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Hafler se-120 Bias

J

jonsand

New Member
Greetings-
I have a Hafler SE-120 amplifier and one side of the amp gets hot even when at Idle. I suspected that the bias needed adjusting. The manual states that the bias should be set at 100ma so I measured the settings according to the service manual and the left channel was spot on at 100ma but the right channel creeps up from 50ma to over 250ma then just peggs the ammeter. It does this regardless of my trying to adjust the bias pot. I have de-oxed the pot to little effect.
FYI the amp sounds fine.

Also, it looks like at some point someone replaced the output mosfets on the side that's getting hot.

Any ideas will help.
 
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Greetings and welcome;

I can't offer much as I've never worked on a hafler but seems like that 'bias' pot needs replacing or the bias circuit needs addressing. some drift upwards is common but again don't know the limits of this unit if settings are safe.
 
You say the right channel has been worked on. Check the leads for the thermistor (it's the device between the output MOSFET's). If they look good, then monitor pin 7 of U1; it should change as the heat sink warms up. Left channel is U1 pin 1. They should change a similar amount and in the same direction.

Oh, and welcome to Audiokarma!
 
Hi!! I just bought a Hafler SE-120
I measure some voltage at the speaker outputs (less than 1 volt) with no speaker or input connected.
Is this normal for Mosfet amps?
Should I adjust the bias voltages ?
Thanks
 
poobs said:
Hi!! I just bought a Hafler SE-120
I measure some voltage at the speaker outputs (less than 1 volt) with no speaker or input connected.
Is this normal for Mosfet amps?
Should I adjust the bias voltages ?
Thanks
Click to expand...

Welcome poobs,

Goe to HifiEngine.com and register. Then download the service manual. That is if you haven't already.

The rest I am going to say without knowing your skill level. So I may come across as condescending or making assumptions. I apologize in advance.

Make sure you're measuring DC voltage (not AC voltage) at the speaker terminals....
1) after the unit has fully warmed up and had time to settle. About 15 minutes is more than adequate, but sometimes things can keep moving a bit.
and 2) make sure you're using a decent voltmeter set on the DC mv setting. (really cheap meters can have a hard time measuring that low and give false readings). And if you leave the meter setting on a higher range it may also struggle to give accurate results below 1v.

Now, if you're measuring anything below 50mv on each channel you should be ok. Above that and you start to get into problematic territory. We want DC offset to be as close to 0 mvDC as possible. Unfortunately the SE120 does not have a built in DC adjustment. If it's out of spec the only real way to address it to my knowledge (I have an SE120 but it's been a long time since I worked on it) is to replace the differential pair of small signal transistors. I can't remember which ones they are on this amp. but if needed we can easily help you figure it out. They would need to be replaced with matched pairs.

And yes, set the bias according to the manual. If the amp is running too hot I suppose it may slightly affect the DC offset. But not usually significantly in my (admittedly limited) experience.

Good luck. IT's a sweet little amp. Somewhere there's a thread where I modded one, but I can't find it now.
 
Thank you very much for your kind reply.

The amp does sound great. I just bough it (used) to replace the same amp that my son let the good smoke out of. I was not able to find replacement output transistors K1056 & J160 nor equivalents. :(

The amp does have 2 pots marked "balance" that I'm hoping are to set the bias voltage. I do have some low voltage at the speaker terminal that I'd like to get down to near 0 if possible.

Looking for ward to your response.
 
poobs said:
Thank you very much for your kind reply.

The amp does sound great. I just bough it (used) to replace the same amp that my son let the good smoke out of. I was not able to find replacement output transistors K1056 & J160 nor equivalents. :(

The amp does have 2 pots marked "balance" that I'm hoping are to set the bias voltage. I do have some low voltage at the speaker terminal that I'd like to get down to near 0 if possible.

Looking for ward to your response.
Click to expand...

Oh yeah, I should have mentioned that it's not in the manual, but you don't want to touch the "balance" adjustments. Those are factory set and are part of the feedback loop (Hafler really should have called them something else). Can only really be set with a scope and a distortion analyzer. The Bias adjustment pots are right next to them. Those are the only ones you can adjust. Follow the directions in the manual.
 
And again, DC offset can only be adjusted by replacing the differential pair.

Changing the Bias adjustment my slightly change DC offset.
As well, if you mess around with the Balance adjustment it may...may slightly affect DC offset, but at the cost of upsetting the optimally minimized distortion levels by throwing the feedback loop out of balance.

Remember, these amps were the smallest Hafler made. I hate to say "Bottom of the line" but it's the reality. And so they were built to a price point and I imagine had a limited service window for the majority of consumers at that price point. In my opinion it's a testament to the design that so many are still out there mostly happily chugging away. But, the reality is there wasn't much expectation that people were going to invest a lot of time or effort into long term service. Most consumers would replace eventually and likely upgrade at the same time. So it was that the larger Hafler's had DC offset adjustments, but not lowest model. Same held true for the DH series (DH-120 vs DH-220) as it is in the later SE series. In fact the earlier DH-200's didn't have DC offset adjustments either.

I'm no Hafer expert. I've just had so many different models pass through my hands over the years that I picked up a few tidbits out of necessity.
 
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Doing some fishing.
Were the outputs changed in the problem channel? If so, are they original parts or replacements. I'm thinking maybe someone put in hexfets (IRF series) which would have thermal runaway problems. Or counterfeits...
 
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Gents,

Everything is stock far as I know.
I bought one brand new because I got tired of cleaning the myriad of pots and switches on my Kenwood KA 8100 integrated amp.
The little Hafler very pleasantly surprised me with its warmth and authority as it handled my old Lafayette Criterions with the Heil Air-Motion tweeters. (!)

I think the tone of the Hafler reminds me of growing up in my father's shop were he fixed old tube gear, mostly radios. Got many electrical shocks as I stuck my finger in stuff- Explains a lot ;)

I'm still bummed out that I can't buy the two output transistors- Any ideas? K1056 & J160

BTW I gave away my good large scope many years ago because it took up half my bed room (joke) ;)
However I do have one of those tiny Chinese scopes several meters including a Fluke 77 and a Signal Generator and I'm waiting on a transistor tester..

Also, a Technician near me wants $150.00 to "try" to set the bias on the Hafler. :(

PS. I have the free version of REW and I was wondering if it can be used to check out Amp freq response.
 
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poobs said:
Gents,

Everything is stock far as I know.
I bought one brand new because I got tired of cleaning the myriad of pots and switches on my Kenwood KA 8100 integrated amp.
The little Hafler very pleasantly surprised me with its warmth and authority as it handled my old Lafayette Criterions with the Heil Air-Motion tweeters. (!)

I think the tone of the Hafler reminds me of growing up in my father's shop were he fixed old tube gear, mostly radios. Got many electrical shocks as I stuck my finger in stuff- Explains a lot ;)

I'm still bummed out that I can't buy the two output transistors- Any ideas? K1056 & J160

BTW I gave away my good large scope many years ago because it took up half my bed room (joke) ;)
However I do have one of those tiny Chinese scopes several meters including a Fluke 77 and a Signal Generator and I'm waiting on a transistor tester..

Also, a Technician near me wants $150.00 to "try" to set the bias on the Hafler. :(

PS. I have the free version of REW and I was wondering if it can be used to check out Amp freq response.
Click to expand...

I think the k1058 and j162 can be drop in replacements and are easier to find. Not 100% sure on that. But worth researching.
 
Hi !! These 2 number parts are sold together in most adds (2sk1058) . Does that mean they are matched or paired in some way ?
Also, they are most likely Chinese. Brand Exicon on the one I'm looking at ..... What do you think ? The 1 amp is toast anyway so there is not much to loose there.
 
Tread carefully on eBay. I filter for US only and even then make sure you are buying from a known US surplus electronics parts seller. They are most like to have the real deal. Chinese counterfeit sellers are getting crafty with mimicking US based sellers.

As far as I know, those parts should only be Hitachi branded if real.

One tell tail sign of fakery is if labelling looks brand new like it was printed yesterday. Legit NOS will look faded even if never opened (usually).

Also, if you see the same date codes across multiple sellers that's a sure sign. Fakers are too lazy to change the stamping.
 
Getting confusing , Ebay has them supposedly made by Hitachi but shipped from China some from Japan and some from Australia !
I can buy loose parts? They don't have to be sold as matched etc ? I've never bought power transistors before ?
 
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Well folks.. I tried setting the bias current on the one good SE-120 (I have 2).

Followed the service manual procedure but the meter doesn't change as I move the bias pot.
Tried AC and DC mili amps but nothing.

I returned the pot to the original position until an answer hits me from out of the blue ;)
 
poobs said:
Well folks.. I tried setting the bias current on the one good SE-120 (I have 2).

Followed the service manual procedure but the meter doesn't change as I move the bias pot.
Tried AC and DC mili amps but nothing.

I returned the pot to the original position until an answer hits me from out of the blue ;)
Click to expand...
Do you have any clip leads? Sometimes the mA part of the meter doesn't work right because of blown fuse internally to meter. Sometimes the turn on surge will pop that fuse. Using a DMM as an ammeter, you have to make sure you are using the right connections and watch polarity too.
Safest (and simpler) way to set bias is to use some clip leads, and put a 1 ohm resistor across the fuse holder, then measure DC voltage drop and use Ohms law. 100 mA will be 0.1 volts...
 
I just found C9 is a bit bloated on top. It is a 100 uf polarized.
Ordered slightly larger capacitance main filter caps too.
Rather than try to hunt down a 1 ohm resistor, I tried another 2 meters and had the same results.
Maybe I'm getting senile. :(
 
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