My Dad assembled a Hafler DH-200 around 40 years ago. Put maybe 25 hours on it (mostly by me) at the time. It's been stored away for the past 35 or so years.
I did a visual inspection and sniff test on both units. Both are low time and haven't been powered up in decades. Didn't notice anything bursting, bulging, scorched or leaking.
I built a DBT and borrowed a variac. A few days ago, I began testing/reforming the DH-200. Using a 100 watt bulb, I began at 5% and three times a day since then, I've been increasing voltage in 5% increments.
At about the 30 VAC, the bulb began glowing which I expected at some point. My expectation was that if everything is intact and functioning, that at some point the glow would level off or subside. It hasn't. I am at about the 70 VAC point and the brightness has increased with each increase from the variac. It is an obvious glow but in no way is it bright.
So after days of using a variac and DBT in an attempt to reform the caps, the amp never powered up. So, I then plugged into the house main and ta-daaa... a pilot light.
I re-checked the outlet on the DBT...120 VAC.
I plugged the amp back into the DBT. Half-dim bulb but the amp will not power up.
???
FWIW: when the amp was powered up, I checked VDC at the speaker terminals (DC offset?)
L: 80 mV
R: 160 mV
The DC offset measurement was just something I jumped on because it was a quick and easy to check. Since the DBT didn't pan out, should I be looking for something else that could be wrong with the amp?
Thank you.
I did a visual inspection and sniff test on both units. Both are low time and haven't been powered up in decades. Didn't notice anything bursting, bulging, scorched or leaking.
I built a DBT and borrowed a variac. A few days ago, I began testing/reforming the DH-200. Using a 100 watt bulb, I began at 5% and three times a day since then, I've been increasing voltage in 5% increments.
At about the 30 VAC, the bulb began glowing which I expected at some point. My expectation was that if everything is intact and functioning, that at some point the glow would level off or subside. It hasn't. I am at about the 70 VAC point and the brightness has increased with each increase from the variac. It is an obvious glow but in no way is it bright.
So after days of using a variac and DBT in an attempt to reform the caps, the amp never powered up. So, I then plugged into the house main and ta-daaa... a pilot light.
I re-checked the outlet on the DBT...120 VAC.
I plugged the amp back into the DBT. Half-dim bulb but the amp will not power up.
???
FWIW: when the amp was powered up, I checked VDC at the speaker terminals (DC offset?)
L: 80 mV
R: 160 mV
The DC offset measurement was just something I jumped on because it was a quick and easy to check. Since the DBT didn't pan out, should I be looking for something else that could be wrong with the amp?
Thank you.
When using the variac did you try inputting a small sine wave signal when the voltage out of the variac is just half the mains voltage ( or thereabouts ) and does the variac have a current meter as well as a voltage one ?
Try a larger wattage bulb, 150W-200W. The amp is probably ok from your offset measurements.
Be sure to fuse the Variac output to no more than the Variac's rated current. It is very easy to
damage a Variac from excess load current.
Be sure to fuse the Variac output to no more than the Variac's rated current. It is very easy to
damage a Variac from excess load current.
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I did not apply any input to the amp. It's a quality Staco variac but no meters. I measured the variac output before I began to get an idea of where 120VAC is reached, which came out to be right around 84% but this was without a load. Don't know if that would make much of a difference.
I've read elsewhere that the closer DC offset is to 0 mA, the better with 40 mA or so being a safe limit. Why would each channel be so different, especially on a low-time unit?
Its not unusual to have different values of offset due to variations in component values in an amplifier .
I have seen worse than that and still working --you can always adjust it down when you have finished repairing or checking it.
If you are worried check it with a dummy load - IE- a high wattage wirewound 8 ohm resistor -- don't ramp up the input ,keep it low to make sure the output is only a few watts and if okay only then slowly increase the volume.
I have seen worse than that and still working --you can always adjust it down when you have finished repairing or checking it.
If you are worried check it with a dummy load - IE- a high wattage wirewound 8 ohm resistor -- don't ramp up the input ,keep it low to make sure the output is only a few watts and if okay only then slowly increase the volume.
I've read later models of this amp had potentiometers to adjust the DC offset but I don't think this one does.
What would that look like? A resistor in series or parallel of the speaker output? How high of a wattage?
I think your right I cant find a schematic--so far though .
Have a read of -
[url]www.diyaudio.com/forums/solid-state/31131-hafler-dh-200-220-mods-print.html[/URL]
A "dummy load " is just a very large wattage resistor -usually wirewound in parallel with a wattage of at least 10 watts but usually very much higher , some professional ones are water cooled JLH had one of those but 10 watts will give you an idea if its functioning under load.
Have a read of -
[url]www.diyaudio.com/forums/solid-state/31131-hafler-dh-200-220-mods-print.html[/URL]
A "dummy load " is just a very large wattage resistor -usually wirewound in parallel with a wattage of at least 10 watts but usually very much higher , some professional ones are water cooled JLH had one of those but 10 watts will give you an idea if its functioning under load.
This is reasonably typical of an amp like that, and nothing to worry about.
There's no output offset zero adjustment needed or supplied.
There is a bias adjustment for DC idle current.
https://hafler.com/pdf/archive/DH-200_amp_man.pdf
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Well I have found a US website where the same question is asked but to get to it I was blocked and had to fill in the name of the current US president ---Biden -- that got me there , its "not secure" - HTTP but it looks okay no malware only Google and a tracker.-
Verifying your page request - please wait! - JLA FORUMS
Verifying your page request - please wait! - JLA FORUMS
Reading down a figure of 240ma is quoted BUT --- the 1980,s version (early ) has on one trimpot -sealed at the factory for that particular circuit board for lowest distortion.
Got it ! --
Yes there is BIAS adjustment =P1 its the standard "amplified diode " (BJT ) type that I am used to in mosfet amplifiers --
Hafler manuals | Hifi Manuals Free: Service Manuals, Owners Manuals, Schematics online for free download
Yes there is BIAS adjustment =P1 its the standard "amplified diode " (BJT ) type that I am used to in mosfet amplifiers --
Hafler manuals | Hifi Manuals Free: Service Manuals, Owners Manuals, Schematics online for free download
