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Philco 640 - Need Some Help - Geoff - 06-29-2016

I have a Philco 640.  A clear schematic can be downloaded here:

https://drive.google.com/file/d/0B-05OmKqYkNgX0R6Zk9PLTJiOTA/view?usp=sharing

I bought it on Craigslist quite some time ago.  The seller said it worked and I did power it up with a variac and was able to receive stations.  Since then I shotgunned it and replaced ALL resistors and ALL capacitors.  I did not touch any of the trimmers.  I went to fire it back up last night and got nothing.  No reception at all, regardless of band.  The only thing I get is the normal slight power hum through the speaker when the volume is at maximum.  I then tested all the tubes on both a Heathkit TC-2 and a Hickok 533 and got mixed results. I won’t post all the results but an example would be the second diode on the 85 tube testing bad on the TC-2 but testing good on the 533.  Regardless, the radio did pick up stations prior to me working on it.  I am not new to radio but and not a guru either.  I shotgunned by father-in-laws Philco 42-1006, my TransOceanic B600 and, as shown in the next link, my Zenith 9-S-262:

http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=197264

I checked all the voltages per the service information and here is what I got:

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/ScreenHunter_52%20Jun.%2029%2014.15_zpsqylgzk4d.jpg]

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/ScreenHunter_46%20Jun.%2029%2011.02_zps4sdmlte2.jpg]

I did nothing to the speaker other then test for continuity.

The one measurement I was not sure of was the screen grid on the 6A7 tube.  The pin location in the service information did not have a location so I assumed it was the cap on top of the tube:

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/ScreenHunter_49%20Jun.%2029%2012.35_zps0xv9tfta.jpg]

When measuring there I got 0V.  The pin cap was a little loose so I retested it on my TC-2 and it tested fine.  I then stuck it back in and measured all the pin voltages:

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/ScreenHunter_51%20Jun.%2029%2012.51_zpsq4ujgfku.jpg]

I tested the voltages on the 3 other tubes that have caps on top and got the following (cap to ground):

78 R.F. = -2.4V
78 I.F. = -0.5V
85 tube = -5.5V

Also, I measured the voltage in and out of the shadowmeter coil and got 270V and 260V, respectively.  Ohms measurement was 1,551 ohms.

Any ideas on why I am not getting any reception?

RE: Philco 640 - Need Some Help - Radioroslyn - 06-29-2016

Hi Geoff,
Voltages all look pretty good. I'd dump a signal down the If  at the cap of the IF amp (78 ) at 460kc and hear what comes out of the spkr. If that goes well then move the generator to the grid cap of the 6A7 and align the IF transformers. Seems like your LO is working as you have negative voltage on grid 1 of the 6A7. Just to be sure take a look at the voltage found grid 1 to the chassis on each 42 tubes. Should see -15vdc or so. Touch the cap on the 85 tube spkr should emit a loud buzz.

GL
Terry

RE: Philco 640 - Need Some Help - Geoff - 06-29-2016

Thanks Terry, I'll give that a try.

RE: Philco 640 - Need Some Help - Geoff - 06-30-2016

The voltages on the 42 tubes are as follows:

1st 42
Pin 2: 262V
Pin 3: 275V
Pin 4: -16.9V
Pin 5: 0V

2nd 42
Pin 2: 267V
Pin 3: 273V
Pin 4: -16.7V
Pin 5: 0V

RE: Philco 640 - Need Some Help - Radioroslyn - 06-30-2016

As for the 6A7 mixer Let me see if I can muddy the waters for you. It's probably more concise to use grid numbers rather than names.

Pin 5 is grid1, the control grid for the oscillator section of the mixer. When the oscillator is operating properly should see a negative voltage there with a hi impedance voltmeter.

Pin 4 is grid 2, some call it the screen grid but it acts as a PLATE for grid 1. The combination of pin 6 the cathode, pin 5, and pin 4 forms a triode which is the oscillator section of the mixer. It must have hv on pin 4 to operate.

