Hello, I am currently troubleshooting a Yamaha MT4X 4 track tape recorder. The issue appears to be coming from the power supply. I'm not getting -5V and on the +5v I am reading +10V. Could anyone hint where I should be looking ? I have access to various multimeters and scope, I have a lot of experience repairing tube amplifiers so getting my hands into the power supply isn't a worry for me.

Here's the schematic:

 

Welcome to AAC!

Post voltages on the dual opamp.

 

The circuit uses a floating ground, using the op amps, with your meter Positive lead on pin 1 of the regulator measure the voltage on pins 3+5 and 2+6 of the op amp,.

 

Hello,

Thank you for your replies. Here are the voltages as requested :

Pin 3 : -4.72v
Pin 5 : -4.73v
Pin 2 : 0.23v
Pin 6: 0.23v

 

Check that R004 (10k) hasn't gone open circuit.

 

Hello,

Thank you for your replies. Here are the voltages as requested :

Pin 3 : -4.72v
Pin 5 : -4.73v
Pin 2 : 0.23v
Pin 6: 0.23v

Measured with respect to what? Where did you out the red and black probes?

 

Measured with respect to what? Where did you out the red and black probes?

As suggested by Dodgydave just above

 

Check that R004 (10k) hasn't gone open circuit.

R004 seems to be okay

 

So the opamp is stuck at positive output, you can verify that by putting your meter lead negative on pin 4 and check the voltage on pins 1+7 and 8, and 3+5 .

 

If the negative supply is zero, or very close to it, and the positive supply reads twice what it should, the first place to check is for an unfortunate low resistance between the negative output and the system common, which should be "ground." It seems like IC002 a 7901 regulator is responsible for the ten volts, and the transistor Q005 is for holding the ground at the midpoint.

This has got to be a really dumb patch for neglecting to specify a center tap on the transformer winding.

 

So the opamp is stuck at positive output, you can verify that by putting your meter lead negative on pin 4 and check the voltage on pins 1+7 and 8, and 3+5 .

While trying what you suggested I've came across the following readings :

Pin 1 : 0.25v
Pin 7 : 0.25v
pin 8 : 9.97v
Pin 3 : 4.98v
Pin 5 : 4.98v

I'm guessing the opamp is defective ?

 

Where did you have the negative probe of your meter when you took those readings?

 

Where did you have the negative probe of your meter when you took those readings?

Pin 4

 

Don't think so, if the negative probe was on pin 4 (which isn't ground). The post #11 readings are then what they should be.

 

After putting this aside for a week or so, I finally decided to get back to troubleshooting. I replaced C001 and C002 after testing them out of circuit, now everything is up and running. Now the whole mixer is getting a recap !

 

Congrats on the Fix!!
Many power supply problems are caused by excess leakage of filter caps. NOW, for our education, can the TS reveal the brand name of the two capacitors that were replaced?? I am asking because there was a serious quality issue with some of the the caps that Sony and Superscope were using in about the same era.