Marshall MG100HDFX - No sound

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#41
Yes the stby is either 0 VDC (stby) or a low +DC voltage (operate). Upon turning the amp on TR2 is "on" providing the ground for stby. After C76 charges up, fairly short time, TR2 will turn off and R47 will provide a +DC voltage limited by D14, the 6.8V zener.

Craig
 
#45
Im getting 13V on the front -side of c76 to chassis and 0V on the positive side. I dont think power is passing through that cap. Changed that cap out and now Im getting .025 volts to the standby circuit.
 
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#46
Are you sure D14 is good? Put your new meter in diode function and measure it. Should read approx. .500 one way and nothing the other way. If the diode is shorted it will read the zero in both directions. DO NOT!!!!! turn the amp on without D14 in place. What is the voltage on the other side of R47? What is the voltage at the R46/R47 junction (cathode of D17). Sure looks like D14 is shorted.

Craig
 
#48
I think that you are right. The D14 is reading .071 one way,and .014 the other way.

One side of R47 is getting 13V, the other side is getting .025V. R47 is another culprit (I believe) because it is not holding a steady value of 33K like it is supposed to. The value goes all over the place, it goes from 0 to 30k or so, then slowly back down.

Do you think that I could "borrow" a zener 6.8 from the footswitch section until I can get a pack of new diodes? Doesnt seem like it would matter for the time being since I dont even use a footswitch. Either that or I guess I could just wait until I get some. : )
 
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#50
One more thing, all I have is 1/4w resistors. Is it safe to use this when there are 1/8w resistors in the amp? I believe that the R47 33k res. is shot. Also, what did you think about a temporary "borrow" of one of the zeners in the footswitch area?
 
#51
Sometimes when you measure parts in circuit with a capacitor your meter charge and discharge the capacitor giving you funky readings. With the type of resistors (carbon film) in that amp you can usually tell a bad one because they are burned up. Is D14 shorted? There are several other 6.8v zeners in the amp but they are there for purpose and with your lack of experience I would suggest leaving them alone. Can you find one locally? As for using 1/4w instead of 1/8w there is no problem electronically so if there is room physically go ahead. Is D14 shorted, zero Ohms both ways?

Craig
 
#53
Yep it's shot, isn't that the first thing I told you to check??????????? Live and LEARN. You could try the 5.1V zener, the most that could happen is the voltage isn't high enough and it still won't come out of stby.

Craig
 
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#54
Alright my man. Hope you aren't irritated about it. It is one of the first things you told me to look at. I also, at that time had a multimeter that couldn't check diodes. I learned a lot on this project. I have been building effects pedals and true bypass strips, and this is my first amp repair. I'm not going to count my chickens yet though!! Ill let you know what happens when I change it out. Thanks again Craig. You da man.
 
#55
Don't even think I'm irritated, just here trying to help. Having good tools is one of the best investments you can make. Be on the look out for an o'scope and a signal generator if you plan on doing more repairs. Both are essential, asking for help on this forum and any other forum and have nothing to troubleshoot with is pointless. We all use our own method of troubleshooting and it all requires test equipment.

Craig
 
#56
I will hop in and say try the 5.1v zener also. I think standby only has to come up to about 4v or so, not sure. Worst that can happen is 5.1 is not high enough as Craig said.

But do it with the understanding that you should then go get th proper zenre and eventually replace the 5.1.
 
#57
So check it out guys, I have a 6.8V coming in the mail. When I tried the 5.1 with the output IC board disconnected, I got 4-5 V in the standby section! I reconnected the output section, and turned the amp back on, and blew a fuse. I then realized that the 6.8V is a .5 W, and the 5.1 from Radio Shack is a 1W. So I think that is why I blew a fuse. When I get the new Zener in place, Ill let you guys know how things go.
 
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#58
Ahhhhhhhhh. I feel like I am goings to scream. So anyways I replaced the zener 6.8v and turned the amp on, and pop, went a fuse. When I was checking the signal output of the IC, earlier in the game, my test probe shorted out at the IC and fried it, so I bought some new ones. Here is the strange part, when I disconnect the input IC from the main board, the fuse does not blow. Any suggestions Craig?
 
#59
Since it doesn't blow the fuse when the output module is disconnected either the IC (TDA7293) is bad or something on the output module PCB is bad. Check all of your solder joints on the output PCB, top and bottom. Did the output module PCB get "charred" when it fried the first time?

Craig
 
#60
The surface of it turned a little bit black, but not really charred. I checked all of the circuits and they seemed fine. No cross continuity. Also, I just changed the IC after I charred. Solder joint look fine. You think I should just get a power module?
 
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