This is my first post having just joined. I'm presently restoring two model 91's. One is a Code 121 and the other is a dual speaker Code 221. The 221 is the problem child. The schematic is beyond understanding. There is a 136 ohm segment on the BC resistor rod but it shows nowhere in the schematic. Where does it go in the circuitry?
Hi LeadFingers and welcome to the Phorum. In the library, Service Bulletin 129 has the schematic for your 91 code 221 and shows the resistor as part 46A and is listed in a resistor table with the values of each section. This is a large wirewound resistor mounted over at the power supply side of the chassis. This resistor determines the bias voltages for the tubes. Hope this helps. https://philcoradio.com/library/index.ph...bulletins/
Thank you for replying to my quest for information. However I'm familiar with the items you mentioned. The problem is with the Philco schematic. There is no 146 ohm section of resistor 46A resistor shown anywhere in the schematic. Without that info one cannot connect that section of 46A to the correct circuit points. From my perspective the answer needs to be an engineered solution. Has anyone solved this problem before?
In it the two sections of 46A are 85 and 205 ohms (and a separate 41A 13K resistor as well). I don't know where a 136ohm resistor would come from unless a) its a replacement, b) it has some shorted section in the wire wind, or c) the original quality control on on the resistor was bad. Unfortunately, Philco also made undocumented changes to their radios from time to time that we all now have to deal with. I have not dealt with this particular issue although I did restore a 91B single speaker version.
Then you know what I'm dealing with - incomplete or missing documentation. No worry. The 136 ohm resistor is mounted on the same substrate as the documented portion of 46A. It's factory original and as you suggest it may have been an undocumented production/design change. No doubt the documentation staff at Philco was short handed and the management short sighted. Philco radio dealers who would have appreciated better service support.
In a later post in this thread I will give the details for others to make use of.
In your set, how many tabs are on Resistor #46A? More than 3?
Does one end terminal go to chassis ground?
Does the other end terminal go to the cathode of the 2 '43 output tubes?
What is the resistance between the cathodes of the output tubes and chassis ground?
This is an interesting setup. Most sets from the early and mid 1930s have all the cathodes at chassis ground potential. A resistor, usually a multi-tapped resistor existed between B+ and the center tap of the transformer secondary that provides the B+. One of the taps of the resistor is made to chassis ground. Grid resistors or interstage transformer center taps go to various parts of this voltage divider between chassis ground (B-) and the center tap, which becomes C-. Sets made around and later than WWII usually had the center tap of the HV secondary at chassis ground and cathode resistors for all tubes requiring a negative grid bias, particularly the output tubes. Your set seems to be a hybrid of both styles.
The schematic shows a 3 terminal resistor with one end at chassis ground, the other end to the 42 cathodes and the center tap to the HV secondary center tap and the interstage transformer secondary center tap, providing some negative grid bias. the resistance between the center taps and cathodes of the 42s provide the rest of the bias.
If there is nothing attached to that terminal, I would not worry about it. Other trace any wires on the mystery terminal as to what tube it goes to, does it go to a cathode or is there a high value resistor between this terminal and either the grid of a tube or to the AVC circuit?
Hope this helps.
Unfortunately, you can't talk to the engineers unless you meet them in the afterlife, in which case, you wouldn't be communicating with us unless we were in the afterlife also.
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I finally stumbled, in my present life, into a workable solution. I connected one end of the 136 ohm section of 46A to the HV winding center tap, 6.3 volt winding center tap, and negative terminals of the B+ filter, to terminal 4 of 46A, junction of the 205 and 95 ohm sections of 46A. The voltages throughout the receiver are ostensibly within tolerance. I have to do audio testing and the alignment, if everything is working properly, when I get back to the work bench.
Regarding the configuration of the the multi-tapped resistor in model 91 code 221 download and look at resistor 46A in the schematic from Nostalgia Air:
You will see two resistor section, 205 ohms between terminals 4 and 5; 95 ohms between terminals 3 and 4, terminal 3 going to chassis ground. The 136 ohm resistor is between terminals 1 and 2 but is not depicted in the schematic, an example of Philco's deficient documentation.
As I stated before I connected the 136 ohm resistor from the negative terminals of the B+ supply filter capacitor to terminal 4 on 46A. That gets the various voltages to the ostensibly correct values as disclosed in the voltage table below the circuitry schematic. The audio output to the speaker pair is working correctly.
I would call this to be a case of user engineering.
I think my problem is resolved. I'm moving on to alignment now. Thank you for your inputs.
One tweak will be necessary. That's paralleling the padder capacitor, which doesn't have enough range, in order to get the dial to read correctly end to end. I forget the value I used in a previous model 91 I restored.
Continuing the Philco 91 saga, I'm offering up a few details FWIW. Working on Philco radios with hard wired to the chassis speaker cables is a royal PIA. It's a recipe for disaster - holed speaker cones, broken tubes, and days in the hospital wondering why you ever wanted to restore antique radios. ?
I used a chassis punch to make a hole in the chassis rear apron and installed a five pin tube socket. For the speaker cable I found a mating five pin cable plug. I could have used a five pin tube base if I had an unusable tube to canobolize.
Sans chassis punch, Drilling, making a nice round hole in the steel chassis has to be a labor of love!
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