Pin 3 grids 3 and 5 act as screen grids. Their job is to speed up the electrons headed for the plate. It to must have hv on it to operate properly. Usually a bit less than grid 2 and the plate.

The cap is grid 4 it is the control grid for the amplifier section of the mixer. This is where the incoming signal is connected to the mixer. In the case of the 640 the signal is coming from the 78 rf amplifier. Some sets don't have a rf amp stage so this grid is connected to the antenna coil.

Think of the pentagrid converter tube as two tubes one on top of the other. The bottom tube is a triode ( oscillator) and the top is a tetrode (amplifier). The input of the amp (cap) is tuned to incoming signal and the output (plate) is tuned to the IF frequency. The oscillator section is oscillating at 460kc above the incoming signal and the result is a signal at 460kc which is amplified in the IF amp.

The G1 voltages on the 42 tubes is good. Won't have any effect on the mixer but call me George I was just curious.

Clear as mud right?

Terry

RE: Philco 640 - Need Some Help - Steve Davis - 06-30-2016

The very first thing to do is, touch the grid cap of the 85 tube with your finger. Do you get a loud hum? Next, place another radio next to the radio under test to see if you can tune the local oscillator of the nonworking radio.

Steve

RE: Philco 640 - Need Some Help - Paul Philco322 - 06-30-2016

Just the detailed presentation of your query I find exhausting!

Paul

RE: Philco 640 - Need Some Help - markmokris - 07-01-2016

(06-30-2016, 10:06 AM)Steve Davis Wrote:  The very first thing to do is, touch the grid cap of the 85 tube with your finger. Do you get a loud hum? Next, place another radio next to the radio under test to see if you can tune the local oscillator of the nonworking radio.

Steve

This is the strategy that should be used with any dead radio - (1) Can you get some hum in the speaker by touching the grid of the last couple stages? Then your audio section is probably working.  Otherwise, concentrate on those last one or two stages.

If the audio is working, (2) determine if the LO oscillator is working.  If not, concentrate on the LO / converter stage.

If the LO is working, then why is the signal not passing through and reaching the audio stages of the radio? This is where you can get out your signal generator and start tracing a signal through, stage by stage.

Mark K8KZ

RE: Philco 640 - Need Some Help - Geoff - 07-01-2016

(06-30-2016, 10:06 AM)Steve Davis Wrote:  The very first thing to do is, touch the grid cap of the 85 tube with your finger. Do you get a loud hum? Next, place another radio next to the radio under test to see if you can tune the local oscillator of the nonworking radio.

Steve

Yes, I get a loud hum when touching the grid cap of the 85 tube.

I will try the radio test tonight.

Thanks!

RE: Philco 640 - Need Some Help - Geoff - 07-01-2016

(06-30-2016, 08:40 PM)Paul Philco322 Wrote:  Just the detailed presentation of your query I find exhausting!

Paul

Sorry, but I dislike being vague Icon_biggrin .

RE: Philco 640 - Need Some Help - Geoff - 07-01-2016

(07-01-2016, 11:59 AM)markmokris Wrote:  
(06-30-2016, 10:06 AM)Steve Davis Wrote:  The very first thing to do is, touch the grid cap of the 85 tube with your finger. Do you get a loud hum? Next, place another radio next to the radio under test to see if you can tune the local oscillator of the nonworking radio.

Steve

This is the strategy that should be used with any dead radio - (1) Can you get some hum in the speaker by touching the grid of the last couple stages? Then your audio section is probably working.  Otherwise, concentrate on those last one or two stages.

If the audio is working, (2) determine if the LO oscillator is working.  If not, concentrate on the LO / converter stage.

If the LO is working, then why is the signal not passing through and reaching the audio stages of the radio? This is where you can get out your signal generator and start tracing a signal through, stage by stage.

Mark K8KZ

Thanks Mark.

