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Odd behavior doing a slow-bring-up of a Hafler DH-500

thorpej

thorpej

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A friend recently dropped off a Hafler DH-500 that he recently acquired in unknown working condition. It appears to be in very good original shape, modulo missing one of the speaker fuses. But because he knows it hasn't been powered up in some time, he asked if I'd do a slow-start on it to avoid letting out any magic smoke.

So, I plugged it into my variac-dim-bulb-volt-and-current-metered rig, and slowly brought it up. It began to spring to life at about 80V mains input ... I began to hear a faint hum, and then the dim-bulb filament started to glow gently. I heard a relay click -- not sure which one -- and the bulb filament started to glow a bit more ... checked the meter and the amp was drawing 1.5A. At this point, the variac was dialed up to 120V but the meter was reading only about 95V at the input to the amp.

So, at this point I shut it all down. No acrid smells, but the fan did not spin (I don't recall if it's temperature controlled on this model or not) and decided to post here for advice.

I suspect the bulb is providing too much resistance for the amp to start up properly. It's a 250W photo flood. It never came on brightly as would happen if there was a problem... just a slight glow of the filament.

I suppose I should take the bulb out of the chain and try with just the variac, and the volt-current meter, but I wanted to get some advice here first.

Thx.
 

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If the DC protect relay is engaging which it sounds like it is then you should be able to check the DC offset and as long as that is good you should be able to take the DBT out of the circuit. Even with only 95VAC input to the amp you should be able to get an idea if everything is okay.

You could pop the DC rail fuses out of one channel and bring the channels up independently on the DBT. This should lighten the load on the DBT and get you closer to 120VAC. If they both test okay then take the DBT out of the circuit and put all of the rail fuses back in and bring it up off the variac.

These amps take a good bit of power from the wall due to the high bias currents and rail voltages. IIRC my XL600 will pull around 200W at idle and the DH500 will be slightly lower. It really depends on what the idle bias current is set for.
 
Just to follow up, was able to do a basic test pass to verify it was in regular working order. Rock solid +/- 85V rails with < 80mVAC ripple at idle. DC offset of < 2mV on each channel. Perfect 1KHz square waves and very reasonable 20KHz square waves. THD @ 1KHz (measuring up to the 5th harmonic) 0.063% at 100W (the rating of my dummy loads), THD+N 0.163%.

Seems like this beast is working perfectly.
 
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