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In Reply to: RE: Repair of Hafler DH-500 posted by neolith on May 16, 2014 at 14:54:50
Neo
I have repaired many Hafler DH200s and about eight DH500s..They are essentially the same amp only larger with more outputs and Hitachi is the only company that makes the MOSFETS that fit these units. The outputs are costly but,it almost always fixes the problem.
A few things here..The transistors will not check like a standard NPN on a diode check of meter.You basically check for shorts and do comparative readings against the new transistors.You still put the leads on the same way but they will read much different.
You don't really have to change them all if they aren't bad but I would put new grease on the outputs and I would check the driver transistors for any leakage, Q12 and Q13 and those are NPN type. I would change all the bypass electrolytics with muse or cerafines..Once you get ready to plug it in,use a variac and go slow as they outputs are expensive.
Honest amplification is better than excessive 2nd order distortion anytime.
Follow Ups:
"Hitachi is the only company that makes the MOSFETS that fit these units."
Hafler used Exicon outputs the last few years of production.
http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
Semelab also sells the same thing (BUZ900/905).
Thanks. I checked the MOSFETs with the standard diode tester and also by applying a charge to the gate and seeing that the drain-source goes from open to closed and the N set seems good, the P set is shot. Q12 and Q13 also check out fine. So the next step is to acquire the MOSFETs.
I applied a 1000 Hz sine wave and square wave to the right channel (no fuses on the left) and got a nice sine wave and square wave at the output with approximate gain of 26db so I am pretty sure this channel is good. However I will follow your advise and grease things up.
I have a set of those MOSFETS, if you are interested.
Disassembled a DH500 some years ago with the intent of rebuilding with 'more audiophile parts'...but never did get the rebuilt going.
Send me an email and perhaps we can agree on terms.
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