MOSFET power amp help

[pollypocket]

Hi;

I was wondering if anyone has had any experience with a project published in Radio Electronics back in the mid 80's written/designed by Reinhard Metz? It was a 250 wpc FET outputs power amp. I tried raising someone at the ATL web site, but no response...I suspect they don't support this amp at all.

Anyone else built one or familiar with the amp? It is a bit of a brute and has no safe guards for the output FETs from shorted speaker leads. I cooked the outputs in on channel (shorting a speaker lead) and need to replace them. They are connected in parallel and are all current matched. I am not sure how far back I have to go in the circuit looking for components that got taken out by the short. Any thoughts?

thanks,

Paul

 

[anatech]

Hi Paul,
Look at the drivers, keep going back until you start hitting good ones.

-Chris

 

[clem_o]

Hi all,

Wouldn't the drivers have a better chance of surviving given the insulated gate of the MOSFETs?

Hmmm, on the other hand if the MOSFET silcon melts down then the gate wouldn't be insulated anyway... oh well...

Cheers!


ps re the amp - my brother's subscription to RE ended about the mid-80's - not sure if I can dig it up! How about posting the schematic?

 

[pollypocket]

Hey Clem_o;

I can post the schematic as soon as I dig it up. Any rules on the forum re: intellectual property for posting schematics?

Paul

 

[clem_o]

Hi Paul,

There are, i.e. if the gear is commercially being produced, if there is good technical support etc.

OTOH, mid-80s would put it at 20 years, probably out of any claim to copyrights, and you haven't gotten any support from ATL...

Chris (anatech), watcha think?

Cheers!

Clem

 

[anatech]

Hi Clem,
There is a ton of other stuff posted. I wouldn't worry about this one too much. The manufacturer can always request it's removal if it came right down to it.

With regard to the mosfet gates shorting. Yup. Sometimes the protective zeners from gate to source (if used) will short instead. In any event, just keep going back until everything tests good. If it was a mid 80's Yamaha home amp, that puts you at the input cap to the diff pair.

-Chris

 

[clem_o]

If it was a mid 80's Yamaha home amp, that puts you at the input cap to the diff pair.

OUCH!!!!

Thanks Chris!

 

[pollypocket]

MOSFET Power Amp Help

Hi Guys;

I will scan and post the schematic as soon as I can find it. I am in the middle of moving and I think it has been packed and put into storage already. I will look around a bit more, but if I can't find it, I will post in September.

thanks,

Paul

 

[anatech]

Hi Clem,
I left out the worst part. No silk screen on the pcb, no bottom access. Warranty claims.

-Chris

 

[clem_o]

Hi Chris,

Some manufacturers can really make a service tech's life difficult! I'm surprised they didn't pour silicon rubber or epoxy to seal up the thing!




I'll have a look if I can locate those RE magazines.... that is, if the termites haven't feasted on them yet!

Paul, I think you can give it a go even without the schematics anyway - expert guide is here (Chris)...


Cheers

Clem

 

[pollypocket]

Mosfet Power Amp Help

Chris;

Do you have a shop in G.Town?

I live in Waterloo....

Paul

 

[clem_o]

Paul,

Suggest sending Chris a pm...

Cheers!

 

[pollypocket]

Clem;

Thanks, I am still green WRT forums.

Paul

 

[clem_o]

Hi Paul,

Ditto - still finding my way through the user interface here..

Try:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=80961&highlight=

And somewhere near the bottom (post #8) is his e-mail...


Cheers!

edit: or, you could click on the person's user-name, the one that appears in each post on the left column, to get more info on that poster...

 

[pollypocket]

Gents;

I am finally getting back to this thread. It took a while to locate the file I had for this amp. I have attached the schematic for our discussion.

As indicated; I shorted the outputs on one channel and have no sound as a result.

Although I built the amp from scratch, I am not a tech or engineer. My best bet at trouble shooting is a shotgun approach; keep replacing components until it works. I am guessing that Q21-28 are gone as a result as well as Q19 - Q20 and D12 - D13.

Suggestions gratefully accepted.

thanks,

Paul

 

[anatech]

Hi Paul,
Have you any idea how much I dislike NTE and ECG parts?

Normally you should repalce all teh outputs and possibly the driver transistors. Some designs will blow stuff up further back too. The protection zeners on teh gates and some of the resistors are most certainly gone as well. Normal bang.

Don't replace components a bit at a time. Remove and check them or replace all before appliying power. A variac comes in very handy around this point in time.

-Chris

 

[pollypocket]

Chris;

Thanks for the advice. I am not a fan of the generic components either. I spent a lot of time matching components and buying the best products I could for the project and had to use the ECG's as prescibed in the design. I do have a variac and it has come in handy. I actually need the variac to start the amp...I have big caps in the PS and it will blow a 15amp fuse if I turn it on cold.

Regards,

Paul

 

[anatech]

Hi Paul,

I actually need the variac to start the amp...I have big caps in the PS and it will blow a 15amp fuse if I turn it on cold.

You need a soft start circuit then. That will greatly increase the life of your power switch if you didn't use a variac and larger fuses. Your variac can then be put to better uses.

Have a look at the On Semi selector guide for "real" transistors.

-Chris

 

[pollypocket]

Chris;

The schematic has an inrush filter on it, but I could never find the part. I do have an external inrush for the amp now.

Can you give me a little direction/detail on the On Semi guide?

thanks,

Paul

 

[anatech]

Hi Paul,
On Semi has a selection chart or tree on their web site, older manuals from Motorlola have a similar guide in the book.

http://www.onsemi.com/PowerSolutions/home.do

I find the web based selection chart much harder to use than the book version.

-Chris