Help with DC on volume pot

[Mooly]

That probe I am using under the tip itself there is a metal ring, can that ring be isolated with tape?

For the ground can we use any part of the chassis, or should it be center-tap?

Alligator clip that goes to ground, can it be prolonged by another piece of wire so I have more maneuver space?

I was going to mention that when we were ready, honest

The metal ring is ground and if you have no attachments for the probe then it might be best covered for safety. If it touched anything with voltage on it, it would short that point to ground.

You can extend the ground lead for the frequencies we are dealing with.

Which ground to use, well for tiny signal measurements it has to be correct (to eliminate noise and ripple) so lets say around the "bottom" end of R516 as that is the preamp ground for the faulty channel. Maybe make a little loop of wire and solder it to the ground and connect the croc clip there, or solder a wire.

Then with the scope on 1V/div to begin touch the probe tip to the collector of Q506 or C518 (the output of the two stage preamp). You should see a straight line on the scope (no signal). Make sure the scope input switch is AC coupled (or else the trace would move off screen with the DC voltage present). If there is no signal then turn the scope setting (V/div) down toward the 2m (2 millivolt setting). You should still see "nothing" just a straight line showing no signal (or oscillation).

So lets see

 

[mkusan]

The horizontal axis is time.
Timebase uses values like 5ms/cm or 2us/cm (div = cm square)

The vertical axis is voltage
Voltage uses values like 2mV/cm or 5V/cm

The two Trim Pots next to the vertical adjustment knobs look like vertical (voltage) trimmers.
We need to try and find a horizontal trimmer, so that a 50Hz signal fits the correct number of div (cm squares).

Let's investigate what the vertical adjustments do.

Set your 1Vpp test signal height by using the switch setting of 1V/cm it should jump up and down 1 square. Set the switch to 2V/cm. It should now jump up and down by half a square.
set to 0.5V/cm, gives 2quares
set to 0.2V/cm, gives 5squares. You may have to adjust the position pots to keep the test signal on the screen.
If you set to 0.1V/cm, the square wave test signal should go at least partly off screen. You should have 8div (cm squares) in the vertical direction, which on the 0.1V/div gives a total range on the display of 800mVpp. Just a bit less than the 1V test signal.
The vertical position adjusters should be able to bring one of the horizontal voltage back on to screen.

Check out this part of the operation and come back and confirm you understand this and the scope does as expected.

Hi Andrew,
I did per your instructions and the scope does as expected. With 0.1V/cm setting the signal did go off screen and with vertical adjustment I was able to bring back only one horizontal line at a time back on the screen so I would say they are probably 8 and 1/4 squares apart. I took some pictures of the scope belly, after that I can only say we can be happy with the results we got.

 

[mkusan]

I was going to mention that when we were ready, honest

The metal ring is ground and if you have no attachments for the probe then it might be best covered for safety. If it touched anything with voltage on it, it would short that point to ground.

You can extend the ground lead for the frequencies we are dealing with.

Which ground to use, well for tiny signal measurements it has to be correct (to eliminate noise and ripple) so lets say around the "bottom" end of R516 as that is the preamp ground for the faulty channel. Maybe make a little loop of wire and solder it to the ground and connect the croc clip there, or solder a wire.

Then with the scope on 1V/div to begin touch the probe tip to the collector of Q506 or C518 (the output of the two stage preamp). You should see a straight line on the scope (no signal). Make sure the scope input switch is AC coupled (or else the trace would move off screen with the DC voltage present). If there is no signal then turn the scope setting (V/div) down toward the 2m (2 millivolt setting). You should still see "nothing" just a straight line showing no signal (or oscillation).

So lets see

I believe you

Remember that C506 hack you wanted me to do, since I already had a wire there going from ground of R516 I used that for alligator clip.

I soldered a piece of wire from C518 that went to the tip of positive probe of the scope, and later did the same for collector of Q506.

Both tests with 1V/div and 2mV/div gave straight line showing no signal.

If it sounds complicated, I have pictures

 

[Mooly]

Soldering wire on the probe... Eeek

Those results seem to show all is OK. (The amp was on wasn't it ? )

We'll start at the other end and work back. We know the VU meter shows some kind of reading so lets see what that is.

Now do this.
1. Leave the ground wire as it is.

