[dconant]

So much information here to absorb. Not sure what to do now so I'm starting over.

Turned radio on this morning with poor reception with outdoor antenna and ground attached. AVC is at -2.2 volts. Removed the ground wire and reattached. Good reception now. Tuned to 1180kc (strong local station) with AVC at -3.5 volts. Tuned to dead spot on dial and get -2.2 AVC volts. Now back to 1180 with good reception. Played for about one hour. Lightly coupled sig gen set to 1180 at 8mv. AVC is cycling between -2.2 and -2.4 volts. I hear the tone cycling as well. Is this because mixing with the 1180 station. Don't forget reception does not go away completely it just weaker. I have to turn the volume up quite a bit to hear the station.

Tuned radio and sig gen to 1100kc 8mv (no station there) and get -2.3 AVC with sig gen lightly coupled. Next I set sig gen 160mv and that gives me the -3.5 AVC the same as what I get on 1180 with good reception.

Shouldn't I be testing with sig gen giving me -3.5 AVC like good reception on 1180kc? Maybe should set sig gen lower if I direct couple to antenna and or IF.

Voltage measurements on 6A8 with no antenna or ground using a VTVM.
Grid cap -2.2
Pin 3 235 volts
Pin 4 95 volts
Pin 5 -13 volts
Pin 6 200 volts.

6K7 tube
Grid cap -2.2 volts
Pin 3 235 volts
Pin 4 95 volts
Pin 5 0 volts

6H6 tube
Pin 3 -2.3 volts
Pin 4 -1.8 volts
Pin 5 -2.3 volts
Pin 8 -1.8 volts

[dconant]

I don't know what I am doing wrong but 1180 with good reception is now -5.5 AVC volts. Maybe this another issue or I'm not tuning in the station at it's peak?

[morzh]

One thing is, your AVC is working, this is why one station could give you a different AVC voltage vs the other, while you still have good volume. Could be a stronger signal at 1180. You could de-tune just a bit or tune back, and while the perceived volume might be similar, the actual output of the tuned circuit will be different, and so will be the V-avc.

Now, you removing the GND wire / reattaching and getting bad/good reception piqued my curiosity.
Anything you could look at in there?

[dconant]

Not that I can see. The A terminal goes directly to the antenna coil and the G terminal is chassis ground. All ground rivets has been fortified with wires going to new screws added to hold new terminal strips. I am satisfied grounds are good. I have wiggled the wire going from the A term to the coil and get no changes. Don't we think the coil is flaky?

Currently the radio is running on sig gen direct to A and G terminals set to 1100kc at 2mv (yes 2mv) giving -4 volts AVC. Going for a walk and hopefully return to some changing.

[dconant]

Last test has not changed anything. Before we started on this I had placed a want ad on ARF and got an immediate response. So I purchased the coil and just finished installing it. When I turned the radio on I got no reception. Did the remove the ground and placed it back and got reception so we can rule out the antenna coil. That was my best hope. Man this is a tough one.

[morzh]

You get no reception with GND?

The tests with crapping out reception before, did you do them with or without GND?
Would it flake out without GND?

And, no, coil, while possible, was not the usual suspect....

[tbone]

I did some searching around and I realized I have a Zenith 10-S-155 set in my collection that needed work. It had a double peak on the strongest stations which turned out to be the 6A8 tube. Anyway, back to the matter at hand.

My radio uses the same basic tube lineup. 6H6 Detector, 6K7 IF, 6A8 Osc/Conv, but it also has an 6K7 RF amp, which yours does not. Regardless, the 6H6 detector and AVC circuit has the same -2.5V feed from the candohm and dropping resistors before it gets to the preceding stages for AVC. Yes, the resistors are different values, but the AVC takeoff point is the same. I took some readings that could be used for reference which gives you a good starting point on how it operates.

I used a 1mv (-47dbm) modulated signal at the antenna jack. My AVC test point is the junction between R5&7 in my set. See attached schematic for test point. I also measured voltage on the grid caps of the 6K7 IF and the 6A8. Here is what I found.

AVC test point -7.7V w digital meter, 6K7 cap -4.6V, and 6A8 cap -4.6V.

Keep in mind anytime you touch a meter probe to a grid cap you are de-tuning the stage, but the voltages are still reasonable.

