Mosfet Question (Trace Elliot)

[sweatyk]

I’m working on a Trace Elliot AH350SMX that just went silent in the middle of a gig. I cannot find a proper schematic for it. However as a guess I pulled the mosfets to check them out. Based on proper testing using my DMM they are indeed bad. Being somewhat obsolete I have found a few on EBay however I came across this thread on diyaudio that suggested a suitable replacement. I copied this from that thread……..

“In my amp the K135 and J50 broke down: the MOS gates were leaky, apparently the protection diodes did not work so well after all.

I used the BUZ901 and BUZ906 parts. However expensive, they are far more robust. I never had any problems ever since, but maybe the price is a bit of an overkill for a sub amp.”

Does anyone see a problem substituting the Hitachi K135’s and J50’s with the Semelab BUZ901’s and BUZ906’s? Let me know if there is any more info that I am forgetting to provide. I did notice a thread on here that was related to this but it wasn’t clear if they were good to go for direct replacement.

[gbono]

BUZ901 N Channel TO-3 200V/8A
BUZ906 P-Channel TO-3 200V/8A

Looks like a very good match to me.

Price = +$8 (BUZ906)

BUZ906 - SEMELAB - P CHANNEL MOSFET, -200V, -8A, TO-3 | Newark element14

[J M Fahey]

The BUZ are good substitutes, in fact nowadays I guess they are exactly the same.

May be wrong, but from reading Exicon/Profusion pages I'm under the impression they manufacture an excellent MosFet chip or die (imagine a 6mm square piece of Mica) which they sell "raw" for others to make their own Mosfets, calling them whatever they wish, and also sell them ready to use, packaged and labelled with what I call "parametric names", as in, say, "N1610" meaning N channel 160V 10A (made up example) and so on, so as suggested above, as long as specs match or surpass original ones, you are fine.

That said

Based on proper testing using my DMM they are indeed bad.

, can you please post those measurements?
Enhancement MosFets (vertical or lateral) are weird devices which are open until bias forces them closed, have no junctions in the conventional sense and to complicate matters, usually have a "parasite" diode (created during the manufacturing process) which is no problem under regular operation, becauuse in that case is reverse biased, but Techs find a low resistance path (the forward biased diode) while testing and worry about it.

So please post measurements, here in Argentina we say: "4 ojos ven más que 2" (4 eyes see more than 2), I bet here in the Forum we are talking about a couple dozen eyes

[sweatyk]

This amp has 2 J50’s and 2 K135’s. I looked at a few videos on testing them to make I do this correctly. Using my DMM set on diode mode. I may have over tested but here is what I came up with. I grounded all pins to clear them as the first step……

J50 (1) Pos on the drain and neg on the source = .242V
Touching the gate with the pos then pos on drain and neg on source = .245V
Touching the gate with the neg then pos on drain and neg on source = .005V but fluxuates all around.

J50 (2) Pos on the drain and neg on the source = .253V
Touching the gate with the pos then pos on drain and neg on source = .240V to .570V
Touching the gate with the neg then pos on drain and neg on source = .003V to .250V

K135 (1) Pos on the drain and neg on the source = Non Conductive
Touching the gate with the pos then pos to drain and neg to source = .001V
Touching the gate with the neg then pos to drain and neg to source = Non Conductive

K135 (2) Pos on the drain and neg on the source = Non Conductive
Touching the gate with the pos then pos to drain and neg to source = Non Conductive
Touching the gate with the neg then pos to drain and neg to source = Non Conductive

[The Dude]

Some odd measurements there to be sure. Were the FET's out of circuit when you measured? Was the amp blowing fuses or are there any burnt resistors? If any of the FET's were shorted, I would think something would have burnt up. Did the amp still power up? I'm reserving judgment on calling anything shorted until we know more about what's going on with the amp.

Edit: Another question: If you do have the FET's removed, what are the voltages on the pads where they were soldered?

[sweatyk]

Some odd measurements there to be sure. Were the FET's out of circuit when you measured? Was the amp blowing fuses or are there any burnt resistors? If any of the FET's were shorted, I would think something would have burnt up. Did the amp still power up? I'm reserving judgment on calling anything shorted until we know more about what's going on with the amp.

Edit: Another question: If you do have the FET's removed, what are the voltages on the pads where they were soldered?

Yes the FETs were out of circuit. There were no blown fuses. No signs of foul play physically anywhere in the amp. Everything powers up as normal with the exception of no output. During a gig he noticed that the volume dropped a bit and then went to nothing. That was how he described it to me. I will test the voltage at the pads and report back.

[Jazz P Bass]

"No output" should not mean dive right into the output devices,

Before going there it should be verified that :
-Is the mains draw within reason at idle?
-Are the power supply rails good.
-Does the input signal reach the output device gates?

