MCS 3847 Integrated Amp Noisy Left Channel

[KilgoreCemetery]

The noise has been very intermittent this morning. An average of 20 seconds on and 20 seconds off probably isn't too far from the truth.

Dorke: While the noise is gone, I get near identical voltages when comparing the A798s. When the noise is present, all four collector pins have about -36.5VDC. The voltage for the base pins do differ quite a bit from one transistor to the other, however. On the right one, I get 22mV, but the left transistor fluctuates in the -500mV range. Emitter pin on the right transistor is at 610mV, left one fluctuates in the 300mV range.

At this time, I can't get the noise to return, but I did include the pictures you requested just in case. I tried to get as little glare as possible, but this was very difficult since the bottom of the case is quite reflective. If it's not clear enough, I may have to try something else.

Edd: I happen to have a brand new Nichicon 47uF capacitor that I was able to connect between Yellow A and the ground next to it. At first, the noise vanished completely, but I wanted some confirmation so I played around with the knobs/switches. It eventually came back while I was messing with the Mono/Stereo knob. Then it vanished, and came back, etc..

Right now, even with the 47uF cap removed I can't get the noise to come back by messing with the knobs/switches. I don't know if it was the Mono/Stereo knob all along or just the vibration and luck.

When I first started diagnosing the noise problem, I sprayed all the knobs and switches with Deoxit hoping that was all it needed, but no amount of twisting, flipping, or bopping made any difference. If I "listen in" at the relay, for example, I can turn the Speaker knob from A to B to A+B to Off, and there is no change in the noise whatsoever. Same goes with Bass, Treble, Balance, Volume, Input, Tape Dubbing/Monitor, etc.. I tried them all with no variance in the noise at all.

There aren't any IC's anywhere in this amp. I checked the pink area specifically and found none on either top or bottom level.

I'll be testing this unit on and off all day to see if the noise returns.

 

[dorke]

I would rather catch that noise,and pin point it.

But looking at the bottom side of the board the area of the bad channel looks much dirtier than that of the good one.
All that dirt should be removed with rubbing alcohol and cotton buds.
Cleaning alone can cause the noise to vanish for good.
Look at the red areas,you get the point.In blue is the Tr.
Clean the board well ,and post a photo of the clean board.

Look at the green arrow as well,could be a bad solder.

 

[KilgoreCemetery]

I'd much rather pinpoint the problem too, but I'm not sure how to make the symptoms reappear. I've messed with all the knobs and switches and used a plastic tool to tap and prod components inside the unit. So far the noise is still absent.

I did clean up the board as you requested, Dorke, and included pictures. The potentially bad solder joint is not my best work, but that was from when I lifted the legs on the left channel's transistors. I removed the old solder and flowed in new, but the noise persisted whether those transistors were removed or not. That step was actually done prior to starting this thread, I believe.

Now that I'm comparing the before and after pictures from cleaning, I can see what you mean about it looking dirty, but looking at the board directly makes me think that this is more of an indication of the limits of my camera than an actual dirty board.

One of these days, I'll have to show you some before and after pictures of this JVC receiver that I cleaned up. Night and day difference. Took forever, but I still use it from time to time.

 

[KilgoreCemetery]

After several hours of listening, the static has returned in spurts. *sigh*

 

[dorke]

Well,
Please measure (with volume knob at minimum !!!) the DC voltages relative to chassis GND on the 5 pins Trs
Tr202 vs. Tr201 without disturbing anything.
Start with Tr202(the good channel).
While measuring, listen to the noise,if it disappears when you touch something,it is important to note what was the point you touched,it may be an important clue.

The board looks much better(pink area).,
but still needs cleaning(yellow etc.),only after we can catch that bug...!

Your soldering skills need improvement(Red).for that take your time going through this.

 

[KilgoreCemetery]

I've never claimed to be a professional with a soldering iron. I just wanted to lift the legs and put them back. The problem with this unit existed long before I even warmed up my iron, though.

