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 Post subject: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Sat 26, 2016 3:57 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
I'm currently in the midst of my first project, recapping a Marantz 2230 and meeting quite a few difficulties along the way. All feeding into a steep 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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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Sat 26, 2016 5:57 pm 
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Joined: Jul Sun 09, 2006 3:11 am
Posts: 4672
Location: Aurora, CO
It's always good to check bias and DC offset anytime you have the cover off. Components age and the adjustments can get out of spec.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Sat 26, 2016 11:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16001
Location: Albuquerque, NM 87123
The service manual is on Hifiengine; if you aren't a member you have to join but it's free. Instructions are on page 12 (service manual pages numbers may be different). You adjust trimmer rheostat R765 to get 5mv across R774. It says to do the same for the other channel but doesn't give R numbers.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Mon 28, 2016 4:13 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
Thanks for your suggestions, they imply that something good will happen to the receiver by doing it so I'll do that.
Still, could anyone enlighten me a bit about the reasons behind it? I wish I understood what "bias and DC offset getting out of specification" actually means. Actually, the straight answer may prove a bit too scientific for me so I'd rather be simply told how the "getting out of specification" affects the receiver's functionality. Is the sound output distorted, is there a possibility for severe malfunction developing or something similar?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Mon 28, 2016 5:39 pm 
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Joined: Dec Thu 31, 2009 4:28 pm
Posts: 4189
Location: OR
Well, to put one problem simply, there could be a DC voltage applied to your speakers. Not a good thing for several reasons. Have you ever heard a SS amp pop when it is turned on?

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Tue 29, 2016 2:07 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
phlogiston wrote:
... Have you ever heard a SS amp pop when it is turned on?

No, but it doesn't sound too appealing. I'd still rather not.

So the getting out of spec may manifest itself in many ways, one of them being a DC current straight to the speakers?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Wed 30, 2016 2:50 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
Much as I tried to avoid it, I still managed to fry something in the process of adjusting the bias and dc offset.

Started by attaching alligator 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 one way to see how the reading will be affected and understand 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 seemingly dead now. As in no lights, no reading between chassis and J808 or between J757 and J758.
There seems to be power coming through though, with measurable tension between chassis and some of the junctions of the two P750.
As about fuses, I checked continuity and they're ok. Wish one of them was blown, but no, not that easy way out.
Could anyone suggest a way of identifying the damage? By using the multimeter, preferably.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Nov Wed 30, 2016 3:44 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 371
Location: Fayette County, Pa
What Happened?

Common problem when adjusting bias on an old amp; The adjustment control is dirty from sitting in the same position for years. Thus one should always note the setting, then before applying power, clean the control making certain to return it to the exact same position after rocking it end to end. Failure to do this can cause the control to go open as you make your adjustments, just like a noisy volume control. Bt what happens here is that many times this open condition causes the voltage across the control to put excessive bias on the outputs causing them to draw high current. You wind up blowing the outputs and possibly driver transistors.

DC offset adjustments are similar; they should be cleaned before adjusting. The difference here is you make that adjustment without a speaker connected so you usually don't smoke the amp (although if the speaker is connected you could certainly smoke it!) You simply adjust so there is 0 VDC on the bootstrap before connecting the speaker. A dirty control here may cause spikes of DC when it is adjusted but with no load that usually isn't problem as long as you confirm the DC offset level before connecting the load.

The above two statements are generic, applying to any amp that provides for these adjustments. Some amps do not therefore it won't matter for them as you won't be tweaking anything. (I don't have a schematic of a 2230 so I can't be more specific as to exact setup or steps to take to fix the damage. I would begin by checking the outputs / drivers first. Also emitter resistors on the outputs and go from there.))


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 02, 2016 5:00 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
CaveRat wrote:
What Happened?

...The adjustment control is dirty from sitting in the same position for years. Thus one should always note the setting, then before applying power, clean the control making certain to return it to the exact same position after rocking it end to end. ...

DC offset adjustments are similar; they should be cleaned before adjusting. ...


Many thanks CaveRat, I was completely ignorant about the very sensible points you're making. I'll certainly do that from now on.
Luckily there's a second chance for me, the damage to my receiver being a single blown 1,25A fuse.
Yes, it was that ridiculously simple: a blown fuse, the main one on the outside of the unit. I was getting all worked up about looking on the inside that I completely forgot to check the most obvious of things that could have gone wrong. I know, I know...
I'll get a replacement and see how things move from here using the precautions you're suggesting.
Would you say Caig Deoxit D5 is safe for cleaning those tiny pots?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 02, 2016 8:02 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 371
Location: Fayette County, Pa
Probably safe to use. Any control cleaner should work, but since I can't say with certainty use care as you would with cleaning any other control. As soon as one says "perfectly safe" along will come that oddball plastic that turns to mush when the cleaner hits it. These trim pots are just like any other control so what works for one should work for all. Also note that all adjustments should be made slowly since they do affect currents and voltages in the output and caps may take a second to bleed down causing spikes that otherwise would not be good.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 03, 2016 12:51 am 
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Joined: Dec Thu 31, 2009 4:28 pm
Posts: 4189
Location: OR
Use the De-Oxit in the green labeled can (Fader Lube"). The regular (black label) will remove the resistive coating on some controls.

