Marshall MG15dfx - no sound on clean channel plus part I.D.

[gpateluk]

Hi,
I just picked up a Marshall MG15dfx for £10. The overdrive channel sounds great but the clean channel is dead. The effects part of the amp is dead too. I researched online and I read it is a good chance it is the fuse to the channel. There should be 2 on the circuit board I also read. I opened up the amp and saw what I thought looked like the fuses. I tested them with a multimeter for continuity. The first beeped showing the fuse was ok. The second didn't make a noise. So I took it out and tested again, still nothing. I also got the schematic online.

The part is connected to C47 and is labelled 1000/25V on the schematic.

I took a picture of the fuse (if it is a fuse) and the markings on it. F2P1 33RJ H5N.

I've looked online and can't find anything. Is it a fuse? Any help would be very much appreciated.

Thanks

[MarkusBass]

This is plain resistor clearly marked on the schematic as R47, 2W, 33 Ohms. Not a fuse. If it's open, replace it. But there must be some reason why it's open. Maybe IC4 (7805) has failed too.

Mark

[Jazz P Bass]

Mouser Electronics has the resistor: http://www.mouser.com/ProductDetail/...F9iVa9ood8Q%3d

And the 5 Volt regulator: http://www.mouser.com/ProductDetail/...QOlwJydEoyhc9b

[g1]

Measure it with your meter set to ohms and see what it reads. The meter may not beep at higher resistances like 33 ohms, the other one you checked was probably a lower value that beeped.

[Enzo]

And the individual channels do not have fuses anyway.

Step back a moment. You have one channel working and not the other. You heard there ought to be a pair of fuses somewhere on the internet. There are no pair of fuses for your channels. As far as I can tell the only reason you removed that part was that you thought it was a fuse. But it isn't, it is a resistor, and chances are it is OK anyway.

Since the distortion channel works, that means most of the amp works, and that includes anything fuses might be involved in. On the working channel, do the digital FX work? If they do, that means the 7805 and that resistor were OK.

The clean channel is very simple, it is a little bypass around one part of the distortion channel. The rest of the amp is used for both channels. That tends to mean that if the clean makes no sound, but the distortion channel works, our problem is most likely limited to:
C49
The clean volume control itself
channel switch S1b

And the solder under each. That is it, nowhere else to look. Oh I suppose the circuit board could be cracked or broken or otherwise damaged, but doubtful.

[olddawg]

Listen to Enzo... These amps have no footswitch. People tend to kick the front panel channel switch with their foot while playing. Guess what happens?

[gpateluk]

Thanks for this. I do tend to get narrow minded so took take a step back is good.
The resistors I took out were good. The digital FX do not work at all. I checked the 7805 and the IN was 21.5V and the OUT was 5V. I also noticed the LED's 1 and 2 do not light up either.

I checked C49 by taking it out and it read good. The clean volume control does not seem to work properly as when I checked with a multimeter the readings did not move as I turned the knob. The other A5K pot did change readings as I turned the knob. So is the clean volume faulty?
The channel switch I tested with the continuity setting and when the switch was pushed I could read the connections had moved (if that makes sense?)

So the next step is to swap out the clean volume control and see what happens?

I looked at IC1 and tested the pins. Pins 4 and 8 read +15V and -15V but the other pins I get nothing. Also on CN3 (the part that connects to the FX board) I get a reading of +5V on the 3rd pin and not the 2nd as it shows on the schematic.

Anyhow I really appreciate all the help given to me. I'm gradually learning more and more and your help is fantastic, thank you.

[gpateluk]

I took out the clean volume control and took it apart. Gave it a clean and now the readings go up and down as I turn the knob. So I put it back in and its the same problem, no clean channel. So time to go and look around to find what else could be wrong.

[Enzo]

Did you understand my earlier suggestions? The entire channel is just those few parts. The way to do it is systematically go along the signal path through those parts and see where the sound stops. The whole channel is nothing more than that cap, the volume control, and the switch. Either one of those is defective, OR a connection between them is broken.