Right now, I can't get the noise to go away, but here are the readings I was able to get from TR202 and TR201

TR202
B: 21.2mV
C: -36.85V
E: 614mV
C: -37.0V
B: 23.2mV

TR201 (with noise)
B: 360mV to 560mV depending on the amount of noise
C: -36.6V
E: Constantly changing faster than my DMM can keep up with
C: -36.9V
B: 550mV to 625mV depending on the amount of noise

 

[KilgoreCemetery]

Noise stopped suddenly while I was across the room.

TR201 (without noise)
B: 18.0mV
C: -36.8V
E: 599mV
C: -36.9V
B: 21.4mV

I think it's time to start thinking outside of the box. I know that things like ceramic caps rarely fail, but I feel like I've already replaced the stuff that commonly fails.

Could a different type of capacitor or a failing resistor cause something like this? maybe a diode going bad?

 

[KilgoreCemetery]

I've cleaned up the Mono/Stereo pot and sprayed deoxit on the volume and loudness controls again since it seems like the noise returns while messing with the controls on that daughter board. I even went so far as to reflow all the solder joints on it just in case there was a bad one that I missed

Noise is still hit and miss.

I removed the Left, Right, and Ground wires from the amp and was still able to hear noise from the speaker terminals.

However, I can also hear a quieter version of the noise from the Left, Right, AND Ground wires I disconnected.

Is this a power issue?

 

[KilgoreCemetery]

It has occurred to me that maybe I simply didn't fix the problem by replacing the electrolytics because I followed in someone else's footsteps. I didn't have a schematic (still don't), but there was one oddity during the recap process. C126 and C125 had different values on a board that is otherwise mirrored. They both were rated for 25V, but C126 had a 220uF cap and C125 had a 47uF cap.

It seemed odd at the time, but without a schematic to compare to, I just went with it. Could the difference in capacitors potentially cause issues like this? If one of them is wrong, how do I know which one?

I suppose I could try matching first one value, then the other and see if it fixes the noise...

 

[KilgoreCemetery]

I've now tried 47uF caps in both spots and got the noise, then 220uF caps in both spots and still have the noise.

 

[dorke]

Well,
The "guides" for old/vintage audio restoration would say "replace all electrolytic capacitors",needed or not,that doesn't usually solve problems,it is more of a preventative thing.

No, I don't think it is a power issue simply because most power is common to both channels.
Some of the possibilities may be a grounding issue,a bias issue,a noisy device.and more.

Please point out C125 and C126 on the PCB,note that there are parts not belonging to the PA channels.
I wouldn't touch/swap anything blindly, it is mostly a waste of time and effort and may add more problems.

Please match the measurements on #26, #27 to the blue 5 pin Tr markings I have put on #25 this is crucial!

Another important thing : please positively verify that the measurements were all taken with the volume knob at minimum.
The volume is in most cases the last point before the signal leaves the pre-amp /tone control circuit to the PA board,please verify it is so in this case as well(physically).


More measurements:
Please measure the DCV between both bases of TR210 and it's neighbor (TR212?)
And the same between the bases of TR209 and it's neighbor(TR211?)
The markings are not seen on your photo,apart from TR210.
With and without noise.

 

[73's de Edd]

Sir KilgoreCemetery . . . . .

You come up with 1XX series numbered parts as C126 and C125 caps, but we have only been shown the C2XX series of caps being on the PA board via the photos.
If it is like other circuitry the C125 will be related to the Left channel, and part C126 for the Right.

I have already figured out the other boards numbering series, so I might relate these C1XX parts as to either being located within the units post or preamp boards circuitry.

Check for me and confirm if one of the C125 cap leads goes to the emitters of two 2SA762F's AND ALSO a 100K resistor.
The other C125 cap lead goes to a 2.2K resistor, PLUS a 240K resistor and PLUS a 47 picofarad capacitor.

