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Readjusting bias current and DC offset - Marantz 2230

bibota

bibota

New Member
I'm currently in the midst of my first project, recapping a Marantz 2230 and meeting quite a few difficulties along the way. All of them feeding into the learning curve, to put a positive spin on it.

I keep reading that after recapping the tuner boards, a "readjusting of the bias current in the output stage and of the DC offset" is the thing to do. Still, I cannot find anywhere a plain explanation why this readjusting is actually needed.
Is the operation meant to improve the sound (implying that now the amp's output is somewhat distorted by the recapping) or there's something else altogether that's affected and needs to be put right again?

Many thanks,
Cris
 
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it may have been adjusted with out of spec components so needs to be checked/reset to factory after any work is done .
i always check before starting any work so i know if i have caused any problems along the way .
 
Setting the bias and offset are done after working on the power amps - not tuners - and generally again after all work is completed. The reason for setting them is that the refreshed cap values can make those settings drift and they need to be touched up. If they are too far off, they can cause distortion at best and damage to components at worst. The procedures for setting them are spelled out in the service manual under "audio adjustments". You have to set them separately for each channel. Attach mini-grabbers from your DMM to the proper pins while powered down so you don't accidentally short something out and cause damage.

On a separate note, if you work on the AM tuner board, be aware C183 has the wrong polarity silkscreened on the board. Check polarity of old cap before removing.
 
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I believe dc offset does not apply to your unit because is cap coupled.
You can adjust bias to 5mg (r774/r765) and set r810 to 35.6v.DC
 
Ah, yes. Forgot it was cap coupled. To measure across R774, measure between J757 and J758 and adjust trimmer R765 to 5mvDC. To set the power supply voltage KT referred to, black lead on chassis, red lead on J808, adjust trimmer R810 to 35.6vDC. Again, attach leads with power off to prevent damage.
 
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Steven Tate said:
...be aware C183 has the wrong polarity silkscreened on the board. Check polarity of old cap before removing.
Click to expand...
Thanks Steven, been there done that and many other silly things. Luckily there were no critical mistakes so the receiver is still alive, and I've learned quite a bit by trying to put them right.
KutzlerTrans said:
I believe dc offset does not apply to your unit because is cap coupled.
You can adjust bias to 5mg (r774/r765) and set r810 to 35.6v.DC
Click to expand...
So no need for DC offset, thanks KutzlerTrans.
 
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Steven Tate said:
... the refreshed cap values can make those settings drift and they need to be touched up. If they are too far off, they can cause distortion at best and damage to components at worst.
Click to expand...
Speaking of drifting in values, I wonder whether this may be responsible for a few things happening now:
-a buzzing noise when the first RCA is inserted into its socket,
-a self-resonance phenomenon while the Bass knob is in the higher range with the volume at about 12 o'clock.
 
bibota said:
...
-a buzzing noise when the first RCA is inserted into its socket,
-a self-resonance phenomenon while the Bass knob is in the higher range with the volume at about 12 o'clock.
Click to expand...

The first is a normal phenomena of connecting a conductor (the center pin) into the input with no relative ground, hence it acts like an antenna and picks up the power line hum. Once the ground is made the input is no longer floating so the hum goes away.

The second sounds like your just clipping the output possibly.

There is actually an adjustment that is the equivalent of a dc offset adjustment in a direct coupled design. It sets the center voltage point so that the maximum power can be achieved. It is best set with an oscilloscope and audio signal generator with a variable output and is in the service manual. It can be rough set with a common voltmeter (dc volts) by measuring the voltage on the filter capacitor and adjusting the pot for half that voltage as measured at the input side of each channel's output coupling cap.
 
rBuckner said:
The first is a normal phenomena of connecting a conductor (the center pin) into the input with no relative ground, hence it acts like an antenna and picks up the power line hum. Once the ground is made the input is no longer floating so the hum goes away.
Click to expand...

Thanks rBuckner,, I was expecting this but it seems that what goes on now it's a bit too loud to be just that.

rBuckner said:
...
There is actually an adjustment that is the equivalent of a dc offset adjustment in a direct coupled design. It sets the center voltage point ...
Click to expand...