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Mon 05, 2016 3:01 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
CaveRat wrote:
...As soon as one says "perfectly safe" along will come that oddball plastic that turns to mush when the cleaner hits it. ...caps may take a second to bleed down causing spikes that otherwise would not be good.


And so it went.

Have I read somewhere that a blown fuse is never as simple as this but only a sign that something else must be wrong?

After getting new fuses and cleaning R810 it was really easy to set voltage to 35.6vDC and I thought I was on top of the game again.

Well not quite, as it turned out: I moved to setting bias on the P750s and started by cleaning the two pots R765.
Everything went haywire on the right hand P750 board after powering up, with smoke coming out of R765 and various resistors glowing red hot all over the board.

Here's a list of parts affected:
R765- trimmer, the one to be adjusted and possibly the source of trouble in the first place
R766, R767, R768, R769, R770
Thermistor+resistor fabrication, as described in Marantz 2230 Service bulletin_2.
.........................................

So yeah, replacing said trimmers and resistors I guess?
Took a look on Digikey, many available so could anyone recommend the most suitable in terms of:
-manufacturer (Bourns, Copal, TT, Vishay)
-resistive material?
-number of turns?
-Wattage? Don't quite get the "B" in the trimmer's Wattage specifications. Wonder what that means

-I take that the lowest 10% tolerance would be best


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Mon 05, 2016 3:39 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
This series caught my eye: they seem reasonably priced, made by good manufacturer, 10% tolerance, 25 turns fine adjustable from top, sold by unit not bulk.

http://www.digikey.co.uk/product-search ... ageSize=25


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Tue 06, 2016 3:18 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 371
Location: Fayette County, Pa
No easy fix when that happens, I have seen many amps come in with multiple components burnt.

When I get an amp that takes out numerous semiconductors I always replace all transistors that are direct coupled . That would be outputs, drivers, and any bias control transistors. You will also replace any overheated resistors as the condition may have changed their values. I don't have the schematic so I have to speak in generalities here, but you will also find one or several diodes that are attached to the heat sink. These are there to reduce bias voltage as they heat up preventing thermal runaway. (Some amps use transistors for this purpose, not sure about yours) These also must be replaced with the same type to maintain proper control action.

As far as the pots, I would go with whatever fits the circuit board. Multi turn pots are usually not needed as the range is not that great when everything is working right. My guess is the pot didn't cause your problem, rather you had a transistor that was breaking down / leaky. It finally gave up the ghost and took a bunch of stuff with it. Only way to be sure you get the culprit is shotgun everything that might be affected.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Tue 06, 2016 3:46 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
CaveRat wrote:
...replace all transistors that are direct coupled: outputs, drivers, and any bias control transistors....any overheated resistors...diodes that are attached to the heat sink


Thanks CaveRat, my initial intention was to replace pots and the resistors which I could see being smoked.
I had a nagging feeling that that may not be all and whether a certain transistor on the board may be the cause and also needing replaced. Thanks for confirming that.
There is indeed a diode attached to heat sink and would be a good moment to replace that as well. However, the old one seems to be an obsolete model. The ones I find at Digikey and else are thin, long and rounded cylinders, looking nothing like the brick-like old part.

Attachment:
heat sink diode.jpg
heat sink diode.jpg [ 9.17 KiB | Viewed 861 times ]

Here's a schematic of the board if you're kind enough to take a look.
Attachment:
p750.jpg
p750.jpg [ 51.58 KiB | Viewed 861 times ]


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Tue 06, 2016 7:06 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 371
Location: Fayette County, Pa
Parts to change:

H001
H002
H755
H756
H752
M005 Critical value, is actually a dual diode to get the right barrier voltage

R774 R775 if open (They may well be)

H753 754 are probably OK UNLESS R756 757 are burnt / open If so they will need changed too.

If any other resistors smoked you should see where they are in the circuit and additional work will be needed before powering up.

Set R765 to minimum (lowest bias), then power this up through a variac with output transistors disconnected and check E/B voltages supplied to them. (Measure E-B, not to ground) Raise the AC voltage slowly while monitoring the E-B voltages. It should stay under 1 volt as you raise voltage to about 75 VAC line voltage in. If they go over one volt STOP, you have other problems and installing the outputs would likely free the smoke monster. Go no higher in this config. Also keep an eye out for any transistors heating up. That should not happen unless something else is bad. Don't connect speakers; not necessary for this stage of testing. There is also a large cap between the bootstrap and speaker terminal not shown on the schematic. Make sure it hasn't shorted. (Usually they don't but if it did it would take out the speaker and probably the outputs again.)

