Help on Lead 12 combo repair

[steve_m]

Since I'm more of a tube guy, I don't even begin to know where to start fixing this amp.

I have a Marshall Lead 12 combo. I bought it used a few years ago to gut and build into something else, but it sounds pretty good as is and makes a nice light practice amp. The stock speaker really sucks and makes the amp sound crappy. When the crappy stock speaker is replaced with a good Celestion or similar, it really opens up the amp's tone and makes it sound much better. The mini stack head was used by a lot of the LA guys in the mid to late 80s in rack setups as the tone generating machine, IIRC.

It has never really worked right. When I got it the low input channel was intermittent, it would cut in and out while playing the amp. The high input is fine. I tried reflowing the solder on the low input jack, but this made the problem worse. Now it squeals when the gain is dimed and the volume is turned up.

Any suggestions where to begin? There aren't that many parts connected to the low gain input: a jack, a resistor and an opamp.

Any ideas where to start to fix this thing?

Schematic is here:

http://www.drtube.com/schematics/marshall/3005.gif

The combo is model# 5005, the mini stack is model# 3005.

TIA,

steve

[turbolx5oh]

I'd post it in the diyuadio.com Musical Instruments forum.  Might get a few more looks at it.

[steve_m]

Thanks for the reply, I'll try that.

steve

[indyguitarist]

Looking at the schem, if the high input works fine but the low input is the only one having trouble, I'd just replace the jack and R3 (resistor). That's the only thing it could be.

bw

[steve_m]

Update:

I got three Cliff jacks from AES in the mail today on the advice from a tech who posts on the Ampage.

I replaced all three jacks with new ones and replaced the 33k, 68k, and 330k resistors connected to the input jacks.

The amp now works and the squeal is gone. Oddly enough the low gain input is louder than the high gain input with the same volume and gain settings.

Thanks to all who replied.

steve

[joecool85]

Awesome :-)  And the low gain jack should be louder with the same input.  The reason why is because the high gain jack has an extra resistor to cut signal so it is essentially low gain... I think.

[teemuk]

Something to do with switching arrangement: No plug in high input jack grounds the signal of low input through both R1 and R2 (parallel connection so resistance is about 24k). I can't read value of R3 (68k?) but this one forms a voltage divider with R1 and R2.

When the plug is inserted to high input the jacks's internal switch mechanism (note the arrow) is broken and the voltage divider is now R2 and R3 (only small attenuation). Are you sure you wired the jack's switching arrangement properly?

[joecool85]

Ah, thanks for clearning that up, regardless its great that the thing is working.

[steve_m]

Thanks for the reply.

I'm pretty sure I got everything wired correctly. The jacks & resistors are PCB mount. I probably should pull the PCB again and check my solder connections with a magnifying glass.

Oddly enough, the low gain input sounds much better than I remember. Usually it is kind of thin and anemic sounding; now it is instant Angus Young

The High gain input is saturated and compressed sounding compared to the low gain input, but not as much volume.

Regards,

steve

Something to do with switching arrangement: No plug in high input jack grounds the signal of low input through both R1 and R2 (parallel connection so resistance is about 24k). I can't read value of R3 (68k?) but this one forms a voltage divider with R1 and R2.

When the plug is inserted to high input the jacks's internal switch mechanism (note the arrow) is broken and the voltage divider is now R2 and R3 (only small attenuation). Are you sure you wired the jack's switching arrangement properly?