The PHILCO Phorum
37-350 Hot chassis - Printable Version

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37-350 Hot chassis - planigan - 06-02-2006

Aside from my getting good voltage checks but no reception, Ive now discovered that the chassis to ground on the set reads 73VAC. Does anyone have any suggestions as to where I may be getting it. I checked power xfmer to ground on the meg scale VTVM and found no obvious short to the frame. I checked to see if any wires were rubbing against or near chassis but again found nothing. 73 VAC is a big leak, I caught it the easy way, when I went to remove the gnd after hooking up ant to look for reception I noticed a spark as I removed the ground. On prior attempts with ant I apparently turned radio off before removing ground and never caught it. Well, easier than getting across it to find it! Pat

- Ron Ramirez - 06-02-2006

Hi

First of all, there is no such thing as a Model 37-350. Now if you mean 37-650, that uses a power transformer and does not have a "hot" chassis.

This model, as well as most AC-only (using a power transformer) Philcos, does have a double line bypass capacitor. The originals were contained in one of Philco's famous (infamous?) bakelite blocks, and contains two .015 uF capacitors. One end of each of the two capacitors are joined together and connected to ground. The other free ends of each of these two capacitors connects to each side of the AC line. With the set plugged in, you will probably read AC voltage between each side of the AC line and ground, IF the chassis is grounded.

These capacitors should be replaced with new capacitors with an "X-Y" safety rating. For additional safety, use a smaller value, like .01 or .0068 uF. The X-Y safety caps are stocked by various parts vendors. With a lower capacity, you will have less voltage from line to ground.

- planigan - 06-03-2006

That was a typo, should have been 37-650 as you point out. I did replace the two line caps with the X-Y safety caps, .015uf, when I re-capped. What I meant by a "hot chassis" is that I'm reading 73 VAC from the chassis to house ground. I don't believe I should be getting any reading but then thats why I posted. I have no idea where the voltage is coming from. The xfmer does not show any reading on the ohm meter meg ohm scale from windings (primary and secondary) to xfmer case. Any ideas? Pat

- Ron Ramirez - 06-03-2006

Hi Pat

Okay, now that we're clear on what's going on... Icon_smile

Remember, a capacitor blocks DC but passes AC current. Nearly all Philco chassis are slightly "hot" due to the AC line bypasses used in them. And why you will see a slight spark when you connect a ground wire to the GND terminal. Terribly simplified, yes, but I apologize as I don't really have time to elaborate - I'm sure Chuck or someone will jump and and elaborate further. Anyway, this is why I use new X-Y units at .008 uF instead of .015. Most of us today have agreed that for safety's sake, you really shouldn't use anything larger than .01 uF as an AC line bypass due to the leakage current increasing with larger values of capacitance.

If you were to test further you would find that the voltage may be 73 VAC but the current would be very minimal. Remember, it is current that kills!

37-650 - planigan - 06-03-2006

Ron, thank you for the information. I thought I had a serious short somewhere in the set. I'm going to order the .008mfd X-Y safety caps and replced the .015's. It's a pain as they are in a bakelite block, on the other hand I imagine they are physically small and I will not have to put grease on them to get them in like I did with the 015's! Thanks again, Pat