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Model 650 Tone Control Question - Printable Version

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RE: Model 650 Tone Control Question - Hamilton - 06-30-2023

I checked the AC voltages and they all seem good but I’m getting some strange DC readings. 

The first 42 output tube, red circle is giving me approximately 120 dc on both plate and grid. I say approximately because it seems to fluctuate. I have switched the 42’s around but the readings stay similar. The other 42 output tube, green circle, is giving me 365 dc on both plate and grid. No fluctuating there! The 42 driver tube is 246 on both the plate and grid.

The 6A7 is 250 on the plate, 86 grid and 2.45 cathode, 78 IF is 261 plate, 86 pin and 1.68 cathode, the 78 RF is 122 plate, 86 grid and 2.34 cathode and the 75 is 171 on the plate. 

Hopefully this provide some needed information for those of you that are much better at troubleshooting then I am. Please feel free to comment. I don’t have much experience but I do have thick skin so any advice is appreciated.

RE: Model 650 Tone Control Question - MrFixr55 - 06-30-2023

Hi Hamiton,

If you get half of a post from me, fear, not, I will complete it. My computer mouse has lost its mind!

OOps, read my post, then reread yours.



Are you talking about the screen grid voltage being the same as the plate or the control voltage being the same as the plate. If you are talking about the Screen grids of the 42s, this is normal! These tubes are connected as Triodes on the 650. The screens are tied to the plates on all 3 42s (the "driver" and the 2 "Push Pull" output tubes), therefore the screen and plate voltage will match on each of the tubes. the "control" grids" (the input to the tube) should be at a negative voltage, especially for the driver) when measured against chassis ground. If the control grid was at plate voltage, the plates would glow red, melt, blow the output transformer, etc.! the Screen/plate voltages on the 2 42s that are output tubes should be the same.

If you rewired the speaker wiring, recheck it. If you miswired, you may have B+ directly to one output tube through the field coil, 1 leg of the output transformer on the other output tube and the other leg of the output transformer feeding B+ (modulated by the output transformer) to the rest of the radio.

With the power off and unplugged and the filter caps discharged, you should get 350 Ohm between the plates of the 2 output tubes. If you get more like 1.5K Ohm then it is miswired.

After correcting, if there are still issues, see the rest of my post.

Try this "divide and conquer" technique:
1. Power down the radio.
2. Remove the grid cap connection from the "75" detector / 1st Audio tube.
3. Power up and allow 1 minute to warm up.
4. Touch the grid cap of the 75. You should receive a very loud hum, especially if your house wiring is not armored cable and you have old school fluorescents in your house.
5. If you get the hum, troubleshoot the RF, IF, detector, volume and tone control circuits and recheck wiring. If no hum, troubleshoot the AF section (1st AF, Driver and output stage).

There are 2 or 2 caps around 50 -110 pF between the detector and 1st AF. If these caps short, this will kill the sound but you will pass the "Hum" test as described above.

Most of my radios have a "phono" jack installed, either from the factory or added by me. This way, if I don't like what I am picking up on AM or want a cleaner, more "HiFi" sound, I can play an MP3, FM Tuner or computer through the radio and call up an "old time radio" (OTR) website or channel, or pipe the Big Band channel from the cable company through the radio. The amplifier in this radio should be a real good performer!

RE: Model 650 Tone Control Question - Hamilton - 06-30-2023

Thanks Mr Fix. 

I was measuring Pin 3 to get my screen voltages. When I measure Pin 4, which on Frank’s is Grid #1, I’m getting -29 dc on the 2 - 42 output tubes and -13 dc on the 42 driver tube. Is that the control voltage you referred too.

The speaker and it’s wiring has not been touched and is original. When i put the probes from Pin 2 onto Pin 3 of the 2 42 output tubes I get a very high reading. Am I doing that correctly.

The radio does work and picks up the local stations but low volume that doesn’t increase with the volume control. The one 42’s output tube’s voltage is very low from the 300 dc that the schematic indicates it should have.

RE: Model 650 Tone Control Question - Hamilton - 06-30-2023

I forgot to include that I did remove the grid cap from the 75 and got a loud hum. I will recheck the sections you mentioned but I’m not experienced enough to recognize problems at this stage. Hopefully something obvious pops up.

Hamilton - Hamilton - 07-03-2023

Something obvious popped up and I now have proper volume. I’m enclosing a pick of before and after. I think it turned out well for a rookie.

RE: Model 650 Tone Control Question - DaleHCook - 07-03-2023

That is very nice work for a rookie. Although I have some knowledge of, but little experience in, cabinetry, I have always avoided sets which would require significant cabinet repair. I am planning to get into some cabinet work this summer, beginning with a 1946 Motorola farm radio which needs little surface work but which does have a largely separated frame. The surface work needed includes one area in which I have a bit of experience - touching up relatively small areas of worn or damaged photo finish.

RE: Model 650 Tone Control Question - Steve Davis - 07-03-2023

Great job!

Steve

RE: Model 650 Tone Control Question - Hamilton - 07-03-2023

Thanks Dale and Steve. Much appreciated. I can do cabinet repairs. I did my first radio cabinet restoration during covid for a fellow and that when I took up this hobby. I have no electrical experience and that’s why I try to get help from this Phorum. 

