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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 08, 2016 3:41 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
cri_sta wrote:
Am I wrong believing that by M005 you mean H758-759...?
...someone recommends using 1N3595 as a straight replacement....Old one is brick-like whereas 1N3595 is thin cylindrical.


Looking closer, I realise you were spot-on right. I was looking at and talking about something completely different: H758-H759 are two small diodes located on the board, whereas H005 (RB) and H006 (LB) are the singular, square ones attached to the heat sink.
The critical ones.
Attachment:
H005.jpg
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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 08, 2016 4:22 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
CaveRat wrote:
I assume first of all you tried ordering direct from Marantz / Denon / Superscope.


Tried, but it seems that the world has moved on from that way of doing things. Here's the essence of the reply:

Dear Gabriel,
Just search in the Web for this type of unit as part are no longer available .
Kind regards / Mit freundlichen Grüßen
Pardi
Sound Systems Technology, Authorised Service Centre for the following brands :
B&O - Boston - Denon - JVC - Loewe - Marantz - Panasonic - Teac - Yamaha


CaveRat wrote:
...I have seen them (diodes) 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.

Replacing with fresh parts would be my first option, but since there seem to be no such diodes available or even guiding parameters to reveal how they're working or what I should be looking for, I think that a reasonable thing to do is making sure that the existing ones really need replacing.
Moreover, there seems to be no talk online of someone having replaced them, which seems to imply that this parts are of the diehard kind. Is there a test I could employ to determine whether they're still functional?

Speaking of replacing parts, I'm tempted to go for scatter gun on trimmers, resistors, diodes and transistors, as well as the thermistor/resistor fabrication.
The fried resistors seem to be all of the solid type while the carbons appear to be unaffected. Is there a reason, is carbon more resilient?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 08, 2016 5:41 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 409
Location: Fayette County, Pa
Admittedly the diodes could still be functional, I just replace them because a failure can be catastrophic to a lot of other components. Sort of like failing to change out 50 year old electrolytic caps in an antique radio. You might get away with it , BUT......

They can be tested but you would need to know the design characteristics of the original. Three tests would resolve it. These are the "homebrew Versions" of the qualification tests a manufacturer would put a diode through for this application. First, determine the forward current rating of the diode then using a DC power supply and a resistor (value calculated by Ohm's Law and the supply voltage) place the resistor in series with the diode so it will draw the rated current of the diode through it for 1 minute. You monitor voltage drop across the resistor watching for any sudden changes. If the diode goes either open or shorted for even an instant, trash it. Forward current test)

Next test is Peak Reverse Voltage. Use a 100K resistor and a DC power supply capable of reaching the design PRV of the diode. Connect it so it is reverse biased and raise the voltage while monitoring the voltage across the resistor. That voltage should remain zero since the diode is reversed biased and no current should flow. If it does not the diode is leaky and is trash.

Next test is the temperature response test. For this you also need a digital thermometer that can measure the surface temperature of the diode / heat sink. Again a DC power supply and resistor in series with the diode, forward biased. The resistor is a 470 Ohm, the power supply regulated and set to 12 VDC. Mount the diode to a heat sink (any large aluminum panel will work) Use a DMM and measure the precise voltage drop across the diode. It will be between .6 and .8 V. Also measure the temperature of the heat sink, it should be room temperature. Next you can use a can of Freeze Spray to lower the heat sink temperature and take both temperature and voltage readings several times as it warms to room temperature. Write these down. Next use a hair drier to warm the panel while measuring temperature and voltage readings. Add these to your list. Run the temperature up to about 150 degrees F, that should cover the expected range the diode would operate at. The resultant curve can be plotted on a graph and would show the drop in barrier voltage versus temperature of the diode used as a voltage sensor. From that you can determine how the amplifier output bias voltage would react at various temperatures as the amplifier heats up. These are the same calculations the engineers went through when they designed the amp. The advantage to this is that you can plug in different component values and compensate for changes. Thus you could make any of hundreds of different component values work and not be limited to some unobtainium part.

I realize this is probably not really feasible, and a lot of work. I just wanted to explain the process involved to do it right. Most people will just get a diode that fits and try it. Truth is that usually works out, maybe things aren't 100% right but often it gets the job done. And to be fair it also sometimes releases the smoke monster. Sort of pays your money and takes your chances.


Based on the two resistors R766 R767 being burnt, I would also replace H753 H754 as there is a good chance the EB voltage exceeded specs and opened the transistors.

The added component looks like a production change to correct an issue with H755 failure Those are a part of the bias circuit and should be replaced or at least checked before powering up.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 09, 2016 4:23 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
CaveRat wrote:
... The added component looks like a production change to correct an issue with H755 failure Those are a part of the bias circuit and should be replaced or at least checked before powering up.


