Philco 70

6/7/2010 9:15:48 AMJohn

My Philco 70 had 60cycle hum so I replaced the original filters but still have lots of hum. The filters that were in the set looked to be original but were grounded and the schematic shows them not going to ground. Don't understand this.

6/7/2010 9:52:55 AMNorm Leal

Hi John

Follow the schematic. Negative of filter connects to center tap of high voltage winding not ground.

Could the original been isolated from chassis by an insulated washer?

These are different versions of the Philco 70. Be sure you have the correct schematic.

Norm

:My Philco 70 had 60cycle hum so I replaced the original filters but still have lots of hum. The filters that were in the set looked to be original but were grounded and the schematic shows them not going to ground. Don't understand this.
:

6/7/2010 11:03:15 AMJohn

Norm
I checked with an ohmmeter for continuity to ground on the filters and they were both grounded. My 70 uses 4-24's, a 27, a 47 and a 80 tube. The schematic for this configuration shows these filters not grounded so I'm still confused.

:Hi John
:
: Follow the schematic. Negative of filter connects to center tap of high voltage winding not ground.
:
: Could the original been isolated from chassis by an insulated washer?
:
: These are different versions of the Philco 70. Be sure you have the correct schematic.
:
:Norm
:
::My Philco 70 had 60cycle hum so I replaced the original filters but still have lots of hum. The filters that were in the set looked to be original but were grounded and the schematic shows them not going to ground. Don't understand this.
::
:

6/7/2010 2:33:22 PMWarren

What looks to be original may not be. And it could be why it still hums. Connect the filters as shown in the schematic. You can't hurt anything doing this. If it now works like it should, your problem is solved.

6/7/2010 10:20:40 PMEdd

Sir John. . . . . .

Examine both the respective schematic and pictorial layouts of the two series . . . being RED and BLUE outlined for differentiation . . . . that I have just poked up and placed below.

Got all of ‘yer B plus-samus-es-es and B minusus-es-es, all clearly designated if in the case of using polarized electrolytics.

In the left of the page series note that the filter items 41 & 43 do share a common connection and then they pass down to then connect with the center tap of the high voltage winding of the power transformer and then the trios circuit path will then flow down the [Green dot] path to the resistor section located between 5-6, and then are able to make it to the ground connection to the right.

But . . . and that’s heavy on the BUT, those filters common negative connections do NOT go DIRECTLY to ground, as they have to pass thru that resistor section in order to develop a negative bias supply, which you see going to connection H, which is then biasing your AF output tube , and if it doesn’t get that voltage, THAT is why its upset with you and growling at you, as well as getting a bit HOT under its collar.

Now should your set coincide with the Blue markups wiring, you will then see that the filter items 50 & 51 have shared positive terminal connections , but there is a disparity in the manner in the respect that the 51 item DOES have its negative terminal going directly to ground.
However, item 50 has its negative terminal connected to the common buss of the center tap of the high voltage winding and then the continuity to ground is left to be made thru the paralleled inductance of 52 and the
series wired pair of resistors 53 and 54.

In which case, the junction of 53/54 provides a voltage divider for acquiring that aforementioned bias voltage of “C” for your AF output tube. Then it won’t growl at you.

Now all up to you . . . to merely compare and see which design case that you have . . . and realize that a fiber washer, the capacitors type of construction, or what have you ? will, be necessary / utilized for the two negative busses if using the “RED” outlined design version.
Orrrrrr, that the negative terminal of item 50 capacitor will have to be floating above ground if using the “BLUE” outlined design version.

Thassit . . . hopefully . . . .

73's de Edd

PHILCO 70 Filter Capacitor Treatments Referencing:

:What looks to be original may not be. And it could be why it still hums. Connect the filters as shown in the schematic. You can't hurt anything doing this. If it now works like it should, your problem is solved.
:

6/7/2010 11:20:22 PMThomas Dermody

If the original filters are mounted as shown, there should be a fiber washer between the capacitor housing and the chassis. Most likely the screw part is made of phenolic material, and this will automatically insulate from the sides of the chassis hole.

