Hello all, 1st post here, what a fantastic resource this site is, thanks in advance for your consideration of my enigma! I have a radio that looks like a Philco 84b (see attached pic), it has the same tube line up, but it has at least 2 variances relative to the schematic shown at this website for the 84b. Unfortunately there are no labels on the radio, save a chassis number of 38-4139, but I don't see any radio at this site that looks like it, except for the 84b. The two known variances relative to the schematic are: the heaters are grounded on my radio tubes, and my power transformer has a total of 9 leads, instead of the 10 shown on the schematic. If anyone has any insight into what radio I have, and how to get an accurate schematic for it, I'd really appreciate hearing from you!
Yes, thank you, we can close this thread! After posting, I found a Service Bulletin in the Library at this website that exactly described the changes I described. Thanks for your response!
Hi and welcome,
Generally there are two different version of the 84B chassis there's a code 121 and a code 122. The 122 was used only in the last of the production and uses octal tubes. The earlier 84B's use the older standard base tubes (this would account for just about all of them). As for the heaters being grounded or not I would look at the run number and circuit updates to get something that matches the chassis.
Cabinet looks to have some extra holes. The two on either side of the dial and the ones near the spkr have been added. The Philco spkr is real close to 8" but doesn't line up w/ an 8" spkr mounting.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 04-24-2019, 11:22 AM by Radioroslyn.)
Thank you for the additional info about the speaker holes etc. The audio output has definitely been altered to accommodate a different impedance speaker, but that speaker isn't even there, so I don't know what all was done to it. I'll need to rework that portion of the guts. I should have stated where to find these changes, what document they are in, in case anyone else has the same question. The document is here at the Philco Phorum, under "Members Only Forums". Choose "Philco (and More) Downloadable Files". Then choose "Philco Files for Free Download". Download the file "changes.zip". One of the many change bulletins summarized in this document is for the model 84, here is the text:
Center Tap Lead (black-yellow tracer) from filament winding of power transformer is eliminated; one side of filaments and pilot lamp are grounded.
Approximate date of change: 2/1/34
Run number: 2
Service Bulletin number: 178
There's two reasons I can think of to ground one side of the heater winding. One cost reduction as there is a bit less copper wiring to do with it grounded And the other hum reduction. THere's a few different ways to go. One is the float both ends and ground the center tap. Another is to ground one end and float the other. And a third is to float both end and twist both ends throughout the set. Perhaps the designers @ Philco determined that grounding one end was cheaper and less noisy than the other methods.
On the 84 it's not particularly it's not critical till you get over to the 2nd 77 tube (detector). It's a hi gain and hi impedance circuit and is prone hum pick up from ac fields hence the tube shield.
Philco sets for the most part use an odd voice coil impedance (abt 1 ohm) as compared to most sets (abt 3-8 ohm). It's not much of a big deal but though I'd mention it. Would recommend use dynamic spkr rather that a pm style for better filter of the hv.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 04-24-2019, 11:20 AM by Radioroslyn.)
Hi GL, thanks for your further comments! It IS a high impedance circuit, I'm amazed at the low values of some of the capacitors. I'm from the solid state era and don't have much experience with these circuits. My 89 year old Dad is still around, and used to listen to this radio when he was a kid. My Dad is savvy on tube circuits from the day, and I'm hoping the two of us can get this thing back on its feet. It was actually Dad starting to trace his way through the schematic vs. the chassis that identified the discrepancies! My two cents, it seems like they went for the cost reduction as you mentioned. Thanks for the discussion on the alternate ways to mitigate the heater hum in other architectures, all interesting news to this solid state kid.
Obtw name is Terry, GL is for good luck.
Am pretty well versed on the 84B so fire away w/any questions.
For starters bakelite encased foil /paper caps need to be replaced along w/ the electrolytics.
Check all carbon comp resistors for tolerance of 20% of stated value. The high value ones around the detector are suspect.
It's fairly common for the ant and osc coils to have an open primary winding. If found to be true it's pretty simple to repair/rewind.
Using a signal generator to do the IF and RF alignment is a good idea. If you just do it willy nilly by ear the performance will suffer.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 04-26-2019, 02:54 PM by Radioroslyn.)
A simple way to measure the primary on the ant coil is to unplug the set from the wall socket. Turn the volume up all the way and measure the resistance from the ant to the gnd posts. If it measures more than 5 or 10 ohms it's open.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
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