2. Unsolder the wire from the probe tip.

3. Make sure the probe is still OK by checking the CAL signal again and that the 1 volt peak to peak signal shows OK.

4. Locate L601 and L602. These are the outputs of the main power amplifier that go through the relay and to the speaker sockets.

5. Set the scope to 1V/div initially.

6. Hold the probe carefully (no trembly fingers) and switch the amp on. The trace will jump as the amp powers up and settles.

7. Now touch the probe tip on L601 and then L602. Does anything other than a straight line (no signal) show up. You should be able to turn the scope V/cm setting toward the 2mv/div sensitivity and still get only a little "thickening" of the trace (noise).

Is that a friend helpng you (picture 2)

 

[mkusan]

Soldering wire on the probe... Eeek

Those results seem to show all is OK. (The amp was on wasn't it ? )

We'll start at the other end and work back. We know the VU meter shows some kind of reading so lets see what that is.

Now do this.
1. Leave the ground wire as it is.

2. Unsolder the wire from the probe tip.

3. Make sure the probe is still OK by checking the CAL signal again and that the 1 volt peak to peak signal shows OK.

4. Locate L601 and L602. These are the outputs of the main power amplifier that go through the relay and to the speaker sockets.

5. Set the scope to 1V/div initially.

6. Hold the probe carefully (no trembly fingers) and switch the amp on. The trace will jump as the amp powers up and settles.

7. Now touch the probe tip on L601 and then L602. Does anything other than a straight line (no signal) show up. You should be able to turn the scope V/cm setting toward the 2mv/div sensitivity and still get only a little "thickening" of the trace (noise).

Is that a friend helpng you (picture 2)

Hi Mooly,
I am eager, but not that "eager", English is not my native language so I can see why you misunderstood my doings. Pictures again.

I think I can skip steps 1,2 and 3 now.

We have to go out now, and it probably wont be until evening I can do those test, but as soon as I do I will report back.

 

[mkusan]

Is that a friend helpng you (picture 2)

Yes, that is a friend inspecting my work, feels safer that way

 

[Mooly]

OK

 

[mkusan]

Now do this.


4. Locate L601 and L602. These are the outputs of the main power amplifier that go through the relay and to the speaker sockets.

5. Set the scope to 1V/div initially.

6. Hold the probe carefully (no trembly fingers) and switch the amp on. The trace will jump as the amp powers up and settles.

7. Now touch the probe tip on L601 and then L602. Does anything other than a straight line (no signal) show up. You should be able to turn the scope V/cm setting toward the 2mv/div sensitivity and still get only a little "thickening" of the trace (noise).

At 1V/div line was straight at 2mv/div my wife started taking pictures

 

[Mooly]

That looks just like white noise (hiss) and a small 50Hz hum component at low amplitude.

Was the VU meter mis-behaving and showing a reading ?




Is C506 still connected to ground ?

 

[mkusan]

That looks just like white noise (hiss) and a small 50Hz hum component at low amplitude.

Was the VU meter mis-behaving and showing a reading ?


Is C506 still connected to ground ?

Actually no, I took pictures with amp off and on. Although when I try to rotate the volume pot with no signal I am getting a reading that corresponds to pot turning but only in left channel now.

C506 is still connected to ground of R516.

One thing I noticed yesterday is that with just ground probe connected I am getting a small reading on the scope when set to 2mV. I don't know if that is relevant, but I took a picture anyway.

 

[Mooly]

The small reading is normal. It can be caused by the scope forming a "ground loop" just like audio equipment does, or it could be the probe just picking up stray field (if it's left open and not connected). If you connect the probe to the ground (which is still connected to the amp) the signal should disappear but in practice may not.
It's nothing to worry about.

So the meters are behaving and we haven't seen anything suspect so far.

I think you have to put the amp back to normal now, reconnecting C506 etc and let us see what the state of play is.

The volume control should be essentially silent in use. On the scope on the 2/mv setting you may see a little disturbance at the amp output as it is rotated but remember that on 2mv the scope is very sensitive and showing every little disturbance.

It would be a big help to get the amp back to the state where the VU meter is showing this unwanted signal if that's possible.

 

[mkusan]

The small reading is normal. It can be caused by the scope forming a "ground loop" just like audio equipment does, or it could be the probe just picking up stray field (if it's left open and not connected). If you connect the probe to the ground (which is still connected to the amp) the signal should disappear but in practice may not.
It's nothing to worry about.