Also can you confirm that my RED RECTANGULAR box in the photo below is using 3XX numbering of its parts and if the BLUE square marked transistors might be 2SA872A's and 2C1775A's

Also there are two BLUE circle marked transistors, 3/4 down on the right center of the looong BLUE rectangle , what Txx numbering has been given to those transistors?

https://s8.postimg.org/mtjdycjw3/MCS3847_Integ_Amp.png

OVERVIEW . . . . . . .
With your info given on posts #26 and #27, with that hefty of an incoming level at the very input of the power amplifier I believe that the origin of this source of noise is going to be within the RED rectangular, post amplifier, if you confirm my 2SA872A's and 2C1775A's presences above. . . . and 3XX parts numbering used on that board.

73's de Edd

 

[KilgoreCemetery]

oops, I forgot to upload the picture of C125/C126, but here it is now. If it helps, it's sandwiched between the board we've been looking at and the front panel.



Dorke: I have been keeping the volume knob at zero at all times. Also, here are the updated voltages for TR201 and TR202:

TR202
B2: 21.2mV
C2: -36.85V
E: 614mV
C1: -37.0V
B1: 23.2mV

TR201 (with noise)
B2: 360mV to 560mV depending on the amount of noise
C2: -36.6V
E: Constantly changing faster than my DMM can keep up with
C1: -36.9V
B1: 550mV to 625mV depending on the amount of noise

TR201 (without noise)
B2: 18.0mV
C2: -36.8V
E: 599mV
C1: -36.9V
B1: 21.4mV

I don't think the volume control is the last in line before the pre-amp. There are three bundles (with three wires each) going from the circuit board with the volume pot to the board above it with the tone controls. Let's call them the Volume Board and Tone Board to help keep things straight. Then, another bundle goes from the Tone Board to the PA board and that's the one we've seen previously. I'll try to label them in the pictures for clarity. The red arrows indicate where the tone board connects to the PA board.

Volume Board:


Tone Board (arrows indicate connection):

More measurements:
Please measure the DCV between both bases of TR210 and it's neighbor (TR212?)
And the same between the bases of TR209 and it's neighbor(TR211?)
The markings are not seen on your photo,apart from TR210.
With and without noise.

I'll be providing this info soon with a new reply. Getting the noise to start and stop on cue is currently impossible.

Edd: There are, in fact, two transistors per side labeled 726 S71F. The positive side of C126 runs through a 100K resistor and to the emitter side of both transistors.

C125 travels a little bit of a different path and is a lot harder to trace. The positive side leads around the outside of the board and right down the middle. There is a 2.2K resistor physically close by, but not directly connected. I have yet to find a 240K resistor or a 47 picofarad cap.

The red section is using 1XX numbering, while the Tone and Volume boards both use 3XX.

I'm not seeing the BLUE squares on your picture, but am thinking red ones instead? I'll update some of this on the picture. I think once you get a better picture of this board, it will make more sense. I'm guessing the actual symmetry is rotated 90 degrees from what you're envisioning.



The BLUE circle marked transistors are TR304 (left) and TR302 (right). They read C1775 and A872 respectively.

 

[dorke]

O.K.
So this unit most probably doesn't have the Volume pot just before the P.A(like most do).
In this case it is clear that the tone control section , which follows the volume pot ,
may also be a possible source for the noise.

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).
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.

 

[73's de Edd]

Sir KilgoreCemetery . . . . .


I'll be providing this info soon with a new reply. Getting the noise to start and stop on cue is currently impossible.

Back when you had the noise presence on the left channel and used the 47 ufd cap from the input of the PA and effectively AC shunted all of that audio signal to ground. Whereupon . . . . you said that the noise was then being gone, that told me that the AF output section was not the ORIGIN of the noise.

Now after my " reading " of the board, for ascertaining the audio signal flow path, the next place to check would be the base of post amplifier Tr303 . .a 2SC1775A I believe . .. ground the negative lead of old faithful 47 ufd with a clip led and probe its positive lead to the base of Tr303 and expect the initial plop.
THEN . . . . if there is the ABSENCE of that noise signal, the noise origin is further towards the FRONT of the audio chain . . .e.g . . .Tr301 in this circuitry's case.
If you HAVE the noise, then we need to seek out from that point towards the AF outputs input.

I can SEE from the shielded wires and foil paths, that the signal passes thru the Tone-Treble and Bass controls and then the High filter and Low filter before entering the AF output terminals.

