dc offset mismatch-hafler dh-220

[stilllovelps]

Guys,

I was wondering if I could be sent on the right direction. This is my situation, I first noticed a small buzz in the speakers on low volume. Then realized that it was only the right channel. Narrowed it down to the Hafler DH-220. I have done some reading here and cannot seem to find a specific answer. I checked dc offset and found the left channel at 20mv but the right channel at 200mv. Anybody have any ideas where I should start looking. I could adjust it out probably but I think I have more problems than that. The music sounds fine with a little increased volume, but at low spots its a little noisy.

Thank you, and I hope this is in the correct forum.

Dan

 

[EchoWars]

DC offset is not the problem. Chances are, the old electrolytic caps need replacing, but the problem may not be limited to that alone.

 

[Jon_Logan]

Yeah, I agree with EW. The main filters are shared by both channels, but there are local filters on the PCB that quiet stuff down even more for the input stages. Check the electrolytic filters on the PCB.

Differential transistors are responsible for making the output DC voltage value equal the input value of DC of zero volts. If you change the caps and get no improvement in DC offset performance, it's a bit trickier on the Haflers as they use a complementary differential stage with no offset control. This means you have to put in two matched transistor pairs to ge the offset down.

 

[stilllovelps]

thanks for the responses, how many caps do i need to replace? the ones on the boards only, i just looked at the boards and i see 2 grey and 1 black cap on each board. are these the only ones?
thanks again, this stuff is new to me. i know what the components are but i dont know the way these are built

Dan

 

[ouimetnick]

On all old electronics benefit from a recap. Google Hafler DH-200 and Hafler DH-220. Plenty of mods and improvements can be done

 

[OverLoad]

......Differential transistors are responsible for making the output DC voltage value equal the input value of DC of zero volts. If you change the caps and get no improvement in DC offset performance, it's a bit trickier on the Haflers as they use a complementary differential stage with no offset control. This means you have to put in two matched transistor pairs to ge the offset down.

I believe the very early releases didn't have offset adjustments but they did add adjustments. The 3 or 4 220's I've had come thru all had offset adjustments with noted board# PC19 revision C.

thanks for the responses, how many caps do i need to replace? the ones on the boards only, i just looked at the boards and i see 2 grey and 1 black cap on each board. are these the only ones?
thanks again, this stuff is new to me. i know what the components are but i dont know the way these are built

Dan

YES, there are all of 3 electrolytics on the boards and certainly replace them. Note that the one is a non-polar in the feedback (next to the bias pot).

 

[Jon_Logan]

I believe the very early releases didn't have offset adjustments but they did add adjustments. The 3 or 4 220's I've had come thru all had offset adjustments with noted board# PC19 revision C.
.

Coo-ool. Thanks for that info. I did not know that. That certainly makes DC offsets less of a problem and takes care of the extra time to match stuff.

 

[stilllovelps]

I believe the very early releases didn't have offset adjustments but they did add adjustments. The 3 or 4 220's I've had come thru all had offset adjustments with noted board# PC19 revision C.

YES, there are all of 3 electrolytics on the boards and certainly replace them. Note that the one is a non-polar in the feedback (next to the bias pot).

so 6 in total. Are the capacitors in question the 2 bigger grey ones and the smaller blue one on each board. see photo.
do you know offhand the values for those 3 caps?

thanks.

Dan

 

[stilllovelps]

sorry i didn't look with a good enough flashlight. it looks like the larger grey ones are 100uf 100v, and the smaller ones are 220uf 10v.
does this sound correct?
my boards are pc-19 rev. b. and they have pots.
in the manual for the pc-19c boards, the smaller caps are shown to have a value of 470uf 10v. which is right? has someone changed these? for any particular reason?

 

[ouimetnick]

Look at the manual
http://www.hafler.com/techsupport/pdf/DH-220_amp_man.pdf
Its hard to tell with the angle and quality of the photo, but you can look at the schematic or the useful diagram on page 13, and match it up.

While you are in there, if you would be able to measure the width and height (I don't need the thickness, just width and height) that would be awesome. I am going to make my own pcb layout to learn some CAD software, so it would he a start.

-Nick

Edit. Rereading your post answers what I said above... well not really... Guess I can't help. I know that they say "Minor variations may sometimes be encountered in value or appearance. These will not affect performance." but that seems bit of a variation.

 

[ryuuoh]

In general, what you pull out of these old beasts is what you put back in.

 

[stilllovelps]

Look at the manual
http://www.hafler.com/techsupport/pdf/DH-220_amp_man.pdf
Its hard to tell with the angle and quality of the photo, but you can look at the schematic or the useful diagram on page 13, and match it up.

While you are in there, if you would be able to measure the width and height (I don't need the thickness, just width and height) that would be awesome. I am going to make my own pcb layout to learn some CAD software, so it would he a start.

-Nick

Edit. Rereading your post answers what I said above... well not really... Guess I can't help. I know that they say "Minor variations may sometimes be encountered in value or appearance. These will not affect performance." but that seems bit of a variation.

sorry about the photo, best i could get without tearing it down. i guess if it works now with the 220uf 10v then it will work after with the same value. Like i stated before, maybe the board revision is the cause for the difference in values. On second look the right channel 220uf 10v looks a little bulged on the top, so i'm hoping thats the problem.

