Koyo KTR-1661 rectifier

[CarlQB]

Hi
New to Videokarma, but have been on AudioKarma for a while.
I just got this Koyo 8-band transistor radio for cheap.
I'm looking at replacing the electrolytics in the power supply and amp section. I have some basic electronic knowledge but this is my first radio. Probably not the easiest for a newbie but this is such a nice unit with a wood case I couldn't let it pass.
It runs on 6V, and has a 2-diodes rectifier as shown in the schematic. I understand how the rectifier works, but can someone explain the purpose of resistor R74, between the 120VAC and +6VDC ?
I don't get it!

Thanks!

[Electronic M]

There's a few reasons for it to exist. One is to limit voltage differential between primary and secondary of the power transformer and prevent it from shorting, another is to RF ground the receiver...The neutral of the most outlets is tied to ground at the breaker panel and you can use either as a ground. If the cord is unpolarized and connected backwards there's still some ground reference for RF reception but there's an extra 10-1000 ohms across the radios transformer and utility transformer windings to ground.

That resistor isn't exactly essential but it's good to have present.

[jr_tech]

Another possibility is to eliminate static charge on the chassis, by bleeding it harmlessly off to “house ground “.

jr

[CarlQB]

OK thanks. I figured it must be there mostly for noise reduction. It was suggested to me (on another forum where I asked) that such a resistor was rather unusual, and while its presence might have something to do with noise reduction, with an unpolarized plug its got more chances creating a dangerous situation of having line voltage on the chassis, and I probably should remove this resistor and make the 6V power supply completely independent from the mains.
Since I'll be working on this radio I think I'll just take the resistor off the board for now, see if it affects the radio in any way, I can always solder it back if needed.
Thanks!

[Alex KL-1]

Originally Posted by CarlQB

OK thanks. I figured it must be there mostly for noise reduction. It was suggested to me (on another forum where I asked) that such a resistor was rather unusual, and while its presence might have something to do with noise reduction, with an unpolarized plug its got more chances creating a dangerous situation of having line voltage on the chassis, and I probably should remove this resistor and make the 6V power supply completely independent from the mains.
Since I'll be working on this radio I think I'll just take the resistor off the board for now, see if it affects the radio in any way, I can always solder it back if needed.
Thanks!

In part, yes, if the resistor voltage rating is less than the maximum voltage breakdown of the PS trafo. If is a common resistor, for sure will survive less spikes than the trafo. But, if is a HV resistor, will survive far more than the trafo, and trafo shortens first (to the secondary).
+1 for the static charge draining cited by jr-tech.

[Electronic M]

If the set has a UL sticker then that resistor was sized such that it would not create a harmful shock even if that side of the line cord was plugged in to the hot side of the outlet.

The static bleed is also right.

[CarlQB]

Interesting, thank you!