MC Preamp no gain when DC Power Applied

[pulsed_power]

With no B+
RI & LI = 75mv pp
RO & LO = 2v pp

With B+ and no change in signal value from signal generator
RI & LI = 3mv pp
RO & LO = 300mv pp

How can a amp circuit create gain with no B+ and then when B+ is applied the gain drops to such low levels?

[TimFox]

When asking about such an apparently absurd result, it behooves you to post more details about your measurement.
1.  How do you connect the generator to RI/LI?
2.  How do you measure the voltage at RI/LI?
3.  What frequency do you measure at, and is the output at the same frequency (i.e., no important noise or oscillation)?
4.  How do you measure the voltage at RO/LO?  Are your voltage measurements true differential, or does it ground LO?

[pulsed_power]

When asking about such an apparently absurd result, it behooves you to post more details about your measurement.

1.  How do you connect the generator to RI/LI? BNC to RCA to inputs
2.  How do you measure the voltage at RI/LI? Scope
3.  What frequency do you measure at, and is the output at the same frequency (i.e., no important noise or oscillation)? 1khz
4.  How do you measure the voltage at RO/LO?  Are your voltage measurements true differential, or does it ground LO? scope

[pulsed_power]

More photos

[pulsed_power]

With power on

[pulsed_power]

Input is shown on channel 1 TOP
output is shown on channel 2 Bottom

[pulsed_power]

75mv in and 2v out with no B+

[TimFox]

Is that a x10 probe measuring the output?
The screen-indicated scale on the CRT indicates the voltage with a x1 input, not having scaled with probe connection.
Therefore, your output may be 10 times higher than the value shown on the scope.

[pulsed_power]

Set on X1

[pulsed_power]

The only change taking place is the B+ being turned off and on

[pulsed_power]

The function of this board is to take the low output voltage of a Moving Coil to the level of a Moving Magnet Cartridge

[pulsed_power]

Spec sheet

[TimFox]

Those specs are incomplete.  What voltage gain do you expect (apparently flat over audio range, with RIAA in the following MM preamp)?
I don't know what is causing your apparent high gain with no DC applied, but I would keep looking for a problem.
Without a transformer, you cannot have a voltage gain > 1 with no DC applied.
For example, what are you using for a generator, what is its output impedance, what happens when you connect the generator directly to your probe without the circuit?
What do you see at the output of the amplifier with no generator, DC on and off?

By the way, using a grounded-base input circuit for a low-Z MC cartridge will work, but there is no improvement in noise performance over a grounded-emitter (normal) amplifier.
The equivalent input noise voltages of a CB and CE amplifier for a given transistor and operating point are equal.
Your circuit is essentially a feedback inverter, with a low-frequency gain determined by R467 and R463 (approximately), and has R467 (180\$\Omega\$) in series with the MC source, degrading its thermal noise.

[pulsed_power]

Those specs are incomplete.  What voltage gain do you expect (apparently flat over audio range, with RIAA in the following MM preamp)? MC is 300uV/10ohm, looks to me like the MM input is 3mV/50k
I don't know what is causing your apparent high gain with no DC applied, but I would keep looking for a problem.
Without a transformer, you cannot have a voltage gain > 1 with no DC applied. It is defying logic at all levels
For example, what are you using for a generator, what is its output impedance, what happens when you connect the generator directly to your probe without the circuit? During those measurements it was at 2v
What do you see at the output of the amplifier with no generator, DC on and off? Zero

By the way, using a grounded-base input circuit for a low-Z MC cartridge will work, but there is no improvement in noise performance over a grounded-emitter (normal) amplifier.
The equivalent input noise voltages of a CB and CE amplifier for a given transistor and operating point are equal.
Your circuit is essentially a feedback inverter, with a low-frequency gain determined by R467 and R463 (approximately), and has R467 (180Ω
) in series with the MC source, degrading its thermal noise. I wish I understood that in greater detail

[TimFox]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.
Your reported 75 mV is roughly explained by the resistors R467 = 3900\$\Omega\$ in series with R463 = 180\$\Omega\$ feeding R461 = 220\$\Omega\$
(3900+180+220)/(220) = 19.6;  2000 mV / 19.6 = 102 mV
Of course, with the power off the other impedances/resistances around that network are not well determined, but that ratio looks suspiciously close to me.

By the way, your photo obscures the output impedance label on the generator panel:  is it 600 or 50\$\Omega\$ , or something else?
600\$\Omega\$ will make a difference, given the low input impedance of your amplifier (roughly 100\$\Omega\$).

