Author Topic: MC Preamp no gain when DC Power Applied  (Read 3863 times)

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Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #25 on: May 31, 2023, 10:42:05 pm »
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.
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #26 on: May 31, 2023, 10:44:21 pm »
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
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #27 on: May 31, 2023, 10:46:36 pm »
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.  >:(
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #28 on: May 31, 2023, 11:13:01 pm »
After all of that where do we stand with this effort?  :-//
 

Online TimFox

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Re: MC Preamp no gain when DC Power Applied
« Reply #29 on: May 31, 2023, 11:23:20 pm »
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 122 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.
 
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Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #30 on: May 31, 2023, 11:31:06 pm »
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.
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #31 on: May 31, 2023, 11:33:13 pm »
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.
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #32 on: May 31, 2023, 11:35:21 pm »
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.
 

Offline JohnG

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Re: MC Preamp no gain when DC Power Applied
« Reply #33 on: June 01, 2023, 12:04:36 am »
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
"Reality is that which, when you quit believing in it, doesn't go away." Philip K. Dick (RIP).
 
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Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #34 on: June 01, 2023, 12:19:32 am »
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.
 

Offline vk6zgo

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Re: MC Preamp no gain when DC Power Applied
« Reply #35 on: June 01, 2023, 03:44:19 am »
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.
 
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Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #36 on: June 01, 2023, 03:18:21 pm »
I have checked the DCV at all transistors and the values are correct per the schematic.
 

Online TimFox

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Re: MC Preamp no gain when DC Power Applied
« Reply #37 on: June 01, 2023, 03:44:59 pm »
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.
 
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Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #38 on: June 01, 2023, 03:47:17 pm »
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?
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #39 on: June 01, 2023, 03:53:52 pm »
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!  :-DD
 

Online TimFox

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Re: MC Preamp no gain when DC Power Applied
« Reply #40 on: June 01, 2023, 03:55:07 pm »
Not shōchū?
 
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Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #41 on: June 01, 2023, 03:57:21 pm »
Not shōchū?
All is possible, and on Fridays they mixed in mushrooms.  :(
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #42 on: June 03, 2023, 05:39:16 pm »
Step by step signal flow
Garbage in = Garbage out
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #43 on: June 03, 2023, 05:57:36 pm »
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?
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #44 on: June 04, 2023, 07:38:13 pm »
Check this @#$!# out!  ??? ::)
 

Online TimFox

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Re: MC Preamp no gain when DC Power Applied
« Reply #45 on: June 04, 2023, 07:44:46 pm »
But, what results did you get when tracing the signal along the PCB?
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #46 on: June 04, 2023, 08:10:13 pm »
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
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #47 on: June 04, 2023, 08:43:29 pm »
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.
 

Offline pulsed_powerTopic starter

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Re: MC Preamp no gain when DC Power Applied
« Reply #48 on: June 07, 2023, 12:31:16 am »
What is the purpose of the Sleeve Shunt on this DC Power Plug?
 

Online TimFox

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Re: MC Preamp no gain when DC Power Applied
« Reply #49 on: June 07, 2023, 02:51:36 am »
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.
 


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