04-04-2014, 04:43 PM
Short read:
I haven’t found any tune up instructions other than Philco Service Bulletin No.352 page 4 which is the same as this page from 1942 Beitrman (same as Rider) except for a bit of description on test equipment set up.
[Image: http://i1356.photobucket.com/albums/q734...e0e624.jpg]
Neither mention the stub between the tuning cap top terminals that I believe controls the amount of 180 degree out of phase (canceling) LO signal injected into the antenna tuning circuit. Is an adjustment procedure floating around or is it run what you got from the factory?
Is there a different (better or worse) tune up procedure floating around?
Has anyone spent any time looking at the 40 to 42 Philco front ends with a spectrum analyzer?
Long version:
I have gotten to know the circuit and parts location better than I had expected to when starting this adventure.
Before firing up I did the normal replace the 3 electrolytic capacitors under the chassis. Used to be I could reform most electrolytics by slowly bringing up the line voltage with a Variac. Seems to me that they are now dried out and now reformation is no longer possible.
Found one of the paper foil .01 MF bad and 1 or 2 resistors out of spec. The OOS resistors probably would have not made any difference in real life, but made me feel better after replacing them. Used a matched pair for the grid resistors on the push pull 41 outputs
Replacing the old hookup wire with the rubber insulation falling off was a real treat on the band switch and on the bottom of the push buttons. Using a dental pick to get the now varnish instead for the original grease off the PB operating rods was a bit of an adventure also.
A part of the B section of the band switch wafer was cracked off and needed a bit of 5 min epoxy and plenty of care to be revived.
Loctal sockets contacts were almost as much fun to get clean as the band switch contacts. Deoxit for the first go and Deoxit with an artist brush for a second time on the band switch.
Added a line fuse in one of the transformer leads and plugged into a Variac and added an ammeter for my first power on. Radio woke up with no power problems so Variac and ammeter were removed.
At first general sensitivity sucked. Not too surprising for the age, but not expected either.
Ensues a week or so trying to get through a normal 20 minute tune up. Twaz LOTS of fun even tried sweeping the antenna tuning circuit which was essentially flat / not effected by the tuning cap. The local oscillator output would fall off and stop on the end of the broadcast band and only function on the top half of the Police band. It would operate at single frequency near 12 MHz on the Overseas band. Nothing I tried seemed to make much difference.
I did question the wire soldered from the top of one section to the other on the tuning cap, but it had been there for years and I had been told that the radio worked fine years ago before it was put into storage so disregarded wiring problem. I wondered a bit about in not being on the print, but then I am used to problems with prints anyway.
Thought it was strange that the LO signal was stronger on the spectrum analyzer than the signal generator looking at the input of the antenna coil.
About the third read of various threads on the Philco Phorum the light finally went on
The jumper on the tuning cap! (THANKS RON!)
The solder joint on the right tuning cap tab appeared original, so removed the solder from the trimmer tab, reinsulated the wire (heat shrink) and stuck it back through the trimmer tab.
Now the ground on the oscillator coil GONE!
WWV also on 2.5, 5 & 10.
LO output transfered to the radio antenna with radio tuned around 5MHz dropped 5 dB wire inserted in tab and 10 dB with the one wrap (wire length limited).
Now the LO signal on the BC band is not noticeable on a Rigol DSA815SG hooked to a square loop antenna 1M on a side and spaced 3 feet from the receiver antenna (HP 8565A still sees it).
The Police band LO signal is also in the noise till about 4MHz @ -60dB and is -39dB @ 7 MHz
The Overseas band LO @ 9MHz is -22 dB, @10MHZ is -20 dB, @12 MHz is -30 dB and peaks @10.4 MHz -19 dB.
LO level readings are comparative only as the analyzers are hooked to an antenna so as not to disturb the input.
I have the feeling that there is a balancing act between canceling to LO on the antenna side (receive desense) and depressing the LO signal to the mixer AKA 1st detector also receive desense
Sweeping the antenna circuit (power off) now has a nice dip following the tuning dial on the Broadcast and Police bands. Not much of a dip on the Overseas band.
