Odd Electrolytic

[jeyurkon]

In making a list of capacitors to replace in a 1949 Sylvania I came across an electrolytic in the AGC section that is rated 10Mfd -12V, 150V. Looking at the circuit I believe it actually can experience both polarities. Other than non-polarized electrolytics I've never heard of such a thing and those have equal voltage ratings.

Have any of you come across these in the past?

I think I'll replace it with a metalized polypropylene if there's room.

John Y.

[Old1625]

Not being able to see the component, nor the schematic to the set, I'd say you're on the right track to go with an NPO replacement.

[jeyurkon]

Originally Posted by Old1625

Not being able to see the component, nor the schematic to the set, I'd say you're on the right track to go with an NPO replacement.

Thanks.

BTW, here's that part of the schematic. Notice the biasing of the AGC amp. The cap in question is C126.

John

[Old1625]

That's an interesting setup for AGC.... Could you offer up more of the schematic so I can see things in a clearer context...?

Other than that if you can scare up the capacitance in a non-polar you should be OK, IMHO.

[Don Lindsly]

In normal operation that AGC line will never go positive. You can use a 25-50 volt positive grounded electrolytic without fear.

[jeyurkon]

Originally Posted by Old1625

That's an interesting setup for AGC.... Could you offer up more of the schematic so I can see things in a clearer context...?

Other than that if you can scare up the capacitance in a non-polar you should be OK, IMHO.

I'm sure I can deal with it. I've just never seen an electrolytic rated this way and wondered if anyone else had.

I've included the schematics for both the chassis and tuner.

John Y.

[avionic]

What numbers or values are printed on the old part? The voltages on the schematic look like to be kind of squished together testpoint voltages or typo error . I would go with 10uf @ 150v based on the schematic.

[nasadowsk]

Wow, I thought Motorola was weird - what year did Sylvania drop the split horizontal and HV power supplies? That's a neat but weird feature...

[jeyurkon]

Originally Posted by nasadowsk

Wow, I thought Motorola was weird - what year did Sylvania drop the split horizontal and HV power supplies? That's a neat but weird feature...

I imagine the same year they realized how much it was costing them.

The Motorola HV supply is similar isn't it?

The September 1949 Sylvania News describes three types of HV supplies.
1. The Fly Back
2. The RF HV Supply
3. The Pulse Type HV Supply. (A blocking oscillator triggered by the horiz. sync)

Do you know of any sets that used the pulse type?

John Y.

[nasadowsk]

Only the 7 inch electrostatic Motorollers are RF supplied, the bigger ones use a conventional flyback type (that in my case, isn't working right, yet )

Wasn't the pulse type used in early European color sets?

For weird cap values, I've seen some in ohms @ frequency. Go figure.

Talk about a bonkers TV though, I think your Sylvania takes the cake!

[jeyurkon]

But I loved it as a kid back when my folks had one. I always remember it working well.

I'm working on restoring a 9T1. It has an 8" CRT but I guess that doesn't count as bigger since it has the same chassis as the 7" units. There are some rather large caps in the HV section to replace. 6KV and 10KV.

Which Motorola do you have?

I think the biggest problem with the RF HV is that if the sweep stops the HV doesn't and you risk burning the CRT.

John Y.

[Old1625]

If you can't find a non-polar then you might want to consider what Don Lindsly has to say; I've now looked over the schematic, and am inclined to agree with him that the AGC line is not apt to go positive in normal op.

[marty59]

Although not very pretty, You can make a non-polar cap by connecting two electrolytic caps together in series and tying the negatives together.

[jeyurkon]

I won't have trouble finding a capacitor. Metalized film caps with that value are quite common.

Looks like I'll have to record the voltage levels there during startup and operation of the set. (Once I get my hands on it and hopefully restore it.)

The fact that Sylvania wanted 150V for the positive rating makes me think that even if it's only a transient that a normal electrolytic wouldn't be very happy living there.

It's great that everyone has been so helpful and thought carefully about this, but I really was only curious about such a wierd capacitor and whether others had ever seen one. I hope the original is still there to look at.

John Y.