[Tubester]

Just picked up a Philco 610B a few months back to begin a new hobby of radio restoration. By way of introucition, I'm a retired electrical/mechanical engineer from the auto industry and getting it re-capped brought back memories from my early years working in instrumtation groups. Also have fond memories of checking tubes from the early TV's using some of the tube testers locally where I lived at that time. But I digress! I have completely cleaned and recapped my chassis. I have created an 'actual' schematic which compares very well with the 'later production' schematics out there showing phonograph wiring. I say actual, because there are some differences that don't make sense to me. For example, the 6A7 tube cathode is grounded with the filament ground. The pair of 51 and 25 k ohm resisters are, instead of being connected to the 6A7 cathode, are wired to the 78 tube grid 3, which is also wired into the BC resistor no. 57. Additionally, the oscillator has some descrepences with the later production schematic. The oscillator seems to be wired correctly for the standard and police boradcast bands (positions 1 and 2 on the bandswitch), but position 3 is another story. The secondary side seams to be ok, but the primary side does not seam right. For example, there is a wire which connects the shortwave band primary input into the 6A7 grid 2 anode directly, although there also is also a feed from the switch 1 connection when in position 3 for shortwave. Also, there is a common side to the shortwave primary/secondary's at the 2250 pF and compensating condenser nut 13 shown in the schematic, which is not there in my unit. So my questions relate to these variances from the later procution schematic. Although the radio does power up and I can play stations from band 1 and 3, there is noticable static or hum. Do any of you experienced vacuum tubers have any thoughts? Should I just rearrange a few wires to agree with the oem schematics + documented service bulletins and call it a day? Thanks in advance from Dan the Tubester.

[Tubester]

   
I've included a picture of the 'actual' schematic I created. I double checked several times and it appears to be accurate. General layout is as appears from the bottom view, with the front of the chassis on the left. Areas of clarification needed are around the oscillator with the wire lines in question highligted with red. I've numbered the oscillator bottom solder tabs clockwise starting at 12:00 oclock, shown as 1,2,3,4 & 5. On the top of the oscillator (hidden by the can) are two solder tabs numbered T4 and T5. These two tabs have wires feed to them from the bottom of the chassis wiring into the center of the oscillator and up to the two solder tabs (shown with an 'x' with a circle around the 'x'). I have identifed the ohms of the coil primaries and seconaries but have not shown them. (Note: to measure them you have to measure the ohms of your test leads and subtract them from your readings.) I believe the shortwave coil primary (brought in circuit by band switch position 3) is across T4-B3 @ 0.5 ohm and the secondary is across B4-B5 @  0.1 ohm. For reference, although not in question because it agrees with oem later production schematic,  broadcast and police wave coil primary (brought in circuit by band switch position 1 & 2) is T5-B1 @ 0.4 ohm and the secondary is T5-B2 @ 3.7 ohm. I am looking for someone to confirm the oscillator solder tabs with respect to the ohms noted and to confirm the connections of the tabs into the circuit. Thanks.... Dan

[morzh]

If the wires look like they were this way from the factory (very neat and very dull soldering joints), I'd leave them be.

The hum could be just that, AC hum, also from the devices around. You could try to switch off things nearby that have converters in them. Some hum might be undefeatable.

[MrFixr55]

Hi Dan,
Divide and conquer. First off trry to differentiate between 60Hz and 120 Hz hum. 120 Hz hum has a higher pitch. Pull out the converter tube (the 6A7). Obviously, there will be no stations. We are checking for background hum in the radio here. start with the volume control all the way down. Is the hum objectionable with the volume control all the way down? If so, check the power supply. If good, raise the volume. There will be a little more hum, but only a little. Again, is the hum objectionable? If yes, where is the grid cap wire for the detector / AF tube? it needs to be shielded. In early Philcos, the cable should be within the tube shield. If outside, it will pick up hum as the volume is increased.

If all of these tests pass, reinstall the converter tube. Tune between stations. Is the hum objectionable? If not, if there is only hum with stations, the cause is the environment.

You HAMs and other experts chime in, but for some reason, sets with Ferrite loopstick antennas (late 1950s and later) seem to have less interference form modern electronic pollution (Old School Fluorescents, CFLs, LEDs, computers, IoTs (the Internet of Things), carrier current meter reading, etc. In my experience, currently at my house, any radio requiring an external antenna seems to have its signal modulated by the environment. Even battery operated radios in my house give hum. I need to move the radio outside and away from the house to get less hum, even the outside long wire doesn't help if it comes into my office. If I pull the lead-in ack outside the house, the signal to hum is much better. It's funny that a lot of audio equipment now use switching power supplies. their output may be clean, but they sure dirty the AC line.

[Tubester]

Thanks for the replies. It's not the really the hum that I am after. Just trying to understand why only the shortwave band part of the osciallator is different from oem schematic. Most of the wiring looks original, although there was evidence of some newer (50-60's) capping. OBTW, mine has only 5 bakelite blocks, so it appears to be a later production unit. Was hoping someone with a 610B could weigh in and comment on my oscillator wiring comparted to theirs. Will eventually change the oscillator wiring to oem schematic and see what happens. Then, will tackle replacing the speaker cone. Is does work but has been heavily repaired. Does anyone know a good source for the cone? Looks to be about 5-1/2" od x 1" deep. Thanks

[David]

Some info from Beitmans says late production. David      

[Tubester]

Thanks David! I do have that same later prodution model 610 schematic. I've also studied the Philco service bulletins for upgrades. Service bulletin 217B (beginning with run #15) talks about improving oscillator action at 6MC, but only mentions removing resistors (17)/51kohms and (18)/25kohms. In my chassis those resistors are connected to tube 78 grid 3 (plate grid) instead of tube 6A7's cathode. There's no mention of oscillator wiring changes. Therefore, my question is about the wiring at item (8), oscillator. My chassis wiring agrees with the upper part of the schematic regarding the coils at the item (10) and (11) variable trim caps. However, my chassis wiring of the lower coils associated with item (12) and (13) variable trim caps and item (14) fixed cap do not agree, as shown in my 'actual schematic' noted earlier. Trying to confirm the later production Model 610 Philco schematic is the way to go. Probably just need to envoke Occam's razor and trust the Philco schematic. Any other thoughts would be appreciated. Dan

[Radioroslyn]

Regarding the oscillator circuit which is comprised of the 6A7's cathode, control grid, and screen grid. These elements are configured as a triode, so more or less the cathode should be @ 0v, control grid a small amount - voltage like -3 or -4v, and the screen grid a high + potential like +80v. If the control grid is reading a + voltage the oscillator is not running. The voltage must be measured w/ a high input impedance meter so the meter won't load the circuit. 

#s 13 and 14 have to be in the circuit to resonate the SW section of the osc coil. If I counted my 0s right it's a .002250mmfd or pf. If you had a .0022 ceramic disc cap you could chuck that in so #8 can tune 6-18mc.

#17 and 18 are the grid leak resistors for the osc. 17 is for the BC/POL band and 18 for the SW band. They too need to be in the circuit. As for the POL band I need to do a little research cause it looks like voodoo...

https://philcoradio.com/library/download...20217B.pdf