I just picked up a working Philco 70. (1) what's the pilot lamp bulb? Mine is only marked 3.2 volts (2) After downloading all the paper work from NA, my chassis layout matches the one for "below B-22,000", but my chassis is stamped 201,052.......so what's with that? (3) What are the mfds for the 2 filter caps? Mine aren't copper and the replacement cans are 8mfd 450V. (4) Will .22 mfd be OK for the .25 caps? That's all for now, thanks in advance....DENNIS
Forget the serial number question!
Hi Dennis,
(1) I have a number 41 bulb in my 70. (2) You have an early model 70, before the alphanumeric serial numbers began. (3) According to the schematic, the electrolytics should be 6MFD. 450v. Are your electrolytics marked Sprague by any chance? I have two 70's with Sprague electrolytics and they appear to be original parts. I only have one 70 with the Mershon copper caps. (4) You should be okay using .22 for .25 caps. Usually, if you stay within 10% of the listed value you are okay. I don't think exact tolerances are too critical in the early sets, my Tech college instructor used to tell me that the early stuff was "Brute Forced"
!
Hope some of this helps and good luck with your "70"!
Kevin
Quote:Are your electrolytics marked Sprague by any chance?
Mine are Solar and the electrical tape over the connections would make them replacements.
Quote:the electrolytics should be 6MFD.
I only have 10mds, +/- 20%, on hand. No prob?
BTW, my cabinet is stamped JUN 26 31
DKinYORKpa Wrote:Quote:Are your electrolytics marked Sprague by any chance?
Mine are Solar and the electrical tape over the connections would make them replacements.
Quote:the electrolytics should be 6MFD.
I only have 10mds, +/- 20%, on hand. No prob?
BTW, my cabinet is stamped JUN 26 31
10mfd caps should work fine. I'm not an expert, but the increased capacitance should just mean better filtering to a point. Philco probably used the lowest possible value that worked well to save on costs.
Agree, the 10mFd should be ok. Personally, I do not like to exceed the original values in the set by a whole lot.
Hi Dennis,
Agree with above posts. The reason you don't want to use a filter cap that is large is that it will increase the HV. Back in the 1920's and 30's the technology wasn't developed enough to make a cap that was more than a few mfd. That's why on these older set in the power supply circuit you'll find three or four small caps, 1.0 to 3.0mfd and a few chokes and the field coil to filter out all of the 60 cycle ripple. These are usually paper and foil wound around each other. Two pieces of foil with the paper as an insulator. It is the makeup of non polarize filter condenser. With this cap there is no + or -. Most of the time it's mounted in a metal can with tar to secure it in place.
Fast forward a few years and now we have electrolyte caps. These are in an aluminum tube or copper (Mershon) with some chemical inside and metal coil. Don't know how these work but they are polarize with the outside metal can being the - and the wire or terminal at the bottom being the + connection. For the most part these caps are 20mfd or less.
Things changed when small ac/dc sets with pm speakers came in to being. No more field coil on the speaker to use for filtering.That gets replaced by a resistor which doesn't have any ability to filter just couple the HV to the next large cap.These set use rather large filter caps in the order of 40 to 80mfd. We 'er talking the late 1940's.
So long story short the issue is the size of the filter. + 20 percent is OK and as a trick you can use is the input cap is the one that is going to have the most effect on the output voltage. That would be the one closest to the filament of the 80 tube. You can use this cap to set the HV level. The other filters won't have of ability. to raise the voltage.The others will won't have much effect on the voltage will increase the elimination of the 60cy hum.
I was working on a model 20 awhile back and put a 20 mfd caps for all three of the filters. The outcome was I nearly burned up the BC resistor because the HV was much higher and put more of a load across the resistor.
The early and late model 70's are pretty different as the late model as AVC and the earlier one doesn't. Haven't seen very many of the late models. As I type this my right is resting on a 370 (chairside version of the early 70) keep my elbow warm from the heat of the 80 and the 47 tubes.
Terry
ps
Pardon my rambling, I need to get a life.
Are there some changes to the early Philco 70, other than what's with the schematics? Mine works great and I'm recapping it. Since it works great, I could just ignore this, but I'd like some confirmation. The 5K resistor shown on cap block 12 goes over to cap block 5 on mine.The schematic and the parts layout show that 5K on the cap block 12 from one side to the other. Has this difference come up before?
On someone's radio web site with a photo of the chassis, the above mentioned 5K resistor was located like mine. So can I take the no response to my question as "leave it alone"?
Probably.
Philco issued two different Service Bulletins for the early 70. The earlier one, as I recall, had additional mica capacitors across many, perhaps all, of the trimmer condensers; the later "early" version did not.
You should order the complete package from Chuck, he should have both versions of the early 70 service info. And the nominal cost is well worth it for what you receive.
http://www.philcorepairbench.com/schematics.htm