So far, so good!

RE: Model 650 Tone Control Question - MrFixr55 - 07-03-2023

Hi Hamilton,

Sorry for not catching your PM.  I will be happy to do so.  Unfortunately, I do not have facilities to easily scan a document since I retired, so it will take me some time to draw and scan a specific one.  I will try to do so.  However, let's try this:

The pic below is your volume control from one of your posted pix.  Note the numbers.  BTW, the connection noted as number 1 should be well grounded to the chassis.

   

The next pic is a portion of the Philco 650 Schematic showing the volume control.  The numbers correspond to the numbers on the picture of the volume control.

   

To add a Radio - Phono Switch and Phono connection, do the following:

Obtain a single pole double throw switch and a chassis mount RCA Jack and some shielded cable.  Mount these in a convenient location on the chassis, but preferably away from the power cord and power transformer.  If there are no convenient holes on the chassis and you do not want to drill holes, then make a metal bracket.  SPDT switches are usually marked C for Common, 1 and 2, or something similar.  If not marked, the middle terminal is usually the common.

Disconnect all wires from terminal 3 of the volume control.  Connect these to one of the end terminals of the switch using a length of shielded cable.  Connect the common terminal of the switch to terminal 3 of the volume control using another piece of shielded cable.  Connect the RCA Jack to the remaining terminal of the switch.      Connect the shield at only 1 end, unless the switch and jack are not mounted to the chassis, or if mounted on a bracket, if the bracket is mounted to the cabinet and not the chassis.  In that case, ground both ends of the shield, one to the chassis, one to the bracket, but do so for only 1 shielded cable.  This is to prevent "hum" from "Ground Loops".

This jack will handle a ceramic phono cartridge or "line level" inputs such as fro a computer, CD Player, etc.  You could connect a guitar to the radio using this connection and the appropriate cable or adaptors.  It will not be really loud or distorted but will have a warm tone.

BTW, seriously nice job on the cabinet!  A beautiful radio! Icon_clap

RE: Model 650 Tone Control Question - MrFixr55 - 07-04-2023

Hi Hamilton, 

Regarding voltages and pinouts for the 42 "Driver" and the 2 42 "Push Pull" Output tubes:

Below is a diagram of the connections to the 42 as viewed from the underside of the chassis.  Please note that the heater connections, Pins 1 and 6 are larger in diameter than the other pins.
   


Below is a copy of the Tung Sol Spec Sheet for the 6F6, 42 and 2A5.  Other than pinouts (6F6) and heater voltage, current (2A5), these tubes have the same characteristics.  This lists "maximum" voltages.  For the output stage use Class AB, triode connected, cathode bias.
.pdf 2A5 6F6 42 Tung Sol Spec Sheet.pdf Size: 222.18 KB  Downloads: 65


Below is the section of the Philco 650 schematic showing the output stage only, for clarity.
   

The Philco 650 Schematic shows all 3 "42"s connected as triodes (This was common; RCA did this in several sets using 2A5s).  Of course, Philco had more than 1 variation for several chassis and these may be connected as pentodes.  However, the plate and screen voltages should substantially match between the 2 output tubes.  In addition, the control grid voltages should also substantially match between the 2 tubes.

Hope this helps!

462ron - 462ron - 07-04-2023

Hamilton what an exquisite refinishing job you did on that cabinet. Do you have spray equipment or did you use rattle cans? I ask because I have a 37-116 cabinet in need of refinishing sitting for about 5 years as I procrastinate starting on it because not sure I want to do it with rattle cans!

Ron

Hamilton - Hamilton - 07-04-2023

Thanks MrFix for the time to explain the hookup for the phono plug. Clear and concise. Much appreciated. I’m at a cottage for a couple of days and will check out the 42’s when I get home. I do know however that pin 2 and 3 are jumpered together for all 3 of them. Thanks again!

462ron. There was a lot of damage to the veneer in spots and the fact that I had to make a new tilt-top piece since it was impossible to straighten the original piece, I went with Teak oil only. I put around 5 thin coats on and thats what ended up giving it the sheen. Some of the trim that originally would have used a toner was badly gouged in spots and was a bit easier to hide the gouges by painting it black. I know that not for purists but getting Mohawk toners here in Canada is difficult and expensive. I realize that doesn’t help you out but thats the procedure I used with this radio. Cheers

462ron - 462ron - 07-04-2023

Thanks Hamilton, actually it does help. It gives me thought for an alternative method! Icon_thumbup

Ron

RE: Model 650 Tone Control Question - DaleHCook - 07-04-2023

Hamilton, we are always here to provide advice on electronic repairs.

RE: Model 650 Tone Control Question - Hamilton - 07-04-2023

Thanks Dale. 

I appreciate your note. I know I frustrate some of the more experienced members with my inability to fully understand schematics. I’m 70, took this up at 68 have no previous electronics experience and have tried to read and comprehend as much literature as I can and being dyslexic does not help. 

I was a firefighter for 37 years and didn’t need much schooling on how to do that. I believe I’ve made great progress considering Covid stopped everything cold so all I had when I started was Youtube videos, this forum and whatever I could understand from reading recommended vintage radio books. 

It’s all good, I thoroughly enjoy the hobby and plan to improve my skills.