[pulsed_power]

I have thought about that

[pulsed_power]

Internal wiring photo

[wasedadoc]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.

I agree.

On the 6 pin connector the two outer pins are outputs inputs, the two centre are power. The remaining two are inputs outputs.

Edit: Corrected after looking at the photos using a desktop PC with large screen instead of a diminutive phone. 

[pulsed_power]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.

I agree.

On the 6 pin connector the two outer pins are outputs, the two centre are power. The remaining two are inputs.

This is where a sketch is worth a thousand words.

[TimFox]

If you think you have connected the generator to the input properly:  with the power off,  take the probe (from the other CRO input) and move it to the circuit board and see what's on the junction of R461 and C461.

[TimFox]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.

I agree.

On the 6 pin connector the two outer pins are outputs, the two centre are power. The remaining two are inputs.

On the conductor side of the circuit board, the outer pins are labeled LI and RI (inputs) and the intermediate pins are labeled LO and RO (outputs).
Tracing the circuit to see if those labels are correct is left as an exercise for the reader.

[pulsed_power]

R461 doesn't junction to C461

[TimFox]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have better documentation?
If not, you have to trace the circuit yourself.

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have good documentation?

Those damn Yamaha schematics are very inconsistent from page to page. I have the high res version from Analog Alley.

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have better documentation?
If not, you have to trace the circuit yourself.

Please also see reply #15

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have good documentation?

Those damn Yamaha schematics are very inconsistent from page to page. I have the high res version from Analog Alley.

The Analog Alley files are the best available anywhere. Believe me I have tried.

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have good documentation?

Those damn Yamaha schematics are very inconsistent from page to page. I have the high res version from Analog Alley.

The Analog Alley files are the best available anywhere. Believe me I have tried.

I have tried to attach the PDF file here but the forum won't accept it. File size

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have good documentation?

Those damn Yamaha schematics are very inconsistent from page to page. I have the high res version from Analog Alley.

The Analog Alley files are the best available anywhere. Believe me I have tried.

I have tried to attach the PDF file here but the forum won't accept it. File size

An additional agrivation on this forum is it will not accept a Screenshot. 

[pulsed_power]

After all of that where do we stand with this effort? 

[TimFox]

OK--you can't show me any decent, self-consistent, or believable documentation on the layout and schematic.
Therefore, you must try troubleshooting it yourself, following what you can see of the circuit.
I suggest you connect your generator to what you believe to be "LI" = left input, and also to Ch1 of the CRO. MCAmp.pdf (17.34 kB - downloaded 123 times.) , and start with power off.
Take your x1 probe (or x10) and start with where you think LI is on the PCB, then go to the emitter of the input transistor.
You should see almost the same thing when you probe those two points on the PCB.
If you pass that test, then you can turn the input level down to a reasonable level for this amplifier, apply DC power, and probe from that point to further points down the two-transistor amplifier chain (CRO AC-coupled).
That would be a good start.
I contend, given the information given me, that you have accidentally connected the generator to an output of the amplifier.

[pulsed_power]

OK--you can't show me any decent, self-consistent, or believable documentation on the layout and schematic.
Therefore, you must try troubleshooting it yourself, following what you can see of the circuit.
I suggest you connect your generator to what you believe to be "LI" = left input, and also to Ch1 of the CRO. (Attachment Link) , and start with power off.
Take your x1 probe (or x10) and start with where you think LI is on the PCB, then go to the emitter of the input transistor.
You should see almost the same thing when you probe those two points on the PCB.
If you pass that test, then you can turn the input level down to a reasonable level for this amplifier, apply DC power, and probe from that point to further points down the two-transistor amplifier chain (CRO AC-coupled).
That would be a good start.
I contend, given the information given me, that you have accidentally connected the generator to an output of the amplifier.

I have tried to give you everything at my disposal, please see comments above.
As you have recommended I will dig deeper with the Scope tomorrow.
I don't have the signal generator connected to an output, please see my posted photos.

[pulsed_power]

OK--you can't show me any decent, self-consistent, or believable documentation on the layout and schematic.
Therefore, you must try troubleshooting it yourself, following what you can see of the circuit.
I suggest you connect your generator to what you believe to be "LI" = left input, and also to Ch1 of the CRO. (Attachment Link) , and start with power off.
Take your x1 probe (or x10) and start with where you think LI is on the PCB, then go to the emitter of the input transistor.
You should see almost the same thing when you probe those two points on the PCB.
If you pass that test, then you can turn the input level down to a reasonable level for this amplifier, apply DC power, and probe from that point to further points down the two-transistor amplifier chain (CRO AC-coupled).
That would be a good start.
I contend, given the information given me, that you have accidentally connected the generator to an output of the amplifier.

