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1956 FOMOCO tube type car radio "hash" suppression

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cowboybob

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In the snips below (from a "Sam's PhotoFact" manual),

View attachment 63679 View attachment 63683 View attachment 63684

we have a vibrator driven boost circuit, with L7 listed as a vibrator "hash" (arky-sparky "noise" common to DC vibrator choppers) suppressor.

After a simple clean-up and a vibrator points filing, on power-up the radio worked remarkably well (tuned to a local AM station), but with some minor static on the audio output that, over time (maybe 20 minutes) increased in volume until the audio was overwhelmed by it. During this time, C15 became "warm-ish".

My reading of the schematic is that C15 (a paper/wax 0.04 μF, 1000 volt (and maybe C16 and C17, as well, but neither have a parts descriptor like L7 & 8 [see above]) is also part of a "hash" suppression scheme. No inductance values are given for L7 & 8.

Voltages were nominal. Cutting C15's ground leg made no difference (i.e., the static got no worse or better and voltages remained nominal). I should note that the vibrator frequency is ≈115Hz.

Also not that C15's picture shows a thin black band indicating that it is polarized, although it is not an electrolytic (note C1A & B's symbol, vs C15's) as I understand the term.

(Finally the questions):

1. Am I right in assuming the function of C15?

2. If 1. is correct, what best modern cap to replace it with?

I know I'd be better off replacing the vibrator with a bistable 555 driven FET (or something like that) but I'm interested in keeping the current rig as "pure" as is practical.

Any other thoughts/suggestions are welcome.
 
I'd probably replace C15 with a Y type interference safety cap. See http://www.justradios.com/safetytips.html

The withstand voltage is 1250 for this cap: https://www.electro-tech-online.com/custompdfs/2012/04/ABD0000CE2.pdf which Digi-key sells.

Although I never tried it, it might make some sense to use a gas discharge tube across the interference cap. http://search.digikey.com/us/en/cat...ube-arrestors-gdt/655426?k=gas discharge tube

There are solid state vibrator versions available: Search for Vibrator.

Back in about 1980 or so, I designed a solid state replacement for a Blaupunkt radio. These were not out yet. I used two 2n3055's, a 555 timer and a transistor to do the inversion. I put it on a big heat sink. The radio was hump mounted on the slide out 8 track tape mounts of the era with the power supply portion sitting between the bucket seats. The heatsink just sat in a cardboard box.
 
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Thanks, KISS.

Once again you've provided unparalleled advice.

BTW, the solid state vibrator is cheaper that the stock replacement. ;)
 
This is a fairly simple solidstate viabrator replacement circuit. Also C16 .5uf in your circuit is basicly across the points of the viabrator, if its faulty O/C can cause exess arcing on viabrator contacts & hash.
 
I've mentioned it before, I'll mention it again - I binned loads of brand new vibrators years back, they were left in stock from before I started work.

Makes you wish you'd kept them now :D
 
Thanks, debe. Dremeled the can off the vibrator (to do the points) and there is mucho sparkin' going on. C16 first. Also, thanks for the schematic. Think I'll give that a shot.

Nigel, how is it that the minute we decide something is no longer needed, and trash it, a need for it pops up the next day...??? Some sort of cosmic "gotcha" law.
 
Thanks, debe. Dremeled the can off the vibrator (to do the points) and there is mucho sparkin' going on. C16 first. Also, thanks for the schematic. Think I'll give that a shot.

Nigel, how is it that the minute we decide something is no longer needed, and trash it, a need for it pops up the next day...??? Some sort of cosmic "gotcha" law.

I think it was more like 20 years than next day :D but same principle.
 
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