http://www.philcorepairbench.com/tips/svctip24.htm
:See the Service Tips section of my site....
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:http://www.philcorepairbench.com/tips/svctip24.htm
The receiver employs conventional automatic volume control. This is the type which is tapped off of the detected audio, and is sent back to the grids of the RF tubes through a resistor. After this resistor(s), it is filtered by a small value condenser(s). This condenser(s) is usually about .05 MFD. If you would like me to point out these components, I will.
The quiet automatic volume control taps its signal off of the detected audio as well. The grid of the controlling tube in this circuit is what is connected to the signal. It is connected through a resistor and is filtered with a condenser, so the voltage after this resistor is esentially AVC voltage--it fluctuates with the average signal strength, not with every audio vibration. The plate of this tube is connected to a 1 meg resistor along with the screen grid of the first audio frequency amplifier tube. The screen grid of the QAVC tube is connected to the variable resistor, which varies how much positive current the QAVC screen grid will receive. When strong stations are received, the AVC voltage is quite negative. The grid of the QAVC tube is driven negative. During strong stations the QAVC tube actually does nothing (unlike regular AVC, which is actually supressing loud stations). When quiet stations are tuned in, the AVC voltage is quite minimal. The grid on the QAVC tube is thus driven negative by a minimal amount. Current starts to flow from the cathode to the plate of this tube. This current loads down the 1 meg resistor which is supplying the plate with B+. Since the plate of this tube shares the same resistor as the screen grid of the first audio tube, voltage to the screen grid of the audio tube is reduced, as well, which reduces over-all output of the audio tube. This quiets the background noise. The variable resistor which supplies the screen grid of the QAVC tube (not the screen grid of the audio tube) with positive voltage is adjusted so that the QAVC tube only cuts out the desired amount of background noise. Now when you open the switch for the QAVC tube cathode, you shut this circuit off. Opening the switch stops cathode to plate current flow. Without this the QAVC tube cannot load down the screen grid resistor on the first audio tube. When you open this switch, background noise will become louder if the QAVC circuit was originally set to make it quieter (QAVC screen grid potentiometer set to anything higher than zero volts).
Now regarding your tone control, those selections will have a more dramatic effect when the volume control is turned at less than 1/3 full on, though they should have an effect throughout most of the volume control's movement. If they have no control then your condensers are either leaky or open. I'd suspect the former, though the latter is entirely possible. The one that may still be working is the condenser that goes to the plate of the first audio amplifier tube. This one will make a popping sound when it is engaged.
If your radio has leaky condensers at the tone control, chances are really good that there are tons of other leaky condensers throughout your radio. Leaky condensers will spoil AVC operation and will put distortion into the audio, especially after the set has been operated for more than 20 minutes. Many of your condensers (like the tone control condensers) will be inside of metal or bakelite containers. The tar must be melted out of these (or taken out with lacquer thinner or gasoline as some suggest....sounds better than baking in an oven with all that smell). Then new condensers may be soldered inside of the containers. Both ceramic and metalized film condensers fit well in most of the cans. Some cans may favor one over the other.
Regarding condenser leakage, a condenser has unacceptable leakage when it has ANY leakage. A condenser to be tested should be removed from its circuit before testing. This removes leakages caused by other bad condensers and other resistors (resistors always pass current). All that is necessary is to remove one of two leads from the circuit. With bakelite condenser blocks and such, you'll have two or more condensers in one block. Normally you have one terminal which is common to both, and the other two are for each separate condenser. If there is a third in there, things can get fishy. Your resistance meter set to its most sensitive setting (X10,000) should spring up a small amount and then back down to EXACTLY where it started. With condensers smaller than .001 MFD, it may not spring up at all. With ALL LEAKY condensers, however, the needle will spring up by how much the condenser leaks. It will not fall back down. If the needle does not fall back down to EXACTLY WHERE IT STARTED, then the condenser is leaky by an unacceptable amount, and should be discarded.
Please note that you shouldn't touch both meter or condenser leads at the same time with your hands, or you'll throw off readings. Your hands have enough leakage to do this. Touch only the insulated part of your meter leads. Also do not touch the chassis. If you touch the chassis, you may cause unwanted leakage through another component.
If leaky condensers are found within your radio, your radio should not be operated until all leaky condensers have been removed. It is actually best to replace all condensers just for reliability's sake. Mica condensers do not have to be replaced, but should be tested for leakage just the same.
Thomas
Thomas,
Thanks for the explaination of the QAVC circuit. I now understand how it is supposed to work which will help me preform any necessary trouble shooting. Thanks also for the advice on the tone control. I tried by-passing the tone control capacitors with good ones so I know they're not open. I suspect they're leaking. I've recapped most of the circuits but not the tone control since the capacitors are difficult to access. Just prior to this restoration, I restored a Philco model 18. It has an almost identical four position tone control and, like the model 16, there is no noticable change in tone for the first three positions. Suspect both radios have the same tone control leaky capacitors.
If the QAVC "action" seems weak, that could be the set needs a good alignment and or a longer antenna to make the normal AVC action better to begin with.
Note: the Model 16 has VERY specific alignment procedures.
Also, bypass cap(s) on the AVC line may be leaky and need changing. ALL wax-paper caps should be changed anyway and all resistors checked for proper values (+/-20% max.), as a matter of course.
Chuck,
Thanks again. I've replaced all the wax-paper caps and I use a 100 foot outdoor antenna as well as a good ground. Think I'll consider an alignment after I check all other possibilities because the radio's performance is probably better than any other I've restored and I'd hate to degrade it by inducing a bad allignment.
Well I checked everything I could think of in the QAVC circuit and found no problem areas except that the QAVC/Interstation Noise Suppression did not appear to work. I then switched to each of the short wave bands and discovered the circuit functioned properly on each of the four bands; just not on the standard broadcast band. I haven't figured out why the circuit would work on the higher frequency bands but not on the lowest frequency band.
Well, it's now eight years later and I'm restoring my second Philso Model 16B. The first was the Baby Grand style and this one is the earliest tombstone style 16B; both have the earliest Code 121 circuits. Some things never change. Once again, the QAVC noise suppression did not work though all indications were that it should. Though the QAVC Type 78 tube tested good, I replaced it with another. To my pleasant supprise, the QAVC works fine with the replaced tube though I have no idea why this is so.
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