Ts4d Motorola

[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 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.

[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.

[dieseljeep]

Originally Posted by timmy

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

I'll have to look at the schematic to order all the HV caps and the rest of the capacitors to complete the restoration.
This set is one of the portable styles and was complete with the antenna and base and in reasonable condition. The CRT seems to be good, but the true test is when the set is restored.

[Zenith26kc20]

I'm not sure this applies but the date is close.

[timmy]

Originally Posted by Zenith26kc20

I'm not sure this applies but the date is close.

Well this helps if it were max volume but mine is minimum volume hum.

[jr_tech]

mine is minimum volume hum.

A few questions:

1. Does the hum actually get louder as you turn the volume control down?

2. Do you observe any change of hum as you change the volume control when the grid of the 25L6 is disconnected?

3. Are you absolutely sure that the cap across the 330 ohm resistor in the cathode circuit of the 25L6 is connected correctly? Properly grounded?

4. Do you have a few more 25L6s to try?

jr

[timmy]

Originally Posted by jr_tech

A few questions:

1. Does the hum actually get louder as you turn the volume control down?

2. Do you observe any change of hum as you change the volume control when the grid of the 25L6 is disconnected?

3. Are you absolutely sure that the cap across the 330 ohm resistor in the cathode circuit of the 25L6 is connected correctly? Properly grounded?

4. Do you have a few more 25L6s to try?

jr

with the grid off the hum is louder but don't change when the volume is raised or lowered. The cap across the 330 is correct and grounded. I tried a new 25l6 or 2 and even a good one but tests weak and again no change. This set is nuts I can't imagine this.

[timmy]

I bridged the power supply caps made no difference but the 2 300 v caps rated at 100uf and 120 uf I had put in 2- 450 v 470 uf caps in place of the original and I thought maybe this was a problem but it shouldn't as it's just Alittle harder on the diodes when turned on.

[jr_tech]

1. Does the hum actually get louder as you turn the volume control down?

2. Can you measure the voltage of the hum at the speaker?

Originally Posted by timmy

the 2 300 v caps rated at 100uf and 120 uf I had put in 2- 450 v 470 uf caps in place of the original and I thought maybe this was a problem but it shouldn't as it's just Alittle harder on the diodes when turned on.

Sometimes increasing the capacitance value of of a filter cap too much can cause 60hz current spikes that will propagate throughout the entire ground system of the set and cause hum problems. More is not necessarily better.

jr

[timmy]

Originally Posted by jr_tech

1. Does the hum actually get louder as you turn the volume control down?

2. Can you measure the voltage of the hum at the speaker?



Sometimes increasing the capacitance value of of a filter cap too much can cause 60hz current spikes that will propagate throughout the entire ground system of the set and cause hum problems. More is not necessarily better.

jr

The hum just appears when the volume is lowered to minimum, gradually you can here it the lower it goes, then it's just a nasty hum. Volume at minimum IIRC the voltage was nearly nothing but my Dvom has a hz setting and with the grid off I'm getting a full 60 hz from the speaker. These big caps I used IIRC I used then in one of my other vt71 sets and don't have any problems with hum.

[timmy]

.012 ac voltage at speaker no hz, grid on, grid off voltage is 1.662 ac at 60hz . No DC voltage grid on or off.

[jr_tech]

Originally Posted by timmy

.012 ac voltage at speaker no hz, grid on, grid off voltage is 1.662 ac at 60hz . No DC voltage grid on or off.

1.662 volts with the grid of the 25L6 disconnected? That's quite high! Was there anything at all connected to the grid pin? For example, just a short clip lead will pick up a significant amount of hum.

Do you measure the same voltage with different 25L6s?

jr

[timmy]

Originally Posted by jr_tech

1.662 volts with the grid of the 25L6 disconnected? That's quite high! Was there anything at all connected to the grid pin? For example, just a short clip lead will pick up a significant amount of hum.

Do you measure the same voltage with different 25L6s?

jr

There was nothing on the grid pin when I measured at the speaker leads and got 1.662 volts ac.But I did use a clip wire to measure the grid with the wire on the grid. 25l6s the same. I've been going over this beast and can't find a thing out of place.

[timmy]

Well still cannot find the source of the hum but is it at all possible to get a hum like this from hv, the 1b3 gets its heater voltage from the hv coil so from there I don't know. The Motorola Manuel states that there is a 22 ohm resistor between the crt and the audio amp heaters to keep audio hum down but that resistor is there.