The resistor bit is badly burned (see previous R _0958.jpg photo), I could see it smoking within two seconds of powering up. Could this be a result of H755 already being open or the other way round, the added thermistor-resistor component being designed as a protection for the transistor, therefore acting like a fuse to save the more important part (H755)?

Plenty of head scratching opportunities on the heat sink diode situation. Alternatives seem equally difficult.

Otherwise, got a bit of a setback with the Variac. It was ordered and paid for three days ago and I was initially assured that it was delivered and well on its way, only to be told this morning that the order has been cancelled.
Started looking for another one but annoyed about the time wasted, not to mention the refund procedure which appears to be kind of foggy.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 09, 2016 5:02 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Not having the Variac yet and with plenty of time on my hands, would it be a good idea to de-solder and check the said transistors off the board with a diode test function?

As in here:
If you have a NPN transistor, and have identified BEC set your meter for diode test, and place the positive lead on the base. Place the negative lead on the emitter. You should read a diode drop voltage of about 0.45 to 0.65V. Now place the negative lead on the collector. You again should read a diode drop of about 0.45 to 0.65V (these values are not written in stone...the 0.45 to 0.65V is the most common range however). Now place the negative lead on the base, and the positive on the emitter (called 'reverse biasing'). There should be no conduction. Now move the positive to the collector, and again, there should be no conduction. Lastly, move the negative to the emitter. Again, there should be nothing.

(note: most transistors fail with a dead-short from the emitter to the collector, especially in the later power stages of an amplifier. Knowing this can allow you to check high-power stages quickly for obvious failures)

For PNP transistors, the leads are reversed while doing the same checks as above (negative on the base to begin, positive on the emitter, and then on the collector etc. etc. to read the proper diode drops)


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 09, 2016 9:17 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 409
Location: Fayette County, Pa
You could check the semiconductors just for your own peace of mind, but I would replace them regardles since based on the amount of resistors burnt, etc. it is clear that the circuit was severely stressed. Even though thy may test out you cannot be sure they are not leaky or breakdown under the higher voltages present in the actual circuit. From experience, I have trid to go on the cheap and only change the ones obviously shorted in the past and it often doesn't end good. You release the smoke then you get to change out the new ones you smoked by some other component failing.

The extra diode and resistor added is not to act as a fuse, rather it allows the bias to be reduced in event the driver heats as well as the output. Just guessing here but it looks like early on they had some failures attributed to unbalance on the differential sides and this was a fix.... ( Engineering screw up? ) When I see things like that to me it indicates something unstable in the original design and this was just a work-around to get it going. These are often difficult to repair simply because the circuit is unforgiving of slight mismatches or component value tolerances.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Tue 13, 2016 2:17 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Right, so not a good idea to rely on measuring but it's safer replacing.
I'll go scatter gun then, that will keep me busy for a while judging by the size of the list I've compiled for ordering.
Quite some investigating needed for finding transistors equivalents, btw.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 14, 2016 4:48 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Hi CaveRat,

Just bought this 9kg weight of copper wire on Ebay to help me with resurrection

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Described as:

Variac Current Rating: 8A
Nominal Power Rating: 2 kVA Voltage Rating: 230V
Actual Primary Voltage Rating (AC): 230 Volt Number of Phases: 1
Actual Secondary Voltage Rating (AC): 0-230 Volt Actual Power Rating: 2Kva
Plug Type: Terminals Mounting Style: Stand Alone


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 14, 2016 5:55 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 409
Location: Fayette County, Pa
A bit of overkill there, but assuming your mains voltage is 230 VAC that should do just about anything you need without fear of damaging it.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 14, 2016 10:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16622
Location: Albuquerque, NM 87123
A variac is only the first step. You need to monitor current with an analog ammeter, or digital if one is not available.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 16, 2016 4:04 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
CaveRat wrote:
A bit of overkill there, but assuming your mains voltage is 230 VAC that should do just about anything you need without fear of damaging it.

It clearly is for this project, but thought I'd have some power reserve for whatever the future brings.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 16, 2016 4:14 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Johnnysan wrote:
A variac is only the first step. You need to monitor current with an analog ammeter, or digital if one is not available.