RE: Philco 640 - Need Some Help - Geoff - 07-01-2016

(06-29-2016, 07:52 PM)Radioroslyn Wrote:  Hi Geoff,
Voltages all look pretty good. I'd dump a signal down the If  at the cap of the IF amp (78 ) at 460kc and hear what comes out of the spkr. If that goes well then move the generator to the grid cap of the 6A7 and align the IF transformers. Seems like your LO is working as you have negative voltage on grid 1 of the 6A7. Just to be sure take a look at the voltage found grid 1 to the chassis on each 42 tubes. Should see -15vdc or so. Touch the cap on the 85 tube spkr should emit a loud buzz.

GL
Terry

I do get a loud buzz when touching the grid cap on the 85 tube.

RE: Philco 640 - Need Some Help - Geoff - 07-01-2016

I'll be hooking up my signal generator tonight and testing the IF section. I'll also try the 2nd radio test to see if the oscillator is working properly.

Also, I went in and replaced #22. It was listed in the service bulletin as a 0.01 uF cap and that was what I had installed, but there was an asterisk. The asterisk stated that after Run. No. 10 to use a 250 pF mica cap. I had originally removed that cap and I forgot it was a mica. I then fired it up and checked the SG (pin #3) on the 6A7 and got 42V instead of the original 180V. I don't have my Excel spreadsheet so here are the voltages:

78 R.F.
Point P (pin #2): Expected = 71, last check = 81, tonight's check = 204
SG (pin #3): Expected = 91, last check = 90, tonight's check = 42
K (pin #5): Expected = 2.1, last check = 3, tonight's check = 1

6A7
Point P (pin #2): Expected = 240, last check = 260, tonight's check = 270
SG (pin #3): Expected = 91, last check = 0, tonight's check = 42
K (pin #6): Expected = 2.2, last check = 3, tonight's check = 6.2

78 I.F.
Point P (pin #2): Expected = 242, last check = 266, tonight's check = 272
SG (pin #3): Expected = 91, last check = 91, tonight's check = 42
K (pin #5): Expected = 2.3, last check = 2.3, tonight's check = 0.81

85
Point P (pin #2): Expected = 102, last check = 111, tonight's check = 112

1st 42
Point P (pin #2): Expected = 240, last check = 264, tonight's check = 263
SG (pin #3): Expected = 250, last check = 272, tonight's check = 276

2nd 42
Point P (pin #2): Expected = 240, last check = 258, tonight's check = 268
SG (pin #3): Expected = 250, last check = 272, tonight's check = 276

80
Filament 1 to Ground: Expected = 300, last check = 319, tonight's check = 328
Filament 2 to Ground: Expected = 300, last check = 319, tonight's check = 327

It should be noted that I used a different multimeter for the above readings. I had problems with the one I was using before so I borrowed one from work. It is a calibrated Fluke 787.

Also, I am using a variac with an analog meter so I think slight differences in voltages from the last check can be attributed to not being precisely where the analog needle was last time.

Again, I only changed the 0.01 uF (#22) to a 250 pF mica. I touched nothing else.

RE: Philco 640 - Need Some Help - Geoff - 07-01-2016

(06-30-2016, 07:45 AM)Radioroslyn Wrote:  Clear as mud right?

Terry

My background is not electronics but I do follow what you are saying. It's easier to visualize when I read your notes and examine the tube diagram.

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/ScreenHunter_50%20Jun.%2029%2012.37_zps80iynlvf.jpg]

RE: Philco 640 - Need Some Help - Radioroslyn - 07-01-2016

Just so you know this is just a hobby for me too but have been at it for 50yrs or so.
Your screen voltage for the mixer and if amp is too low. Check #64 too high in resistance, 79 too low in resistance,and 68 leaky or shorted.
If values are ok then pull 78 and check sg voltage then pull the 6a7 and measure the sg voltage. If it goes up significantly then substitute the tube.

Terry