marshall g50r cd resistor value

[nosaj]

I've got a Marshall G50r cd sounds like crap. Looking on the inside I see r69 is burnt. But I cannot seem to find R69 on any schematic any ideas It is by the power chip.
Marshall-G50RCD guit amp.pdf

Thanks,
nosaj

[J M Fahey]

1) we canīt see your file but the infamous white pixel, so rename it or zip it or print it so the paranoid server does not block it with the excuse: "a guest with the same credentials is already inside so you must be fake"
2) ok, where does you mystery resistor go from , to?

Acrobat reader has a "snapshot tool" which nobody uses, it allows you to select an area and when you lift your finger, copies and sends it to the clipboard , then you can paste it as anew image on any graphics package (even humble Paint), edit/write/whatever on it and save it as gif/jpg/png which you can then attach here to your answer.

FWIW what I used to post the corrected Balder schematic as .jpg , main idea being that it appears straight inside the message window and is editable by anybody.

And f*ck pinterest, photobucket, google cloud, etc.

[Jazz P Bass]

I cannot locate that resistor.

As JM asked, you will have to trace what it connects to.

Here is a repost of the schematic: Marshall-G50RCD guit amp.zip

[nosaj]

I cannot locate that resistor.

As JM asked, you will have to trace what it connects to.

Here is a repost of the schematic: [ATTACH]45126[/ATTACH]

Ok. I'll have to remove it from the chassis so i can see the connections.

Thnaks,
nosaj

[MarkusBass]

I've got a Marshall G50r cd sounds like crap. Looking on the inside I see r69 is burnt. But I cannot seem to find R69 on any schematic any ideas It is by the power chip.

R69 is on the first page of the schematic, on the input of IC3:A op amp. But this is in the preamp and not in the power amp. Are you sure that this is correct schematic? Can you post a photo of the board (with this resistor)?

Mark

[nosaj]

R69 is on the first page of the schematic, on the input of IC3:A op amp. But this is in the preamp and not in the power amp. Are you sure that this is correct schematic? Can you post a photo of the board (with this resistor)?

Mark

Here it is. Looks like there are some lifted traces. The bottom side the 2 solder points that are discolored are the resistor looks like it is connected to j17 and c70. I'll see where j17 goes to. It goes to a speaker lead.

Thanks,
nosaj

[MarkusBass]

On the third photo you can clearly see that the version on the board is different from the schematic you posted. This could be the reason why you couldn't find the resistor on the schematic.

Mark

[nosaj]

On the third photo you can clearly see that the version on the board is different from the schematic you posted. This could be the reason why you couldn't find the resistor on the schematic.

Mark

Does marshall Give out schematics? If so I'll call them Mon.

Or does anyone have a different version of the schem?
nosaj

[DrGonz78]

It is definitely a different board version but some components numbers appear to match up. Such as C50, C51, and C71 look like the designations did not change from version A to later version(schematic). Also, the schematic does not list J17 or any of the Jumper leads on the schematic, so just look at it as a trace.

Edit: So what is connected to J17 on your board? List those components as R71 or whatever, but try to put what values the components are as well.

[nosaj]

It is definitely a different board version but some components numbers appear to match up. Such as C50, C51, and C71 look like the designations did not change from version A to later version(schematic). Also, the schematic does not list J17 or any of the Jumper leads on the schematic, so just look at it as a trace.

Edit: So what is connected to J17 on your board? List those components as R71 or whatever, but try to put what values the components are as well.

Ok Red speaker lead >j17>r69>r52 c51>pin3 of the lm3886.

Thanks,
nosaj

[Jazz P Bass]

If it was mine to do, I would print out the schematic & remark the component identifiers to match your board.

Then I would go through and verify the schematic changes.

Edit: "Ok Red speaker lead >j17>r69>r52 c51>pin3 of the lm3886."

This makes no sense.
Why would the IC output go through all of that to get to the speaker +?

[DrGonz78]

"Ok Red speaker lead >j17>r69>r52 c51>pin3 of the lm3886."

So I am reading this as Speaker goes straight to pin 3. R69 is fried but what values are R52 and C51? You need to look closer between schematic and board layout to make more sense of this. My guess is that R69 = R61 and R52 = R60 which is then in parallel with C51 = C53.

Edit: BTW my guess involves all the components that lead up to the line out jack.

[nosaj]

"Ok Red speaker lead >j17>r69>r52 c51>pin3 of the lm3886."

So I am reading this as Speaker goes straight to pin 3. R69 is fried but what values are R52 and C51? You need to look closer between schematic and board layout to make more sense of this. My guess is that R69 = R61 and R52 = R60 which is then in parallel with C51 = C53.

Edit: BTW my guess involves all the components that lead up to the line out jack.

It would correspond with r61. All I had on hand was a 36k So I popped it in. It works fine. If you look at the pics someone replaced the input but it looks like they just broke the board off and soldered in a switchcraft jack. When nothing is plugged into the input jack this thing will self oscillate and then burn that resistor. Keep a guitar plugged in and never a problem turning it on. It is being considered for donation to the local prison music department.

Thanks,
nosaj

[Mrcloc]

I came across this thread looking for information on a Marshall amp I'm going to try repairing.

I see that the burnt resistor looks like the output crossover. It's part of the LM3886 (I know Marshall like to use these) basic schematic. On the datasheet they show a 2.7 ohm resistor.



It could be up to 10 ohms. Who knows?

Ah wait - there it is on the PDF someone posted - R71. It's a 10 ohm.

Ok, so what now? That can only blow with the amplifier oscillating tremendously, or C70 blowing (who knows why that would). The problem could be isolated to the amplifier - it could have overheated, which could have caused something to go wrong before it blew. Or it's not blown - probably not if you hear sound. Crap sound is likely because that resistor was responsible for eating the oscillations, and now they're causing the amplifier to clip at low power.

How to fix the oscillations? That's a mystery. Perhaps replace the burnt resistor and C70. Use a 100V polyester capacitor (100nF) and a 5 W wirewound resistor (10 ohm). That little 0.25 W thing was never going to cut the mustard.

Pitiful that these big name manufacturers create stuff that's going to blow sooner or later, and use low quality amplifiers and sell millions, where a guy like me, who makes ridiculously high quality stuff can't sell one.

You could connect an inductor and resistor to the speaker at the speaker like so:


Use 1mm magnet wire and a 2.7 ohm 5 W wirewound resistor. Wind the magnet wire around the resistor, there and back, which makes about 18 windings. That might help.

Here's what I mean - in the foreground you can see the resistor and wire wound around it.

[Mrcloc]

I would also like to add that I believe the oscillation happens because of that input connector - I'm almost 100% sure that the original connector will short the input when nothing is plugged in.

Amps should be built to never oscillate under any circumstances. Shorting the input to prevent oscillation is pathetic.