Heathkit GC-1195 circuit guru question...

[orange_glow_fan]

Working on this GC-1195....

I need to replace the diodes in the power supply (along with the two electrolytic caps. For some reason Heathkit used different diodes for (D101,102) and (D103,104).

D101,102 are three amp diodes while D103,104 are 1 amp diodes. It's my intention to use three amp diodes (NTE-156) to replace all four diodes. Then self doubt crept in and I am questioning my choice.

Is there any reason you can think of that you can think of, other than cost savings, that Heathkit engineers used diodes of differing current ratings in this full wave bridge? Note that the emitter of Q1 goes off to power the incandescent bulbs of the display segments. Q1 is sourced from the center tap of the transformer.

So, can I simply use 4 three amp diodes or am I missing something??

Thanks

[Nick]

From what I can see, just use 3A diodes all round - the 57-42 is a 1N5403 (300V PIV, 3A, DO201) and the 57-65 is an 1N4002 (100V PIV, 1A, DO204).

Use 4 x 1N5403.

Nick

[gregebert]

I cant think of any reason why 3amp diodes would cause a problem, and the schematic cant account for a 2-amp difference in current. Notice the 100K resistor from the transformer lead; that means almost zero current. If this is a clock, I suspect that's for the line-frequency reference.

I guess Heathkit had extra 3amp diodes in their inventory ?

[Dekatron42]

You can use 3 amp diodes in all positions. Heathkit saved a few cents by using 1 amp diodes for the logic portion and 3 amp diodes for the lamp portion of the design.

This design is used with center tapped transformers and you often see it in linear power supply modules which use the LM723 voltage controller/regulator IC where the beefier diodes are used for the output voltage and the weaker ones for the "logic" surrounding the LM723.

Types like the one below sometimes use this design with different sized rectifier diodes (I'm not saying this exact one does but similar types do - I have some in my storage somewhere).

/Martin