hello, first sorry for my poor english. I will try to be as clear as possible...
I've bought a model 640 code 121 last week in france. it was a model specially design for french user. Everything is written in french on this philco.
I've downloaded the schematic of model 640. I've seen the bias is made with cathode resistor on the 78 RF, on the 6A7 and on the 78 IF.
On mine, it's different. all the cathode of this tree tubes are directly connected to ground. The CAG (you say AGC I suppose ???) is going first to the 78 IF and the 6A7 and in a second part to the 78RF.
Another thing. I've got a different resistor on the negative side of the transformer (the middle point between the to winding). On the original schematic, we've got a double resistor with one side to ground, the middle side is 60 ohms (for the 6A7 bias) and the other side is 160 ohms (60 + 100 ohms) for the 42 bias before the inter-stage transformer.
On mine, I've got a third point between ground and 60 ohms, approximately 24 ohms. There's a wire with start here and go to the AGC to maintain a minimum of -4V on all the grids.
My question is why does my radio is different from the model 640 ? Is there some modification for radio send to france ? The resistor,with the third point is an original part from philco and not an add-on.
Do you know 640 radio with all the cathode to the ground ???
Best regards
frederic ABIGNOLI
Frederic-
Your English is excellent. It is not a problem.
I have two 640's, a 645, and a 650- all are wired to the schematic. They are excellent radios after re-capping and replacing the high value resistors.
The last owner could have been trying to optimize it for one weak frequency. I am not the expert on this series but I have never seen wiring like your 640. It should take only an hour or two to restore the wiring to original. Rewiring would certainly be worth the time.
Hi Frederic
In reading your post, it made me start thinking...and so I decided to look up the schematic for Model 645. It seems to me that your set is similar to, but not exactly like, Model 645.
Model 645's Type 78 RF tube, Type 6A7 det-osc tube, and Type 78 IF tube have all of their cathodes tied direct to ground as you describe.
The resistor you refer to was often called a "B-C" resistor by Philco. A more correct term would be a bias resistor. Anyway, this resistor differs in the 645 from that used in Model 640 - but is not the same resistance as you describe.
Could be that your radio is a late production 640; and in addition, perhaps some undocumented changes were made for the French market? It is quite normal for Philco to have made running changes in production, sometimes without bothering to take the time to document those changes. This makes things somewhat frustrating for us now, seventy-one years on, but for the true Philco Phanatic, it just makes life more interesting.
Thank you for all your answer.
I think the last owner haven't done a good job. There's lot of bad thing in this radio. The push pull is not here (it's an single ended). I will have to find a driver transformer and a push pull transformer and the original speaker.
I don't no for the cathode but I will add resistor and condenser to make it original.
I think the RF trans former is out of order? I will have to rewind it because there's a wire beetween the antenna and the 6A7 grid to bypass the RF amplifier. Not a good news.
I'm happy to own my second philco. The first one was a model 20 with lot of work (the main transformer was totally burned and I have rewinded it !)
If you want to see what I've done to my philco 20 everuthing is explained here :
http://cfp.radio.free.fr/restaurations/p...lco20.html
have a nice day and than'ks for your answers.
Frederic ABIGNOLI
Hello,
the 640 code 121 is working since last night at 3 A.M. !!!
I've made the bias with cathode resistor and cap for the two 78 and the 6A7
The RF transformer was out of order. The long wave winding was unsolder and the primary was broken at 8 turns from the end (hopefully it was then end of the winding)
I've recap everything and re-wire as the original (except one thing, the push-pull.... because the interstage transformer was not here. The 42's are connected plate to plate, screen to screen, cathode to cathode and the link with the 85 is made with 2 capacitors).
I plug the radio and it works.
now, I've to wind a interstage transformer and an output transformer to use the amplifier in push-pull mode.
The 640 is great. a very good sound. The shortwave are marvellous, the vernier is a great piece of gear.
There's only one problem and I've to work on it. On longwave, I've got some king of oscillation and motor-boating stage under 1000M.
I don't know if it's between oscillator stage and Antenna stage, or RF stage and antenna stage... I the grid of the 78 RF stage is grounded, the oscillation stop. SO I think the problem is between antenna stage and something else in the radio. Do you experience problem like that on long-wave ?
After the holidays, I will write an article on these restoration. I've done all the measurement of the inductance in µH of all the winding, if it can help....
Best regards
Frederic ABIGNOLI
Frederic,
Congratulations! I am sure that once you have reinstalled the correct push-pull interstage and output transformers and change the circuit back to push-pull, that you will have an excellent radio.
I do not currently own a 640 so I cannot comment on the oscillation problem you are experiencing on long wave.
Hello,
I've found the oscillation this week. it happened between Antenna and RF stage. i've put a steel plate beetween the two stage and the oscillation stop. This morning, i've seen thaht I've forgot to put a small capacitor of 1800µµF on the primary of the longwave transformer, and I think that's the reason of the oscillation. i will try this evening to wire it and we'll see.
the next step is to wind the 2 audio transformer. I can buy it in US but with the shipping charge.... And winding them is a great challenge.
I start restore the cabinet and I see that the speaker is really a philco model as you said. This speaker sound great, with lot of bottom and it's in good condition.
best regards
Frederic ABIGNOLI
Hello,
The push-pull is singing for the first tome this evening. It works great.
I wind the interstage transformer.
2100 turns of 0.1mm enameled wire (something like 38 AWG for you)
and 2 secondaries of 5250 turns each, with the same wire.
FOr the output transformer, I know... It's a shame... I use a power transformer
I left the primaries unwired, I only use the secondaries.
The speaker is on the 6,3V filaments winding. And I use the 2 high voltage winding of 2 x 280V for the primaries. It works fine. The sound is very loud and clear. With an ohmeter, I see 680 ohms with these 2 winding, like the schematics. That's great.
Everything is working right. lot of works but it was worth it.
Now, I've to work on the cabinet.... But after the holidays :-)
Best regards
Frederic ABIGNOLI