problems with dc offset on marantz 2235b

[marantzman]

Hey guys,
I am setting the bias current and dc offset on my marantz 2235b.
let me tell you the procedure and then I will tell you my problem.
here it goes...
Connect DVM to the output terminals (speaker outputs).
Adjust R713, for 0 VDC.
Repeat for the other channel (R714)
Connect a DVM between J713 and J717.
Adjust R766, for 8mVDC.
Repeat for other channel.

repeat after 30 minutes

Connect a DVM between J804 and J805.
Adjust R806, for 35VDC.

The problem I am having is that in the first step when I connect
the DVM to the right channel (R714) It is opposite of R713,
when I turn it to the right it decreases VDC and when I turn it
to the left it increases VDC. It does opposite of what R713 does.
and either way I cannot dial it in to 0 VDC. The perviuous owner
of the stereo or maybe the factory had marked where the little dials
are supposed to be when they are set correctly, and the left channel
(R713) is still almost exactly where the marks are. If I set the right channel (R714) where the marks is, then the DVM reads about
0.696 VDC. I am using an Extech instruments true rms auto range
multimeter 430. What could be causing this problem? Could it possibly
be bad output transistors? Sometimes, not always when I turn on the stereo with the voulme all the way down I can hear this slight rumble or THUMP
noise in the LEFT speaker. However I cannot hear any distinct difference in the right or left channel while playing music.
I really need some help. I am scared to even hook this thing back until I solve this problem. I have everything else adjusted to specs
except the right channel dc offset.

I had previously posted that the right channel had a slight thump upon power up, but it is the left channel, however the left channel is adjusted correctly

 

[rulerboyz]

If you are ever worried about hooking up a piece of stereo equipment, consider using a dim bulb tester which you can build yourself as follows:

http://audiokarma.org/forums/showthread.php?t=41523&highlight=light+bulb+variac

http://audiokarma.org/forums/showthread.php?t=51853&page=1

It may not always be possible to get the DC offset exactly on 0. I believe that -.696 is pretty good for DC offset. The real problem is when you can't get the bias anywhere close to where it supposed to be.

Could it possibly
be bad output transistors? Sometimes, not always when I turn on the stereo with the voulme all the way down I can hear this slight rumble

IF you are hearing some noise from your speaker when there is nothing playing (ie amp is on and on CD mode, but no CD is playing), there is the possibility that it could be a grounding problem.

A bad output transistor tends to give you problems such as less or no sound at all on the affected channel. Bad driver transistors (the smaller ones on the circuit board close to the power transistors) tend to give you an almost dead, distorted sound.

If you do decide to troubleshoot your Marantz while the power is turned to check DC be careful not to touch anything other than what you are testing with your test leads. If for example you are testing a transistor and accidentally touch the test leads together while at the same time touching more than one lead of the transistor, you will probably blow that transistor and a couple other things such as more transistors and fuses.

 

[hpsenicka]

-.696mV is good , but -.696V is not!

See the DC offset thread in the solid state forum for details
(http://audiokarma.org/forums/showthread.php?t=5634) , but the generally accepted rule of thumb is that a value of less than 50mV (or 0.050V) is considered acceptable.

If the available range of adjustment doesn't let you achieve a value of less than 100mv, I'd suspect a problem somewhere, and it is likely having an audible effect on your amp's performance.

 

[rulerboyz]

Yeah I was thinking of a different scale. it is a good idea just to list the DC offset value in mv to avoid confusion. The fact that the right and left side are not behaving in exactly the same way is a good sign that something is wrong somewhere.

 

[EchoWars]

Replace the DC blocking cap in the feedback path, and you might also want to match a couple of new high-gain transistors for the input differential pairs I use Zertex ZTX 694B's (NPN) or ZTX795A's (PNP) for diff-pairs with excellent results. The stock pots are nothing to brag about either, and perhaps a new 25-turn Bourns would help here too.

 

[rulerboyz]

What value of capacitance do you generally find for a DC blocking cap?

 

[EchoWars]

Anywhere from 22µf to 470µf, with 220µf being somewhat common. If the amp uses a FET input diff-pair, it will be DC-coupled, and there will be no DC blocker cap.

 

[marantzman]

marantz 2235b DC offset follow up

Hey Guys,
I went through the entire Idling current and DC offset voltage procedure,
Here are the results:

Warmed up for 4 hours, cover on, volume at minimum, tone controls set flat,
balance in middle, all options such as loudness off, main speaker switch on with no speakers connected.

