MCS 3847 Integrated Amp Noisy Left Channel

[KilgoreCemetery]

I'm more interested in the "net difference voltage" between the bases of TR209 and TR211 with noise.

I'm getting 2.375V. As the noise increases, the voltage drops. The voltage also seems to drop more and more over time.

I see what you mean about the residue around TR201. I removed it and did some cleaning before re-soldering it. The noise is still there though.

To me, the most interesting part right now is how loud the noise is on the Phono Input. Even when the output is dead silent, it's crazy loud. For most of this testing I've had the input set to Aux because when I started there was no difference.

 

[dorke]

Trying to put # and #41 together:

The voltage drop between the two bases.
TR210 and TR212 Without the noise: 2.41V
TR209 and TR211 Without the noise: 2.398V

If you're wanting the actual voltage.
TR210 and TR209 are around 1.2V
TR211 and TR212 are around -1.2V

With noise:
TR210 and TR212 don't change,
TR209 and TR211 move closer to zero 2.375 and dropping (to?)
I.E. 1.2V drops below 1 and -1.2 goes somewhere between -0.5 and -1.

TR201 is squeaky clean now.

Please repeat the DCV measurements with no input signal (volume at minimum)
on the 5 pins of TR201 vs. TR202 with and without noise. And the measurement of the "net voltage"(i.e. probes on both legs of the resistors)
on R211 vs. R212 ,R205 vs. R206 ,R207 vs.R208 with and without noise.

Do it also on the white big resistors R227(?) vs. R228(?) ,R229(?) vs. R330.
And the brown-grey ones R223 vs.R224,R225 vs. R226.




The phone issue seems non-related to the output noise issue.

 

[KilgoreCemetery]

Without noise, volume at zero, black probe on TR201 and red probe on TR202:
B1: 1.4mV
C1: -3.5mV
E: 10mV
C2: -3.5mV
B2: 1.4mV

And the measurement of the "net voltage"(i.e. probes on both legs of the resistors)
on R211 vs. R212 ,R205 vs. R206 ,R207 vs.R208 with and without noise.

Does this mean you want to know, say the difference in voltage between the southern leg of R211 and the southern leg of R212 or the difference between the southern and northern leg of the same resistor?

Here's the info for the resistors with no noise and volume at zero. Measurements are the difference between the northern and southern leg of the same resistor. If I need to go back and do it the other way, then I will.

R211: -14.8V
R212: -14.8V
R208: -1.13V
R206: -0.99V
R226: -0.61V
R224: 0.60V
R230: 19.4mV
R228: -18.9mV

This is as far as I got before I realized that you probably want it measured the other way

 

[dorke]

Measurements for TR201 and TR202 are with black probe on chassis ground and red probe on each of the legs.
That means voltage relative to chassis ground.

Measurements for the resistors are all with both probes on the resistor itself(difference between both legs of the same resistor).

Sorry if I was not clear enough before.

 

[73's de Edd]

Sir KilgoreCemetery . . . . .

A good read . . . . .

In my delving into and absolutely peeking into the minds of the N.E.C. design team at about the time frame of your units concept.
There were three shifts in different phases of developments.

There is your unit which is using discrete semiconductors in its circuits entirety.

Then the phase step up, which used an audio integrated circuit for the preamp section of the magnetic phono input .
Also there were the use of AF power output modules which were just minimally, being darlington arrangements of power transistors.

The final phase was where the whole Audio output stages were using self contained potted amplifiers akin to what we currently refer to as STK modules .

This info below was based upon NEC units of your time frame and a comparative examining of your units photos.

THE AUDIO FLOW PATH . . . .


