[Michael L. Squires]

I have two Halfer P500's, a DH500, and a pair of P225's which I need to
reduce to at least a couple of working amplifiers.

I have some technical experience, and have access to some test equipment
(Tek 465B scope, HP 339A distortion analyzer, that sort of thing) but
have never worked on this type of amplifer.

One P225 blows the power fuse immediately, which appears to suggest the
power supply itself is shorted - I haven't looked at it except to determine
nothing (to my surprise) is smoked. The other P225 works with crackling
from one channel, which to me suggests it's time to replace capacitors.

I do have the Halfer P225/230, DH500, and P500 manuals; however, the only
maintenance information regards setting the bias. I have a few spares
(no MOSFETS).

One question I have is how one tells a fried MOSFET from a good unit, without
purchasing one of the Hafler rigs that shows up on eBay from time to time?

Any suggestions? I did try Google, but nothing very useful turned up, other
than the manuals which I'd already gotten from other sources.

Mike Squires
mikes at siralan.org

[Trevor Wilson]

"Michael L. Squires" wrote in message
m...
I have two Halfer P500's, a DH500, and a pair of P225's which I need to
reduce to at least a couple of working amplifiers.

I have some technical experience, and have access to some test equipment
(Tek 465B scope, HP 339A distortion analyzer, that sort of thing) but
have never worked on this type of amplifer.

One P225 blows the power fuse immediately, which appears to suggest the
power supply itself is shorted - I haven't looked at it except to
determine
nothing (to my surprise) is smoked. The other P225 works with crackling
from one channel, which to me suggests it's time to replace capacitors.

**Why? I'd suggest you look at the front end transistors, as they are a more
likely cause of such a fault.


I do have the Halfer P225/230, DH500, and P500 manuals; however, the only
maintenance information regards setting the bias. I have a few spares
(no MOSFETS).

One question I have is how one tells a fried MOSFET from a good unit,
without
purchasing one of the Hafler rigs that shows up on eBay from time to time?

Any suggestions? I did try Google, but nothing very useful turned up,
other
than the manuals which I'd already gotten from other sources.

**I use one of these:

http://www.peakelec.co.uk/acatalog/jz_dca55.html

Very handy. However, testing MOSFETs with a multimeter is pretty easy. Set
to the 'diode' position on your multimeter. Connect the leads to Source and
Drain. Use a wet finger to Gate and watch the meter. Reverse the leads.
You'll figure it out.


--
Trevor Wilson
www.rageaudio.com.au



--
Posted via a free Usenet account from http://www.teranews.com

[Michael L. Squires]

In article ,
Trevor Wilson wrote:

**Why? I'd suggest you look at the front end transistors, as they are a more
likely cause of such a fault.

Thanks for the hint.

It's been a long time since I did this kind of thing; the last amp I worked
on extensively was the Dyna MK III when Hafler had just stopped producing
them.

Mike Squires

[harrogate3]

"Trevor Wilson" wrote in message
.. .

"Michael L. Squires" wrote in message
m...
I have two Halfer P500's, a DH500, and a pair of P225's which I
need to
reduce to at least a couple of working amplifiers.

I have some technical experience, and have access to some test
equipment
(Tek 465B scope, HP 339A distortion analyzer, that sort of thing)
but
have never worked on this type of amplifer.

One P225 blows the power fuse immediately, which appears to
suggest the
power supply itself is shorted - I haven't looked at it except to
determine
nothing (to my surprise) is smoked. The other P225 works with
crackling
from one channel, which to me suggests it's time to replace
capacitors.

**Why? I'd suggest you look at the front end transistors, as they
are a more
likely cause of such a fault.


I do have the Halfer P225/230, DH500, and P500 manuals; however,
the only
maintenance information regards setting the bias. I have a few
spares
(no MOSFETS).

One question I have is how one tells a fried MOSFET from a good
unit,
without
purchasing one of the Hafler rigs that shows up on eBay from time
to time?

Any suggestions? I did try Google, but nothing very useful turned
up,
other
than the manuals which I'd already gotten from other sources.

**I use one of these:

http://www.peakelec.co.uk/acatalog/jz_dca55.html

Very handy. However, testing MOSFETs with a multimeter is pretty
easy. Set
to the 'diode' position on your multimeter. Connect the leads to
Source and
Drain. Use a wet finger to Gate and watch the meter. Reverse the
leads.
You'll figure it out.


--
Trevor Wilson
www.rageaudio.com.au



--
Posted via a free Usenet account from http://www.teranews.com



I would add to that that if it is blowing fuses at power up it tends
to suggest an output device gone short circuit, very common - even
with MOSFETs - if the loudspeaker terminals have been shorted. Remove
the supply from one amp, insert a new fuse and switch on.... you know
the rest.


--
Woody

harrogate3 at ntlworld dot com

[Phil Allison]

"harrogate3"


I would add to that that if it is blowing fuses at power up it tends
to suggest an output device gone short circuit, very common - even
with MOSFETs


** However, Hitachi TO3 lateral mosfets, as used in most ( all?) Haffler
amps, fail OPEN CIRCUIT !!

The device's drain pins are connected to the chip's surface by easily fused
aluminium wire links.


The same is not true for other types of mosfet or flat pack mosfets.




........ Phil

[Eeyore]

Phil Allison wrote:

"harrogate3"


I would add to that that if it is blowing fuses at power up it tends
to suggest an output device gone short circuit, very common - even
with MOSFETs

** However, Hitachi TO3 lateral mosfets, as used in most ( all?) Haffler
amps, fail OPEN CIRCUIT !!

The device's drain pins are connected to the chip's surface by easily fused
aluminium wire links.

The same is not true for other types of mosfet or flat pack mosfets.

Repairing an amp using Hitachi MOSFET devices should indeed be pretty
straighforward. Phil's entirely right. I've never seen one of their lateral
'audio' devices fail anything other than open. This means that you're unlikely
to see any kind of cascade failure as you might in a bipolar amp.

Graham