tube in and out, unplugged I get no resistance at all from any of the pins to pin 5 couplate off the grid. Yes a real head scratcher I already ran out of options. From tubes to diodes to capacitors to checking the volume control to the .370k resistor that is supposed to be 330ohm, nothing, it won't give up anything. I already clipped a few ceramic disc and dogbone caps at the ground to see if one may be leaky but so far nothing , clipped still humming. The ones I clipped one by one I believe we're at the heaters to ground. :scratch2:

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.

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.

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.

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!

It's just a typical Bakelite socket.

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.

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.

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.

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.

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. :scratch2:

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.

.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? :scratch2:

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. :smoke:

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.

The 6SQ7 is a high mu tube. Try a 6SR7 tube, that has a lower mu.
The 22 ohm resistor in parallel with the 6SQ7 heater has to be in there, as the 6SQ7 tube has a heater rated at 300ma, directly in series with the CRT, which has a 600ma heater.
I'd be tempted to rewire the socket for a 6J5, which is a half of a 12SN7, but for now the 6SR7 is a plug and play replacement.
I have to look at the schematics again and do some comparisons with the various versions.
I never ran into this version, only the ones using the 12SN7. :scratch2:

Would 6sj7 or 6sk7 be of any use

Sorry, those are penthodes, the 6SR7 is almost the same tube as the 6SQ7, but with a lower mu or amplification factor. A triode with detector plates, all tied to ground or B-.
I looked at the schematic again and the 22 ohm resistor across the heater of the 6SQ7 is located in the ballast tube. Insane!

Yes that resistor is in the ballast or it was now it's separate on the chassis. I mentioned that resistor because the Manuel spoke about audio hum so I double checked that resistor to make sure I didn't put the wrong leg to it but it's good. I went over the Power supply and it all looks good. I'll probably wind up just dealing with this hum which I really do want to I would rather find the cause. If the speaker was 15 ohms the hum would be much less but the original speaker is 3.5 ohms and compared to the higher ohm speaker it would be so much more quite. The speaker I was using is 15 ohms rather then taking the one out of the cabinet.

Well the issue of the audio hum has not been found so I am done with looking for it so instead I placed 2 - 6.3v 680uf in parallel across the speaker leads so when with the volume was down the hum was very noticeable, loud, now the hum cannot be heard when the volume is down and still have good volume. :thmbsp:

There is a post somewhere that shows a schematic on how to put together a ballast for the Motorola vt71. It shows the resistors as well as the caps but the caps shown there rated UF but if they are film caps then that should be NF so I'm just seeing if anyone made one of these and it is for sure the caps are NF .

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.

How did you determine that value, mathematically?
It sounds like a plan!
Your hum problem was a topic of discussion between myself and another VK member at the WARCI swap meet. :scratch2:

Well no math as I'm not good In math so I just found that the lower the rated voltage of the cap and the highest in uf worked best, I guess you can say it was Alittle trial and error. I found that my other motorolas has the same hum but very low and have to get up to the speaker to here it so after putting those caps across the speaker leads it seems to filter out 90% of the hum and at the same time added Alittle bass to the sound, kind of warmer sound since befor I put the caps in the sound was ok but had lots of treble. The lowest voltage caps I had was 6.3v maybe lower would have been better , maybe even a non polar cap could have been better but I only had higher voltage non polar caps so that was out. It didn't matter about polarity since sound is mostly ac. So the topic of the hum you had, any conclusions? :scratch2:

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

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

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

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-.

That's it exactly :yes:

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.

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.