Ts4d Motorola

[dieseljeep]

Originally Posted by 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.

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.

[timmy]

Would 6sj7 or 6sk7 be of any use

[dieseljeep]

Originally Posted by timmy

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!

[timmy]

Originally Posted by dieseljeep

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.

[timmy]

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.

[timmy]

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 .

[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.

[dieseljeep]

Originally Posted by timmy

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.

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.

[timmy]

Originally Posted by dieseljeep

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.

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?

[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.

[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

[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.