I believe that you were BOTH listening for the origin of the "noise " source by signal TRACING with a separate audio amplifier and probing into suspect test points AND also just listening for the "noise " by listening to an attached speaker . . . it being the left one in this case . . .since that was the only channel passing the "noise ".
MY INTENT was for you to just listen for the "noise " with the units left channel speaker.
I do believe that you had the 47 ufd capacitor connected from the input of the left power amplifier and said that there was no "noise " output then. And I believe that if you had left it connected for an hour, that you would still not have heard a peep from that "noise ".
That electrolytic was effectively AC shorting that noise out to ground.
Soooooo . . .. the "noise " derivation / source must be in circuitry further to the LEFT in the audio flow path. I then directed you to the base of TR303.
BUT I think that you made the boo-boo at that point in listening with a probe.
You need to still listen for the "noise " at the speaker after shunting that base to ground with the 47 ufd.
If you heard the noise then, the TR303 or / and its circuitry is the suspect area of your noise sources origin.
If you did NOT hear the noise, then you need to discharge the cap and move it to the base of TR301.
NOW if you still hear the noise, the . . .TR301or / and its circuitry circuitry is the suspect area of your noise sources origin.
If you DON'T hear the "noise ", we be in some deeeep doo-doo, as the minimized volume control setting should have shut down any signal from passing thru it from the earlier 5 discrete transistors . . . .TR101-103-105-107 and 109 being used in the phono preamp stage.
Some amount of gain is going on in it . . .with it having to bring 5MV of input audio, on up to an audio line level input.

LOOK at the start of the post amplifier section and you will see that it has a direct flow path to the input of the power output amplifier. ALSO that means that a turning of the bass and treble controls WILL have an effect on the " tonal colorization " of the "noise" IF the TONE switch has them enabled, and not being in its OFF position, where the signal passes thru with an nonadjustable averaged FLAT response. You should also hear when the high filter is switched in.

LOOK at the audio path BEFORE the post amp circuitry back at its origin at the RCA jacks on the back of the amp.
If you have the FUNCTION switch in the TUNER or AUX inputs positions, the PHONO PREAMP circuitry is not being used and the TUNER or AUX input goes straight into the MODE SWITCH.
If you are in PHONO POSITION, the PRE AMP OUTPUT does get into the amp, down at the MODE SWITCH.
There are some mixing and dubbing features involved but they all get sorted out by the time that their outputs go into the MODE SWITCH.
Now at the Mode Switch, if the noise origination was within the phono pre amp you could switch to MONO and have the noise coming from both L and R channels.
Switch back to STEREO.
You could also, REALLY knock out any earlier left channel noise feed through, by switching the BALANCE to its extreme Right channel position.

Now if " youse guys" done been a' pokin' around in that AF output stage, do you realize that you do NOT now actually have it TOTALLY disconnected from the post amp stages with that mere lifting of the C201 electrolytic coupling capacitor.
You need to just lift and float the left YELLOW shielded wire coming into the amps input terminals.
If you do a vewy-vewy close foil path check, you will be finding that a .00027ufd cap is also shunting that C201, and still being in circuit and that can still pass quite a bit of the HIGH frequency spectrum of your Gaussian type of " noise " source.
Now . . go ye forth . . .and wear out 2 ufds of that that 47 ufd cap some more, at GREEN arrows A B and C to see if you can loose the " noise".

73's de Edd

.

 

[dorke]

Dear Edd,
The input circuit to the PA looks like below(drawing made from the PCB traces).

Removing C203(not C201 !) totally isolates the PA from the feeding circuit.
The test shows no noise at the input to the PA(at the right tip of R201),
with noise at the output of the PA (speaker) .
That absolutely means the noise is generated at the PA itself not before it !
look at #34 and #39.

 

[73's de Edd]

Gentlemen . . . . .

On my # 45 almost to the end . . . .correctly amend . . . ..
" with that mere lifting of the C201 electrolytic coupling capacitor."
Change C201 to being C203 electrolytic .

Ref au #34
Sir Dorke sez . . . . .

In order to verify which board is the source of the noise, I would do the following:
1. Remove C203 from the PA board(red).
(but . . . .BUT . . .BUT BUT BUT . . . the C201 which is shunting it, is still being left in the circuit. )
2. Listen to the noise both on the output and on the marked leg of R201(blue).
In case the noise appears only on the output , the PA board is the source.
In case the noise appears only on the leg of R201, the tone control is the source.

Ref au #39
Sir Kilgore Cemetery sez . . . .

Connecting the negative side of my signal checker to the negative side of the cap and the
positive side of the signal checker to the base of TR303. The noise is there, but it's significantly quieter.

