Author Topic: AVC----Automatic Volume Control  (Read 962 times)

Ken Doyle

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Re: AVC----Automatic Volume Control
« Reply #15 on: September 02, 2019, 02:43:23 PM »
Ok guys, I'm stupid and have no idea what you are talking about?  AVC? I have heard of AFC, but not AVC?   So, if I had an AVC problem how would I fix it?  Is there a simple way?  Sorry for being dumb, but I'm still green at this and still learning.  :-[ :-[ :)

Bill



AVC stands for "automatic volume control".  A better name for it might be "automatic gain control".  All but the cheapest AM radios from the mid-1930s onward have AVC.  It serves two purposes, it makes all the stations about the same volume, and it makes up for fading on far away stations at night.  AVC circuits can a bit confusing and can require careful study when there's a problem.

ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #16 on: September 02, 2019, 03:33:35 PM »
              Well, Bill maybe you have heard of it as AGC, automatic gain control ? instead of AVC. automatic volume control ? I have actually only had two AVC/AGC problems in all my years of messing with receivers.
                One was a shorted AVC capacitor. What the radio would do is, it worked great on weak signals and between stations, but if you tuned in a strong local station it got very distorted as you tuned it in. Louder too, at least a bit louder, but mainly very distorted.
               The other was total loss of between-station background noise and weak stations being barely audible. That was an open 3.3 Meg-Ohm resistor that went between the B+ voltage and the  AGC line in a TV set. The set would eventually white out with no snow but a white raster and no sound, after the TV warmed up.
             Whenever you tune in a station with a Magic-Eye indicator tube, you are monitoring the AVC voltage because that's what the Magic Eye tube's amplifier grid goes to, in I what think is all cases, bar none. If your radio has a tuning meter, that is usually (in a tube set) monitoring the plate current in an RF or IF amplifier tube that is connected to AVC voltage, so it is more indirect a reading. Plus the meter is in a resistor circuit where it hooks up backwards and if offset so that the higher the meter reads, the lower is the tube current.
               I think it is totally legitimate to think of automatic volume control as the float shutting down the water flow in a toilet tank. Maybe a tank with a bad leak in the flush plunger. The float rises and chokes the water flow low enough so that the tank never gets full (since it has that leak). The float position getting higher is the increasing voltage of the diode AM detector. The float lever connecting to the water flow valve is the detector DC voltage connecting to the RF and IF tube grids and cutting their plate current (and circuit gain) down to a lower value. If the plunger leak got a lot worse and let more water past it, the float would drop some and let more water into the toilet tank and hold the level steady.  If you tune in a weaker station, the AVC voltage would go down and the tuber would conduct more current to bring the gain back up.
                How's that ?
           

ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #17 on: September 02, 2019, 03:36:21 PM »
             Sorry Ken, I posted without checking for another post first.

electra225

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Re: AVC----Automatic Volume Control
« Reply #18 on: September 02, 2019, 10:53:42 PM »
By what we have been discussing here, it does not seem to me the problem with my RCA radio is in the AVC circuit.  I have some other problem.  I can go ahead and check resistors and see where they are.  I can run down the AVC bus and make sure everything is okay.  I haven't run across a known AVC problem, either, ever, so I was wondering if I was overlooking something.

Perhaps I'll start a thread under "Radios" and we can go thru the RCA together and see if we can find out what ails it, if anybody would care to participate in that activity.  What say you?
I don't need Google.  My wife says she knows everything.

ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #19 on: September 02, 2019, 11:25:16 PM »
          I was just rereading this topic from beginning to end and it made me wonder if the radio industry in the 30s was just like so many others in the depression era in that the "bean counters" had an iron grip on decisions. So began the attempts to get "adequate" performance with an absolute minimum of tubes and parts. Adequate performance could mean receiving local stations in an urban area for a long enough interval that if the radio pooped out after that, the owner would assume they had gotten a reasonable service life out of it and would repurchase the same brand at the same store. Radios were improving so fast in those years that it made no sense to build something that would last for decades when the radio circuit would be obsolete in a few years. At least not in the low price brackets.
          Keeping all that in mind, it seems almost miraculous that the most expendable radio of all, the AA5 can be restored to like-new performance in most cases.  I'm as curious as anyone as to what is causing what Greg has pointed out about the many radios that lose their sensitivity despite replacing almost every part. Hopefully we will find out that it's something easily fixed and if any forum can figure it out this one can because for one thing, we all work together and share our findings.  I hoping its something like high resistance grounds. Or maybe coils that have slowly absorbed moisture over a long time and needed to be thoroughly baked out for weeks maybe and then recoated with some modern high Q coil sealant to keep the moisture out.
          It would be interesting to make before and after measurements on a very oxidized and dingy variable air tuning capacitor that has been shined up and cleaned. That might have contributed to Greg's oscillator problem too. One of the few parts that is common to the oscillator and the antenna tuning circuit is the variable tuning capacitor.
           It looks like another post arrived while I was typing out this one, so I apologize in advance if I have been repetitive. 
         

ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #20 on: September 02, 2019, 11:36:34 PM »
          I definitely agree with you Greg, a separate thread dedicated to the RCA sensitivity problem would be something we will all learn from.  Just curious, if you tune in a decent station at the very top of the AM dial, something around 1500 kHz or so, does adjusting the air trimmer across the antenna tuning part of the variable tuning capacitor have an obvious sharp tuning peak ? Or better yet substitute your signal generator with audio modulated carrier and just enough signal strength to be barely audible and try and peak that trimmer capacitor. Does it have a sharp peak ?

electra225

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Re: AVC----Automatic Volume Control
« Reply #21 on: September 03, 2019, 12:11:04 AM »
If we are going to get serious about troubleshooting this radio, I really need to start over.  I need to make a chart like I do with Magnavox so I can record as-found readings and then readings as we go along.  That way I don't have to rely on memory and I won't chase my tail and waste everyone's time.  I have recapped it.  And I replaced an IF transformer, since the original has SMD.  It may be the first IF transformer I try to do SMD repair on.  I'm running low on available spares.  I have done at least a precursory alignment.  I got it so it plays.  That's about all. 
I don't need Google.  My wife says she knows everything.

Bill

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Re: AVC----Automatic Volume Control
« Reply #22 on: September 03, 2019, 06:51:57 AM »
Thanks Ken, Ed, and Greg.  I guess I understand some of what you all are talking about.  Being green in this hobby isn't easy, and simple explanations is best for a dummy like me.   :-[ :-[ :( :)

Bill


ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #23 on: September 06, 2019, 11:25:43 PM »
     Hi Greg
               I finally printed a copy of the RCA schematic foe your BC1079F chassis from the RCA service manual that covered the chassis A thru F.
          The only difference that I could spot in the A thru E chassis and the F chassis is that they simplified the wafer switch that selects power, radio, phono and 3 tone positions.
          Also C 6 is a 0.1 on the A schematic and 0.01 on the F schematic but it may be a misprint. C6 goes from the phono jack ground to chassis ground, so it shouldn't be part of the radio function.
           I saw something that may have messed up the radio. I'm sure you recapped both of the 120 uF power supply filter caps. Well, the replacement electrolytic probably doesn't have any kind of rating as to its bypassing abilities at either 455 kHz or any frequency as high up as the AM broadcast band or any frequency as high up as your local oscillator.  Of course it should be a dead short at such high frequencies but usually the actual impedance is all over the place. I would bet that your local oscillator problems and your RF sensitivity  problems would both go away if you would parallel the +85 VDC line with a .1 uF capacitor somewhere near the oscillator mixer tube. Almost every radio I've ever sen has a .1 or so capacitor in parallel with the electrolytic so as to make the bypassing good even at the high RF frequencies but it looks like this RCA radio designer was determined to only use components that were deemed as absolutely essential. Which is funny because some of the circuit areas are fairly elaborate.. Look at all the components around the loudness control taps, for example.

ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #24 on: September 06, 2019, 11:49:12 PM »
           Also, having audio on the AVC line sounds like either a bad .05 uF C8  AVC filter capacitor next to the 3.3 Meg resistor, or maybe the audio is riding on the ground. I don't see where you can ground the signal tracer probe on this radio. The schematic shows the metal IF cans and the tuning capacitor  ground as all being somewhere other than ground. Do you  have a polarized power plug on your line cord so that the ground side of he house current goes to the ground side of the chassis wiring ? Or do you just reverse the plug for minimum hum ?  I se that pin 1 of the three metal tubes goes  to chassis ground and so does the speaker ground. The "real" ground seems to be a bus bar that travels around the chassis and is separated from chassis ground from a 220 k resistor (R18) and a ,1 uF capacitor (C19)
         If you are certain that your ground goes to the ground side of your 120 volt line cord, and not the hot side, you might place your signal probe directly across C8 with the ground lead of the signal probe going to the ground side of C8 and hot side of the probe to the hot side of C8. If there's any audio except for bass tones, you might have a defective C8. Or the audio gain on the signal tracer is so high that what little audio voltage there is on C8 is being amplified enough to be audible.

ed from Baltimore

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Re: AVC----Automatic Volume Control
« Reply #25 on: September 07, 2019, 12:02:13 AM »
       BTW if the loop antennas in these radios are wound on the back panel of the set and if the back panel of the set is a sort of masonite board and if that board has adsorbed moisture and become lossy with years of age and moisture at RF frequencies, it is murdering the radio signal. Can you substitute another loop for that loop from a different radio with the same tuning condenser. ?  I guess they all use the 365 pF condensers ? 

electra225

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Re: AVC----Automatic Volume Control
« Reply #26 on: September 08, 2019, 10:48:50 PM »
I wanted to try something before I responded to the latest post.  I haven't had the opportunity to do that yet.  My idea was to completely remove the antenna from this chassis and use my roof antenna to see if I could even get noise at reasonable volume thru this radio during daylight hours.  I did check the resistors in the AVC circuit in the radio that works well, the 561, and they were drifted higher than the ones in the radio that won't work.  The resistors need to be changed in both radios, but I'd like to tinker with antennae a bit before I do that.  When I got thru the 561 that works, I'm not going to shotgun components.  I'm going to do one at a time, then check operation.   
I don't need Google.  My wife says she knows everything.