Just shooting the breeze Carvin 400FET

[DrGonz78]

Well no dice. Called Carvin and it was as if they've never even heard of the SX400-A. First thing the guy said was "You mean the sx 300 right?"... Very quick call and not on hold very long at all. They quickly transferred me to a service tech and he was searching his database in minutes. So no schematic but the one on the 400FET should prove to be as close as it gets. I guess my main concern was really the bias procedure on the amp, but might have to wing that one now.

[DrGonz78]

Well I am reviving this old thread to ask a little help about biasing the amp. I ended up letting this amp sit for a bit while I had other things to get to first. Now I have it all repaired and it is good to go. However, looking on this schematic it says "adjust P51 for 75ma (+/-25ma) idle current unit warm" and I am unsure where to take my measurements. I can always call Carvin again, but I was wondering if anyone here knows where to measure idle current. Note that I am only concerned about the Ch2 part of the schematic as my amp "SX400" does not have both channels. This schematic is accurate to my amp and will suffice as a reference to the SX400. Oh for the record I have used IRF630 mosfets for the outputs and researching a bunch found those to be really good replacements on this amp. Thanks as usual for all the help.

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[Dr Kaos]

You might need to be very careful there Dr Gonz.
The Fet you suggest IRF630 and the IRFP250 are switching Fets rather than linear Fets and manufacturers can use them for audio power if they are well matched and usually grade them themselves.
If you or I buy a few Fets off the shelf they may have quite a spread of turn-on characteristic and not share the current equally. Also switching Fets have a sharp turn-on making bias adjustment critical.
There has been similar discussion about these Fets on this forum used in other amps, Ampeg,Hartke.

Regards

Dr Kaos

[DrGonz78]

I am reading more related info on threads on this site and I understand why Dr. Kaos has his concerns with (possibly) unmatched switching mosfets. The reason I replaced these with IRF630 is because the original mosfets they used on this amp were MTP8N20 types. The MTP8N20 cross reference sheets match up the IRF630 as a good replacement. I know that TH15N20 will match as cross referenced to IRFP240, so all that make sense. It seems that with all my DIY research and failure to ask for more help has put me in a new situation. Which I find all of this to be a great tool for learning and am happy to stall out to do more research. This is the way I end up learning and hopefully with a working amp at the end of the day.

So there is still the question about how to measure idle current on this particular circuit. I am reading more and learning more about just this very topic. Even doing a bias procedure where it tells you exactly what to do does not really help you learn any theory. I have read Enzo's approach to setting bias on solid state amps where you have no schematic and found that to be interesting. In this case I have instructions on the schematic, but I just have no clue which resistor I am measuring for a voltage drop. Obviously an emitter resistor is what I need to look for and there are three wire wounds on this board. So it looks like I need to keep on reading and analyze this board some more. I can also try calling Carvin but I really want to understand what I am looking at better first.

I am gonna look at the date codes on the transistors to see if they are at least close in that department...

Edit: Note that I say an emitter resistor but that is not necessarily related to the wire wounds or is it? Wire wounds are R70 R71 R73 repectively on the schematic. Emitter resistor for each mosfet leg is independent and possibly related. I mistyped by saying it was an emitter resistor I guess.
Also, my schematic is really more related to the other version of the schematic I posted on my original post to this thread. There are six mosfets not four. Still the schemas are very closely related.

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[Jazz P Bass]

It would help you if you keep in mind what the bias in a Class A/B amp is used for.

Specifically, it is used to remove the crossover distortion that results in a true Class B design.

That is all, that is it.

A Class B amp will idle 'stone cold', because both output halves are off.

So, if you monitor the line current going through the amp, you can see when the amp goes into the A part of Class A/B.

At turn on, with the bias set stone cold, the amplifier will pull X watts from the line. (20-40 watts)
As you slowly raise the bias voltage you will observe that at some point the current will start to climb.
Bingo, you are now in Class A/B.
Turn the adjuster back (towards colder) a notch & call it a day.

You can also monitor the output load & 'watch' as the xover notch disappears. (which requires running a signal through)

edit: Credit goes to Enzo for pointing out this method.

[52 Bill]

If you look at the schematic, there is a boxed number 9 located at the point where the power supplies connect to the circuit. Legend number 9 is the note about how to set the bias, so it would make sense that there two items are related.

