4140 Power amp Bias Issue

RWood

Slowly Speeding
Subscriber
Hello,

I'm working on a 4140 and have a spare parts unit. Two of the channels on one side work fine and bias up well. The other two need attention. I'm aware of the errors on the schematic (thanks Catrafter). In short, I can't get the bias voltage down to the required 10mA. The lowest I can get on one channel is about 85mA and on the other, around 115mA. I've tried adjusting the balance pots still can't get close.

What I've done so far - I've extended the leads on the bottom of the board so it can be removed enough to access the components for testing voltages. I've removed the four power transistors as well as the next eight transistors, testing for voltage drops and shorts. All test good BUT when I put in a pair of power transistors from the parts unit, that side's voltages got more stable and the VU meter from that side, which was bouncing, quieted down to an almost negligible bounce.

I've replaced the two 220uF capacitors with 470uF 50V caps.

I'm getting good voltage readings on the output transistors although the emitters on H001 and H004 do fluctuate quite a bit, but are generally in the low range as indicated.

I am used to working on tube gear and although my soldering and testing skills are good, I am still fuzzy about the transistor circuits, NPN vs PNP and what is happening in general. In addition to repairing this unit for my brother I 'd like to learn some theory as well. For instance I am not clear on how the bias voltage is developed and which components control it.

Any help appreciated.
 
Check the resistors relating to the adjustment pot for accuracy.
You can find a service manual in the digital docs forum

If you did not replace the differential pair, you should. Order a matched pair, or buy 20 and match the HFE with a dvm that has that capability.

They are the first set of transistors and they are shrink wrapped together with thermo compound between them.

Make sure your rail voltages are the same. (Opposite polarities) and make sure all transistors get new thermo compound and all thermistors are contacting heatsinks.
There are some improvements you can make in that dept. Search the forum for bias issues. Some great threads on this. Good luck and learn everything you can.

If you understand tube theory, then transistors should be a breeze. As with a tube needing the grid to stay at a negative voltage to the cathode to protect electrons from flowing backwards,
A transistor needs a certain base voltage to keep the transistor operating with the correct on time to keep the signal on for at least 180 degrees on one polarity (npn)and the opposite polarity(pnp) mated transistor to do the same.
+
Bias adjusts the balance of the base / emitter to keep the base voltage about .7 v above the emitter voltage to ensure the proper Q point.(Collector current)
 
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A PNP will have the arrow on the schematic representation pointing in --- Points iN Proudly
An NPN transistor will have the arrow on the schematic representation pointing out, NPN --- Not Pointing iN.
Transistor conforming to Electronic Industries Association of Japan (EIAJ) standards will have part numbers 2SA and 2SB for PNP types and 2SC and 2SD for NPN types.

In Class AB power amps, 'bias' is used to keep the output transistors slightly 'on' to reduce distortion.
There are many methods of applying (and controlling) so called bias. In the 4100/4140, a small 'idle' current is supplied to the bases of the driver transistors (H611,H613) (H612,H614) to keep them and the output transistors (H001,H002),(H003, H004) slightly conducting. There is a multi junction diode (H581), (H582) usually mounted to the heat sink that senses the temperature of the outputs so that the bias can be regulated as the temperature increases to prevent thermal runaway. A thermistor (H627), (H628) is also involved as is the rheostat (R628?), (R629?) used to adjust the bias. Look for about +1.2VDC on the base of (H611), (H612) and about -0.6VDC on the base of (H613), (H614), reference chassis ground, volume all the way down.

There are many folks here who understand this a lot better than I, perhaps they will chime in.
Tom
 
Thanks for the replies, gents!

If you did not replace the differential pair, you should. Order a matched pair, or buy 20 and match the HFE with a dvm that has that capability.

I'm have not. In fact, I don't see a pair that meet your description. I've attached a schem of the 4100 amp.....could you let me know which these are?

Look for about +1.2VDC on the base of (H611), (H612) and about -0.6VDC on the base of (H613), (H614),

Ok here's what I measured last night.
Base of H611: .995v
Base of H612: 1.131v
Base of H613 -.3v and fluctuates quite a bit
Base of H614: -.415v

The rail voltages seem pretty well balanced at 31.37v and -31.46v.

I tried the amp out with my phone as the signal source and a pair of headphones for the output . On the right side, the one I'm trying to repair, the rear channel does pass signal and sounds fine while the front channel does not pass signal at all. I have not traced the schematic to see if this is the even- or odd-numbered side of the power amp board. I suspect the bad channel, the front, is the odd-numbered side.
 

Attachments

  • Marantz 4100 Schematic.pdf
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h601/603 & h602/604
they should be bonded together:idea:

make sure your ground pins are soldered good. check all solder joints for loose legs. if in question, resolder it.

I would put the diff pair in before waisting a ton of time on it. get a can of dr scholls freeze off and start cooling parts while monitoring the oscillation . it may help track it down faster.

what is the EBC Voltages of H609 and the base v of h605.?
backtrack the fluctuation and see if you can track it down.

check for ACv while your in there.
Caution!:whip:
i can not stress enough how important it is for you to protect your test leads with tape right to the tip. not being careful about that can really ruin your day.:yikes:
 
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Excellent info; thanks for that.
I will check those voltages tonight and report back.

Thanks; I use Pomona mini-grabbers.
 
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They are side-by-side on this amp.
Are these what you use: ZTX795A
I notice there is a different pinout than the original 2SA640L
 
I generally use KSA992 from a reputable supplier like Mouser.
You can buy a bunch and Hfe match them in pairs with a cheap DMM that has that function.

Tom
 
Ok thanks Tom. My Fluke doesn't do HFE but the Harbor Freight does- go figure.