So the meters are behaving and we haven't seen anything suspect so far.

I think you have to put the amp back to normal now, reconnecting C506 etc and let us see what the state of play is.

The volume control should be essentially silent in use. On the scope on the 2/mv setting you may see a little disturbance at the amp output as it is rotated but remember that on 2mv the scope is very sensitive and showing every little disturbance.

It would be a big help to get the amp back to the state where the VU meter is showing this unwanted signal if that's possible.

Thank you Mooly,
I will reconnect C506 to its proper position.

Just to be clear, the reading while rotating the pot was visible on the left channel vu-meter, I was not referring to the scope reading.

Just out of curiosity, on a scale from 1-10, 1 being simplest how complicated do you think this problem is?

 

[Mooly]

Scale of 1 to 10
I'd say well off scale probably

We have had a lot of posts since the beginning so lets just confirm what problem/s we are chasing.

One specific fault that should be traceable is the VU meter reading,

"I left the amplifier turned on over night and in the morning the right vu-meter needle was closer to zero, imagine my vu-meters have a scale from 0 to 10, immediately after powering on my left channel is on 1 and right on 3. In the morning left was on 0 and right on 1, and this time needles were jumping up with volume up or down with volume down (without any input). Looks like what ever DC is there it was lower after the amplifier was on for eight hours.


Is this problem still present ? I read this to mean that the meter was showing a reading even if the volume was turned down (or up) and the control untouched.

The other issue is the noise of the switch.

Maybe it would be worth connecting it all back up and re testing with speakers again and confirming that the problems are still present.

The tests up to now haven't shown much amiss.

You could connect the scope across the speaker output and try and "see" on the scope how much of a click the switch causes. Try and see what the amplitude is. It helps to slow the scope timebase right down so the spot is moving more slowly.
You just connect the scope earth to the negative speaker output and probe tip to positive.

 

[mkusan]

Scale of 1 to 10
I'd say well off scale probably

We have had a lot of posts since the beginning so lets just confirm what problem/s we are chasing.

One specific fault that should be traceable is the VU meter reading,

"I left the amplifier turned on over night and in the morning the right vu-meter needle was closer to zero, imagine my vu-meters have a scale from 0 to 10, immediately after powering on my left channel is on 1 and right on 3. In the morning left was on 0 and right on 1, and this time needles were jumping up with volume up or down with volume down (without any input). Looks like what ever DC is there it was lower after the amplifier was on for eight hours.


Is this problem still present ? I read this to mean that the meter was showing a reading even if the volume was turned down (or up) and the control untouched.

The other issue is the noise of the switch.

Maybe it would be worth connecting it all back up and re testing with speakers again and confirming that the problems are still present.

The tests up to now haven't shown much amiss.

You could connect the scope across the speaker output and try and "see" on the scope how much of a click the switch causes. Try and see what the amplitude is. It helps to slow the scope timebase right down so the spot is moving more slowly.
You just connect the scope earth to the negative speaker output and probe tip to positive.

Off scale or on scale, we shall prevail. It even rhymes

C506 back in and the VU-meter reading is still present, I took pictures for you to see. I believe the reason why the needle did not zeroed in immediately after we removed C506 is because the amp was upside down all along. I will leave amplifier on overnight to see whether the readings will be lower in the morning. You understood right, no input and the control untouched and the reading is there.

Noise of the switch is still there, more so in the right channel, more so when the unit is colder. Louder pop (or thump) is heard while the switch is being depressed then while being pressed in.

I took AC reading with DMM, just ground probe on chassis gives me 3.99V, normal or not?

On which v/div setting should I do scope test, 1V?

Thanks
Marko

 

[Mooly]

Lets work on the VU meters first. Also is it possible that the VU meters are sticking ? Just trying to cover every posibility here. When you power up they probably flick up a little. If you tap the meters do they settle to a lower reading ?

We will prove if there is a problem or not

Diagram shows where to connect scope probe. We will use the correct ground for each meter circuit and look at the output of the opamps. The scope should show no AC signal. If you also measure with your DVM on DC volts there should be NO voltage across either C631 or C632.

Those two checks will prove conclusively whether the VU meter reading is caused by an AC signal, a DC problem around the opamp or the meter physically sticking.

The meter reading just probing the chassis is normal if the other meter lead is just floating around. It's just stray pickup.

Some other checks you can do.
"C506 back in". So the amp is all connected back up as original and ready to go ?

Connect scope across either R529 or R530 in the preamp (depending which channel you are looking at). Connect ground of scope to bottom end of resistor. Set the scope to 0.1V/div initially and the time to 5 or 10m S/div.

If you operate the filter switch can you see the scope jump and show a spike or click as the switch is operated ? Can you see what the amplitude is approximately? Also if you rotate the volume control can you see any disturbance. How big if so ?

 

[mkusan]

Lets work on the VU meters first. Also is it possible that the VU meters are sticking ? Just trying to cover every posibility here. When you power up they probably flick up a little. If you tap the meters do they settle to a lower reading ?

We will prove if there is a problem or not

Diagram shows where to connect scope probe. We will use the correct ground for each meter circuit and look at the output of the opamps. The scope should show no AC signal. If you also measure with your DVM on DC volts there should be NO voltage across either C631 or C632.

Those two checks will prove conclusively whether the VU meter reading is caused by an AC signal, a DC problem around the opamp or the meter physically sticking.

The meter reading just probing the chassis is normal if the other meter lead is just floating around. It's just stray pickup.

Some other checks you can do.
"C506 back in". So the amp is all connected back up as original and ready to go ?

Connect scope across either R529 or R530 in the preamp (depending which channel you are looking at). Connect ground of scope to bottom end of resistor. Set the scope to 0.1V/div initially and the time to 5 or 10m S/div.

If you operate the filter switch can you see the scope jump and show a spike or click as the switch is operated ? Can you see what the amplitude is approximately? Also if you rotate the volume control can you see any disturbance. How big if so ?

You are right, it is possible that VU-meters are sticking. Only left flickers when I power up, if I tap on plastic cover they settle. I played music observing meters and the right one is definitely lazier than the left.

I am getting AC reading on pin 7 and on pin 1 of op-amp to their respective grounds, see pictures. I am also getting DC reading of 1.1mV across C631 and across C632.

"C506 back in" means returned to factory default position We are ready to launch.

Scope across R530 with 5mV/div and time of 5 S/div:
pressing in 1/4 of a square up
depressing 2 squares down

Scope across R529 with 5mV/div and time of 5 S/div:
no visible jumping

If I rotate volume pot in both channels I can see what you described once commenting the picture I took as a white noise with the amplitude of about 2 squares with 5mV/div and time of 5 S/div scope setting.

 

[Mooly]

The meter measurements are OK although it does sound like they are sticking a little.
On that basis we have to say there is no electrical problem showing and that the meter "problem" is mechanical.

R529, the channel with no jumping. Is that channel silent when speakers are connected and the switch is operated ?

Now you have a scope it might be worth re checking some things.
If you remove C518 does the scope still jump when measuring across R530.

Does the position of the "High Filter" have any effect on the reading ?

 

[mkusan]

The meter measurements are OK although it does sound like they are sticking a little.
On that basis we have to say there is no electrical problem showing and that the meter "problem" is mechanical.

R529, the channel with no jumping. Is that channel silent when speakers are connected and the switch is operated ?

Now you have a scope it might be worth re checking some things.
If you remove C518 does the scope still jump when measuring across R530.

Does the position of the "High Filter" have any effect on the reading ?

Thanks Mooly,
I really appreciate the effort you are putting into this, and I apologize for not realizing myself that the needle is sticking thus perhaps misleading you

I will do those tests and get back as usual.

 

[Mooly]

No problem

It's quite a subtle problem to try and resolve really but we will try all we can.

 

[mkusan]

The meter measurements are OK although it does sound like they are sticking a little.
On that basis we have to say there is no electrical problem showing and that the meter "problem" is mechanical.

R529, the channel with no jumping. Is that channel silent when speakers are connected and the switch is operated ?

Now you have a scope it might be worth re checking some things.
If you remove C518 does the scope still jump when measuring across R530.

Does the position of the "High Filter" have any effect on the reading ?

R529, LEFT channel with no jumping is silent with speakers connected and the switch is operated.

With C518 out of circuit the scope still jumps measuring across R530, amplitude is smaller, but I am seeing a spike while depressing switch which I don't recall seeing yesterday.
I also measured across R530 with DVM now without C518, I am getting 00.1mV of DC, before it was 14mV if you remeber.

Position of "High Filter" does not have any effect on scope reading.