Also, a new development, there is significant noise on the left channel where the Phono input
connects to the Preamp (I think) Board. The noise there is loud even when the the noise at the
speaker connects is quiet. The red arrow is the noisy pin. This wire goes straight to the Phono Input
RCA jack. I re-flowed the solder on the Left, Right, and Ground pins both to the circuit board and to
the wires with no change
Ergo . . . .
We do have significant noise back at the post amp (or preamp circuit ), in accordance with the SETTING of the units FUNCTION SWITCH, with it still being able to get INTO the POWER amp via the still circuit existent C201 cap.

NOW . . . .

Sir Kilgore Cemetery . . .
Don your surgical gloves and break out your enbalming tools, as we are now needing you to perform a circuit component analysis / autopsy , with the principal point of contention being the C201 plastic family of capacitor.
My eagle eyes *** see the C201 capacitor as shunting the now pulled C205 electrolytic, vice, it having one of its leads being grounded . Reference to the Yellow Brick Road path . . . . or dots.
Alternate . . . . . removal of the incoming audio from the Power amp via the shown wire being lifted, and then confirm the minutes, hours that the left channels speaker is then silenced of that experienced Gaussian noise.

Foil Path . . . . Being All Blowed Up . . . . .




73's de Edd

.

 

[dorke]

Edd,
There is a photo of the print side(#21),no need for eagle eyes...
a "blind bat" can trace it better

Lifting C203 was intentional (although pin 7 was indeed my first choice! )
1.Easier to do and replace,(square old pin and wire) and been done before on this board.
2.No need to isolate that floating wire if pin 7 would have been removed.

We are beyond that point,the PA alone is definitely the source of noise!

 

[KilgoreCemetery]

As a continuation from post #43. Volume at zero, no noise, comparing voltage from one leg to the other of the same resistor:
R228: 14.4mV
R230: 14.4mV

R227: 13.5mV
R229: 13.8mV

R223: 0.59mV
R225: 0.60mV

R224: 0.59mV
R226: 0.60mV

As for re-checking the same components WITH noise, the problem has become too sporadic to be able to get any sort of reliable reading. It comes and goes randomly. Sometimes it swells in volume over the course of 10-15 seconds, but most of the time it crackles for 1-2 seconds, stops, then does it again 5 minutes later.

 

[dorke]

Those intermittent problems...
Try measuring the above when the problem is present,that's the only way to pinpoint the problem.

 

[73's de Edd]

Sir KilgoreCemetery . . . . .

To me, the most interesting part right now is how loud the noise is on the Phono Input. Even when the output is dead silent, it's crazy loud.

Look back at my post #45 and the illustration of the signal flow path.
Assuming that you are now having the LEFT input isolated by C203 cap being of of circuit, it needs to be hooked in again.

All you have to do is switch back into PHONO input position and advance the volume up and if the noise origination back at where you suspicioned it . . . . . is present . . . . . it should be coming in like GANGBUSTERS . . . .with the volume control advanced.
I see in other photos that you have had both the shielded wires R and L inputs to the power amp inputs lifted . . .as well as their common center ground. My preference would be to leave the C203 in circuit and just lift the L input wire up and away from its normal L input pin.

Sometimes it swells in volume over the course of 10-15 seconds, but most of the time it crackles for 1-2 seconds, stops, then does it again 5 minutes later.

Listening back to your INITIAL sound sample being submitted, I was just hearing a continuous Gaussian type of noise, akin to a FM radio signal when tuned off from any station.
Your PRESENT situation sounds like what one might expect from a noisy / avalanching transistor in the output stages. That could sound akin to a snap, crackle or pop or a short frying type of noise.
A given:
AND if you had the L input wire lifted out of circuit, there would only be the few semis in the output stage attributable to this noise source.

Is this intermittent noise PRESENTLY loud enough, when listened for with the left channel speaker, for you to be ADDITIONALLY puttering around in the room, doing other things . . . .without you having to be hovering over the speaker to be able to hear it ?

73's de Edd

 

[KilgoreCemetery]

Honestly, the nature of the noise has changed so much since I started this thread that I think a new (better) video is in order.