So, if our noise is originating at Tr303 or Tr301 and IF you have the tone controls switched into circuit and being active with the TONE . .ON-OFF switch . . .being in its ON position. . You should be able to detect the change in that noises " tonal colorization " as you adjust the bass and treble controls to their extremes. . . . and also . . . the High filter switch says . . me too !
Low filter . . . will have NO or MINIMAL effect.

NOW if you lost the " NOISE " by the AC grounding of the base of Tr303, you need to lift the caps + lead and ground it to DC discharge the cap completely and move the + cap lead to the base of Tr301, expect the plop, and then see if that makes the " noise " disappear, if so it looks like Tr301 and Tr303 are not being involved. . . . .. but just passing the noise on thru themselves . . . .we will then need to move to the pre- amp board.

Standing by to see what you find.

73's de Edd

 

[KilgoreCemetery]

More measurements:
Please measure the DCV between both bases of TR210 and it's neighbor (TR212?)
And the same between the bases of TR209 and it's neighbor(TR211?)
The markings are not seen on your photo,apart from TR210.
With and without noise.

I'm assuming that you want to know 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

The noise has become more of a start and stop sputter, so getting any readings while it's acting is tricky. When it's happening, TR210 and TR212 don't change, but the voltages of TR209 and TR211 move closer to zero. I.E. 1.2V drops below 1 and -1.2 goes somewhere between -0.5 and -1. The change is faster than what my multimeter updates at.

 

[73's de Edd]

Sir Kilgore Cemetary . . .


The noise has become more of a start and stop sputter, so getting any readings while it's acting is

In my test situation above, the amp and left speaker only have to be listened to for the typical time that the noise has been recurring. In which case you would want to temporarily solder tack the cap in place. WITH one caveat of NOT letting any soldering iron heat be applied to those transistor base leads. Instead track down the SAME connection point shared by a co joined components lead or foil path. No heat then reaches your transistors lead.
Because ?
In semiconductors, that are being noisy, sometimes thermal shifts . . .hotter or cooler . . can speed up or slow down their undesired effects.
Possibly even showing a loss / or / temporarily loss of their internal noise generation.
So,if you start at that first stage and then do not hear the noise again within the typically experienced rates of recurrence, or hours. Then the noise must be sourced from yet an earlier stage.

 

[dorke]

I'm assuming that you want to know 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

The noise has become more of a start and stop sputter, so getting any readings while it's acting is tricky. When it's happening, TR210 and TR212 don't change, but the voltages of TR209 and TR211 move closer to zero. I.E. 1.2V drops below 1 and -1.2 goes somewhere between -0.5 and -1. The change is faster than what my multimeter updates at.

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


Please remove C203 as per post #34,
what happens?

 

[KilgoreCemetery]

Sorry for the delay with the replies. The holiday season has really upturned my normal schedule. Hopefully, I can get back on track very soon.

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).
2. Listen to the noise both on the output and on the marked leg of R201(blue).

We have a result! No noise on R201, but there was an avalanche of noise on the output. I re-soldered C203 for the next tests.

Now after my " reading " of the board, for ascertaining the audio signal flow path, the next place to check would be the base of post amplifier Tr303 . .a 2SC1775A I believe . .. ground the negative lead of old faithful 47 ufd with a clip led and probe its positive lead to the base of Tr303 and expect the initial plop.

TR303 is, in fact, a C1775. With the 47ufd cap, I'm assuming we're talking about the mismatched C125. I connected 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

 

[dorke]

"We have a result! No noise on R201, but there was an avalanche of noise on the output."
So the noise definitely comes only from the PA, not from anything before it !!!

Looking at TR201 there is something suspicious around it.
Look at the REd area there is a "yellowish residue" area and what looks like light blue corrosion near E.
You should inspect that ,underneath the black TR body it should be clean.
If it isn't gently remove the TR and clean it well as well as the board area.
If possible please post a closeup photo of the location board and the bottom of the TR body itself.

Still waiting for the bases measurement with noise, as per #38.