Rebuilding a Philco Tropic 42-730EZ - Printable Version
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RE: Rebuilding a Philco Tropic 42-730EZ - Mondial - 10-16-2014
I would probably leave out the .003 uF caps, as you mentioned Ron. They would be an unnecessary risk for failure given the high voltage across them.
The only reason I can think of for using them would be to reduce the RF harmonics produced as each diode suddenly stops conduction on the AC waveform. You sometimes see these caps with silicon diode rectifiers in solid state receivers. I could see the need for them if this radio covered the LF 200-400 Khz band, where the RF noise generated would be the greatest. Perhaps a similar model had a LF band and they carried over the power supply design?
RE: Rebuilding a Philco Tropic 42-730EZ - cewrightiii - 10-16-2014
I would say the caps are filter caps and the resistors are load resistors...so its a Capacitor Input Filter
Smooth out ripple to produce less "noise". That's my best guess on what I remember from college!!"To smooth the output of the rectifier a reservoir capacitor is used - placed across the output of the reciter and in parallel with the load.. This capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as the rectifier voltage falls, the capacitor provides the required current from its stored charge.
It should be remembered that the only way discharge path for the capacitor, apart from internal leakage is through the load to the rectifier / smoothing system."
RE: Rebuilding a Philco Tropic 42-730EZ - cewrightiii - 10-16-2014
Checked with engineer here at work (electronics company), he agreed and said they are also to make the circuit more resistive...and to help bleed #67 and #68. So if you omit the resistors, you should be fine....just don't run the radio then unplug it and grab the plug!! You may get a little bite!
RE: Rebuilding a Philco Tropic 42-730EZ - morzh - 10-16-2014
hate to rain on your parade,
but the analysis is incorrect. Those are NOT reservoir capacitors. As they are NOT placed across the rectifier's output. And not big enough to be a reservoir for this kind. They have to be about 3 orders of magnitude larger for that.But...this is not really important.
RE: Rebuilding a Philco Tropic 42-730EZ - cewrightiii - 10-16-2014
LOL! Ill tell our engineer he blew that one! I thought they were just filter caps, he add in the reservoir part....and I took him at his word on that...sorry for that!!
(You would think the engineers here would know, since its an RF company!!!)
RE: Rebuilding a Philco Tropic 42-730EZ - morzh - 10-16-2014
Oh.....RF engineers.....
No, no, they are great folk, and myself I do not know the first thing about RF, this is a black magic for me, but as I remember those I worked with, power supplies for them is as much of a black magic In fact when each diode conducts the respective capacitor connects in parallel to the last filter "reservoir" cap (40uF) WHILE the first filter cap (16uF) is being charged. However 3nF to 40uF is 1/13,000 times difference and this is how much extra "reservoir" action you get.
However the RF interference that gets through the capacitive barrier between Primary and the Secondary, usually as a Common mode noise, will appear on every single lug of the transformer's secondary windings (and will be approximately the same).
the main reservoir cap (16uF) will not filter it out as it is common mode.
And the Ground (Chassis) is decoupled from the transformer (from the centerpoint) by the Field coil acting as a filter, so the second filter cap (40uF) will see it now as differential, BUT as an electrolytic it is too inductive to filter it well, so it will appear on the B+ relative to Ground.
This is where the common mode filter comes in, these two small caps will short the Ground and the transformer at RF frequencies, effectively eliminating the common mode becoming a diff. mode at B+ relative to Ground due to the Field Coil.
Now usually these caps are placed in the primary circuit, tying both L and N wires to the Chassis through 3-15nF caps. This however requires Y-safety caps (unknown at the time, there were no Y or X rated caps, hence the Death capacitors). So maybe this was an ingenious way of defeating Death cap problem by placing all caps at the secondary side.
Of course the rating of these caps had to be much higher, as each half can easily provide 350VAC when unloaded.
RE: Rebuilding a Philco Tropic 42-730EZ - mikethedruid - 10-16-2014
From what I can see in the schematic, caps 73 & 74 go from each plate of the rectifier tube to ground. They are isolated from the mains by the power transformer and so cannot short out across the mains. I would use regular 600 or 1000 V poly caps for them. I believe they chose to use the RL circuit with R78 and R79 on the primary (mains) side of the transformer for just that reason, and to also provide filtering there. It is the only explanation I can think of for their being there.
RE: Rebuilding a Philco Tropic 42-730EZ - morzh - 10-16-2014
Yes, exactly. No Y or X rating needed, I would use 1000V DC or 600VAC rated caps.
RE: Rebuilding a Philco Tropic 42-730EZ - Ron Ramirez - 10-17-2014
I'm enjoying the discussion, folks. It's great that we can have a good debate here without it deteriorating into fighting as sometimes happens on, well...you know, that other place.
But I'm still going to leave (73) and (74) out.
If I have time this afternoon, I am going to reinstall the electrolytic cap cans and the audio output transformer, hook up an AC cord, and try it out.
If not, I'll do so tomorrow afternoon. We are expecting a visitor this evening, so I don't really have much time for tinkering today.
RE: Rebuilding a Philco Tropic 42-730EZ - morzh - 10-17-2014
I do not think you need them to be honest, as I said in the beginning, but in the days of yore you might.
resistors too
If you find out you have some interference....you could always add them later. In front (Y-caps) or where they are on the sch.
RE: Rebuilding a Philco Tropic 42-730EZ - Ron Ramirez - 10-17-2014
Well, I've done everything but attach the speaker connector to the new speaker wiring harness. Electrolytics reinstalled, audio output transformer reinstalled, new AC cord installed. I've tried it out with the 80 removed and everything lights up as it should.
Photos later. I'll probably try it out tomorrow afternoon.
RE: Rebuilding a Philco Tropic 42-730EZ - morzh - 10-17-2014
I agree, there should not be any 80 next to a Tropic
RE: Rebuilding a Philco Tropic 42-730EZ - Ron Ramirez - 10-17-2014
The latest photos:
[Image: http://www.philcoradio.com/images/phorum/42-730EZ/Image00010.jpg]
[Image: http://www.philcoradio.com/images/phorum/42-730EZ/Image00011.jpg]
[Image: http://www.philcoradio.com/images/phorum/42-730EZ/Image00012.jpg]
I hope it works as well as it looks...and if I may say so myself, I think it looks pretty good.
RE: Rebuilding a Philco Tropic 42-730EZ - morzh - 10-17-2014
Does too!
RE: Rebuilding a Philco Tropic 42-730EZ - NostalgiaRadioTime - 10-17-2014
Indeed it does, Ron! Excellent work
Seeing your dial glass, with the typical rotted and crumbling gaskets under the mounting clamps, reminds me of how I've been fixing that problem for quite some time now. I use rubber bands! A standard rubber band is the right width, so all I do is cut it apart, cut it into strips and apply with a light coat of spray adhesive. Works like a charm.