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48-1262 - Printable Version

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Pages: 1 2

48-1262 - ke5yum - 10-08-2013

Good afternoon all

I am getting back into my chassis of my radio after a near six month break, and I have question about a resistor that reads 0.

Section 4 of the troubleshooting page shows R400. When I started restoration the original resistor read 0. But after I replaced it with the correct value, it still reads 0. Continuity is fine.

C402 appears to be "parallel" to the resistor, terminating to the same terminal.

Question: Can a bad capacitor be making the resistor read zero? Replacing the capacitors here are not an issue, but I am wanting to make sure I don't have other problems further back. Note: I don't have a capacitor tester.

Thanks,
Terry
KE5YUM

RE: 48-1262 - klondike98 - 10-08-2013

If the external aerial lug in the diagram is connected to the chassis for loop antenna operation then R400 will read as 0 ohms since both ends terminate to the chassis. You can not always measure the resistance while the resistor is still in the circuit. If there is an alternate path for DC current to flow around the resistor you will get a faulty reading.

RE: 48-1262 - BrendaAnnD - 10-08-2013

If your external antenna wire is connected to chassis ground, your resistor will read zero ohms, because it's being bypassed to chassis ground on both ends. This is normal.

RE: 48-1262 - ke5yum - 10-09-2013

Thanks for both replys. Glad to know my situation is normal.

RE: 48-1262 - ke5yum - 10-22-2013

Good morning, all

I am still working on the chassis of my 48-1262. I feel that I am getting close, but I have two issues that need resolving: a hum with no reception and the oscillator test shows + voltage instead of -.

Trouble shooting guide points to bad C103A and B if abnormal hum is present, but these are new and were checked for correct capacitance. R102 was also a suspect, but it tests just fine.

Voltages are good except test point D is a little low. Voltages are correct on the 14AF7 as well. The oscillator coil test at 3.0 ohms instead of 3.8.

Any thoughts as to where I should check next? I only test equipment is a Fluke Multimeter.

Terry

RE: 48-1262 - Radioroslyn - 10-22-2013

Hum
I would double check the wiring of the filter caps. Some may have the minis side connected to the chassis instead of B- which is the HV center tap.
Osc
If the osc is working the grid should read -. Check the bandswitch and be sure it good and clean. Same with pins on the tube and socket.
Terry N3GTE
ps got my 80mtr ant backup the other day maybe I'll get back on the air!!

RE: 48-1262 - Radioroslyn - 10-22-2013

Sorry just looked at the diagram for your set. I thought it was a set with a power trans and SW. You got the economy model with a voltage doubler and series strung heaters.
http://www.nostalgiaair.org/PagesByModel/614/M0013614.pdf
Hum
Have much do you hear? A lot or just a little?? I would suspect that this set may have a little bit hum just from it's design.
Osc
Check the pins on the tube and socket. I'm presuming that you've replaced all the paper caps? C 403 or the mica C405 could be your culprit. Did you measure the resistors when you had the caps disconnected??
Terry

RE: 48-1262 - ke5yum - 10-23-2013

Terry,

There is quite a bit of hum. I have had a friend of mine who is familiar with tube gear look at the wiring, and he says that the electrolytics are correct. I have replaced all the paper caps, and I too suspected that mica 405 may be a problem. I can replace that easily enough. Will keep trying.

Terry

RE: 48-1262 - ke5yum - 10-30-2013

Well, I'm stumped, and I hope that someone can tell me if I am missing something.

Voltage in Section 1:

Test point A - 126 (should be 167)
Test point B - 204, but dropped to 178 (should be 214)
Test point C - 90 (should be 181)

Electrolytics are new with correct values and properly wired.

Section 4 Oscillator Test

+3.8 and then dropped off. Can't figure why it isn't negative.
Coil ohms at 3.0

Section 4 Voltage Check of 14AF7

Pin 3 111
Pin 6 125 (then dropped to 107)
Pin 7 2.6 (should be 4.2)

I replaced C405 with no change.

Last item: How do I perform a audio transformer check? I hear scratching in the speaker but no sound except a hum when I turn the volume up. No audio whatsoever.

Terry

RE: 48-1262 - Radioroslyn - 10-30-2013

Well part of your issue is that test point B is where you should be hooking the - lead of your voltmeter. Test point B is the negative side of the power supply. So measure A,C,and D against B and tell me what you get.
The other
Terry

RE: 48-1262 - ke5yum - 10-31-2013

Terry,

Most of the issues have been resolved I think. First, I have been placing the - lead of the VM on Test point B. I did not mention this before not thinking that it mattered but those power checkes were done with the unit attached to a dim bulb tester. It occured to me that the power will be low with that arrangement, and it was. My power check is as follows:

Test Point A 180 (high)
Test Point C 255 (high)
Test Point D 129 (low)

Voltages on the 14AF7 are

Pin 3 120
Pin 6 126
Pin 7 7.9

I was also conducting the oscillator test incorrectly. It measures -4.2, so I am happy with that.

The audio transformer is ok. I tested it by touching a wire to it and listened for a pop. I heard one.

But I have no audio. The only thing I hear is scratching in the speaker that sounds like a nearby thunderstorm heard on the AM dial. There is a hum that gets louder as I turn the volume up.

Any thoughts? Could the unit be out of alignment that it receives no stations?

Terry

RE: 48-1262 - Radioroslyn - 10-31-2013

Good. The reason I mentioned test point B was you posted that it had 204vdc on it so I was confused.
I would check the continuity of the primary and secondary of both IF transformers. Do you have a signal generator??
Terry

RE: 48-1262 - ke5yum - 11-01-2013

No signal generator yet, but it looks like I may have to buy one.

Concerning the IF, I checked their continuity a while back and I don't have the numbers in front of me. If I recall correctly, all ohmed out ok except one which was a little high. I will double check and post that as well. It will be Tuesday before I can do that.

Thanks for all your insight.

Terry

RE: 48-1262 - ke5yum - 12-13-2013

Well, I am still not oscillating. I've replaced every capacitor except C401 in the oscillator circuit, and I hear nothing buy a dull hum that is controlled by the volume control. I quickly checked voltages this morning, and they appear ok, but when I touched pin 6 of the 14AF7, I heard a very loud, scratchy, static sound. The volume was turned up all the way, so it was very loud! That is the pin that connects to the IF. Any thoughts about that?

I still don't have an SG. The one I borrowed does not work, so I am on hold there.

RE: 48-1262 - Radioroslyn - 12-15-2013

Give us a shout when you get a SG and then we can give this set a good going over. Also a signal tracer wouldn't be a bad piece of test equipment to have too. You should be able to find these for around $15-20 each. Heathkit stuff is pretty good stuff and is pretty well documented.
GL and Merry Christmas!!
Terry