If you get the variac up to about 75 VAC in without the E-B voltage going over 1 volt, measure the supply voltage from ground to J755 point. Then measure the bootstrap at J758. The bootstrap should be about half the B+ supply voltage. (You may need to tweak the centering pot R760 to do this)

If all checks out, install the outputs and repeat the variac test. Measure all transistor voltages as you bring it up watching for anything out of normal. By the time you get to 75 VAC you should be able to get audio through it. reconnect speakers and test that. It may be a bit distorted, but both channels should be similar. If so bring it p to 120 VAC and monitor for anything out of the ordinary. Gradually bring bias current up to specs and you should be good.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 07, 2016 4:06 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
Hi CaveRat,
I seem to be going ever deeper down the rabbit hole but happy that I'm learning lots in this free fall. Couldn't help thinking there's the easy option of replacing the whole P750 board (or another receiver, for that matter) from ebay but that would mean missing the fun bit, wouldn't it?

Now, it will take me a good few readings to take on board all your advice so many many thanks for taking the trouble to deliver it.
I'm starting by building up the took kit and gathering the parts. I've ordered the Variac last night (yes, I was one of those people that didn't have one) and now I'm looking up replacements for resistors, trimmers, transistors, diodes.
Speaking of which,
CaveRat wrote:
M005 Critical value, is actually a dual diode to get the right barrier voltage

You put the fear of God into me with this but I can't find M005 anywhere.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 07, 2016 4:56 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 371
Location: Fayette County, Pa
I assume first of all you tried ordering direct from Marantz / Denon / Superscope. This is an old part but always an outside chance it is still available. (Probably not, but who knows?)

There are ways around that one but the devil is in the details. First, is the mounting, how is it attached to the heatsink? I have seen them as a single epoxy block with a hole through the center, clamped in, and in some cases two separate diodes under a single clip or even just stuck to the heatsink with silicon compound.. The replacement needs to be the same configuration and mounted in the same location. This is not because the diodes get hot, rather they monitor heat sink temperature and adjust their barrier voltage to prevent thermal runaway.

Once you determine this here is where it gets tricky. You can get individual diodes and build a module up your self BUT!!!! You must find diodes with the same heat response curve as the originals and that data is usually not available. Build up a diode set with 2 diodes and try it under test to see if the bias control will adjust properly. If not, there is another option but it is more work.

What I have done to make it work is if the circuit uses 2 diodes like yours I build up a module with three is in series. That will make it overly responsive to heat (Better over than under!) Set the bias pot to minimum, then bring it up slow while measuring bias current. Careful though, the extra diode will cause an increase in current, you must use the variac method I mentioned previously to ensure you don't draw too much and smoke the outputs. if you can get the bias to specs with some adjustment range you are home free. It depends on how much tolerance was designed into the circuit.

If you can't get the current low enough, you will need to shunt the diode set with a resistor. The value must be determined by testing. Start with 1 K and go down. Probably between 100 - 300 ohms you will find the bias current drops when you install the resistor. When you find a value that allows the control to set the proper current you found the value you need. Install that one. If it hasn't worked, go no lower than about 50 Ohms, the lower value will negate the temperature characteristic of the diode set. You will need to find a 2 diode pair that will work.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 07, 2016 5:21 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
Thought I'd upload a few pics to illustrate my case
I can detect by sight 5 fried solid resistors: R766, 767, 768, 769 and 770.
Looking like this
Attachment:
R_0962.jpg
R_0962.jpg [ 22.36 KiB | Viewed 794 times ]

Attachment:
R_0961.jpg
R_0961.jpg [ 9.55 KiB | Viewed 794 times ]

There's also obvious damage done to this
Attachment:
R _0958.jpg
R _0958.jpg [ 44.18 KiB | Viewed 794 times ]

which is the resistor end of this
Attachment:
R_Therm_res.jpg
R_Therm_res.jpg [ 26.12 KiB | Viewed 794 times ]

Meant to protect transistor H755 like this
Attachment:
R_H755.jpg
R_H755.jpg [ 16.22 KiB | Viewed 794 times ]


Last edited by cri_sta on Dec Thu 08, 2016 3:18 am, edited 1 time in total.

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 07, 2016 5:44 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 58
Location: London, UK
Am I wrong believing that by M005 you mean H758-759, the 1N60 diode?
I see someone recommends using 1N3595 as a straight replacement. I may be naive again, but at a first look I can tell the 1N3595 is very different from the old part in terms of size and shape. Old one is brick-like whereas 1N3595 is thin cylindrical.

This is how the fabricated construction is placed on the board

Attachment:
R_Therm_res.jpg
R_Therm_res.jpg [ 24.04 KiB | Viewed 794 times ]


Could there be a direct relation between the heat sink diode to the resistor and finally to the H755 transistor? As in one of them malfunctioning and sending the others into overdrive so to speak.


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