Hafler Dh-500 no sound after 20 minutes...

[Tom Pogue]

I bought a Hafler DH-500 amplifier last spring, it had been recapped and a new power switch and fan were installed. Was working great for the last 6 months, used it in an outdoor system driving some Infinity RS4B's. Runs barely lukewarm, with the fan running normally. But last week it started shutting down after 20 minutes or so, power light is on but no sound. If I turn it off and then back on, sound returns. No heat issues at all.

Any ideas?

Thanks in advance.

Thomas

 

[Oerets]

IIRC there is a thermal switch on the heat sink. That might be the culprit. Hair dryer blowing on it might be a way of testing.


Barney

 

[Tom Pogue]

Are these the thermal switches? Located under the large aluminum heatsink?

https://www.ebay.com/itm/NOS-HAFLER...e=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649

Thanks
Thomas

 

[Oerets]

Here is the manual, http://www.hafler.com/pdf/archive/DH-500_amp_man.pdf
Manual states,
""If it is blinking at about 3 times a second, this indicates that the
thermal safety breaker on one channel has shut down the
amplifier because of excessive temperature.""

Looks like the sensors are in the power supply and control the fan. After looking over the SM. I thought there was on the heat sink with the outputs. Must of been another amplifier.

Barney

 

[Tom Pogue]

Power light is not blinking. I had the amp disconnected for a few days and have set it up in a different system this morning, working fine for 4 hours. don't understand \\ I hate intermittent problems.

Thomas

 

[ouimetnick]

Pull the cover off and check the voltage off the filter caps. Should be +90V and -90V. If it isn’t then, we can work back at the power supply. Otherwise work forward with the amp.

Really sounds to me like you have a faulty intermittent thermal switch.

 

[sregor]

I don't see anything in the manual that will make the light blink. There are thermal switches on each channel. If either one opens, the power is disconnected from the transformer, so there will be no power on the filter caps, but still goes to the light and the fan circuits.

 

[Tom Pogue]

Thanks, I will give it a go and replace the thermal switches. But today, everything has been okay and it's been on for 6-7 hours without cutting out.

Thomas

 

[nj pheonix]

I don't know much about thermal protection. That is , I don't know if they fail intermittently like you described.
I'm going to throw another idea out there.
Perhaps a cold solder joint that develops a high resistance as it heats up. Just an uneducated guess.
I've seen stranger things happen

 

[Tom Pogue]

I thought of that, also. But once it cuts out, I can turn it off and back on and it's up and running. If it was a solder joint that lost connectivity due to heat, it seems that it wouldn't come back on so quickly. Plus the amp seems to run barely lukewarm and fan speed is low. I am going to leave it on today. Intermittent problems always have a way of returning.

Thomas

 

[nj pheonix]

Good point

 

[dshoaf]

There's a speaker relay with contacts that can be causing the problem. It is a 24 volt relay and I seem to recall that the folks who bought all the old Hafler stock still sell them there.

To completely eliminate heating as the root cause, open the case by pulling the top. Then, have a look at the heat sink system. It is actually 2 separate pieces, one for each channel. Using the back of one of your fingers curled into a fist, check each side of the heat sink system for equal warmth. If one is hotter than the other, you've likely got a bias problem with that channel.

If not, the next thing to figure out is if one channel is causing the safety circuit to open the relay because it detects a too-high DC offset. Both channels will be disconnected but only one may be causing the problem. The fact that you can turn it off and reset the problem says the relay is involved in some way.

Cheers,

David

 

[sregor]

Quick check - if the problem is the thermal switch, there will be no DC on the filter caps when the unit isn't working. If there is still normal voltage, I would be look at the relay also.