If you are using replacement filter capacitors, merely wire as shown in the schematic. As the schematic Edd posted shows, connecting the filter capacitors to the chassis while they are otherwise wired as shown in the schematic will short across vital filter components.

Also, the chassis is not ground. It is merely the chassis. It is used as a common tie point, but this electrical reference point isn't necessarily ground or the most negative point in the set. It is merely a reference point. If you study the schematic thoroughly, and think of the chassis symbols as merely interconnecting wires, you will better understand how the circuitry works. Some sets use the chassis as the most negative point, and others don't. Having the chassis as the most negative point means that there must be cathode bias resistors, etc., for each tube requiring such circuitry. With the chassis not being the most negative point in the radio, all of the cathodes may tie directly to the chassis, and a few resistors between the chassis and the most negative point in the circuit can serve to both provide negative bias voltages and integral filtering points.

6/8/2010 6:56:36 AMJohn

Well, I goofed. When checking from the filters to ground I used a DMM and just noticed it was showing conduction but didn't notice it wasn't measuring zero but just a low value. Both original filters appear to be mechanically connected to the chassis when looking from the bottom but a closer look shows two feed through style connections from the bottom of the chassis where wires go through to the top and then connect to the cases of both filters and so this is the negative connection for the filters and they are not connected to ground. Set works now but gets nothing below about 1000 kcs. Don't know why yet. Thanks for all replies.

:If the original filters are mounted as shown, there should be a fiber washer between the capacitor housing and the chassis. Most likely the screw part is made of phenolic material, and this will automatically insulate from the sides of the chassis hole.
:
:If you are using replacement filter capacitors, merely wire as shown in the schematic. As the schematic Edd posted shows, connecting the filter capacitors to the chassis while they are otherwise wired as shown in the schematic will short across vital filter components.
:
:Also, the chassis is not ground. It is merely the chassis. It is used as a common tie point, but this electrical reference point isn't necessarily ground or the most negative point in the set. It is merely a reference point. If you study the schematic thoroughly, and think of the chassis symbols as merely interconnecting wires, you will better understand how the circuitry works. Some sets use the chassis as the most negative point, and others don't. Having the chassis as the most negative point means that there must be cathode bias resistors, etc., for each tube requiring such circuitry. With the chassis not being the most negative point in the radio, all of the cathodes may tie directly to the chassis, and a few resistors between the chassis and the most negative point in the circuit can serve to both provide negative bias voltages and integral filtering points.
:
:T.
:

6/8/2010 8:52:32 AMNorm Leal

John

Good to hear you figured it out.. Look for shorting plates in the tuner if nothing is received below 1000. If stations just become weaker probably need alignment.

Norm

:
:Well, I goofed. When checking from the filters to ground I used a DMM and just noticed it was showing conduction but didn't notice it wasn't measuring zero but just a low value. Both original filters appear to be mechanically connected to the chassis when looking from the bottom but a closer look shows two feed through style connections from the bottom of the chassis where wires go through to the top and then connect to the cases of both filters and so this is the negative connection for the filters and they are not connected to ground. Set works now but gets nothing below about 1000 kcs. Don't know why yet. Thanks for all replies.
:
:
:
::If the original filters are mounted as shown, there should be a fiber washer between the capacitor housing and the chassis. Most likely the screw part is made of phenolic material, and this will automatically insulate from the sides of the chassis hole.
::
::If you are using replacement filter capacitors, merely wire as shown in the schematic. As the schematic Edd posted shows, connecting the filter capacitors to the chassis while they are otherwise wired as shown in the schematic will short across vital filter components.
::
::Also, the chassis is not ground. It is merely the chassis. It is used as a common tie point, but this electrical reference point isn't necessarily ground or the most negative point in the set. It is merely a reference point. If you study the schematic thoroughly, and think of the chassis symbols as merely interconnecting wires, you will better understand how the circuitry works. Some sets use the chassis as the most negative point, and others don't. Having the chassis as the most negative point means that there must be cathode bias resistors, etc., for each tube requiring such circuitry. With the chassis not being the most negative point in the radio, all of the cathodes may tie directly to the chassis, and a few resistors between the chassis and the most negative point in the circuit can serve to both provide negative bias voltages and integral filtering points.
::
::T.
::
: