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Philco 645 Hum-m-m-m - Printable Version

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RE: Philco 645 Hum-m-m-m - Winky Dink - 07-03-2014

Maybe I screwed up something the last time I was fiddling with the caps to get rid of the hum. This will give me a chance to use the capacitor checker I got last month. Many thanks.

RE: Philco 645 Hum-m-m-m - morzh - 07-03-2014

Yes sounds like motorboating.

Not sure but because it has to do with tuning and the IF and RF do not really depend in their stability on the passing frequency, try to look at the oscillator first. Maybe it becomes unstable at a certain point.

RE: Philco 645 Hum-m-m-m - Winky Dink - 07-04-2014

Thanks. I'll redo the alignment first to be sure I didn't screw that up, then follow up with potential oscillator issues, etc. Happy Independence Day.

RE: Philco 645 Yet Another Question - Winky Dink - 07-05-2014

One thing leads to another. I was checking some of the caps for leaks, shorts, and grounds; and I noticed that I've placed a 630v cap where the voltage is supposed to be 680v. I've run the set for quite a few hours with no problem. I suspect that the actual voltage is higher than 680v, but my multimeter doesn't go above 600v.

[Image: https://lh4.googleusercontent.com/-lNfXaQC-Hhk/U7hta0m4AfI/AAAAAAAAMx4/Mu7XPSOSpf8/s500/355%2520Philco%2520645.jpg]

So I have two questions:
1. Should I replace the cap with one of higher voltage rating? The only solution I have on hand is to use two .2uF/630v caps in series. This is in a two-cap bakelite block, and I can squeeze three caps in if I'm really careful about shorts.
2. To check the actual voltage on the (specified) 680v line, can I put the transformer on 60 VAC from the variac and expect 340 volts?

RE: Philco 645 Hum-m-m-m - morzh - 07-05-2014

1. There are no such DC voltage in this radio as 680V. While tubes re cold the voltage can possibly spike to 500V for a few seconds.
2 the wire you show has 0V on it, not 680V.
3. The working voltage will be about 250V or so.

4. I think your cap is fine the way it is.

RE: Philco 645 Hum-m-m-m - Winky Dink - 07-06-2014

My question betrays my lack of basic knowledge--but I'm really trying.
This picture is from a few months ago. When I checked the transformer voltages, I got no reading from the green/yellow wire, so assumed the voltage exceeded 600. Of course, since it's actually 0v, that would also explain no reading and explain why I haven't fried the capacitor.

[Image: https://lh5.googleusercontent.com/-D9Z_yahdd00/Ur_KcN0BGaI/AAAAAAAAMoI/h9rSFrGhvxI/s500/45%2520Philco%2520645.JPG]

Thanks. Now I can safely put the set back together and proceed to my next blunder or success.

RE: Philco 645 Hum-m-m-m - morzh - 07-06-2014

Well, it can be a volt or two WITH the load (rect neg is connected to the GND via small resistor) but for practical purposes it is zero.

RE: Philco 645 Hum-m-m-m - Winky Dink - 07-06-2014

Thanks again. Another oversight of mine is that I never figured out what "8-10" means in the transformer voltage table ("Center tap 8-10"). What means "8-10"?

RE: Philco 645 Hum-m-m-m - morzh - 07-06-2014

I have no idea. Maybe pins.

680V is the AC voltage between the plates of 80, and this is what's important, the centertap to each plate is exactly half, 340V.

RE: Philco 645 Hum-m-m-m - Winky Dink - 07-07-2014

I put the bakelite block back in and turned on the power. Waited about 20 seconds for warm-up, then noticed that one of the rectifier plates was totally glowing orange--the whole plate! I wanted to get a picture of the tube, but decided to cut the power immediately. This is what I found under the chassis:

[Image: https://lh3.googleusercontent.com/-L8YainDh-AQ/U7scus7ducI/AAAAAAAAMyY/SMvqvIvnbvc/s450/360%2520Philco%2520645.JPG]
I don't know where that washer came from. The tube is toast, but there's no other apparent damage.

I removed the 12-inch wire I was using as an external antenna, and that eliminated the motorboating. Since the last alignment the reception is very good without any antenna.

Someday I'll look into the hum again, but for now the electronic restoration is complete, and I'd like to thank all the forum members who contributed to its success.