marshall lead 12 3005 no crunch

[jologman2000]

hi all,

just got an old marshall lead 12 from a trade. i remember that a friend's lead 12 had very nice crunch tones. mine doesn't have any overdrive even at full gain. the previous owner told me that the amp got a bit weak. i've cleaned the pots to no avail.

is this normal for an old unit? also any components i should look at? there doesn't seem to be any burnt stuff or bloated caps. i'd like to learn amp electronics with this unit since the schematics look easy.

thanks.

[jologman2000]

update:

i put in a socket so i can change the opamp. i tried an rc4558p. plugged in my guitar, then i could only play for a few seconds till the amp slowly loses volume. then only crackling sounds when i move the volume or gain pots. strange. checked my socket for continuity or any shorts with a meter, it's fine. although some resistors near the gain pot seem flaky with the ohm readout. i'll figure this out in between my busy work sched.

further question though: i noticed there are no clipping diodes, so i'm wonderin how do i get the crunch tone? i do remember that this specific head does have the marshall crunch.

regards.

[jologman2000]

3005 schematic

if it's any help, here's a schematic:

[oc disorder]

While I'm not familiar with this model a good place to start is the power supply.
Check the +/- supplys to the preamp ie with one meter probe on the chassis
put the other first on pin 8 and then on pin 4 of the IC in the preamp.
Seeing you have fitted a socket it would be easy to remove the IC and measure there incase the probe slips and shorts to other pins.
According to the schematic you should get +16 volts dc and -16 volts dc.
It may not be exactly that ie 15.3 v or similar is in the ball park.
As I would suspect R 11 & R 6 check they have aprox 19 volts on one side
and 16v on the other or another way measuring across the resistor should
give a reading of 3 volts the amount "dropped" by the resistor.

If thats all ok the next parts I would check are the emitter resistors R26 &
R 27 which should measure like a short circuit as they are only .33ohm.

However if one of these was open the amp would sound quiet with the wrong
kinda crunch....!

Hope thats a good start.

[jologman2000]

Thank you!

cool! will check out those parts. thanks.

regards.