I have a Philco 90 that I finished recapping, including electrolytics, and replacing most of the resistors. I have a hum problem that I cannot seem to trace. The hum seems most pronounced when the volume is all the way down against the stop. As the volume is turned up or a station is tuned in, the hum gets less objectionable but is still present. My serial number is 949101 and it is the model with one 47 output and AVC. Can anyone help me with this problem. I have worked on this problem for over a week, all volatges look good at the tube sockets and it's beginning to drive me batty. Are there known trouble spots with this set? I'm mainly a Zenith person and this is my first Philco. Help!

Did you replace 56 and 57 exactly according to the schematic? Notice how one connects its negative to the chassis, while the other connects to the transformer center tap. Also, the one going to the tap has its positive prior to the field coil. The one going to the chassis has its positive after the field coil. Do not alter these connections.

Also be sure that every other condenser is connected exactly as it should be. Did you modify any wiring? The #47 filament and its winding center tap must be connected exactly as shown in the schematic, or else it'll hum and malfunction. Be sure that you're looking at the 90 schematic, as, on this web site, there are also schematics for I believe the 70 and 71, which show up with the 90 schematic when you try to look at it.

Thomas

:Did you replace 56 and 57 exactly according to the schematic? Notice how one connects its negative to the chassis, while the other connects to the transformer center tap. Also, the one going to the tap has its positive prior to the field coil. The one going to the chassis has its positive after the field coil. Do not alter these connections.
:
:Also be sure that every other condenser is connected exactly as it should be. Did you modify any wiring? The #47 filament and its winding center tap must be connected exactly as shown in the schematic, or else it'll hum and malfunction. Be sure that you're looking at the 90 schematic, as, on this web site, there are also schematics for I believe the 70 and 71, which show up with the 90 schematic when you try to look at it.
:
:Thomas

Thomas,

Thanks for the advice. Although I have been at this almost a week, I will check specifically what you mentioned. It will help a bunch. Thanks again. Anyone else have a suggestion?

David R

:Did you replace 56 and 57 exactly according to the schematic? Notice how one connects its negative to the chassis, while the other connects to the transformer center tap. Also, the one going to the tap has its positive prior to the field coil. The one going to the chassis has its positive after the field coil. Do not alter these connections.
:
:Also be sure that every other condenser is connected exactly as it should be. Did you modify any wiring? The #47 filament and its winding center tap must be connected exactly as shown in the schematic, or else it'll hum and malfunction. Be sure that you're looking at the 90 schematic, as, on this web site, there are also schematics for I believe the 70 and 71, which show up with the 90 schematic when you try to look at it.
:
:Thomas

Thomas,

I double checked the wiring and it seems to match the schematic. I did not make any wiring modifications however the symbol you thought was the speaker field is actually a choke across the positives of the electrolytics. The speaker field is shown in the upper right hand corner of the schematic. However, there is something strange, the choke across the electrolytics has a .09 cap across it in my chassis but it is not shown on the schematic. I have heard that Philco was notorious for making changes and not documenting them. Could this be the problem? Help!!

Hi David

I would remove the .09 mf cap. This cap was used to make a tuned circuit reducing hum. If value isn't exact hum will be increased.

Using 10 mf filters and removing the .09 mf cap should eliminate the hum problem if it's from the power supply.

Here is a nice schematic:

http://www.philcoradio.com/tech/images/90b.jpg

There are other versions if this schematic doesn't match, two 45's or 2 47's for output.

Norm

::Did you replace 56 and 57 exactly according to the schematic? Notice how one connects its negative to the chassis, while the other connects to the transformer center tap. Also, the one going to the tap has its positive prior to the field coil. The one going to the chassis has its positive after the field coil. Do not alter these connections.
::
::Also be sure that every other condenser is connected exactly as it should be. Did you modify any wiring? The #47 filament and its winding center tap must be connected exactly as shown in the schematic, or else it'll hum and malfunction. Be sure that you're looking at the 90 schematic, as, on this web site, there are also schematics for I believe the 70 and 71, which show up with the 90 schematic when you try to look at it.
::
::Thomas
:
:Thomas,
:
:I double checked the wiring and it seems to match the schematic. I did not make any wiring modifications however the symbol you thought was the speaker field is actually a choke across the positives of the electrolytics. The speaker field is shown in the upper right hand corner of the schematic. However, there is something strange, the choke across the electrolytics has a .09 cap across it in my chassis but it is not shown on the schematic. I have heard that Philco was notorious for making changes and not documenting them. Could this be the problem? Help!!