It looks like there are links to open the power supply line and insert a current meter into the circuit. My guess would be that you open the link, connect your meter across the two points and set the bias there. By the way, 75mA, +- 25mA? Not too wide a range there.

[greenmeanh1]

New here. Ended up here chasing FET400 info. I have two of the amps mentioned with Rev A 9008 boards. Mine are a little different than the one in the picture. I have four 4700uf caps where the original posters pic had only two.
I got confused by a schematic i had downloaded which showed a different layout with less N mosfets and of a different type. The earlier amp like mine used 12 IRFP254 n modfets and the later amp used only 8 5n20's. I just downloaded the schematic that was listed in this thread and it ended up being the correct match to what i have. So thanks eh!!
Both my amps are blown out on one side showing 5 of the 6 nfets shorted. Replacements are available online for cheap.. I am also going to replace the 4700uf 80v caps.
One interesting note about my amps is they all have the insulators under every mosfet but one of them has no bolts into the heat sink for the individual fets. Anyway mine are all stripped down for cleaning and repair. Just waiting on parts from China. I would add some pics if i could figure out how.

[g1]

Sorry to be pessimistic, but if you are sourcing obsolete parts and find them in China, there is a high probability they are counterfeit low power substitutes.

[Leo_Gnardo]

Sorry to be pessimistic, but if you are sourcing obsolete parts and find them in China, there is a high probability they are counterfeit low power substitutes.

I wuz gonna say... Good Luck Chollie!

Always best to source thru trustworthy vendors: Mouser, DigiKey, Element14. Prices may not be the cheapest OTOH your time is worth something too, best not wasted by installing counterfeits.

[DrGonz78]

Can someone help explain the specifics on how to match mosfet pairs. Even links to correct info on the subject will be beneficial. I would like to build some piece of test of equipment to be able to match the mosfets. To be honest that SX400a amp, the one I attempted to repaired in this thread, is still waiting for me to try again. I got it up and running but when passing signal there was all sorts of issues. My thinking back then was to get better at tech work then return to it at a later date. Perhaps now is as good as a time to learn how to match mosfets. Greenmeanh1 could benefit too from this as well.

[g1]

Can someone help explain the specifics on how to match mosfet pairs. Even links to correct info on the subject will be beneficial.

Here's a couple:
http://www.geofex.com/article_folder...h/fetmatch.htm

http://music-electronics-forum.com/t24947/#post375805

[greenmeanh1]

I wuz gonna say... Good Luck Chollie!

Always best to source thru trustworthy vendors: Mouser, DigiKey, Element14. Prices may not be the cheapest OTOH your time is worth something too, best not wasted by installing counterfeits.

Digikey and Mouser are my first choices always. Digikey has the IRFP254. I have been in contact with Carvin and they sent me the correct schematic for my unit. Turns out i have the FET900 version even though the box says FET400. I sent them a pic of my board to help confirm. The original mosfets for this unit were STw33n20's. No idea how IRFP254's ended up in both amps. I crossed the W33N20 and got an IRFP250 and 2SK3176. No mention of the IRFP254. I would like to get some thoughts on this as i want to repair both of these on my own if i can.

[greenmeanh1]

Thanks for those links. Got me thinking of fabricating something similar. Does it make any sense to test the mosfets hot, say at 100c and repeat the test cold at room temp?? I was thinking of adding a temperature controlled box to test them in at various temps.
Also wondering about a way to load test them???
Seems also you would have to do better than just match pairs ie Q62.63 or Q60,61 or Q58,55 as in the case of my amp. You would also have to match from left to right channel as well, meaning all 12 mosfets should come out and 12 new matched ones should go in. Here is the schem for mine.
FET 400 Good Schematic.pdf

[Enzo]

Why would two separate channels in a stereo amp need their transistors to match each other? Other than draw from a common power source in most amps, they do not interact other than you hear both channels from the speakers. You match transistors so they share current well with their parallel buddies. Transistors do not share current with the other channel.

[greenmeanh1]

I was thinking of audio left/right parity. I had been using these in the studio when they were working for mixing. Both were pretty bang on in terms of left right power balance. My thinking was slightly differing specs on output devices would have an effect on that but bear in mind i know very little about this stuff. Maybe its enough to set the bias at spec. I am less worried about power as the amps will rarely be pushed to clip out but parity is a concern.