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Memphis 16-MC1000D Ground Short (Newbie 1st Post)
Memphis 16-MC1000D Ground Short (Newbie 1st Post)
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Old 13th October 2021, 07:04 PM   #1
williokeef is offline williokeef  United States
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Join Date: Jul 2021
Default Memphis 16-MC1000D Ground Short (Newbie 1st Post)

I'll start off by mentioning this is my first attempt at repairing an amp.

Much of what I've learned has been from Perry's free tutorial and watching some videos by BareVids. I'm not completely versed in all the "lingo" but I have no issue stepping back and doing some research to understand better.


The amp in question was going into "protect" and immediately shutting down.
I tracked down the issue to a faulty repair by a tech years ago. In which he switched the order of the op-fets/transistor? They placed the IRF640's on both banks at the front and end of the bank. ie: (640,9640...9640,640)

I removed all the op-fets as they were shorted and replaced all the power transistors.
After this I was able to power up the amp without going into protect using both 9V/12V. I then began to check the rail voltage for each back without the op-fets and drivers. I was reading with both a "budget" oscilloscope and a DMM.

With oscilloscope: +53.54V and -47.86V
DMM: +56.5V and -56.5V
On both banks

This is where my issue starts. While checking the voltage on the TL494CN pins I managed to get a reading from pins 6-10 but as I was trying to read pin 5(Oscillator) I could hear a "whining" being emitted from what I believe is the transformer on the power side. I essentially held it on too long and the fuse fault kicked on and the amp powered off. I did try to restart but no luck. I was able to determine a short to ground as when I check for continuity between +12v and GND my DMM "beeped". I also have no continuity between REM and +12v. I did all this with the power driver installed and removed.
I found that if I remove the 2x 40a fuses I would not get a continuity "beep" between +12, REM, and GND. Now I'm not too sure how to proceed in finding this short. Any help would be greatly appreciated.

IC# TL494CN
Pin 1: N/A
Pin 2: N/A
Pin 3: N/A
Pin 4: N/A
Pin 5: N/A
Pin 6: 3.62v
Pin 7: 0v
Pin 8: 11.71v
Pin 9: 4.48v
Pin 10: 4.48v
Pin 11: N/A
Pin 12: N/A
Pin 13: N/A
Pin 14: N/A
Pin 15: N/A
Pin 16: N/A


My tools:
Budget Solder Iron and Solder Sucker
Aukuyee Q15001 Digital Oscilloscope
UEI DM383B DMM

Last edited by williokeef; 14th October 2021 at 12:10 AM.
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Old 13th October 2021, 10:29 PM   #2
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
Refer to the power supply FETs as PS FETs. All large transistors are power transistors.

Why n/a on the 494?

When you refer to beep, do you mean a steady tone for as long as you have the probes on or a short beep?

Did you blow the PS FETs when probing pin 5?

Post a working link to the owner's manual for the meter you have.
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Old 13th October 2021, 11:29 PM   #3
williokeef is offline williokeef  United States
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Join Date: Jul 2021
Quote:
Originally Posted by Perry Babin View Post
Refer to the power supply FETs as PS FETs. All large transistors are power transistors. Will do.

Why n/a on the 494?
I didn't start measuring in sequential order from 1-16. I was only able to get the measurements I posted.


When you refer to beep, do you mean a steady tone for as long as you have the probes on or a short beep?
Sorry. It's a steady tone.


Did you blow the PS FETs when probing pin 5?
Just got home from work and I did blow one PS fet. I just removed it and no longer have a short to ground.

But I have no continuity between +12 and REM.


Post a working link to the owner's manual for the meter you have.
https://www.ueitest.com/ecommerce/site/content/PDFs/DM383/DM383%20MANUAL%20English.pdf
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Old 14th October 2021, 12:13 AM   #4
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
What do you mean by:
But I have no continuity between +12 and REM.

What are you expecting to read between the B+ and remote?

What about if you reverse the probes?
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Old 14th October 2021, 12:30 AM   #5
williokeef is offline williokeef  United States
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Join Date: Jul 2021
Quote:
Originally Posted by Perry Babin View Post
What do you mean by:
But I have no continuity between +12 and REM.
Honestly I should have asked if I needed continuity between the two. But now thinking about it maybe that should not have continuity. Is this correct?


What are you expecting to read between the B+ and remote?
I hope my comment above answers this.


What about if you reverse the probes?
With continuity mode. I get "OL"

I checked in Diode-mode and can see a charge being applied between B+ and REM.
I'm assuming since I no longer have any shorts or open circuits that it's safe to power it up?
I have an adjustable AC-DC that I can use 9V or 12V at 2A
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Old 14th October 2021, 12:56 AM   #6
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
Don't use continuity mode. Use either ohms or diode-check.

Most amps will have problems at 9v. Is the 12v@2A all that you have for a 12v supply?
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Old 14th October 2021, 01:18 AM   #7
williokeef is offline williokeef  United States
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Join Date: Jul 2021
Quote:
Originally Posted by Perry Babin View Post
Don't use continuity mode. Use either ohms or diode-check.
Ok. Is there a reason why I shouldn't use continuity mode? (Not questioning your advice.)

Most amps will have problems at 9v. Is the 12v@2A all that you have for a 12v supply?
I have an ATX PSU that is 12v@32A max.

I did try the 9V@2A yesterday and the amp powered up and the relay clicked on but the protect light stays on.

I did not wait a long time to see if the protect light would go off.
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Old 14th October 2021, 01:37 AM   #8
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
Continuity mode tells you virtually nothing. It's good to tell you if you have a break in a wire or if an incandescent lamp is good but not much else.
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Old 14th October 2021, 04:28 AM   #9
williokeef is offline williokeef  United States
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Join Date: Jul 2021
Should I use the ATX PSU to power the amp @12v? I'm assuming it's not too safe as I have no control over the current. But for now that's all I have. Or stick with the Ac-Dc?

Any thoughts or ideas on why pin 5-6(TL494)would cause a whine and go into protect mode?
As mentioned earlier I believe the whine is emitted from the transformer behind the ps driver. Could it have a possible short in the winding?

I can supply pics if needed.
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Old 14th October 2021, 05:04 AM   #10
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
The 2A may work.

The ATX can be used with a low-rated fuse or with a resistor in the B+ line.

The meter was likely affecting the frequency. I don't know why it went into protect. Shorting an FET should have shut the 2A PS down.

Did you have it in DC volts mode?
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