Hafler Amp Output Transister Replacement

[Baldrick]

Hi folks. I've got a Hafler P1000 that has channel 2 with a clipping light on and sounds fuzzy.

I was going to replace the output transistors and looked up the part number, but the original (IRF9540N) are obsolete. I can get a lot of new variations of the original, but I can't match the specs.

How important is matching the Drain Source Resistant and Gate Source Voltage?

From Mouser's website, the original has a DSR of 200 mOhms and a GSV of 20V. I can find a DSR of 200, but the GSV is 10V. Or, I can find a DSR of 117 mOhms and GSV is 20V.

Any ideas?

Thanks.

[petehall347]

Re: Hafler Amp Output Transister Replacement

what makes you think its the outputs at fault ?

[R_J]

Re: Hafler Amp Output Transister Replacement

I agree with petehall, mosfets usually short, I don't see a problem replacing them with the irf9540N (P-Channel) as long as you replace both. the irf540H (N-Channel) have an even lower rds(on)

I suspect if one of the P mosfets shorted, either R97 or R69 would be open as well, same for the N mosfets, check R92 and R70

[eccerr0r]

Re: Hafler Amp Output Transister Replacement

The IRF9540N and IRF9540NPBF (ROHS) are close enough, you're not running them saturated. However agree with petehall347, I would start the look elsewhere to find why it's clipping - as far as I can tell a bad MOSFET will not cause the clip light to turn on.

(And yeah having an o-scope would be very fun to debug this...)

[redwire]

Re: Hafler Amp Output Transister Replacement

MOSFETS are optimized for high speed switching, not linear audio operation. If you pick a substitute based on RDSon or VGS threshold you're gonna have a bad time. Almost all switching MOSFETs fail in heavy linear mode use. Use the SOA curve as a guide. IRF9540NPbF is still in production anyhow.
Best to confirm it's not the output mosfets that have failed. It could be something else.

[truclacicr]

Re: Hafler Amp Output Transister Replacement

There is a circuit on pages 16 and 17 of the manual.

[Baldrick]

Re: Hafler Amp Output Transister Replacement

Thanks all.

The only reason I was going to replace the transistors is because that's my go-to fix for a bad amp. I've done it before several times, but yeah, the channel was always dead - not clipping.

I'm great with a soldering iron, but not troubleshooting.

Any pointers on where to look first? I guessing somewhere in the input stage???

Thanks.

[eccerr0r]

Re: Hafler Amp Output Transister Replacement

Chances are without an oscilloscope, you will have to go through a lot of guess, desolder, check, resolder.

You could try to go through all the transistors one by one and see if any are shorted with an ohmmeter or diode checker.

Perhaps with some more information, someone can deduce which transistor is bad. Without an oscilloscope and it's very hard to understand sound by typing descriptions out in a message forum, perhaps information can be gleaned from the clip lamp.

What is the status of the clip lamp, is it constant on, flickering?

Can you get the lamp to turn off ever? Will it turn off with a shorted input? Specific input voltage or volume level?

[petehall347]

Re: Hafler Amp Output Transister Replacement

Clipping
Monitors the DRIVE SIGNAL and lights when the drive signal voltage exceeds the maximum level for
linear operation of the output MOSFETs.

[petehall347]

Re: Hafler Amp Output Transister Replacement

i would be checking around the op amps .

[Baldrick]

Re: Hafler Amp Output Transister Replacement

Thanks folks for all of your help!

I printed out the wiring diagram and realized I didn't know much of what I was looking at... but, there are a few things I noticed. There's a row of six transistors on the same heat sink. The first two are the IRF9540's, the next two are IRF540N's, and then there's one each of a LM337SP and a LM317T.

From the tiny bit I know, it looks like the last two are the output transistors and the others are farther up the chain. I pulled one of the IRF9540's and it tested bad.

So, I'm going to replace the first four and see what happens. These first four btw, look like they were replaced at some point judging by the flux on the board (I got it used). Hopefully, there's not something else wrong that's taking out these transistors.

Thanks again all for giving me the confidence to continue. I'll let you know what happens (I'm sure you're all on the edge of your seat).

Merry Christmas!!!

[petehall347]

Re: Hafler Amp Output Transister Replacement

LM337SP and LM317T. are voltage regulators

[eccerr0r]

Re: Hafler Amp Output Transister Replacement

/me somehow is feeling a "still doesn't work" post for some reason...
/me hopes the feeling is wrong however...

[budm]

Re: Hafler Amp Output Transister Replacement

"I pulled one of the IRF9540's and it tested bad."
Tested bad in what way?