I might be off-base with this, but sometimes a hum is caused by excessive (conductive) bias on the output stage. If there is too much current flowing while the stage is idling, the power supply ripple will be heard in the speaker!

Disconnect the the G1 coupling capacitor to verify.
I could be the coupling cap itself leaking, thus causing an undesireable bias shift.

The cathode bypass might have leak issues to cause the bias change as well.

just some random thoughts...

:Hi David
:
: I would remove the .09 mf cap. This cap was used to make a tuned circuit reducing hum. If value isn't exact hum will be increased.
:
: Using 10 mf filters and removing the .09 mf cap should eliminate the hum problem if it's from the power supply.
:
: Here is a nice schematic:
:
:http://www.philcoradio.com/tech/images/90b.jpg
:
: There are other versions if this schematic doesn't match, two 45's or 2 47's for output.
:
:Norm
:
:::Did you replace 56 and 57 exactly according to the schematic? Notice how one connects its negative to the chassis, while the other connects to the transformer center tap. Also, the one going to the tap has its positive prior to the field coil. The one going to the chassis has its positive after the field coil. Do not alter these connections.
:::
:::Also be sure that every other condenser is connected exactly as it should be. Did you modify any wiring? The #47 filament and its winding center tap must be connected exactly as shown in the schematic, or else it'll hum and malfunction. Be sure that you're looking at the 90 schematic, as, on this web site, there are also schematics for I believe the 70 and 71, which show up with the 90 schematic when you try to look at it.
:::
:::Thomas
::
::Thomas,
::
::I double checked the wiring and it seems to match the schematic. I did not make any wiring modifications however the symbol you thought was the speaker field is actually a choke across the positives of the electrolytics. The speaker field is shown in the upper right hand corner of the schematic. However, there is something strange, the choke across the electrolytics has a .09 cap across it in my chassis but it is not shown on the schematic. I have heard that Philco was notorious for making changes and not documenting them. Could this be the problem? Help!!

Gentlemen,

I removed the .09 cap across the choke and it made no difference. I'm using 8uF electrolytics for the power supply. I'm going to short the grid of the 47 to ground to make sure it is not coming from upstream of the 47. Please keep the suggestions coming. I'm ready to trash this thing, I have never been so stumped with a set.

:I might be off-base with this, but sometimes a hum is caused by excessive (conductive) bias on the output stage. If there is too much current flowing while the stage is idling, the power supply ripple will be heard in the speaker!
:
:Disconnect the the G1 coupling capacitor to verify.
:I could be the coupling cap itself leaking, thus causing an undesireable bias shift.
:
:The cathode bypass might have leak issues to cause the bias change as well.
:
:just some random thoughts...
:
:
:
:
:
::Hi David
::
:: I would remove the .09 mf cap. This cap was used to make a tuned circuit reducing hum. If value isn't exact hum will be increased.
::
:: Using 10 mf filters and removing the .09 mf cap should eliminate the hum problem if it's from the power supply.
::
:: Here is a nice schematic:
::
::http://www.philcoradio.com/tech/images/90b.jpg
::
:: There are other versions if this schematic doesn't match, two 45's or 2 47's for output.
::
::Norm
::
::::Did you replace 56 and 57 exactly according to the schematic? Notice how one connects its negative to the chassis, while the other connects to the transformer center tap. Also, the one going to the tap has its positive prior to the field coil. The one going to the chassis has its positive after the field coil. Do not alter these connections.
::::
::::Also be sure that every other condenser is connected exactly as it should be. Did you modify any wiring? The #47 filament and its winding center tap must be connected exactly as shown in the schematic, or else it'll hum and malfunction. Be sure that you're looking at the 90 schematic, as, on this web site, there are also schematics for I believe the 70 and 71, which show up with the 90 schematic when you try to look at it.
::::
::::Thomas
:::
:::Thomas,
:::
:::I double checked the wiring and it seems to match the schematic. I did not make any wiring modifications however the symbol you thought was the speaker field is actually a choke across the positives of the electrolytics. The speaker field is shown in the upper right hand corner of the schematic. However, there is something strange, the choke across the electrolytics has a .09 cap across it in my chassis but it is not shown on the schematic. I have heard that Philco was notorious for making changes and not documenting them. Could this be the problem? Help!!

