Marshall MG100hdfx Problem

[Alphadog69]

Hello
I have a Mg100hdfx that When used in the clean mode has a low volume level, when you press crunch you get blasted into the next room.

While using the amp on channel 2 OD1. I have a low volume level The same as the volume level when used in clean mode, Then pressing the OD2 button you blasted into the other room.

I am trying to find out why such a difference in theese volume levels.
All of the effects work. I can use the amp in clean or channel 2 OD1 but have to turn up the gain and turm up the master volume almost all the way but this seems to be about 1/4 the volume of the crunch or channel 2 OD2 Modes.
I have traced this down to possibly an op amp 4558DD but could not locate a data sheet for it.
If any of you help me out here it would be appreciated.
Thankyou
Alphadog69

[izzybell2810]

Sounds like one side of Ic4 (4558) has has gone bad!

http://www.farnell.com/datasheets/36657.pdf

[Enzo]

Look in any large parts house for the base number 4558, use filters for integrated circuits and "op amps" or "amplifier ICs" and see what you find. Each manuafacturer adds his own letters, so KA4558, NJM4558, RC4558, etc are all the same part. DO watch out for hte letters after though, because you probably want DIP style parts - the ones that fit in holes through the circuit board, as opposed to the tiny "surface mount" parts or SOIC type. Electrically they are all the same, but one won;t physically fit in place of the other.

All are the same, pins 8 and 4 are the + and - power pins, and pins 1 and 7 are the outputs. You often cannot measure signals right at the input pins.

[Alphadog69]

Gratitude

Thankyou both for your help and Knowledge.
After staring at the schematic last night for a while, this ic was my first choice at trying to solve the problem. I studied electronics in college many moons ago and still have a little retention. I always find it a good idea to ask others for their opinion and I appreciate the advice from people who are more knowledgeble.
I will replace this part when it arrives and post the result
Thankyou both once again!

[Alphadog69]

I just recieved The MC4558CN op amps today and replace Ic4, I istalled a socket to make life a little easier next time.
I still have the same condition. Do you think the issue could be in DG212CJ?.
Can you offer me any advice on where to look next.
Thankyou for your Time and expertise.
Alphadog69

[Enzo]

I wouldn;t think so, but anything is possible.

The DG212s do channel switching. IC2c, IC2d are for the clean channel. When they turn on, d completes the circuit for Clean channel out, and c mutes the OD gain stage of IC6a. Then IC2a is the OD channel output enable, and when the other two turn off, they opens the circuit from the Clean and also let the OD gains stage operate.

I am having a hard time wrapping my head around your symptom. There are two strips through this amp, the Clean and the OD channels. if Clean/crunch worked OK and both OPDs were weak, that is easy to follow. And if both ODs work but not the clean or crunch, that too makes sense. But when one of each of those pairs goes down, it makes me think there are two problems. Which is certainly possible, but less likely.

Crunch is just a couple switches on the clean that bypass resistors. SO I can see if R89 or C110 were open for example, you might have weak clean but strong crunch. More likely broken free than open, but...

And the OD selector is one simple swutch, so again if there were an open in the C103 C104 segments, I could see it.

But those two things are not related, and would have to be independent problems.

I doubt it is the DG212 because they don;t switch at all during the OD1-2 switch or in the clean-crunch switch.

APply a steady signal and trace it through the channels with a scope or AC voltmeter. See where it vanishes. Might as well treat it as two problems, If they wind up related, then solving one will probably solve the other.

[RAZ]

Marshall MG100HDFX Problem

I have an MG100HDFX with a similar probelm, clean mode has a low volume level, when I press crunch I get blasted, probably 3x the volume. I replaced IC4, 4558, and also checked out the resistors and capacitors in the clean/crunch circuit. Everything checks out OK. I'm scratching my head on this one, can anyone offer some suggestions?

[Enzo]

I'd say compare it to another unit. The crunch is after all a substantial gain boost, so I expect it to be a lot louder. I don;t know from 3x or whatever, but a bunch louder.