimes, I check a pot by connecting some signal across it from my signal generator. Then scope from one end to the wiper. Turn the pot. The wiper signal either smoothly goes up and down in amplitude or not. if not, then one way or another, the pot is "dirty." If ti won;t clean up, replace it.