I have a radio that should be working well. All new caps and resistors. Great tubes, and traded with some from working radio. BC works very well but shortwave reception is weak in continuous bands, and non-existant in the spreadbands. I tested next to another TO.
I can align the radio with a signal generator through the antennas, in all settings, except that the L15, 16 & 17 have no effect, but do have continuity. L21-23 and L9-11 do have effect and I can align "fairly" well. The filament voltage is correct and I pushed it up to the safe maximum to insure oscillation. I rang out the cleaned switches to related circuits and all seems okay. The oscillator coil tests good for continuity across HG, EF, AC, DB.
I am setting up for signal tracing, but am not 100% ready yet.
Norm has, in the past, pointed out a potential issue with the oscillator in shortwave and I want to check it out on this radio.
What is the best way for me to verify that the oscillator in the converter section is at the proper beat frequency for the selected band and signal frequency? As I understand it, on BC and the two continuous coverage bands it should be at 455kc higher than the signal frequency. On the four spread bands it should be 455kc lower than the signal frequency. I believe that this should happen as a function of the original IF setting, and the switch selection. I want to verify it and have something more concrete than using another radio to pick up this "beat frequency" (A great idea. However, I am a visual person and have to actually see it with my peepers). I have a good frequency counter. (Am currently shopping for a good used reasonable replacement ocilloscope as mine recently died of old age). I have a good DMM and signal generator. I thought it might be accurate to set the radio at say, 17.8Mc with the signal generator. Move the frequency counter to pin 2 of the converter, and the neg to B-. Am I thinking correctly to assume I will see 17.8 less 455kc or 17.344Mc at this point? I could also do it in the continuous band range and look for a 455kc higher frequency.
Thanks in advance for your help.
JeffW
Jeff:
1. Anytime a coil has no effect there is something wrong.
2. I think you also may have another trouble in the local oscillator circuit. A good way to test this is to measure the DC voltage on the control grid of the oscillator. Your scope is not a good candiate for this and neither is your frequency counter due to circuit loading issues.
3. Use a very high impedence meter, at least 10 megs or more, and make this osc. grid dcv measurement on the broadcast band where you say it plays good. This will be a negative DC voltage. Record it.
4. Change bands progressively and repeat the above, compare the dc osc. voltages as you go up --if they diminish by very much then your trouble is indeed in the oscillator.
5. You need to find the trouble in your "no effect" coils. Try new slugs first. Then replace the coils with ones from another set for trial. I have seen these coils have a short circuited turn which damps out oscillations but is completely undetectable with an ohmmeter check. You can check them with a good scope however, but that is another session. Luck.
Hi Dr.T.
Thanks for your input.
1. I replaced one coil. No change in that band.
2. I agree that Norm probably pointed me in the right direction on the oscillator circuit.
3. Using high impedence meter Fluke 8022B.
4. Done results are:
I have on BC as measured to B-
pin
1 +2.2
2. +102
3. +93
4. -1.8 (no antenna connected and off station goes to -2.4 just dialing a strong station no antenna)
5 +44
6. +2.2
7 +4.3
On continuous band
All the same except pin 4 is +2.7
On shortwave band
All the same except pin 4 is +2.2
5. Changed one coil assembly. No effect.
Now I need to understand the difference so I can isolate cause.
JeffW