Eden Nemesis NC320

[fredcapo]

I see there is some recent activity on this amp on the forum...I have one in my junk pile that was dead.
So seeing the schematic posted here by Jazz I decided to open it up. Found that the -15v zener (cheapo looking) was shorted, so I replaced both + and - with some motorolas I had...amp works now but the speaker output is fizzy fuzzy. The phones output sounds good and clean. So this seems to leave the mosfets as the problem. They are kind of pricey to just yank them all...so how do I test them? Is there an in circuit test, though since they are in parallel cant figure...probably need to pull all and resistance test? Thanks.

[Jon Snell]

Check the bias current. The FETS will fail catastrophically and not make fizzy noises.
Schematic would be useful.

[nevetslab]

Check the bias current. The FETS will fail catastrophically and not make fizzy noises.
Schematic would be useful.

I've never actually checked the MosFET's in the one Nemesis NC320 I have in the shop. Now, in servicing Ampeg SVT3-Pros thru SVT6-Pro amps, those are all closely matched at the factory, and I've been buying tubes of 25pcs or more and batching/matching them in sets so I have replacement sets, as well as some that might closely match what's in a given amp. Often with the Ampeg SVT-Pro's, you'll loose one side completely, though not always. Over-bias the MosFET's if you're not monitoring the ones running the highest current in the set, they WILL fail if you turn the bias up too much. I'll be curious to hear your result, measuring the voltage across the Source resistors and calculating the idle current of each device.

[fredcapo]

I printed a copy of the schematic from a post from “waspclothes” (not Jazz P Bass) that was in schematic requests shown under the similar threads section...but I cant seem to locate the post now... i will get the scope on it and make sure that the headphone output is as clean as I thought. The headphone output feeds directly into the 3 mosfet pairs that drive the speakers

[g1]

This schematic?

[Jon Snell]

I've never actually checked the MosFET's in the one Nemesis NC320 I have in the shop. Now, in servicing Ampeg SVT3-Pros thru SVT6-Pro amps, those are all closely matched at the factory, and I've been buying tubes of 25pcs or more and batching/matching them in sets so I have replacement sets, as well as some that might closely match what's in a given amp. Often with the Ampeg SVT-Pro's, you'll loose one side completely, though not always. Over-bias the MosFET's if you're not monitoring the ones running the highest current in the set, they WILL fail if you turn the bias up too much. I'll be curious to hear your result, measuring the voltage across the Source resistors and calculating the idle current of each device.

These Toshiba MosFets have a very high gate impedance and are easily mistaken as short circuit when if the gate discharged, they are open again and work correctly. Another good reason for anti static protection when dealing with MosFet.
MosFet transistors are like a valve with no heaters.
A valve if over driven will heat its anode and the electron flow will reduce, compensating for the over current and compressing the signal.
Unlike silicon transistors, a MosFet will reduce its gain as the MosFet gets hot, effectively compressing the signal, going a long way towards eliminating the need for close matching. Not that matching is not a good practice of course. It is always good to have a level playing field.

[fredcapo]

Thanks g1 thats it. I will take some measurements at the output stage.

[nickb]

Call me blind as a bat but I can only see a 10V zener. Are you sure it was 15V? Not that I think that is the problem.

[fredcapo]

Nick. Look at the schematic posted by g1 a few posts above...+ and - 15v to supply the circuit.

Yes the zeners not the cause of the distortion fuzz problem.