Amazing how some knowledge came back to me

[ToddW]

OK, so my dad wanted me to help him fix an old AM superhet tube radio, being as I have a degree in electrical engineering.  But, hadn't really done any EE since I graduated in 87 since I veered into programming, controls and various other stuff.  Off the top of my head I remember V=IR and not much more.  So me and my dad decided to take the plunge the last few weekends.  I had to convince him to buy a scope and function generator.  He asked how they would help fix a radio and my only reply was - not really sure, but we used them all the time in school.  Also, this was to be a learning experience and a fun project so he shelled out the cash.  Got a decent Siglent SDS1202X-E and a $70 Chinese FeelTek FG.

Radio would not tune to any stations, you could hear something while turning the dial, but just garbage, and this constant irritating hum under all conditions.  OK so got out the scope -I have not touched one in 35 years.  After about 10min, a lot came back to me and used it to check the B+ rail.  Horrible, noise everywhere and a distinct square jump at 230 Hz. Dad got a electric vibrator to replace the mechanical one that operated at 115Hz.  Figured FWR 115 * 2 = 230 was the constant hum, so began the capacitor replacement, including the filter caps, and hum was gone.  Showed my dad the scope before and after the cap replacement screen shots and the difference was obvious.

Still couldn't tune into any stations.  I found the FG had AM modulation so set it up for 1Mhz carrier and 800Hz modulation.  I then ran it into the scope and was able to use it to see the AM modulated waveform and then use the FFT math to do a frequency plot showing the carrier peak and sidebands!  Next, coupled the FG signal into the RF section, tuned to 1Mhz and heard a beautiful 800Hz tone.  Then sent an mp3 of Natalie Merchant song "Carnival" into the external FG VCO as the modulator signal and it played just fine on the radio.  Antenna to speaker works, but still no stations will play.

At some point for some reason, we hooked the radio to a 6V battery and bingo, found multiple stations.  Dad also bought a $70 Chinese switching power supply and it is so noisy, it drowned out the RF section.

Anyway, I did a lot of things I hadn't done in decades:  FFT, AM modulation, general scope and FG use, etc.  Even learned how a superhet works with its LO and IF systems that mix the original RF with LO to get a constant carrier frequency IF of 260Hz which the rest of the stages are tuned to and also learned the basics of tube operation with the plate/grids/cathode functions.  Its was slow going and all still kind of fuzzy, but step by step and a lot of youtube and me and my dad got the radio working.

Next up, I am going to check the amplification gain between the stages and do a frequency sweep across the IF using the FG and external sync (or just see, or maybe align) the frequency selectivity and whatever the hell else I can think of.  I think there is a way to do bodie plots with what I have.

Guess I just wanted to post here that I have not done any of this stuff for so long I had my doubts I could really do much at all but pleasantly surprised as I did more it all started coming back.  Also, it was a fun project with my dad and he is already talking about finding another old tube radio to see if we can fix it.   

[radiolistener]

Old tube AM radio can be very useful in the coming times 

[ToddW]

Yeah, been thinking about developing my SHTF skills.

[radiolistener]

[CaptDon]

I thought car radios had a 262khz i.f. and most other a.m. radios were 455khz?
You may want to be sure about that 260 vs 262 before you try to align something?
You can actually pass the 262khz right into the antenna jack for tweaking the i.f.
and be sure to shield your test leads!! I used to have an old pontiac car radio with
a gonset S.W. band converter hooked to it. That combo could receive a.m. s.w.
broadcasts like Radio Norway, Swiss Radio International, BBC, VOA and so forth
and sound better than any Shortwave receiver I ever put it up against!! The front
end of that car radio was HOT!!! The Gonset Converter must have been of good
design also because my combo could pull in weak stations that troubled purpose
built shortwave receivers!!

[ToddW]

Where are the little hamsters running in their wheels to make the electricity?

[ToddW]

Hmm, will have to double check the schematic.  I thought it said the IF was 260.  Took the radio outside (from the basement) today and it worked pretty good.  Still getting a little fuzziness in the sound even on a strong station.  Dont know if that is something that can be tweaked on or just the nature of AM on an 80 year old tube radio.

[CaptDon]

Chances are good that you have some grid coupling capacitors that have
a few microamps (or more) of leakage and it is throwing off the grid bias
on your audio stages. I find this to be true of almost every wax capacitor
in these old radios and I replace most of them whenever someone brings
me an old radio for repair. The last one I did was a 1926 Wells. Big floor
model with twin 12" speakers and a 6U5 magic eye tube.