LEad Mosfet 100 nearly no sound

[Tahelos]

Hello all,

since I have almost no electric guitar experience and only play acoustically, I bought an electric guitar a while ago and played it with a cheap modelling combo.
By chance I found an old Lead Mosfet 100 of my uncle from the 80s in the attic.
So I bought a 1x12 cab for home. After a while the amp started to drop out, I had to turn master and volume up to 10 to get a "reasonable" sound out of it, even though level 3 was already too much for the flat.

What could be the problem? Can I repair it myself (I am somewhat experienced and my grandfather is a former telecommunications technician) or would it be better to take it to a workshop? Can anyone recommend one in Germany that also repairs transistor amps?
Or is it probably no longer worth it?
Since i can't play with this amp at the moment, I have switched to a "flat / wife-friendly" DSL1HR, but would like to keep my uncle's old amp in the family (playable if possible).

Thank you for your feedback

 

[fitz]

By chance I found an old Lead Mosfet 100 of my uncle from the 80s in the attic.

After a while the amp started to drop out, I had to turn master and volume up to 10 to get a "reasonable" sound out of it, even though level 3 was already too much for the flat.

Start with some simple maintenance before heading off to the repair tech - and yes, the amp is worth saving.

If it's been sitting an attic for a long time, try cleaning the speaker and loop jacks with some Deoxit. Some cleaning of the input and all the pots may also be in order while you're at it, but a bad contact in the loop or speaker jacks would be my first suspicion of fading output signal.

 

[Kuga]

Make sure you are using a speaker cable on this 1x12 cab.

 

[Tatzmann]

It's just the the loop-cliffjacks. They need professional cleaning, not just drenching the loopholes in contactcleaner.

They are "switchingjacks". If nothing is inserted
the switchcontacts close and pass the preampsignal further to the poweramp. The contacts are getting tarnished over time through environmenta. They are not selfcleaning.

Search "cleaning cliffjacks". I explained the procedure many times.

 

[Tahelos]

Make sure you are using a speaker cable on this 1x12 cab.

I got told to never use an amp without a cab, so thats for sure.

Search "cleaning cliffjacks". I explained the procedure many times.

Thank you, i will try this out and report back.

 

[TassieViking]

If it is a dirty contact in the return jack then plugging a loop cable from the send to the return jack should work, a small cable like you use between stomp-boxes is the perfect length but any guitar cable will do.
If that fixes the problem then you need to clean the jack sockets.
Don't forget to clean the input jacks as well while you are at it.
Which model Mosfet amp is it ?
Most likely a 3210 but you never know till you check.
There are several different Mosfet models of heads and combos out there.

 

[neikeel]

Transistor amps and the Mosfets are much more forgiving than valve amps re no load but you should really use decent speaker cable for minimal impedance and current handling capacity.
Do the things suggested above, if they work = great. If not you need to do some static tests of components and then an oscilloscope.

 

[Tahelos]

If it is a dirty contact in the return jack then plugging a loop cable from the send to the return jack should work, a small cable like you use between stomp-boxes is the perfect length but any guitar cable will do.
If that fixes the problem then you need to clean the jack sockets.
Don't forget to clean the input jacks as well while you are at it.
Which model Mosfet amp is it ?
Most likely a 3210 but you never know till you check.
There are several different Mosfet models of heads and combos out there.

U/ps, forgot to mention. A 3210.
Cleaned the jacks, tried the cable. Still nothing :/

Transistor amps and the Mosfets are much more forgiving than valve amps re no load but you should really use decent speaker cable for minimal impedance and current handling capacity.
Do the things suggested above, if they work = great. If not you need to do some static tests of components and then an oscilloscope.

I used a decent cable for it. For now nothing worked, maybe open it up and check if something is burned or damaged.

 

[TassieViking]

Knowing the model is 3210 is a good start, I have only seen 2 different schematics for the 3210 model.
There were upgrades as times went along with most amps.

 

[Tahelos]

Hello everyone,

haven't heard from me for a long time.
A quick update on the 3210:
After having absolutely no time to take care of the amp for a long time, I took it to a specialist workshop this week because I don't know enough about it myself and really don't want to break things any more, but want to finally play on the 3210.
Hopefully it won't be too expensive.
I can keep you up to date if there's anything new.

 

[Tatzmann]

Good luck getting it fixed, the 3210 Mosfets are nice sounding amps, better then alot of stuff they came out with afterwards, valve or transistor. A little noisy though. If you got a good tech you could ask him to do something about it, idk if it's enough to change the opamps for higher quality ones to cut down on the noise floor/hiss, especially the reverbcircuit introduces alot of it. Ask him.

 

[anitoli]

Lose the 1458's and replace with TLO72 and all the SS hiss will go away.

 

[PelliX]

Lose the 1458's and replace with TLO72 and all the SS hiss will go away.

You have a lot of faith in TL072's, hehe. Let's just say OP should be aware there's a pretty broad spectrum of those available and the really, really, really dirt cheap ones can be ropy at time. In general a decent TL072 should be better, though - agree.

 

[anitoli]

Texas Instruments TLO72CP. These are the good ones. I used these religiously.

 

[PelliX]

Texas Instruments TLO72CP. These are the good ones. I used these religiously.

Yup, probably the best there is. Not to nitpick, but if OP goes googling, it's T-L zero seven two, not T-L-O. Phonetically it works for Brits with the O, of course. Is that a thing in the States, too?

 

[anitoli]

Yup, probably the best there is. Not to nitpick, but if OP goes googling, it's T-L zero seven two, not T-L-O. Phonetically it works for Brits with the O, of course. Is that a thing in the States, too?

Yes you are correct, i have always typed this in wrong, lol. It's TL072.

 

[TassieViking]

The mosfet power transistors can be hard to find, but they are around if one really searches.
Just don't buy fake ones on fleabay or any cheap Chinese shops

 

[Tahelos]

I think the guys in the workshop know what they are doing, they have been around for over 20 years and the reviews for them are consistently positive.

 

[Tatzmann]

A youtube classic?



Remember watching that vid the year it was uploaded on yt at 240p...guess I'm old.

 

[anitoli]

The mosfet power transistors can be hard to find, but they are around if one really searches.
Just don't buy fake ones on fleabay or any cheap Chinese shops

If the T03 case MOS FETS are too hard to locate there are suitable T0220 case ones that with a bit of work will fit.