Sanyo MBC-775 Diode Identification

[nauseous]

Hi all,

Have a Sanyo MBC-775 on the bench for repair. Have managed to resolve issues with 5v and 12v rails, but the 22v rail is being problematic. In the process of isolating components for testing one of the following diodes legs detached when removing from circuit. Could anyone help me ID a suitable replacement...?

Unfortunately both fail on component tester & multi-meter.

D2 = "TD 4J"
D4 = "TD 4K"

 

[GiGaBiTe]

It looks like a SOD-57 or SOD-64 package diode, which doesn't narrow it down much. It's probably not a TVS diode, since those are usually labeled as such on the PCB, so it could be a standard recovery diode or fast recovery diode/avalanche diode. The 4J and 4K markings most likely denote the voltage/current ratings of the diode, but it's not exactly helpful when the manufacturer isn't known.

https://www.newark.com/search?st=sod%2064%20diode&gs=true

Where are these diodes on the power supply?

 

[nauseous]

Thanks for your reply.

This is on a centre tapped transformer within the power supply. Being used for Vcc rectification to the power regulator. I was going to try 1n series diodes with regulator isolated and see if Vcc comes up...

 

[GiGaBiTe]

Is the transformer a regular mains transformer, or is it a SMPS design? You can probably get away with a normal diode if it's just a linear supply, but if it's a SMPS design, you'll need something like a Schottky barrier diode.

But the question is why did the diodes fail? It's entirely possible they failed from age, but something farther down the line could have been drawing excessive current and burned them out. You may try putting an ammeter on the +22v rail to see if there's excessive draw. Maybe also put a fuse in-line at the rated current of the rail for protection.

 

[nauseous]

Great advice! And yes this is a linear power supply in this machine.

The 22v rail is responsible for powering the inbuilt CRT. I haven't started going over the chassis yet but I think this will be the culprit. I will test crt using Bench power supply with inbuilt protection once power supply is complete.

Thanks again!

 

[Hugo Holden]

Basically there are two universal Sinterglass diode replacements that you can use that are easy to get and work in practically every circuit where there is an unknown one requiring replacement, either in SMPS supplies, high or low voltage and in flyback supplies and H output stages in TV sets & VDU's, and in both low voltage low frequency and very high voltage high frequency applications. The reason why they can act as a universal replacement is that their rated PIV voltage exceeds practically all other versions of these diodes.

They are the BY448 and the BY228, easily recognized as different; the lead wires on the BY228 are larger in diameter, because it has a higher current carrying capacity. It looks like your diode has the thick lead wires so replace it with a BY228.

The reasons these diodes fail is that oxidation around the lead wires where they enter the package expands in volume. It is not an uncommon problem where metals interface with ceramic or glass. The pressure builds up until the glass fractures. In the case of these diodes the leads simply fall out of them because of the taper of the body. Interestingly there are some ceramic resistors which do this too.