Marshall Lead 100 Mosfet - crackly, loss of vol

[wagdog]

Greetings all;

I picked up a busted Marshall Lead 100 Mosfet (3210 head). It sounded pretty bad - loud hiss, poppy, pretty much unusable. Since the amp was an '86, I went ahead and replaced all the electrolytics, that didn't seem to help but the originals were definitely past their prime. By trial and error I narrowed down the noise to IC1 and replaced that and now the amp sounds great.

It has one glitch that I don't like though; when I turn it on it makes a very loud pop in the speakers.

I admittedly know much more about tube amps then solid-state. Can someone give me a direction to start looking in to help with this? Maybe it's normal? The amp is nice and quiet after that (except when played - this amp can bring the loud!).

Hard to read, but apparently the only schem to be found is at: http://drtube.com/schematics/marshall/3210.gif

Thanks,

Wag

[Steve Conner]

Solid-state power amps are not guaranteed not to pop. It depends on capacitor tolerances and so on. If it bothers you, the easiest solution might be to install a muting relay.

Loudspeaker Protection and Muting

If you turn the volume to zero before switching on, does that help?

[g1]

It's probably normal but you might check that the +/-15 rails are balanced. Check all 4 of those 2 watt resistors in the low voltage supply. A DC offset could make the pop worse than normal.

[Gtr_tech]

The CA3046 is a common failure in these. I see it all the time. Causes the symptoms you describe. Fortunately, they are cheap and commonly available.

CA3046 NPN Transistor Array-The Electronic Goldmine

[wagdog]

If you turn the volume to zero before switching on, does that help?

Thanks for that link. Forgot that I could just unplug the speaker cab when I turn it on... just used to tube amps!

Turning the vol all the way down makes no difference in the pop when it comes on.

It's probably normal but you might check that the +/-15 rails are balanced. Check all 4 of those 2 watt resistors in the low voltage supply. A DC offset could make the pop worse than normal.

Will do. It is quite a pop when it comes on.

The CA3046 is a common failure in these. I see it all the time. Causes the symptoms you describe. Fortunately, they are cheap and commonly available.

CA3046 NPN Transistor Array-The Electronic Goldmine

Thanks for that link, will pick some up. I might as well change the rest of the op amps just for grins (there are 3 more).

Amp sounds pretty good cranked up. Never heard this amp before I bought it. Figured if I couldn't get it working I could just build an 18w into the chasis. Might do that anyway as it looks pretty cool.

[wagdog]

Does anyone have a better, more clear schematic for this amp?

After really looking mine over, there is a cap missing on the board (C42) that was there at some point (there are two legs in the holes where it was). I can't find C42 on the schematic, and I've seen pics online of other lead 100 mosfet's that do have a cap there... and a pic of another one that doesn't.

Anyway, no progress on the loud pop when it turns on. Checked the 4x 2w power resistors and all of the diodes (including the zeners) in the amp, all good. Just been unplugging the cab when I turn it on. Having all vol controls down to 0 does not make a difference.

[J M Fahey]

The pop is normal.
My solution, when I donīt want to bother with a relay and associated circuit, is simply to add a "standby" switch (on the front or the back panel, your choice) which just opens the wire that goes to the "hot" speaker jack pin.
Works like a charm.
You just turn the amp on, and 5 seconds later the standby switch; when turning off reverse the procedure.
Personally I would build the 18W in its own chassis and enclosure, and give this old workhorse its proper use: a *loud*, quite good sounding paint-peeler.
It takes pedals very well too.
Of course itīs your choice.
I find any small (25/50W) SS combo a much better candidate for an 18W, an AX84 or similar gut transplant.
jm2c

[wagdog]

The pop is normal.
My solution, when I donīt want to bother with a relay and associated circuit, is simply to add a "standby" switch (on the front or the back panel, your choice) which just opens the wire that goes to the "hot" speaker jack pin.
Works like a charm.

Thanks for that suggestion. I'd actually thought about doing that, but wanted to be sure the pop was normal and putting a speaker disconnect switch in wouldn't just be masking some other problem.