Hafler DH-500 overheating

I had the blinking power light happen to me on my Hafler DH 500 today. Googling turned out to be at that it's the thermal sensors need replacement. i might as well fit a new fan in there while i'm at it.

Only problem is I don't know which parts those are?


source : FAQ

Additionally I found this out when deciding i should probably check the amp's DC output. Is this one trim knob the only one I need to use to fix that? Not sure how to do if if I have an older revision that didn't just have a single pot to change that. DC offset adjustment on a Hafler DH-500? | Audiokarma Home Audio Stereo Discussion Forums

I have a multimeter but if someone could give me a total dummies guide on checking my DC offset I'd really appreciate it. In the meantime I am trying to find what part I need to replace that thermal sensor.
 
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You can, as described, adjust the bias current but NOT the DC offset.
See pages 14 and 15. Download the Hafler DH-500-Kit manuals for free - Hifi Manuals

Thanks Jon. I did take a quick look at the manuals and noticed bias current but not DC offset. Does this mean this amplifier does not have configurable DC offset? This is what i am confused about. I was going to try and measure the offset but all I can find it the bias current adjustment. Thanks again Jon

I did take a look at the service and owners manuals(have hard copies of both here) but couldn't find out what the two thermal sensors actually were so I don't know what too look for should I replace them.

To be fair i had never seen the overheating shut off before though, and it was while i was testing my Dynaco PAT5 i just brought home, which I rather quickly realized puts out an insanely strong signal. A tiny turn on the volume knob on the Dynaco is equivalent to turning my other NAD or Adcom preamps to half full volume?
 
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Well it just did a thermal shut-off for the second time in a row today, not using the Dynaco today, so I'm thinking something else is wrong with the Hafler itself. Saw the blinking power light again.

if anyone can help me figure out what part I need to order to replace the thermal sensors on the DH 500 I would massively appreciate it. The service manual just says "Thermal Sensor"

I apologize if i am not supposed to double post. I tried clicking the edit button but it didn't let me edit my previous post.
 
That doesn't sound right. The line stage gain is 20dB, just average.
The phono gain is 37dB, slightly higher than average.

I'm sorry I have two consecutive threads going, but thank you so much for helping with both.

I used my trusty old NAD preamp that's never been a problem and I got about 3 hours of listening in before the Hafler's thermal protection kicked in again.

From the Hafler manual:

If the pilot lamp should ever blink (about 2-3 times a
second), this signifies that a protective thermal breaker
has shut down the amplifier because ofexcessive heat sink
temperature. The fan will be operating at its highest speed,
and as soon as the heat sink temperature has declined, the
amplifier will automatically return to normal operation.

Such an occurrence is extremely unlikely, and if the
amplifier shuts down again and the lamp flashes, you
should check for inadequate ventilation, or an excessive
input signal, an extremely low load impedance, or an input
which may have dangerous signal content (such as
osciIlation). Failing evidence of this, the fan or the amplifier may
have malfunctioned, Because of the very effective fan
cooling, any normal signal will not cause the amplifier to
overheat

Both times I shut it off as soon as I saw the blinking light so I didn't give it a chance to see if it would "cool off" and turn back on.

It may be a coincidence but a couple times today I thought it just sounded ever so slightly distorted, but wasn't sure if that was the recording or my amps to blame.
 

rayma

Member
2011-04-29 8:37 pm
I used my trusty old NAD preamp that's never been a problem and I got about 3 hours of listening in
before the Hafler's thermal protection kicked in again.

The timing of the thermal problem when using the PAT-5 is probably a coincidence,
especially since it also happened with the NAD. There's an input DC blocking capacitor in the amp
that prevents source DC from causing problems. Unfortunately, amplifiers can and do have problems
because of internal component failures, especially after many years.
 

ticknpop

Member
Paid Member
2005-05-28 9:43 pm
toronto
Old Hafler inventory including thermal sensors is on Ebay. I'd replace both thermal sensors ( and the fan) if your going to keep it for a while. That's if the amp is OK, otherwise you'll have to fix it first then do the fan and sensors.
If you haven't changed the big filter caps their due for replacement soon, as are the electrolytics on the 2 driver boards and the small protection board. The amps due for a major overhaul while you have it apart.
 
The timing of the thermal problem when using the PAT-5 is probably a coincidence,
especially since it also happened with the NAD. There's an input DC blocking capacitor in the amp
that prevents source DC from causing problems. Unfortunately, amplifiers can and do have problems
because of internal component failures, especially after many years.

I am definitely going to give it a full re-cap and look-over sooner or later.

The only thing I can't figure out is what the heck part do i buy to replace the thermal sensors...

Is this all I need? :
Hafler DH 500 Others Thermal Sensor Set No 55c 60C SF025 | eBay
But i would rather buy something new from Mouser, Digikey, etc rather than New Old Stock if possible

I was going to replace the internal fan with a normal PC case fan unless the voltages do not match up. I have not yet done enough research to find out if this is a possibility. I have a ton of very nice 120mm fans around and it would be convenient to use one or two of those.

Many thanks again to all for all the help...
 
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If you haven't changed the big filter caps their due for replacement soon, as are the electrolytics on the 2 driver boards and the small protection board. The amps due for a major overhaul while you have it apart.

So far my list of things to do:

1. Replace the power cord with a grounded one
2. Replace all capactiors except the two huge 20k MFD capacitors
3. Replace the fan

I have two spares of the 20k MFD capacitors around already but they are a little old. If i really have to replace them I just might get all new ones. but i really don't want to because those 2 will be very expensive.

Can I use these in place of the Hafler SF025 thermal sensors? I strongly prefer to use new parts if possible and I will by buying lots of caps to recap the Hafler anyway.
http://www.digikey.com/product-detail/en/cantherm/SDF-DF077S/317-1125-ND/1014754 55c
http://www.digikey.com/product-detail/en/cantherm/SDF-DF084S/317-1126-ND/1014755 60c
 
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The Hafler continues to work but the amount of time before the thermal auto-off happens is shorter each time. The fans are working properly and the unit is perfectly cooled so I am thinking the thermal sensors are the problem.

Will the ones I linked above from Digikey work? I don't want to buy NOS parts if they're going to die on me again,
 

phase

Member
2004-10-04 11:59 pm
What I have done to sniff out bad sensors on various cars, appliances, is to check the resistance with a meter while warming them up to see if they match the datasheet specifications, or come close for that matter.
Maybe you could access the wires for the sensors without taking it all apart by lifting them from the relay board?
Then use a hair dryer or heat gun to warm them up a bit.

Oh, be carefull inside that thing, are kind of a mess, and have some pretty (lethal)high voltage on some of those wires while energized.
 
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