I also tested the voltages in the chart and found a big difference on the grid caps when I used my Simpson 20K/volt meter. This is obviously expected. I think your -2.5V on the grid caps with no signal would be closer to -0.5V as in the manual if you used an analog meter like what was called out in the voltage chart.

I also experimented with 6H6 tubes from my stash. A couple of them started out good, but either dropped down low or went erratic as they warmed up. Tube testers are a good ball park but should not necessarily be taken as gospel. Testing in a radio while monitoring voltages is the best method.

I went further and measured signals off the air from my longwire and was a little surprised. My strongest local was -30dbm or 7.5mv on the analyzer. Weaker stations were in the 80-100uv ranges. So yes, you can conceivably receive stations in the mv signal range.

10-S-155 Schematic

[morzh]

It seems to be the same radio with an extra RF stage and then push-pull audio out.
The core of it, the det-osc, 1st IF and the 2nd Det are exact same circuit.

[dconant]

When just using radio stations (no sig gen) I have always used the ground. Reception does drop while the ground is attached or not. Just removing the ground and placing it back restores reception. Actually just removing the ground restores reception but noisy. Putting the ground back on cleans up the noise. Reception is quite good with the ground and very noisy without the ground while the radio is playing normally. Also tested without the ground and the reception will drop then as well.

Just to see, I got out my Simpson 260 and measured the AVC at the 6A8 grid cap and it read the same as my RCA VTVM. The difference was the VTVM does not alter the signal but the 260 did to the point of much weaker reception much like the problem I am having.

[morzh]

OK. Going out on a limb here.

What you describe looks like the oscillator still oscillates but not very well.
And you somehow jolt in into going back to good oscillation.

1. Forgot, your 6A8, do you have a know good strong tube? Not "it works in another radio" but a known good tested, strong in both Pentode and Triode (Osc) sections?
If you do, try it.

2. R3, 10K. Try to reduce the value to, maybe, 7K to 8K.

[dconant]

Well I don't know for sure my 6A8s are known good. Buying one or two on ebay does not guarantee a known good one. Do you have one you are willing to sell? I'll work on the R3 change. It sure looks to me like a 6A8 problem. Should I try and get a new osc coil from same guy I got the ant coil from and maybe the IFs?

[Chas]

Does this volume drop out occur on all bands?

If only on one band, measure the resistance of all the RF/Osc coils. A corroded winding will always have a higher resistance, use the Simpson. Most of these coils will be around 20 ohms or less. If you know which coils are layered secondary over primary. disconnect a lead from each winding and measure resistance of the one winding to the other looking for leakage caused by the layer insulator corroding the windings. It should be infinity, use the high ohms scale so 9-volt battery is in play in the 260.

I have found several strands of litz in an IF coil open at the solder joint. Connecting all of them lowered the resistance and improved the "Q" so the winding could peak properly.
Thus, another clue is a alignment setting that does not peak (or null) as expected yet works. Indicated an issue with that circuit.

If the volume drop out occurs in all bands, concentrate similar coil measurements at the mixer/IF circuits right up to the volume control.

Another alternate method is to use a spot heat gun or a freeze spray and methodically try to either induce the issue or clear the issue. Keep in mind that separating the AF from the RF at the volume control helps clear one of the three sections of the radio, the others are the RF and power supply.. The Candohm, if still present and the output transformer primary. The output transformer bypass cap and tone circuits can be problematic in this model.

New components can be faulty, as I stated failing "Shoopage" in a modern poly cap and intermittence welds in modern resistors.

But stage isolation must be done or its just a crap shoot to find a bum component.

YMMV

Chas

[morzh]

No. At this point buying the magnetics, to me, is a waste of money.

I do not have a 6A8 that would qualify as a "bogey."
All those I bought did have a weak Pentode section and borderline oscillator, but they seem to work in where they are.

Start with the R3. This is a low hanging fruit.
You do not have to yank it.
Measure it in-circuit; if it is 10K, add 30K in parallel to it, it will give you some 7.5K.

[dconant]

Ok Mike, Did the C3 change to 7.5K still craps out. One wire was easy to pull. Seems like it is failing faster now but could just be my imagination.

Chas I just tried SW. While the radio was in its reduced state I found one station but very weak. Removed and replaced the ground wire and that bought that station in much stronger.

[morzh]

You mean, R3.
Why did you pull the wire? What value did you use? By itself or in parallel?