I have tried to give you everything at my disposal, please see comments above.
As you have recommended I will dig deeper with the Scope tomorrow.
I don't have the signal generator connected to an output, please see my posted photos.

I have turned the signal generator output down to the noise floor and the input vs output stays linear.

[pulsed_power]

OK--you can't show me any decent, self-consistent, or believable documentation on the layout and schematic.
Therefore, you must try troubleshooting it yourself, following what you can see of the circuit.
I suggest you connect your generator to what you believe to be "LI" = left input, and also to Ch1 of the CRO. (Attachment Link) , and start with power off.
Take your x1 probe (or x10) and start with where you think LI is on the PCB, then go to the emitter of the input transistor.
You should see almost the same thing when you probe those two points on the PCB.
If you pass that test, then you can turn the input level down to a reasonable level for this amplifier, apply DC power, and probe from that point to further points down the two-transistor amplifier chain (CRO AC-coupled).
That would be a good start.
I contend, given the information given me, that you have accidentally connected the generator to an output of the amplifier.

I have tried to give you everything at my disposal, please see comments above.
As you have recommended I will dig deeper with the Scope tomorrow.
I don't have the signal generator connected to an output, please see my posted photos.

I have turned the signal generator output down to the noise floor and the input vs output stays linear.

One more interesting fact is I have two of these MC boards, therefore 4 channels and they all act exactly the same.

[JohnG]

Here are some random things you can try, just for troubleshooting:

1) Connect both probes to the same point on the circuit at the same time. Make sure they measure the same.
2) Without B+, but with the applied signal, measure the voltage at B+ to make sure you are not rectifying any input (long shot, very unlikely, but hey, it is possible to rectify the input signal and get DC power).
3) When B+ is off, disconnect from the power supply and connect the B+ to ground with a 100 ohm or smaller resistor, then redo the measurement and see if anything has changed.
4) Send a photograph of the probes to show exactly how they are connected to the PCB when making the measurement. Are they grounded properly, etc?

John

[pulsed_power]

Here are some random things you can try, just for troubleshooting:

1) Connect both probes to the same point on the circuit at the same time. Make sure they measure the same.
2) Without B+, but with the applied signal, measure the voltage at B+ to make sure you are not rectifying any input (long shot, very unlikely, but hey, it is possible to rectify the input signal and get DC power).
3) When B+ is off, disconnect from the power supply and connect the B+ to ground with a 100 ohm or smaller resistor, then redo the measurement and see if anything has changed.
4) Send a photograph of the probes to show exactly how they are connected to the PCB when making the measurement. Are they grounded properly, etc?

John

This unit does have some very strange ground loops and is very critical to these grounds. I have chased it through all boards with great difficulty. This MC has been removed from the unit and is running external at this time.

[vk6zgo]

With no B+
RI & LI = 75mv pp
RO & LO = 2v pp

With B+ and no change in signal value from signal generator
RI & LI = 3mv pp
RO & LO = 300mv pp

How can a amp circuit create gain with no B+ and then when B+ is applied the gain drops to such low levels?

As the circuit shown is really two separate preamps, one for the left channel & the other for the right channel, with only the "common" rail in common, it would seem unlikely that both channels have the same problem.
This increases the likelihood that it is a testing error.

[pulsed_power]

I have checked the DCV at all transistors and the values are correct per the schematic.

[TimFox]

As I stated above, you need to trace the signal from the input to the output with the same probe.
DC voltages only indicate that your DC power connection is correct.
Since the documentation available to you (that you shared here) is woefully inconsistent, there is a limit to what others can diagnose remotely.
Specifically, the schematic is inconsistent with the PCB layout drawing, which in turn has duplicate part references that do not correspond to the circuit diagram.
Unfortunately, that means that you have to trace the circuit yourself in order to get reliable documentation, and then trace what is actually happening.
Rule of thumb:  when the apparent results are obviously physically impossible, start by verifying the actual connections.