[drewl]

As JM mentioned, the mosfets will switch back and forth between conducting and not conducting as you move your meter leads to the different terminals.

The voltage from the meter is turning them on and off in a way.

You have a scope?
see if the signal gets to the output amp and check your power supply voltages as mentioned.

[sweatyk]

"No output" should not mean dive right into the output devices,

Before going there it should be verified that :
-Is the mains draw within reason at idle?
-Are the power supply rails good.
-Does the input signal reach the output device gates?

It's more instinctive on my part to go to them first. it's just the way I'm wired I guess. As for your 3 suggestions will the power PCB need to be back in the circuit with the outputs reinstalled? I can reconnect at of the wiring to it but it's a booger to get to mounted in the chassis. Anyone that's been inside one of these Trace Elliots already knows that. But you have to admit that the mosfets are reading funny out of circuit. Seems like out the four only one was reading normal as per the instructional videos I have viewed. I also do not own a scope for those that asked. I will reconnect the power board and report back. Also the only schematic that I have found that even resembles this gets grainy when you magnify it which really makes it hard for me to identify certain labeled parts.

On a side note I spent some time and removed all of the crusty crappy solder connections and flowed fresh solder. I actually had to take some light sandpaper to the leads to get the solder to adhere to them. It was time consuming. Did it more as a general purpose thing. There were no broken connections.



GP12 SMX Series 350W Power Amp.pdf

[Jazz P Bass]

As opposed to a BJT output section, the MosFets do not need to be installed to check power supplies & signal At the gate.

Of coarse, no load at this time.

While the readings do appear odd, MosFets typically fail shorted.
Not that this proves yours are good, simply saying.

[sweatyk]

OK I soldered the power wired back to the power output PCB. With the mosfets out I read the voltage at the mosfet pads on the PCB and here is what I got.

K135 gate = 350MV and would drop
K135 drain = 240MV and would drop
K135 source = 230MV and would drop

J50 gate = 390MV and would drop
J50 drain = 250MV and would drop

While I was in there I noticed that the 12AX7 did not appear to light up so I read each pin and here’s what I got.

1. 7.6 MV
2. 7.2 MV
3. 5.8 MV
4. 152 MV
5. 3 MV
6. 5.1 MV
7. 5.3 MV
8. 6.6 MV
9. 6.8 MV

So I go to the 4700uf cap that supplies the + V to that board and just like all of the other readings it was low MV’s. I backed up and looked at the bridge rectifier. Now correct me if I’m wrong…..when you check between the 2 AC tabs on the BR with the DMM in diode mode either should not read anything in either direction right? Now that is in the circuit at this moment so does that make a difference? I’m getting ready to unsolder it and check it out of circuit to be sure.

[DrGonz78]

So I go to the 4700uf cap that supplies the + V to that board and just like all of the other readings it was low MV’s. I backed up and looked at the bridge rectifier. Now correct me if I’m wrong…..when you check between the 2 AC tabs on the BR with the DMM in diode mode either should not read anything in either direction right? Now that is in the circuit at this moment so does that make a difference? I’m getting ready to unsolder it and check it out of circuit to be sure.

Yes measuring the two terminal AC positions on the BR should yield OL on your meter or equivalent to overload, while in diode mode. In other words infinite resistance as far as the range to the test meter in hand. You should be able to test that in circuit most of the time. Trace the AC from the plug and to everything leading up to the AC inputs of the BR.

[The Dude]

The AC tabs on the BR should measure......well.......AC. There should be some transformer secondary wires that work their way to the BR. Did you have any AC there? If you measured resistance/continuity/diode check across the AC tabs on the BR in circuit, it would read low or near shorted because you are measuring the transformer secondary. Again, what you need to find out is if there is AC supplied to the rectifier.

[sweatyk]

Please disregard post #11 completely. I had the amp positioned to where I could not see the back very well. The power switch is paddle style..............but so is the "earth lift" switch. I feel like a complete idiot And some of you are agreeing as you read this. I will post the CORRECT readings on the next post. If possible can a moderator remove post 11, 12, and 13 for they have no value considering my mistake.

[sweatyk]

OK now that I have arrived at common sense land here are the reading on the gate pads for all 4 mosfets

K135 = -11.42V
K135 = -11.42V
J50 = -12.12V
J50 = -12.08V

They seem fairly consistent.

The +/- coming into the board is:

+ = 80.3V
- = -80.7V


Now testing for signal at the gate pads. This is with the mosfets out, injecting a 400hz tone into the input jack. I got nothing but a buzz…..not even the good kind. I checked the signal at the jack and the same buzz but no tone signal. Perhaps we have a worn out input jack?