Here are the voltages Kutzler asker about:

H609
E -.06 to -.2
B -.1 to -.3
C -.6 this pin fluctuates the least

H605
B -29.47 measured at collector of H601


For AC voltages I'm getting around .1 v or so at most points, a little higher on the bad side than the good.

Does any of this indicate a problem?
 
Is it oscillating across the h623 diode?
Confirm grounds and solder joints and cap direction/ correct values just to be sure. Also make sure the base of h710 is stable.
The problem could be created by the feedback curcuit, or its responding to a real output oscillation.

Check r657, R643, c618(for an open) or jump it with another same value cap, and last but not least, is the h004 emitter stable or not.

I had an amp oscillate because i had a cap in backwards.
 
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Thanks again for the help. Here's what I found:

Is it oscillating across the h623 diode?
Voltage definitely IS oscillating across H623's compliment on the bad channel, H624 (it is the "upper" channel on the schematic that is has no audio, although the bias voltage - my perceived original problem - is high on both channels).

Confirm grounds and solder joints and cap direction/ correct values just to be sure.
The only terminals to ground I see are J623 and J624, and they are both grounded.
The only caps I've changed are the large 220uF caps in the center, and they are oriented correctly.

make sure the base of h710 is stable.
I don't see an H710. H610 maybe? Or, on the bad channel, H609 whose base is unstable.

Check r657, R643, c618(for an open) or jump it with another same value cap
They all seem to be OK. I didn't jump C618 (or 617) but measuring voltage on both sides they are not open.

last but not least, is the h004 emitter stable or not.
H001 emitter, the one on the bad side, is most definitely oscillating, going several hundred millivolts from positive to negative.
H004 emitter reads high, but it fluctuates only from around 140 to 145 mV.
 
I have looked at the Schematic again and the outputs are wired wrong. as you can see, both are NPN Transistors and the rail voltages go to different legs. I believe the rail should go to the emitters, and if i am right, H001, H003, h005 and H007 are PNP and not npn like it says here.

So, since i was looking at the wrong amp, h002 is the one i was wondering about. Now your saying its also on the other side of the amp.
Oh, I am used to marantz amps having a P700 # and messed up the number.

The open cap would not show up open because a resistor parallels it, but with you saying H001 is acting up, then disregard checking it. I just meant to tag another cap right on top of it to confirm it has nothing to do with it.
I'm not sure if this will work, but it would be nice to find out which end its coming from, so if it was my amp and i didn't care if i smoked it:D... I would ground the base of h609 to see if the output fluctuation is the cause or effect.
I am not sure if that would cause any undesired effects, but i would make sure i used a higher ohm resistor to snub it to ground . Then i would do voltage drop checks on every resistor around it. If the voltage oscillates across any resistor on your lists of suspects, then its coming from one side of the resistor. Maybe it will give you a popcorn trail to follow. if the resistor is low ohms, it may not be enough current to show up in a voltage drop.

I do think it would be nice to trace that oscillation backwards on every transistor base to see were it originates.
Did you replace the differential pair yet?? See my note below..
That"s the first thing i would do.
Its a feedback system and a loop and could be a catch 22.
If the base is stable and the emitter or collector is fluctuating, then pull the transistor and check it. at least you will narrow its origin.
A bad transistor or cap might do this.
When i get into a stumper like this, i stop and write out each transistor value like this,
H601
E 0v
B 0v
C 0v

H602
E
C
B
Etc.
That way your Not in the dark and can see were in the amp your voltages start getting out of whack and take a downward turn.
NOTE: If it was my amp, I wouldn't waste another minute on it till i put all new electrolytics caps, Diodes, and Diff pair.
Output transistors, you can rob off another amp for a test.BUT...
If you find any overheated resistors or shorted transistors, do your amp a favor and spend $18 on a new set of MJ21195/96 for that amp.
Do not forget to use the DBT!!! If you make a mistake or the parts are junk, then you could really fry something.
I am guilty of thinking my work looks good and had a few "now who in the hell put this in backwards" surprises, so just remember
" We are only as good as the parts we use,... or worse"
Buy a cheap Chinese transistor like this
http://www.ebay.com/itm/12864-Mega3...237498?hash=item4af681207a:g:sBMAAOSwa-dWhWzn

I would see if it oscillates on the dim bulb tester. Then use that until you fix it .

Hopefully someone smarter than i can assist you.
 
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Thanks. I haven't ordered parts yet as I was hoping to make only one order (yeah, when has that ever happened?). I was also hopeful that I would be able to ID the culprit(s) and go to the source and replace them rather than a shotgun approach, but at this point that's probably the way to go.

I like the idea of tracing the oscillation backwards but I don't know enough theory to know which direction to go. I don't know if it follows the power supply voltage or the signal itself, and I can't follow it the way I am used to with a tube circuit. Maybe the feedback circuit you mention is confusing things for me.

There are some schematic errors on the 4140 so that is why we are using the one from the 4100. The power transistors are all NPN; they are 2SC897.

Thanks for the transistor tester and all the other tips. I do have a DBT.
 
You can check your resistors on the board. Compare same colored ones against each other.
If you find a resistor that reads higher than it should, its bad.
Most resistors drift high, not low.
You will get lower than spec readings on the ones that are parallel to something, but since you have good boards to compare the reading to, it shouldn't be a problem

You have a ground leg and two power legs. If these are stable, which they should be, any instability you find in the amp itself, relating to direct power feeds and grounds, would tell you that there is a crack in a track.
A solid wire cannot have one voltage on one end and a different or unstable voltage on the other end.
If you right down all you ebc voltages going from right to left, h614 - h 601 ,the path my lead you to the rotton apple.
Read again Catrafters post about transistor base bias. :thumbsup:
 
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