Sony TC-152SD DC-DC Diagnostic

If you have a dead TC-152SD, or one that does not produce sound, then you need to diagnose the fault. This page is of course only for engineers, and the ones who received a pass examination at the institute.

The focus should be on the power supply board, which has the dc-dc module. The pins indicated are at the output-end of the dc-dc module. These pins should provide +24 V, +9 V, 0 V, and -24 V respectively, and the PCB has voltage markings for the first three pins.

Using a digital meter, I checked the voltages and noted that the +24 V rail was very low at around +10 V. Just to make sure that it was not loaded down, I isolated that pin from the rest of the circuitry and measured the voltage again, and it was still +10 V indicating that the DC-DC module was not functioning to its maximum capability. The rest of the voltages +9 V, 0 V, and -24 V, were present in the other pins.

To verify the premise I decided to inject +24 V DC at 100 mA, from a bench power supply, directly to the +24 V rail, and the tape recorder came to life and worked perfectly proving that it needed the correct voltage level. In such a situation, there are two methods of repairing the circuit. One obvious method would be to remove the dc-dc module, open the sealed metal can and look for any faulty components. However opening the sealed can may be difficult. The faulty components may be partially failing and difficult to identify, and opening the can may damage the circuit and the rest of the voltage rails that are working correctly may be lost.

The second option was to build a new dc-dc converter circuit that could step-up +6 V DC to +24 V DC. This circuit then connects in parallel to provide the +24 V rail, bit like a piggyback heart transplant, when one heart is not powerful enough to pump the blood, a second heart connected in parallel helps. I knew I should have been a doctor!

DC-DC Module

Here is the dc-dc module in a sealed metal can. Click on the image above to see exactly where it is located on the board.

Circuit Diagram

Here you can see that the +24 V pin can be isolated by removing the 10 mH coil and the 47 µF capacitor. It is always a good idea to check for the 6 V DC at the output of the rectifier and input of the dc-dc module.

The design of the +6 V DC to +24 V DC converter circuit will be in part 2 of this article, assuming I can find the time to write it up.

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