Ts4d Motorola

[bandersen]

Did you check between all the pins on the socket - not just to pin 5 ? Your meter might not be putting out enough voltage to detect any leakage. I don't suppose you have a megaohmeter or insulation tester (AKA Megger) ?

[timmy]

Originally Posted by bandersen

Did you check between all the pins on the socket - not just to pin 5 ? Your meter might not be putting out enough voltage to detect any leakage. I don't suppose you have a megaohmeter or insulation tester (AKA Megger) ?

I did check the socket very closely and the Dvom I have is very sensitive, voltage from the meter I don't know but the socket looks new for its age.

[timmy]

Maybe I should look at the other sockets maybe another one may show some carbon track, who knows at this point. For a hum I would think it would be obvious if the socket was bad but much higher voltages I would assume would show something on a socket but this problem don't seem like much ac is getting in somewhere because it's only affecting audio and not an image problem.

[dieseljeep]

Originally Posted by timmy

Maybe I should look at the other sockets maybe another one may show some carbon track, who knows at this point. For a hum I would think it would be obvious if the socket was bad but much higher voltages I would assume would show something on a socket but this problem don't seem like much ac is getting in somewhere because it's only affecting audio and not an image problem.

I would think a socket would carbon-track only if water was spilled on the chassis and then it would have to be operating at the time.
After all this discussion, I decided to pull the 7" Motorola off the shelf. I never restored the set, but I did the preliminary checking of the condition of the set.
Of course, the set works very poorly and I don't remember what I did to check the basics. IIRC, I bridged the 'lytics and installed silicon diodes to replace the seleniums. I didn't do anything to it but plug it in. It hummed and it has a dim display on the screen.
BTW, this set was dated Feb, 1949 and uses 1/2 of a 12SN7 for the 1st audio.
More much later!

[timmy]

Originally Posted by dieseljeep

I would think a socket would carbon-track only if water was spilled on the chassis and then it would have to be operating at the time.
After all this discussion, I decided to pull the 7" Motorola off the shelf. I never restored the set, but I did the preliminary checking of the condition of the set.
Of course, the set works very poorly and I don't remember what I did to check the basics. IIRC, I bridged the 'lytics and installed silicon diodes to replace the seleniums. I didn't do anything to it but plug it in. It hummed and it has a dim display on the screen.
BTW, this set was dated Feb, 1949 and uses 1/2 of a 12SN7 for the 1st audio.
More much later!

Some of the early ones did use a 12sn7 and did use half for the audio and the other was for the verticle oscillator.

[robert1]

Originally Posted by timmy

I did check the socket very closely and the Dvom I have is very sensitive, voltage from the meter I don't know but the socket looks new for its age.

Is that socket a "wafer" type socket? I have already experienced dealing with two bad sockets of that type developing leakage between the wafers. as a last ditch effort, why not try replacing the socket.

[timmy]

Originally Posted by robert1

Is that socket a "wafer" type socket? I have already experienced dealing with two bad sockets of that type developing leakage between the wafers. as a last ditch effort, why not try replacing the socket.

It's just a typical Bakelite socket.

[bandersen]

Huh ? You can use whatever units you like for any capacitor, pF, nF, uF, etc.

The original Motorola ballast schematic I've seen used 10uF AC capacitors. They need to be rated for 125VAC or better. When I did the math myself, I found that 8.2 uF is better for modern line voltages of around 120 VAC. I've built them with both values and they work very well.

[timmy]

Originally Posted by bandersen

Huh ? You can use whatever units you like for any capacitor, pF, nF, uF, etc.

The original Motorola ballast schematic I've seen used 10uF AC capacitors. They need to be rated for 125VAC or better. When I did the math myself, I found that 8.2 uF is better for modern line voltages of around 120 VAC. I've built them with both values and they work very well.

I thought I read that they are film caps which are not electrolytic where the UF comes in and film are typically NF . So if I use the calculator for 10uf and convert it to NF then could the cap be rated for 630v. I think that conversion would be 10,000 nf

[bandersen]

Yes, it would be 10,000 nF. None of the American distributers I'm familiar with use nF. Mouse, Digikey, Allied will all use uF for their film caps in the online catalogs.

You want a capacitor rated for AC not DC. Also 8.2 works better than 10uF

I suggest you use these.
https://www.digikey.com/product-deta...183-ND/5876926

[Tom9589]

Just for grins, I looked over the schematic. Where is the voltage divider resistor connecting B++ to B+ ? Definitely not a SAMS schematic.

[dieseljeep]

Originally Posted by Tom9589

Just for grins, I looked over the schematic. Where is the voltage divider resistor connecting B++ to B+ ? Definitely not a SAMS schematic.

I was looking high and low for the way they did the voltage drop. As Tom C stated, they used other stages, probably the IF or other signal stages.
I'm always used to seeing the audio output tube as the voltage divider.
Admiral and many other firms did the same thing, a common practice.

[Electronic M]

IIRC on these sets, they don't actually use a resistor based divider to get B+ from B++...How it works is they actually use the tube stages as a divider (take a tube stage filter it's rails and you can model it as a resistive load to the PS). Some stages are across B++ and B+ some are across B+ and B- (those together form a divider to create B+ form B++) and others that need more voltage are across B++ and B-.

[bandersen]

That's it exactly

[Tom9589]

The audio output stage was the first place I looked. It was common to get about 150 volts from the cathode of the audio output tube. While this practice eliminates some parts, anything that affects the bias on the audio output tube could affect the 150 volt bus.