[R-390] Coming: Real In-rush Protection / Step Start

[Drew P.]

A "Wise" man wrote:

"Combine the approaches.

Thermistor as delay element, and the relay shorts it out."

This approach makes the most sense. This would give you the advantages of the NTC thermistor, a gentler start, and remove the heat disadvantage.

Something obtusely similar, but with a different purpose, is done in the degassing circuit of CRT computer monitors. Here, in the degaussing coil, a large initial current is allowed, which then decays as the series posistor heats up. A series relay then disconnects power to the degaussing circuit entirely. Yes, I know it is accomplished differently from that which we are advocating here, but there are similarities.

Dave went on to comment:

"On the other hand, it makes me laugh to hear the CL90
called a fire hazard when the R-390 came with two 6082's."

Installing a CL-70 in my '67 EAC, I left the leads long, sleeved with teflon tubing, and soldered it in.  Gets somewhat hot, but works.  Before the ICL installation, powerup produced a loud "Click-Tuuuuuuunnnnnnnggg!!, but now there is a just a quiet "ZZhhink-click" as the antenna relay pulls in gradually. The new sound is much more satisfying and appealing, especially as compared to the previous sound which implied that the transformer had wanted to bust at the seams.

On those horrendously hot 6082's used as pass elements in the R-390/URR regulated power supply, I know someone whose R-390's 6082's set fire to some schematic prints that the radio was sitting on atop his workbench.