Has anyone else done measurements to compare against what I found? I don't want to bother trying to fix what ain't broke.
Part 21 / 21A – dual 370 uuf mica - a square cap with 3 leads mounted on the side of the band switch is an interesting part. I thought I had seen all types of caps, but this is a new one on me. It is used in PB mode only and appears to also be a 50 / 50 RF voltage divider to feed the cathode from the center of the C in the LC for the LO in PB mode. The service bulletin doesn't give a tolerance for Pt 21. It seems to me that picking a pair that has close to matching values and having a total value of 185 pf for resonance with the PB coils is the idea. LLX interelectrode capacitance around 5pf would not be an issue.
Has anyone been bitten by replacing part 21 with a pair of 390 mica - 5% high if not sorted for a pair with lowest values and probably within the original tolerance? If I had to buy them I would go with 360pf. Guess if the PB coils run out of adjustment the question would also be answered.
Aside: pf = puff, aka uuf / mmf = micro micro Farad in 50s general speak Mickey Mikes.
Soap Box comment: I was a young whipper snapper when I earned my Novice license in 1960 and my Elmer's / mentors were 40 to 50 at the time so were born in 1910 to 1920. They (and Heathkit - gimmicks only) were quite specific about what "gimmicks & stubs" were. A gimmick is 2 wires twisted to form a few pf cap. Gimmick capacitance is determined by the wire length, the tightness of twist and insulation thickness. A (coupling) stub is what the tuning cap on the Philco has on top. A (matching) stub is what is used on an antenna feed point for impedance matching. Nope, I do not remember seeing this in a text book. One Heathkit (I built way back) manual went into some detail on the "gimmick" how to construct it and tune it in whatever the kit was, maybe a SB10. No experience with tuning stubs mostly UHF and above IMHO.
Pie in the sky: Never tried boiling a Loctal tube to separate the bottle from the base. Don't know if there is any point in this anyway. Has anyone separated a LXX bottle from the base hooked up the filament only for the glow and hidden a MOSFET in the base and rewired the chassis to up the receiver performance?
I haven’t found any tune up instructions other than Philco Service Bulletin No.352 page 4 which is the same as this page from 1942 Beitrman (same as Rider) except for a bit of description on test equipment set up.
[Image: http://i1356.photobucket.com/albums/q734...e0e624.jpg]
Neither mention the stub between the tuning cap top terminals that I believe controls the amount of 180 degree out of phase (canceling) LO signal injected into the antenna tuning circuit. Is an adjustment procedure floating around or is it run what you got from the factory?
Is there a different (better or worse) tune up procedure floating around?
Has anyone spent any time looking at the 40 to 42 Philco front ends with a spectrum analyzer?
Long version:
I have gotten to know the circuit and parts location better than I had expected to when starting this adventure.
Before firing up I did the normal replace the 3 electrolytic capacitors under the chassis. Used to be I could reform most electrolytics by slowly bringing up the line voltage with a Variac. Seems to me that they are now dried out and now reformation is no longer possible.
Found one of the paper foil .01 MF bad and 1 or 2 resistors out of spec. The OOS resistors probably would have not made any difference in real life, but made me feel better after replacing them. Used a matched pair for the grid resistors on the push pull 41 outputs
Replacing the old hookup wire with the rubber insulation falling off was a real treat on the band switch and on the bottom of the push buttons. Using a dental pick to get the now varnish instead for the original grease off the PB operating rods was a bit of an adventure also.
A part of the B section of the band switch wafer was cracked off and needed a bit of 5 min epoxy and plenty of care to be revived.
Loctal sockets contacts were almost as much fun to get clean as the band switch contacts. Deoxit for the first go and Deoxit with an artist brush for a second time on the band switch.
Added a line fuse in one of the transformer leads and plugged into a Variac and added an ammeter for my first power on. Radio woke up with no power problems so Variac and ammeter were removed.
At first general sensitivity sucked. Not too surprising for the age, but not expected either.
