Hello, I found a schematic here but I would like the service manual that would have the parts list and locations as well as alignment instructions. Can someone help me out with that? I found another one on Tom Bavis's site but the alignment instructions are not there.
I just recently picked this radio up and it appears to have all original tubes (globes with Philco paper label) and they test very good. The cabinet needs a lot of veneer work. Looks like it got wet. The speaker is frozen. I will do a separate post for that.
It will a while before I get to this radio. Looking forward to it though.
Thank you,
Dan
(This post was last modified: Yesterday, 01:08 PM by dconant.)
Hello, I have begun the restoration of my Philco 91. I have been looking over the parts and the very first one I checked is way off. It is a 160K resistor connected to pin 4 of the 44 (RF) tube. It looks factory. That resistor does not exist on any of the schematics I have for that radio. The schematic shows connection from pin 4 of the RF tube to pin 4 of the IF tube with nothing in between. The other end of the resistor connects to block 9. Also the connection between pin 4 of the RF and IF is through a switch. This switch is on the same post as the on/off switch. It will be the bottom switch in the picture.
I found a schematic on Nostalgia Air showing the switch as being a frequency switch. There is only one band on the radio. The NA schematic is for code 126-226. The schematic is here http://www.nostalgiaair.org/pagesbymodel...013884.pdf. Off to a crazy start already.
I can't explain this specific resistor but I have seen the use of a switch to apply B+ voltage to screen grids of rf tubes to give a boost in gain to pull in distant stations. The switch was called a range or local/distance switch. Does the resistor connect to the ground terminal of block 9? It may be there to reference the grids to ground when the B+ is switched off. That would make the tubes run as triodes.
The resistor connects to pin 4 and the ungrounded side of block 9. From pin 9 of the RF tube to one side of the switch, from the other side of the switch to block 9 and then to pin 4 of the IF tube. It seems to me the resistor is in parallel with the wire. With the switch off the resistor is in circuit and when off it is not. So with the switch in one position the two pin 4 connections are directly connected and the other way they connect through the resistor. If what you say is correct then where is the B+ coming from? I just realized pin 4 of the RF tube connects to block 27. Parts list says this is a .01 cap but the block has two caps or one cap and resistor in there as all three connection points used. I don't find another 27 on the schematic.
I have changed the title of this thread from Philco 91 Schematic.
I have gone through most of the cap replacements. I am working on the replacement of the E caps and wiring of the output stage. There is a capacitor (#47) in the plate circuit of the output tubes. Schematic says it is a .001. What I think is the cap is a part mounted to the back of the chassis. It looks like compensating capacitor but without the adjustment screw. Is there some reason why it is not just a plain paper cap or one in a bakelite block?
Something seriously must have have happened to this radio as the speaker field coil and output transformer are open and the voice coil is pushed way out therefore damaging the spider. The speaker is useless and I will be using a permanent magnet speaker and Hammond output transformer. Also just about every resistor is way out of tolerance and the canedohm resistor has an open section (2-3).
I have two schematics for this radio one shows the 47 block (126-226) and the other (121-221) does not. My radio does not have the 47 block. The compensating cap I show is between the two 42 tube plates. I'm sure that is #47. Neither chassis layouts show the compensating cap. It is mounted on the back of the chassis between the E caps.
Anyway what is the purpose of that cap? I believe my cap is open. My M-Tester says unknown or damaged part.
Well, this cap you showed cannot possibly, due to the way it is constructed, be 1000pF (0.001).
It can be in tens of pF.
Now the purpose of it really depends.
One schematic shows it in series with the resistor. That could be a snubber, that protects the primary from high frequency transients with the capacitor allowing them to go to the resistor and be dissipated as heat. This stabilizes the stage and reduces voltage spikes.
Another schematic, also in our library, will show just the capacitor. I think it also acts as a High frequency filter and stabilizes the output, preventing some unwanted oscillations, or spikes; the parasitic of the transformer may act instead of that resistor.
It also depends on the frequency range, as the equivalent circuit of the audiotransformer is very frequency-dependent.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Thanks Mike. I at least know why it is there if I don't fully understand it. I see where you talk about a cap and resistor in series. That shows as 47 and 47a in the NA schematic (code 126-226). Parts list says the cap is .006. Anyway I think that part is sorted out.
I remember somewhere reading that the cannedohm 900 ohm (pins 1 & 2) section may be wrong and should be 9K. The person did some tinkering and settled on 5K. I remember the person said the screen grid voltage was way too high on the 44 tubes and the 2700 ohm section was running way to hot. The change bought the temp way down and bought screen grids down to reasonable level. What is your thought on that?
I still need to get my output transformer and speaker wired in and a resistor wired in to replace the field coil. What wattage would I need for the coil resistor?
If you know the fc resistance you can use 120 ma nominal current through the resistor to get the voltage drop then calculate the wattage. Probably 10 watts or more.
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