DC voltage:
Right channel: Dc 0.695 V, after 10 minutes stabilizing : DC 0.692 V
Left channel: DC -001.5 mV, after 10 minutes stabilizing: DC -000.8 mV

Idling current:
Right channel: DC 020.1 mV, after adjustment : DC 008.2 mV
Left channel: DC 008.0 Mv, didn't need adjustment

As you can see the right channel bias was not set right, it is supposed to be
8 Mv DC on each channel. all the adjustments EXCEPT the right channel
DC offset are set correctly. The right channel DC offset is still reading
the same, and the little dial is a mirror image of the left channel, both are set
almost exactly where the previous owner or technician had marked them,
the same goes for the Bias, they are "mirror" images of each other.
I visually inspected the stereo all over inside and could not see any leaky caps. After adjustments I have noticed a better sound, I haven't turned it up
loud yet because I have been doing this all night since 9:00pm last night
got done about midnight, went to sleep, and am now getting up at 4:30am.
Everyone else has been asleep since 10:00pm.
I can't sleep, part of the reason being this damn Dc offset crap, The marantz
is the only working unit I have right now, and I have an excellent pair of
Boston Acoustics Vr-975's That I am scared to use, I don't want to thrash them. I listen to my stereo system more than I watch TV, so for now
I am using my old JVC party speakers. This is really perterving me.
How much would an audio repair shop charge to go through the entire stereo
and replace any bad caps, transistors, etc... ??? I have an oscilliscope
that was a gift from a buddy of mine, but I have no user manual for it,
and have never used one. He is supposed to be coming to visit thanksgiving,
Is there any way we could trace whatever is causing the problem???
I really love the 2235b and could only imagine the difference in sound once this DC offset crap is fixed and I can use my bostons. I am looking for a nice pair of JBL L-100's or Pioneer HPM-100's and will trade my Bostons for them staright across. They have 3 full years left on the 5 year parts/labor warranty
from Boston Acoustics. If interested PM me.
BTW: in my first thread on this topis I said that there was a slight thump
upon power up in the right channel, but it is the left channel that has a slight thump sometimes upon power up. It is odd because the left channel bias and DC offset are set correctly. Whats up with this thing???

 

[EchoWars]

The root of the problem can range anywhere from the differential pair being unmatched, to a burnt resistor somewhere, to a transistor that is leaking current from C to E when it should not (always a joy to locate).

I've never opened up a 2235b...some Marantz's are a breeze to work on, others have you cussing like a sailor on shore leave as soon as the cover is off. Wish I could steer you in the proper direction, but it seems to me that you've done all a layman can do, and then some. My advice at this point is to find someone you trust to track down the trouble and put it right. If that seems to be difficult, there are a number of good techs who frequent this board and would be willing to get it straightened out for you, provided shipping the receiver isn't something you have a problem with.

 

[marantzman]

Yeah, at this point I have done almost all I can do, plus I have no training
in this stuff, everything I know has been self taught. I am 17 and find this stuff very interesting and fun, although sometimes frustrating. When I talk
to my friends at school about this stuff most of them don't even know
what watts or ohms are let alone bias current and DC offset. Although I do
not fully understand it all, I have a basic understanding and with the help
of the internet, you guys, and manuals I am able to learn more, and
do some of this stuff myself. I appreciate help from people like you and
others on this board. I have tried to get help over at audiosylum forums,
but so far you guys have been able to give me more info than those guys.
What do you think the going rate would be for a technician to go through
the stereo and track down and fix the problem would be? Will I have much luck even finding a tech that repairs vintage stereo's?

 

[EchoWars]

Will I have much luck even finding a tech that reapir vintage stereo's?

This board is chock-full of guys who fix old gear, including myself.

What'll it cost? Who knows...at least until the problem is located. Most guys can find a way to keep costs to something everyone can live with.

While no one is expecting you to dump hundreds of dollars into a nice (but somewhat 'close-to-the-low-end-of-the-scale') 30 year-old Marantz, if you focus on the $$ and not the joy that the gear is capable of bringing, then you miss the point of the hobby.

 

[rulerboyz]

I have dumped a couple hundred dollars into my KR-4140 which is a receiver that isn't worth more than $20. It's my experiment/test unit, which means that I've changed almost every component at some point (ordinarily not a good idea). I enjoy chasing down it's problems and learning how and why certain components fail. I've found that it is important to not keep your expectations too high or lose your patience when you are working on trying to get it to work the way you wish it would. You can also put it in a box and get back to it later. If you are simply interested in having a unit that works, the effort it takes to fix might be better spent finding another marantz that is in good condition.

 

[dr*audio]

Since you are interested in this stuff, I would like to encourage you to learn more about it. Perhaps there is an electronics class you can take at your school.