Model 81 woes.... - Printable Version
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Model 81 woes.... - Guest - 03-12-2006
Greetings to all in Philco land!! I am brand new to this site, but have been rebuilding old radios since i was a teenager. Its a labor of love! I have run into one of "those" that is the oppositte of keep it simple stupid. I have an 81, that will not play for anything! I have completely recapped the unit, using chuck's method of rebuilding the blocks. I replaced all the dog bones, and filter caps. The tubes are very strong, and I have checked all my transformers and all windings are within 3% of dc resistance specs. Soo y does it not pick up stations??? I know my AF amp is fine, having injected a signal into the top cap of my detector/af amp. I checked, I have an oscillator running, and doin the math, it is right on the money frequency wise. I have noticed that the amplitude of my oscillator diminishes as I get towards the high end of the bc band. I have put my signal generator on it, and i have my if signal going into my if transformer (18). It is of adequate p-p amplitude. Coming out and going to my detector, I have almost nothing. With generator maxed out, i can get a barely disernable amt of audio out of the speaker. I replaced my 4 meg resistor (19) as well. What obvious in front of my face problem have i overlooked?? Has anyone else run into this problem? I would greatly appreciate any help on this one.
Rick
- Guest - 03-13-2006
OK.. made a few more checks and wo is me.. i had an open antenna coil primary. I took the coil apart and discovered someone else has been here before me. I rewrapped the primary after fixing two breaks, and now i can get it to tune stations, but volume is very very low. Again, I put a signal directly into the 2nd det grid and i have gang buster volume from an external audio source. I have tried alignment and it does not help. One note I will mention, is that when i inject my 460khz modulated rf signal into the grid of my det/osc my volume is extremely low. It acts like it has a dead IF ckt. I followed the suggestions on the web page and changed the 4 meg res (19) to the recommended 2 meg and it does not help. Where am i going wrong. I have subbed out my new 78's with the old ones, and other ones,, and still audio is low. I removed the IF transformer (18) physically inspected the windings, and checked the dc resistance.. all good. Again,, thank you in advance.. and any help is appreciated.
Rick
- Chuck Schwark - 03-13-2006
Also look thru the various SERVICE TIPS for the Models 80, 81 and 84 on my site (URL below).
- Ron Ramirez - 03-18-2006
You mentioned changing out 78s? The 81 uses Type 77 tubes, not Type 78. This could be a key to some of your problems? Check it out.