Marshall Mosfet 100- HISS

[woodstock1]

ebay seller: 'the guy HE bought it from said it had a hiss.....but honestly, he could not hear a hiss'.......bloke is deaf or a blasted liar.............snake city..........anyone have a clue.....?!........

[Enzo]

You tell us. Do ANY of the controls have ANY effect on the hiss? If the volume control turns the hiss down, then the hiss is coming from before that control, as an example.

[woodstock1]

hiss is loud and constant.....volume control does not seem to affect it........

[woodstock1]

i will do a complete control check tonight.....

You tell us. Do ANY of the controls have ANY effect on the hiss? If the volume control turns the hiss down, then the hiss is coming from before that control, as an example.

i had spent about an hour on it earlier today......will give a detailed rundown tomorrow on each control......thanks.....jim

[J M Fahey]

Just curious: does the heat sink heat up even if not playing?

We might be hearing a subproduct of some nasty HF oscillation.

Whay happens if you set *all* conrols to 0, including Master?

What if you insert a shorted plug in the loop return?

[woodstock1]

Just curious: does the heat sink heat up even if not playing?

We might be hearing a subproduct of some nasty HF oscillation.

Whay happens if you set *all* conrols to 0, including Master?

What if you insert a shorted plug in the loop return?

hmmmmm- I will be checking it again late tonight for the sink heat......and the loop return trick.......when all controls are 0- no sound....no hiss......actually, when volume or master controls are advanced pass 1, the hiss arrives quite loud.......stays fairly constant but does increase somewhat past 5.......when tone controls are all at 0- no sound, no hiss.......even if master and volume are turned up.........plenty of volume........good speaker.....good cords.........your note was much appreciated-thanks..... any ideas are welcome...!.....jim

[52 Bill]

when volume or master controls are advanced pass 1, the hiss arrives quite loud.......

So you have isolated the problem to something in the circuit before the Master Volume control.

Bad transistor or opamp chip?

[J M Fahey]

Plese post schematic here so we all use the same.

A closeup readable preamp pcb picture would help woodstock find which part is where, faintly remember part designations were silkscreened on pcb.

If memory is right, all gain was through op amps, transistors were switchers, some inside a transistor array IC, one or two discrete.

Also: does switching clean/dirty channels affect hiss?

Which is the noisy one?

This is an early amplifier, not that high gain, about what a Dist+ or TS or DS1 would provide.

[g1]

Is this the lead 100 (3210), reverb twin (5213) or something else?

[woodstock1]

jm- heat sink stays cool when not playing.......plugging a guitar into the effects return jack yields no hiss....no hum......no controls affect the sound or volume.........shorting the plug yields no sound.....and no hiss at all.......bill- this is the 3210 head..........I use library computers.......unfortunately, I have no clue on how to post a diagram on these........I will check the 2 channels and the hiss factor tonight....and post results tomorrow PM.........your help and ideas are much appreciated.......jim

[g1]

Here is the 3210 schematic, is it the same version as you are using?

[g1]

Or this one?
http://www.drtube.com/schematics/marshall/3210-iss7.gif

[woodstock1]

g1- why thanks for the print....yeah- this is the one that I dug up.......the noisier channel is the drive channel.......I did put an mxr gate into the effects loop.......it works well.......but the hiss does attach itself to each note when playing......oh well......I will experiment with a couple of my usual outboard dod distortion effects.......perhaps the hiss will be buried........jim

[J M Fahey]

Not a fan of shotgunning, but in this case try to replace IC1 , a TL072, with a fresh one, just to check , it might be bad.
Friends here might suggest an alternative lower noise type, still available in DIP8 package.

If you dare, there are modern definitely lower noise ones, but sadly available only in miniature SOIC8 package, which would need an adapter.

That said, your dirty channel should not be noisier than an MXR Dist+ , a DS1, a RAT, etc, because that's what it basically is.

[woodstock1]

thanks- will try it......