Ensues a week or so trying to get through a normal 20 minute tune up. Twaz LOTS of fun even tried sweeping the antenna tuning circuit which was essentially flat / not effected by the tuning cap. The local oscillator output would fall off and stop on the end of the broadcast band and only function on the top half of the Police band. It would operate at single frequency near 12 MHz on the Overseas band. Nothing I tried seemed to make much difference.
I did question the wire soldered from the top of one section to the other on the tuning cap, but it had been there for years and I had been told that the radio worked fine years ago before it was put into storage so disregarded wiring problem. I wondered a bit about in not being on the print, but then I am used to problems with prints anyway.
Thought it was strange that the LO signal was stronger on the spectrum analyzer than the signal generator looking at the input of the antenna coil.
About the third read of various threads on the Philco Phorum the light finally went on
The jumper on the tuning cap! (THANKS RON!) The solder joint on the right tuning cap tab appeared original, so removed the solder from the trimmer tab, reinsulated the wire (heat shrink) and stuck it back through the trimmer tab.
Now the ground on the oscillator coil GONE!
WWV also on 2.5, 5 & 10.
LO output transfered to the radio antenna with radio tuned around 5MHz dropped 5 dB wire inserted in tab and 10 dB with the one wrap (wire length limited).
Now the LO signal on the BC band is not noticeable on a Rigol DSA815SG hooked to a square loop antenna 1M on a side and spaced 3 feet from the receiver antenna (HP 8565A still sees it).
The Police band LO signal is also in the noise till about 4MHz @ -60dB and is -39dB @ 7 MHz
The Overseas band LO @ 9MHz is -22 dB, @10MHZ is -20 dB, @12 MHz is -30 dB and peaks @10.4 MHz -19 dB.
LO level readings are comparative only as the analyzers are hooked to an antenna so as not to disturb the input.
I have the feeling that there is a balancing act between canceling to LO on the antenna side (receive desense) and depressing the LO signal to the mixer AKA 1st detector also receive desense
Sweeping the antenna circuit (power off) now has a nice dip following the tuning dial on the Broadcast and Police bands. Not much of a dip on the Overseas band.
Has anyone else done measurements to compare against what I found? I don't want to bother trying to fix what ain't broke.
Part 21 / 21A – dual 370 uuf mica - a square cap with 3 leads mounted on the side of the band switch is an interesting part. I thought I had seen all types of caps, but this is a new one on me. It is used in PB mode only and appears to also be a 50 / 50 RF voltage divider to feed the cathode from the center of the C in the LC for the LO in PB mode. The service bulletin doesn't give a tolerance for Pt 21. It seems to me that picking a pair that has close to matching values and having a total value of 185 pf for resonance with the PB coils is the idea. LLX interelectrode capacitance around 5pf would not be an issue.
Has anyone been bitten by replacing part 21 with a pair of 390 mica - 5% high if not sorted for a pair with lowest values and probably within the original tolerance? If I had to buy them I would go with 360pf. Guess if the PB coils run out of adjustment the question would also be answered.
Aside: pf = puff, aka uuf / mmf = micro micro Farad in 50s general speak Mickey Mikes.
Soap Box comment: I was a young whipper snapper when I earned my Novice license in 1960 and my Elmer's / mentors were 40 to 50 at the time so were born in 1910 to 1920. They (and Heathkit - gimmicks only) were quite specific about what "gimmicks & stubs" were. A gimmick is 2 wires twisted to form a few pf cap. Gimmick capacitance is determined by the wire length, the tightness of twist and insulation thickness. A (coupling) stub is what the tuning cap on the Philco has on top. A (matching) stub is what is used on an antenna feed point for impedance matching. Nope, I do not remember seeing this in a text book. One Heathkit (I built way back) manual went into some detail on the "gimmick" how to construct it and tune it in whatever the kit was, maybe a SB10. No experience with tuning stubs mostly UHF and above IMHO.
Pie in the sky: Never tried boiling a Loctal tube to separate the bottle from the base. Don't know if there is any point in this anyway. Has anyone separated a LXX bottle from the base hooked up the filament only for the glow and hidden a MOSFET in the base and rewired the chassis to up the receiver performance?
