Hello All,
I have replaced all paper and filter capacitors. I hear nothing from the output. But when I connect my tracer to the wiper of the volume control, there is faint audio. When I tune a station I can hear it on the tracer. So I am pretty sure that the tuning section and detector are ok.
Also, 1/2 of the primary of the speaker transformer crackles (when I put a tube in on that side, the voltage coming from the speaker transformer drops down to 20 from 200. I am sure though, that the other half works fine (voltage doesn't drop when a tube is connected). But I hear very little through the speaker. Just a faint hum. I am only using the 47 tube that is on the working side of the output transformer. I should, at least hear something, even if it's distorted. Nothing.
Where should I check. I have the service data and have been checking resistors, and all connections, but cannot find anything wrong.

Please help.

Thanks,

Dave...

If I find the answer before I get a reply, I will post where the problem was incase someone else has this problem.

If the voltage at the other side doesn't drop at all when you put in a tube, it is very likely that this winding is completely open. The other one is obviously arcing open. I'd check the output transformer thoroughly for continuity and proper DC resistance. If this all checks out fine, check below.

Did you check the output transformer for shorts to ground or something like that? Voltage at the driver tubes? Absolutely sure that when you replaced condensers in the audio section that they are reconnected to the same tube pins? Most circuit diagrams base the sockets from the underside, so that shouldn't be a problem. Be sure to go clockwise from 1 when under the chassis.

Possible that the cathode bias resistor(s) (if one is used) on the tubes has opened up or drifted severely? This is another point to check on the side that the voltage does not drop down at all when the tube is plugged in. When you plug in the tube, the voltage should drop just a little--from around 10 to perhaps 50 volts. Check your diagram to be sure. When you plug in the 47 tube on the side where the voltage stays high, you should hear a loud pop in the speaker when you remove it, and like I said before, the voltage at the plate really should change a little when the tube is inserted warm and removed warm.

It is obvious that the side that crackles is in fair working order (or at least has power) other than the output transformer having a break in the winding. Check out the side that does nothing really carefully (no voltage change). I think you have a faulty output transformer.

If you don't have a push-pull transformer at hand of the proper inpedance, etc.(you can purchase one from AES www.tubesandmore.com), you can test out each side with a spare single ended output transformer that you may have at hand. This will give you a rough idea. Try to get the DC resistance somewhat similar to the DC resistance of each side of the original transformer. If audio quality is somewhat poor, but mostly agreeable, don't tamper with your set until you receive your exact or near exact replacement push-pull transformer. Especially if the single ended transformer is from a cheap radio (transformer fits quite easily in the palm of your hand, and is possibly from an AC-DC radio of small size), the audio quality may be a bit poorer, and may distort easily. For testing purposes, a simple PA matching transformer can be had from Radio Shack for not too much money. The transformer has various wattage settings for public address systems. Each wattage setting is a higher resistance as the wattage rating goes down. Pick the resistance that somewhat matches your original transformer, and then pick the secondary that plays the speaker loudest. This can be used as a testing transformer if none other can be had.

Thomas

:Hello All,
: I have replaced all paper and filter capacitors. I hear nothing from the output. But when I connect my tracer to the wiper of the volume control, there is faint audio. When I tune a station I can hear it on the tracer. So I am pretty sure that the tuning section and detector are ok.
: Also, 1/2 of the primary of the speaker transformer crackles (when I put a tube in on that side, the voltage coming from the speaker transformer drops down to 20 from 200. I am sure though, that the other half works fine (voltage doesn't drop when a tube is connected). But I hear very little through the speaker. Just a faint hum. I am only using the 47 tube that is on the working side of the output transformer. I should, at least hear something, even if it's distorted. Nothing.
: Where should I check. I have the service data and have been checking resistors, and all connections, but cannot find anything wrong.
:
:Please help.
:
:Thanks,
:
:Dave...
:
:If I find the answer before I get a reply, I will post where the problem was incase someone else has this problem.

Also.....you may be wondering then, if the transformer is blown, how you are getting hum. I looked at what I think is the circuit diagram for your radio. What was listed was either 10--a single speaker unit, or 10C--a dual speaker unit. No 10B was listed, but I assume that your 10B falls somewhere with the 10 and 10C. Well, the reason you hear a FAINT hum is because your field coil is connected across the power supply. The field coil will receive power as long as the power supply is in good condition. It will induce the hum it has into the voice coil. At first glance you may not realize this, but the voice coil is actually in a complete circuit by being connected to the output transformer. So because currents are being induced into the voice coil, it will vibrate just slightly in the field of the field coil. Actually, depending on how ruggedly the field coil is mounted to the frame, it may cause hum anyway just by it vibrating on its own.

Thomas

Tom,
Problems solved. One of the #47 tubes had loose shorting elements. The speaker transformer is good. The detector coil was open so I had to make a new one. Also, the 500 ohm section of the voltage divider resistor was open. I had already replaced it before I sent the first post.
Anyway, the Apex model 10 B is a US Radio & Television model 10 because it has 10 tubes.
Unfortunately, the meter movement is damaged, probably because it pegged at some point. Also, the coil is open. I could have rewound the coil but since the movement is in such bad shape, I left it as it was.
This set works great and is extremely sensitive. I also have a model 8 to restore.

Dave...
:Also.....you may be wondering then, if the transformer is blown, how you are getting hum. I looked at what I think is the circuit diagram for your radio. What was listed was either 10--a single speaker unit, or 10C--a dual speaker unit. No 10B was listed, but I assume that your 10B falls somewhere with the 10 and 10C. Well, the reason you hear a FAINT hum is because your field coil is connected across the power supply. The field coil will receive power as long as the power supply is in good condition. It will induce the hum it has into the voice coil. At first glance you may not realize this, but the voice coil is actually in a complete circuit by being connected to the output transformer. So because currents are being induced into the voice coil, it will vibrate just slightly in the field of the field coil. Actually, depending on how ruggedly the field coil is mounted to the frame, it may cause hum anyway just by it vibrating on its own.
:
:Thomas
:
: