2N6027 PUT Replacement

[kahlerpa]

I have a RC Timer circuit that has been reliable for years using 2N6027 PUT from On Semiconductor, the part is discontinued and nearly impossible to find. I bought some Central Semiconductor 2N6027's as a replacement, many of the boards I build do not seem to be firing when the RC charges, it is frustrating as it is intermitted, any ideas on how to make the circuit more stable with the new "replacement part"? Any help would be appreciated.

Regards,
Chris

 

[ronsimpson]

schematic, (that part of the circuit)
Scope pictures.
more information
Have you tried 2n6028?

 

[kahlerpa]

I can get them tomorrow don't have it in front of me now, I'm more concerned with why one device work consistently and the other does not. The only big difference that jumps out at me is the valley current in the specs, I'm firing and SCR S401 of the UJT. I played with the voltage divider and can see the 2N6027 fire on a non-working unit but it doesn't seem to be firing the SCR, could this be that the 6027 isn't firing long enougn to make the 401 fire?

Thanks, More tomorrow.

 

[ronsimpson]

firing long enougn to make the 401 fire

what is a 401?

I had a project at the university in 1970+/- using a 2N6027 triggering TTL. It just did the job. Changing the digital gate to a slightly different part made a great difference. Changing the temperature of the UNI made a difference. Just unstable. I remember the pulse was very fast and the levels were not good. etc.

 

[cmartinez]

According to their datasheet's both Central and On Semi have different valley current vs supply voltage curves. Maybe you could add a resistor to compensate for that?

 

[KeepItSimpleStupid]

ON parts: https://www.utsource.net/sch/2n6027

I've used the company before.

 

[kahlerpa]

The S401 is an SCR to turn on a relay.

 

[kahlerpa]

According to their datasheet's both Central and On Semi have different valley current vs supply voltage curves. Maybe you could add a resistor to compensate for that?

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You are correct, its been many years since I have done work like this, I worked it out mathematically and it is working I just have to screw with the values on the rc to get the timinr correct again. Thanks.