Good point, since there's no dial I'm actually thinking of making one by marking set voltages on a disc so I know where I am while using the beast.
Hold on... are you saying ammeter? Didn't think it was an issue but it's most probably just my lack of knowledge. Could you go a bit into details please.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 17, 2016 3:57 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 409
Location: Fayette County, Pa
You won't really need to monitor the line current for this project since the power supply appears to be functioning. You will need to monitor the output transistor base current when you do the set-up, but this can be done indirectly by measuring the voltage across the two emitter resistors (the .2 Ohm ones) If the voltages are within spec, the current has to be assuming the resistors are not open (Ohm's Law)

What you might find useful though as you set up your variac is a an AC voltmeter across the output. That will save you from having to continually take line voltage readings if for example you want to run the load at 75 percent instead of the full line voltage.

Just to set the record straight you mentioned the 230 VAC variac. Am In correct in assuming your line voltage there is 230 Volts and that the 2230 is set to operate on that same voltage? I need to know that to ensure I give correct advice regarding the amp. Wrong advice would not be good!


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Mon 19, 2016 5:04 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
CaveRat wrote:
Just to set the record straight you mentioned the 230 VAC variac. Am In correct in assuming your line voltage there is 230 Volts and that the 2230 is set to operate on that same voltage? I need to know that to ensure I give correct advice regarding the amp. Wrong advice would not be good!

Thanks for that CaveRat, your question is spot on: initially I had my eyes on a 110V variac, not realising that we do have indeed 230V on this island.
It has arrived in one piece and I have it with me now but looking at it in confusion: which way to hook-up?
Too many possibilities.
Attachment:
variac.jpg
variac.jpg [ 25.06 KiB | Viewed 1416 times ]

Hope I'm not wrong, what I understood by reading online is that
-neutral goes 1 and 1
-ground goes bottom left

Completely lost on where live input and output though.

Attachment:
variac1.jpg
variac1.jpg [ 25.64 KiB | Viewed 1416 times ]


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Mon 19, 2016 7:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3951
Location: Boston, MA USA
My interpretation of the wiring diagram is this:

Earth as you have indicated

N (input and output): 3

L (input): 1

L (variable output): 4

Terminal 2 looks to be a step-up so an incoming 115V line can become a 0-230V output.

-David


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Mon 19, 2016 11:28 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 409
Location: Fayette County, Pa
I read this as :
line in on 1
Common / Return on 3
Wiper (variable output 0 - 230VAC) on 4
Earth on the screw as you indicated (Case Ground)
Pin 2 is a 115 VAC tap that would allow you to derive a fixed 115 Volt output between 2 and 3. You could also use this as a step up from 115 to 230 VAC within the current limitations of the variac. You would need to derate the total wattage to half of the stated value if you used in in this manner.

I don't see any mention as to which side of the winding each go to, if you get those reversed the variac will operate backwards. (CCW will be max output, CW will be minimum.)This should not hurt anything but would be contrary to normal where turning a control CW increases output. Check it under minimal load and if you get this effect, reverse the line and common leads.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Tue 20, 2016 3:09 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Hi CaveRat and David, I've got good news:

I'm alive after wiring the fat boy as you suggested; moreover, it delivers smooth volts. Thanks for yet another good advice.
Now I'm working on a makeshift dial and possibly start replacing some of the transistors and trimmers on P750.
I'm sure you'll wonder whether I thought of this, so yes I will use thermal paste on the transistors, this:
http://www.ebay.co.uk/itm/172429023476? ... EBIDX%3AIT

Really proud of myself for finding about it, will see how it goes since I've never done this before.

Regards,

Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 21, 2016 3:35 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Have to cut down on the pride thing apparently: not sure at all about using thermal paste on the TO39 type transistors. As in neither how nor if.
As they are, all three of them are sat on ceramic pedestals. H755 and H756 have also got crown-like heatsinks but there appears to have been no paste added to that.
However, the crowns seem pretty well attached to the original transistor bodies and that makes me suspect some clear coloured (therefore invisible) compound may have been used initially.
I'm very tempted to add some.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 21, 2016 6:38 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
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R760 is a 25 turns one, for R765 couldn't find similar so only 1 turn

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Couldn't resist temptation and took the plunge using the compound. Did my best to clean excess but still looks a bit messy.
Also not very happy about the way the 18ohm resistor looks wrapped in the red isolation. Shrink wrap is recommended but haven't got any. Could it be safe as it is now?


Last edited by cri_sta on Dec Wed 21, 2016 6:57 pm, edited 1 time in total.

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 21, 2016 6:55 pm 
Member

Joined: Feb Sat 12, 2011 2:29 pm
Posts: 409
Location: Fayette County, Pa
As long as the clip type heat sink is tight and firmly grips the transistor it is not needed on the driver transistors, but the fact you put it in won't hurt anything. Just make sure it doesn't act like lubricant and let the heat sink slip off easily.


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