I'll have a go at adjusting the bias and see what, if any improvement may result (sort of half-reminding that I read somewhere the pot adjusting is to be done with a plastic screwdriver??)

Then try to learn a bit about setting the center voltage point, the rough, "no oscilloscope" version. I may come back with a question or two if you don't mind.
 
Doesn't look very good I'm afraid.
Started by attaching leads to chassis and J808 with power off.
Then powered on. Can't remember precisely but I think the initial reading was something in the region of 37.2vDC or so.
Powered off and slightly adjusted R810 to see which way I should go towards 35.6vDC.
Then powered on again to read the result.
After just a couple of seconds I noticed a faint trace of smoke coming out of the Right side P750 and quickly turned off power.
It wasn't fast enough though, the unit is dead now. As in no lights, no reading between chassis and J808 or between J757 and J758.
All I could venture checking were JP11 to JP15 fuses, which are good.
Any suggestion what and how to investigate further?
 
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Not good news. You would normally make that adjustment with the unit running -- just carefully and preferably with a non-conductive screwdriver. Hard to believe a small movement in the trimmer would change enough to cause that damage. If it were mine, I would try to put the trimmer back where I thought it was originally, DBT it and see if it passes. If it does, then start looking for fried parts. At some point, you will have to set that voltage while the unit is running. I'm not sure if you should do that first or work on the damage first. Someone who knows better will chime in, I'm sure. Good luck!
Steve
 
Best bet is to find where the smoke came from, I bet it wasn't the P750 (amp) board but the 800 power supply. I've had these supplies go full tilt unregulated and not hurt anything so my best guess is that your test connections were shorting to something. Most likely the transistor H802 is blown. But if there's no power / no lights and the wall outlet still has power, there has to be a fuse somewhere that's out either inside or on the back panel. Hopefully the power transformer primary didn't fry open!
 
rBuckner said:
Best bet is to find where the smoke came from, I bet it wasn't the P750 (amp) board but the 800 power supply.
Click to expand...

I'd say it's a very slim chance of a short between test leads because I was really cautious installing. They are properly isolated alligator clips and the situation was static at the time. Moreover, I've clearly seen smoke coming out of the P750 on the right and I could still smell a whiff of it there, almost 10 hours later. There's no obvious burn mark or sign of bulging though.

rBuckner said:
But if there's no power / no lights and the wall outlet still has power, there has to be a fuse somewhere that's out either inside or on the back panel. Hopefully the power transformer primary didn't fry open!
Click to expand...
Although the lights are dead, there seems to be power coming through. There is measurable tension between chassis and some of the junctions of the two P750.
As about fuses, I cannot find any apart from the bunch in the attached photo. They seem ok.
 

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Got technical and checked continuity on fuses, they're all good. I wish I had that easy a way out but no.

Could anyone suggest a way of identifying the problem by using the multimeter please?

Speaking of possible reasons why the damage occurred, is there a certain sequence of operations while adjusting bias and DC current? Should have I started with adjusting the bias on the P750s instead of the other way round, which I did?
 
The sequence didn't matter.

I'd start with seeing if the switched outlet on the back is connected when the unit is powered on. This checks the mains fuse and the power switch on one side at the least. Then I'd measure for AC voltage at the transformer secondaries. Then move on to the power supply if all is well.

With no lights, which are transformer secondary powered at 8Vac, either the primaries are not getting power or at the least the lamp supply secondary is fried open.
 
its probably something simple. follow your wires and maybe you will get lucky and find a main power wire busted inside the insulation. gently pulling on the end should separate it.

make sure you have AC voltage going to your rectifier, then follow it from there, unless your switch smoked. Randys answer of checking your unswitched 120v line out is a good start.
Make sure you actually have voltage at your wall outlet :D
 
KutzlerTrans said:
its probably something simple. follow your wires and maybe you will get lucky and ...
Click to expand...
It was simple, idiotically simple: the main 1.25A fuse on the outside was blown. Seems that I was getting all worked up looking at the myriad of possibilities on the inside and completely forgot about the outside.
Thanks KutzlerTrans

Now, while we're at it: I wonder how do you mean "having AC voltage going to your rectifier". It isn't a euphemism, is it?
 
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