Hey Guys:
I picked up a model 60 cathedral and have it all recapped and its working pretty good but with anything more than a 2' piece of wire for an antenna and it is way over loaded signal wise, lots of double images and lots of distortion also on the shortwave band as well. Anyone have any thoughts on this?
Thanks
Gregb
Are you getting proper AVC control voltage to the grids of the converter and IF tubes? How much DC voltage out of the detector when tuning in signals?
Getting AVC for sure though I don't know a proper value for this radio. With the 1' antenna goes from -.08 V to grd off station to -.65 on station, with a 20' antenna goes from-.12 V to grd to -4.95 to grd.
Gregb
AVC voltages seem a bit low, but it really depends on how strong the signals are in your area. Is the voltage about the same at the detector output as it is at the bottom ends of the ant and RF coils (after the filtering resistor)? Sometimes you can have a gassy tube which will put a positive voltage on its grid although the AVC voltage at the detector will still be negative.
I would try swapping the 6A7 and 78 with known good tubes and see if it makes a difference.
Yup, tried tubes, no improvement
What is the DC voltage on the cathodes of the 6A7 and 78? Should be about 3 V to gnd. If this voltage is low then the tubes will overload easily.
Are the other tube voltages normal?
Hi Mondial
The 78 is 3V to grd and the 6A7 is 3.3V to grd. As for the other voltages I don't have the correct voltage chart for this chassis so I am not sure about the 6A7 and 78 but everything else looks good.
Gregb
Hey Mondial:
I grabbed another similar radio and checked the AVC voltage on it and its much more so seems that is the issue. Now to figure out why its to low. The 1' antenna on the working set gives me -5V to grd at the AVC filter as compared to -.68 for the 60 chassis. I should check with Chuck and see if he has the correct schematic and voltage chart for this version. I will keep looking.
Gregb
Greg, did you check here?
http://www.philcoradio.com/tech/60evol.htm
Ron has just about all the variations of the 60 schematics. I believe you have the later version if the 6A7 and 78 have individual cathode resistors instead of the earlier single combined resistor.
Voltage readings should be similar on all models. I would expect about 250 v on the plates with 90 to 110 on the screens of the 6A7 and 78.
Do you notice a big difference in AVC voltage on each side on the 2 meg AVC filter resistor? There may be something loading down the line on the side connected to the coils and tube grids. With a 2 meg series resistor it doesn't take much to pull the voltage down.
Thanks Mondial, I will check when I get home. I have the very last schematic version on the link you sent.
Gregb
Hey Mondial:
I have found the problem with the 60 and it now works perfectly. This is a strange one and I don't pretend to understand why this is but here it is. When I did the alignment I set the signal generator to the IF frequency and checked with a counter to be sure I am accurate and I set everything up and did the RF side all looked good dial wise but the overloading issue. Like you said the AVC voltage seemed low and compared to another radio it was. It seemed to me like the if alignment was out so I checked it the old fashion way, by ear tuned to a local station. 1st, 2nd, and 3rd were bang on, the forth was way off. so I put VOM on the AVC line and tuned it to the clearest reception and highest AVC voltage, -9.8 volts. All is good now.
So I went back and tried the generator method again and out to lunch again so retuned using the meter on the AVC and perfect. Any thoughts on this, using the generator method has worked on countless radios before this one.
Gregb
Where did you apply the signal from the generator? I wonder if connecting the generator at that point affected the tuning of that particular circuit? Which trimmer needed to be retuned?
In any case, if you are now getting good AVC voltage and no distortion, then all is well.
The signal was injected at the grid cap of the 6A7 and I tried with the cap wire attached and not attached, made no difference. It was the secondary of the second IF that was giving the grief. Yup works real good now nice clean sound and super sensitive.
Thanks for your suggestions, they got me looking in the right direction.
Gregb
Greg, when you peaked the IF with the generator did you adjust for max AVC or max audio tone at the speaker? I have always found the AVC method the most accurate. Unless you keep the signal level very low, its hard to accurately align for max audio tone.
Early radio alignment procedures always specified the tone method for two reasons. One was that it was applicable to radios without AVC which were fairly common back then. Second very few people had VTVMs, and the 1000 ohms per volt meters which were available loaded the detector too much for accurate results.
But when high impedance VTVMs became popular in the 40's, the AVC peaking method became prevalent. I always do alignments for max AVC regardless of the original specified procedure, unless of course the set has only a manual gain control and no AVC.
I peak for tone because for many years I had no test equipment and by ear was the only way I could it. I will be doing the AVC method for now on.
Gregb