[pulsed_power]

1) Connect both probes to the same point on the circuit at the same time. Make sure they measure the same.
2) Without B+, but with the applied signal, measure the voltage at B+ to make sure you are not rectifying any input (long shot, very unlikely, but hey, it is possible to rectify the input signal and get DC power). Measures 0.00vDC
3) When B+ is off, disconnect from the power supply and connect the B+ to ground with a 100 ohm or smaller resistor, then redo the measurement and see if anything has changed. 0.00vDC
4) Send a photograph of the probes to show exactly how they are connected to the PCB when making the measurement. Are they grounded properly, etc?

[pulsed_power]

As I stated above, you need to trace the signal from the input to the output with the same probe.
DC voltages only indicate that your DC power connection is correct.
Since the documentation available to you (that you shared here) is woefully inconsistent, there is a limit to what others can diagnose remotely.
Specifically, the schematic is inconsistent with the PCB layout drawing, which in turn has duplicate part references that do not correspond to the circuit diagram.
Unfortunately, that means that you have to trace the circuit yourself in order to get reliable documentation, and then trace what is actually happening.
Rule of thumb:  when the apparent results are obviously physically impossible, start by verifying the actual connections.

All is extremely well understood and appreciated. My common humor says that the Yamaha Documentation Team was constantly under the influence of Sake! 

[TimFox]

Not shōchū?

[pulsed_power]

Not shōchū?

All is possible, and on Fridays they mixed in mushrooms. 

[pulsed_power]

Step by step signal flow
Garbage in = Garbage out

[pulsed_power]

Here are some random things you can try, just for troubleshooting:

1) Connect both probes to the same point on the circuit at the same time. Make sure they measure the same.
2) Without B+, but with the applied signal, measure the voltage at B+ to make sure you are not rectifying any input (long shot, very unlikely, but hey, it is possible to rectify the input signal and get DC power).
3) When B+ is off, disconnect from the power supply and connect the B+ to ground with a 100 ohm or smaller resistor, then redo the measurement and see if anything has changed.
4) Send a photograph of the probes to show exactly how they are connected to the PCB when making the measurement. Are they grounded properly, etc?

John

Any additional thoughts based upon my reply to you?

[pulsed_power]

Check this @#$!# out! 

[TimFox]

But, what results did you get when tracing the signal along the PCB?

[pulsed_power]

But, what results did you get when tracing the signal along the PCB?

You were correct all along, I had signal + connected to signal - and vice versus

[pulsed_power]

I have now lost B+ in the Main Amp, I will need to track it down.

I did connect the MC Amp B+ to a 50vdc power supply and it works fine.

[pulsed_power]

What is the purpose of the Sleeve Shunt on this DC Power Plug?

[TimFox]

It's a switch that disconnects when the mating plug is inserted.
These were common on 1/4" 'phone plugs, often used to short an input when disconnected on a guitar amplifier.

[pulsed_power]

I mounted one one the MC Amp Box and another one on the Integrated Amp to get the power to run the MC Amp. If this is shorting the +vdc at the Integrated Amp than that is not good, will blow the Power Supply. I am not getting voltage to the MC Amp.

[pulsed_power]

I am using a two conductor cable with connection from Center Pin to Center Pin and Sleeve to Sleeve. Nothing connected to the Sleeve Shunt. No +vdc at the MC Amp.

[Audiorepair]

What is the purpose of the Sleeve Shunt on this DC Power Plug?

These are commonly used when the unit can either be powered from an external source or an internal battery.
Inserting the DC plug disconnects the battery leaving the power coming from the plug instead.

[pulsed_power]

What is the purpose of the Sleeve Shunt on this DC Power Plug?

These are commonly used when the unit can either be powered from an external source or an internal battery.
Inserting the DC plug disconnects the battery leaving the power coming from the plug instead.

Wow! Now that makes good sense, Thank You for the clear explanation. I was also thinking the Center Pin was for +v, with that configuration it appears the +v should be on the Sleeve?

[wasedadoc]

What is the purpose of the Sleeve Shunt on this DC Power Plug?

These are commonly used when the unit can either be powered from an external source or an internal battery.
Inserting the DC plug disconnects the battery leaving the power coming from the plug instead.

Wow! Now that makes good sense, Thank You for the clear explanation. I was also thinking the Center Pin was for +v, with that configuration it appears the +v should be on the Sleeve?

Not necessarily. Could be switching negative side.

[pulsed_power]

But, what results did you get when tracing the signal along the PCB?

Thank you for all your assistance. As stated before I did in fact find the input/output cause for failure. I have also corrected the power plug issue and all works very well. Input 20mv = Output 400mv