Negative voltage supply issue

Thread Starter

shawnsick1

Joined Mar 18, 2024
7
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:
Screenshot 2024-03-18 at 18.24.36.png
 

Dodgydave

Joined Jun 22, 2012
11,318
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,.
 

Thread Starter

shawnsick1

Joined Mar 18, 2024
7
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
 

Dodgydave

Joined Jun 22, 2012
11,318
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 .
 

MisterBill2

Joined Jan 23, 2018
18,934
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.
 

Thread Starter

shawnsick1

Joined Mar 18, 2024
7
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 ?
 

Alec_t

Joined Sep 17, 2013
14,370
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.
 

Thread Starter

shawnsick1

Joined Mar 18, 2024
7
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 !
 

MisterBill2

Joined Jan 23, 2018
18,934
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.
 
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