[gpateluk]

Thanks. I realised this morning I had not soldered one leg of C49 in (it was late last night). So I soldered it back in this morning and I thought I might as well re-solder the switch too. Then I turned on the amp and the clean channel works!!
I then tried the FX part. That works too but is very faint. If I turn up the amp I can faintly hear the delay. So that's the next thing to look at. Any ideas as to where to start please?
Once again I appreciate the help a huge amount. Thank you.

[gpateluk]

I decided to do the same with the fx part of the circuit as I did with the clean. I took out the effects volume pot and took it apart and cleaned it. Re-soldered the switch and checked a resistor that was connected to the volume. I put everything back and the fx part is working again! Reverb, delay, chorus and flange.
I have to say I'm not impressed with Marshalls build quality. Maybe it's just me but when I'm putting in components I make sure they are straight and upright. A lot of the capacitors are slanted, one leg in deeper than the other. The potentiometers don't seem to be very good as that seemed to be where the fault was. The resistance when turning never changed on the clean volume and effects volume. It was only after I took them apart and cleaned them they were OK. So maybe it's worth replacing them all together. I've had a Peavey Companion 15 amp for over 30 years and it's still running strong.
Finally thank you all for your comments and help. I wouldn't have fixed it without you. So it wasn't bad for a £10 purchase and I didn't have to buy any parts too! Thank you.

[altazepsta]

Need Enzo... Sorry for revisiting this old thread

And the individual channels do not have fuses anyway.

Step back a moment. You have one channel working and not the other. You heard there ought to be a pair of fuses somewhere on the internet. There are no pair of fuses for your channels. As far as I can tell the only reason you removed that part was that you thought it was a fuse. But it isn't, it is a resistor, and chances are it is OK anyway.

Since the distortion channel works, that means most of the amp works, and that includes anything fuses might be involved in. On the working channel, do the digital FX work? If they do, that means the 7805 and that resistor were OK.

The clean channel is very simple, it is a little bypass around one part of the distortion channel. The rest of the amp is used for both channels. That tends to mean that if the clean makes no sound, but the distortion channel works, our problem is most likely limited to:
C49
The clean volume control itself
channel switch S1b

And the solder under each. That is it, nowhere else to look. Oh I suppose the circuit board could be cracked or broken or otherwise damaged, but doubtful.

I am having the opposite problem. Clean works, switches to OD , the LED comes on, the clipping LED's even light up when playing (2 on clean and all 4 on OD), but getting no sound out of it. What would should I be looking to here. I did replace TR1, little FET didn't seem to be gating. no change in Overdrive sound though. Also no sound out of the headphone/emulated pre-amp out on OD, clean good.

I have looked and poked solder joints all day with not luck, so I am going to concede I am not getting lucky this time with a cold solder joint. Don't see any cracks and the board is pretty clean with no burns, cracks etc. DFX are functioning on clean channel as well. I wish I were better at schematics, I keep at it though.

Thanks in advance Enzo... You have been a huge help so many times on my projects, just from your responses to others.

[Enzo]

Hi a, welcome to the forum. Threads are cheap, so please start a new one for your amp. That way no one gets confused which amp we are working on.

Look at your schematic, if the LEDs blink with the music, then IC1 is working. That leaves the OD volume pot itself, a resistor and a cap, plus the selector switch itself. And teh connections between all.

But are we still talking MG15DFX? My schematic does not have a TR1 I can see, it is all ICs.

[altazepsta]

and case in point

Hi a, welcome to the forum. Threads are cheap, so please start a new one for your amp. That way no one gets confused which amp we are working on.

Look at your schematic, if the LEDs blink with the music, then IC1 is working. That leaves the OD volume pot itself, a resistor and a cap, plus the selector switch itself. And teh connections between all.

But are we still talking MG15DFX? My schematic does not have a TR1 I can see, it is all ICs.

Well, this makes your point. I was looking at so many pages, I just noted the MG. Mine is the 100 hdfx. But you have already given me a little bit to work with. Should I go ahead and start a new thread at this point?