OK, here's the latest on the Philco testing. I double checked the capacitors in the audio section for the right value and they are all correct. I replaced several resistors that read a little high or low. They probably didn't really need replacing, but I thought I'd see if it helped.... it didn't.

Next I removed the grid lead from the audio tube (single 47) and I got no sound, but a "normal" soft background old radio hum. I reconnected the grid lead to the 47 and disconnected the grid lead to the 1st AF tube. This is a type 27 tube. The radio now works with so called "normal" hum, although the volume is a little low even with the control cranked all the way clockwise. Here the scary part... there is nothing connected to the grid of the 1st AF tube. I don't even know how this radio is working.

I'm not sure I solved a problem or just created more questions for myself. Can anyone answer these questions and help me restore this set to normal volume levels? Why does this set work with nothing connected to the grid of the tube? I'm really stumped!

I recommend increasing the electrolytics to 20 MFD units. I used these in my Philco 60 with great results (absolutely no hum). You say that you replaced all condensers, so I assume that you replaced the ones in the bakelite block. It is possible to still get some audio with the grid lead disconnected. There'll still be capacitive coupling through the air. Just goes to show how sensitive tubes are. You most likely do have troubles with condenser 23 or one of the two 24 condensers. Could be something else, too, like maybe you mixed up condenser wires when you put them inside of the bakelite blocks. Could be that the grid bias circuit isn't filtered properly elsewhere. I strongly recommend that you increase the electrolytics to 20 MFD units, however.

If you decide to trash the set, send it my way!!!! I'll fix it for sure! Too bad I can't trouble shoot long distance. It is much more difficult to do so when the set isn't right in front of you. Hopefully all of the suggestions everyone has given will help.

Thomas

Dave, nows not the time to trash the set. If the set worked with no hum, low volume with the 1st stage grid disconnected you have isolated the hum problem to that stage. Its got to be in that circuit whether it a mis wire, value of resistance off or a bad cap it shouldn't be to hard to find now. PL

Actually, electrically speaking, it's almost never time to trash the set unless it's terribly rusted. Just about everything can be repaired or replaced. If you follow the schematic, and get all components working as they should, the set will work like new. The circuitry is of good design, or else Philco wouldn't have sold thousands.

Thomas

The testing will continue this afternoon and yes I will replace the 8uF electrolytics with 20uF units. All the caps have been replaced by restuffing the bakelite blocks, I will pull them out of the radio and double check their values and connections. Thanks for the advice, hopefully I will have this set working soon.

I probably misspoke when I indicated I might trash this set. Nothing doing, someone gave it to me and it is in almost perfect condition, no rust, no electronic hack jobs and the cabinet just needs a little cleaning.

I'll keep working and let you know my progress. Thanks again for everyones help!

David

I just took a look at the schematic. I agree that it's very weird that with the triod's coupling cap disconnected that you get audio in a PROPERLY operating circuit. But WAIT! this has other paths though that multi-tap resistor. If the filter caps are open or perhaps if those taps are shorted you might be getting enough fluctuation in the DC from the detector stage that it's being amplified as proper audio.

also note that point c is a negative bias node. the ratio of the caps in this power supply must be maintained.