The board size is 11cm X 11cm.

thanks for replying
i'll pick up some new caps tomorrow and post my results when i get a chance to install them.

BTW,Does anyone know what the dc offset should be?

Can i set the bias to 275ma and then the dc offset to 0 volts after they are stable?
or once the 2 channels match after the repair( i hope the caps do this) or should I just leave the dc offset at 20-25mv?

Dan

 

[ouimetnick]

sorry about the photo, best i could get without tearing it down. i guess if it works now with the 220uf 10v then it will work after with the same value. Like i stated before, maybe the board revision is the cause for the difference in values. On second look the right channel 220uf 10v looks a little bulged on the top, so i'm hoping thats the problem.

The board size is 11cm X 11cm.

thanks for replying
i'll pick up some new caps tomorrow and post my results when i get a chance to install them.

BTW,Does anyone know what the dc offset should be?

Can i set the bias to 275ma and then the dc offset to 0 volts after they are stable?
or once the 2 channels match after the repair( i hope the caps do this) or should I just leave the dc offset at 20-25mv?

Dan

From the manual

Offset
A DC voltmeter capable of resolving 10 mV variations is needed. Connect it to the output terminals of the selected chan- nel, and adjust Pl, near the top of the board, to zero volts, + 10 mV. A small correction of the control will produce a large initial change, which will settle down in a second. Repeat these small changes to achieve zero volts.

Simply put. You want to have it as close to 0 mV as you can.

However 20-25 mV is still very good, so if you can't go down much further, don't worry.

 

[Jon_Logan]

While I'm not a real champion of wholesale cap replacement, if you're in there you might as well replace them all. You're gonna have to pay for shipping from wherever buy them from, so just get all of the electrolytics for both channels.

Just be careful when you put them in (soldering and polarity, etc.) and everything should be fine. As mentioned, make sure you a get a bi-polar/non-polar cap for the one that's on there (NFB shunt cap)

Then, hope the hum goes away.

Make sure you ID the right DC offset adjustment pot. The wrong pot is bias. You want to make sure you're set up to measure and adjust bias before you turn that pot.

 

[stilllovelps]

thanks , if 20mv is ok maybe i'll just leave it alone. after i fix the problem causing the 200mv, that is.

dan

 

[ouimetnick]

While I'm not a real champion of wholesale cap replacement, if you're in there you might as well replace them all. You're gonna have to pay for shipping from wherever buy them from, so just get all of the electrolytics for both channels.

Just be careful when you put them in (soldering and polarity, etc.) and everything should be fine. As mentioned, make sure you a get a bi-polar/non-polar cap for the one that's on there (NFB shunt cap)

Then, hope the hum goes away.

Make sure you ID the right DC offset adjustment pot. The wrong pot is bias. You want to make sure you're set up to measure and adjust bias before you turn that pot.

Just a question for you, will a cap explode if let's say you replace a bi-polar cap with a cap with polarity? And what about the other way?

 

[stilllovelps]

While I'm not a real champion of wholesale cap replacement, if you're in there you might as well replace them all. You're gonna have to pay for shipping from wherever buy them from, so just get all of the electrolytics for both channels.

Just be careful when you put them in (soldering and polarity, etc.) and everything should be fine. As mentioned, make sure you a get a bi-polar/non-polar cap for the one that's on there (NFB shunt cap)

Then, hope the hum goes away.

Make sure you ID the right DC offset adjustment pot. The wrong pot is bias. You want to make sure you're set up to measure and adjust bias before you turn that pot.

i'm thinking i should be able to get these caps from my local electronics parts store. i'm going to check there 1st. if that fails i have seen complete cap replacement kits for these old things on ebay.
Do the other old film caps etc. have the same problems as the electrolytics?

there are a lot of film's in there and i don't wan to do mess with anything more than i have to in fear of errors and making things worse.

the bias and dc offset pots are marked on the pcb so i dont think i'll mess that up, but caution will be exercised.

i have looked through the manual and read the info on the adjustments

is it correct to adjust bias 1st if i do decide to try?

thanks
Dan

 

[ryuuoh]

If you can get a cap kit from the web for not much money, I'd do that. Films generally don't get replaced for the same reasons as electrolytics, but if you get a kit with them, why not replace them.
Align your amp per the service manual and you should be good.

 

[Jon_Logan]

I'd leave the film caps alone. Most of them don't have any age related issues.

I wouldn't put a polarized cap in the place of a non-polar/bi-polar. Electrolytics can have a little bit of reverse polarity, but a larger amount of reverse voltage can cause them to leak (electrically) or short. With the their placement in the circuit, a short there could/will cause a large DC offset.

NAD used to have polarized electrolytic caps in their feedback shunt circuit. They put a diode across it to make sure not too much reverse voltage would develop across the cap. Not ideal IMO.

You have a good electronics shop if they stock 100V caps. Yeah, get'm locally if you can.

 

[stilllovelps]

well the kits on flea bay are $ 50.00 so that not gonna happen.

if i only need 6 caps why replace 48

i have no idea if my shop has the caps, if i started this venture earlier i would have checked today.

if i can't find them locally digikey has everything.

i'm in vancouver b.c.

anybody know of shops here that would have these. by chance could save me a little time.

thanks again everyone for the info

Dan