I agree that you are very close to a solution here.
it might be a simple as changing that disconnected cap,
but it concerns me that you get decent audio without it which would make ME recheck the power supply as well.

Happy hunting. We're all gonna keep an eye on your dumpster...just in case..LOL

:OK, here's the latest on the Philco testing. I double checked the capacitors in the audio section for the right value and they are all correct. I replaced several resistors that read a little high or low. They probably didn't really need replacing, but I thought I'd see if it helped.... it didn't.
:
:Next I removed the grid lead from the audio tube (single 47) and I got no sound, but a "normal" soft background old radio hum. I reconnected the grid lead to the 47 and disconnected the grid lead to the 1st AF tube. This is a type 27 tube. The radio now works with so called "normal" hum, although the volume is a little low even with the control cranked all the way clockwise. Here the scary part... there is nothing connected to the grid of the 1st AF tube. I don't even know how this radio is working.
:
:I'm not sure I solved a problem or just created more questions for myself. Can anyone answer these questions and help me restore this set to normal volume levels? Why does this set work with nothing connected to the grid of the tube? I'm really stumped!
:
:

Hi David

Now things are interesting. By disconnecting the grid coupling cap to first audio stage you removed low frequency response of the radio. The sound you heard was most likely only high frequencies?

Without low frequency response hum was lost.

Are coupling caps very large values? Try another 27 detector tube. Some of these, especially old types, cause hum.

Norm

:OK, here's the latest on the Philco testing. I double checked the capacitors in the audio section for the right value and they are all correct. I replaced several resistors that read a little high or low. They probably didn't really need replacing, but I thought I'd see if it helped.... it didn't.
:
:Next I removed the grid lead from the audio tube (single 47) and I got no sound, but a "normal" soft background old radio hum. I reconnected the grid lead to the 47 and disconnected the grid lead to the 1st AF tube. This is a type 27 tube. The radio now works with so called "normal" hum, although the volume is a little low even with the control cranked all the way clockwise. Here the scary part... there is nothing connected to the grid of the 1st AF tube. I don't even know how this radio is working.
:
:I'm not sure I solved a problem or just created more questions for myself. Can anyone answer these questions and help me restore this set to normal volume levels? Why does this set work with nothing connected to the grid of the tube? I'm really stumped!
:
:

I am noticing more and more #24 condensers. They filter the bias voltage to the grid of the 1st audio. All of them must be connected and functioning well in order to eliminate hum.

T.

Here were some suggestions I found in an old book for eliminating hum in a Philco 90: They indicated that the 47 tube was too close to the rectifier: they suggested a shield plate between the 47 and the rectifier. They indicated that even though the #27 tube may test good it may still be defective and causing hum. Same suggestion for the #47 tube. Hope this helps

MRO

I wanted to let everyone know what the problem was with this set. After two weeks of driving myself insane with changing this and rewiring that, I discovered the speaker was not original to this set.

With that in mind I started looking at the field coil and how it was wired into the power supply. I discovered the previous owner had wired the field coil backward and it was not cancelling the 60Hz hum, but in fact enhancing it. In hindsight, I wonder now if the radio ever worked after the speaker was changed, or if the speaker was robbed and switched with the current one after sitting on the shelf for many years.

Whatever the reason for the missing speaker being switched, I now have a wonderful sounding Philco 90 to enjoy for the Holidays. Thanks to everyone for the advice.

David R

I have a Philco 90 that I finished recapping, including electrolytics, and replacing most of the resistors. I have a hum problem that I cannot seem to trace. The hum seems most pronounced when the volume is all the way down against the stop. As the volume is turned up or a station is tuned in, the hum gets less objectionable but is still present. My serial number is 949101 and it is the model with one 47 output and AVC. Can anyone help me with this problem. I have worked on this problem for over a week, all volatges look good at the tube sockets and it's beginning to drive me batty. Are there known trouble spots with this set? I'm mainly a Zenith person and this is my first Philco. Help!

Yes, hum bucking systems will do quite the opposite if wired up backwards. Hope your Christmas music sounds excellent!

T.