Got a lead on an early DuMont TV locally

[bandersen]

Yes, it is. I'd be more concern about getting zapped while working on it. The entire metal bell is connecting to HV. It's not lethal but stings.

Good idea to use a 5AXP4 or 8XP4 while on the bench if you have one.

[vortalexfan]

Originally Posted by bandersen

Yes, it is. I'd be more concern about getting zapped while working on it. The entire metal bell is connecting to HV. It's not lethal but stings.

Good idea to use a 5AXP4 or 8XP4 while on the bench if you have one.

I have a 10BP4 tube in another TV that's functional, would that work? Other than that I don't have any Test CRTs.

[vortalexfan]

I did trace out where that filiment transformer went to, it went to the second 5U4G tube. So perhaps a bad 5v filiment winding in the original power transformer? The original power transformer in this unit is a beast, I can't believe the transformer would of fried that easily...

[bandersen]

You can use any CRT that will fit and has the same base pinout. 10BP4, 10FP4, 12LP4, etc. Bigger issue is mounting and supporting it. You'll also need to modify the HV connector.

Both 5U4 filaments should be wired in parallel. More likely there is leakage between windings than it is open. You could check it if you have a megger or leakage tester. Some devices like the Sencore PR-57 have a leakage tester built in.

[vortalexfan]

Originally Posted by bandersen

You can use any CRT that will fit and has the same base pinout. 10BP4, 10FP4, 12LP4, etc. Bigger issue is mounting and supporting it. You'll also need to modify the HV connector.

Both 5U4 filaments should be wired in parallel. More likely there is leakage between windings than it is open. You could check it if you have a megger or leakage tester. Some devices like the Sencore PR-57 have a leakage tester built in.

Unfortunately I don't have a leakage tester, I do have a capacitor checker from the 1930s that tests for leakage in capacitors but I'm guessing that's not the same thing...

Also if the winding in the power transformer for the 5v filiment turns out to be fine (not open, but maybe slightly leaky) would it be fine to just reuse the original transformer's 5v Filiment winding, the less modifications I have to contend with in this set, the better.

[bandersen]

Slightly leaky would be bad. The transformer will get warm (maybe hot) which will likely cause it to get worse until it spirals out of control and goes up in smoke.

Personally, I've never encountered a bad transformer in a vintage TV but of course all things are possible.

I do have a couple Dumont RA-103 parts chassis. If the power transformer is the same, it's yours for the cost of shipping.

[vortalexfan]

Originally Posted by bandersen

Slightly leaky would be bad. The transformer will get warm (maybe hot) which will likely cause it to get worse until it spirals out of control and goes up in smoke.

Personally, I've never encountered a bad transformer in a vintage TV but of course all things are possible.

I do have a couple Dumont RA-103 parts chassis. If the power transformer is the same, it's yours for the cost of shipping.

OK, that's fine. I'm really not sure how I would go aboout figuring out if this TV's Transformer is bad or not if I don't have a leakage tester, as test equipment doesn't come up for sale very often in my area, especially test equipment thats really specialized like a leakage tester, every once in a while I will come across a tube tester or a oscilloscope, but not much else.

[bandersen]

A crude way is to pull the rectifier tubes and power it up. If the transformer gets hot after a few minutes (or less), it's bad.

[vortalexfan]

Originally Posted by bandersen

A crude way is to pull the rectifier tubes and power it up. If the transformer gets hot after a few minutes (or less), it's bad.

OK, thanks for that little tip, I'll give that a shot.

Also another thing I noticed is that this TV has an RCA Jack on the back in place of the 300 Ohm screw terminals for the antenna and then from that there was a 300 Ohm twin-lead that was hanging off the back, I was wondering how the original Antenna terminals would of looked like on this TV so I could maybe try and get it put back right again.

And then I was testing some of the tubes in this TV and I noticed that one of the 5U4G Rectifier tubes measured shorted in both sections, would that happen to have anything to do with the "bad 5v Transformer winding" issue? The other 5U4G tube was fine by the way and it measured into the 90s emissions wise.

[Electronic M]

Most Dumonts from the end of WWII to the Emerson merger used a 72 Ohm (not 75 or 300) coaxial antenna hookup and had an RCA jack as the factory antenna connection. Every RA-103 I've seen and my manchu are setup in that manner.

[vortalexfan]

Originally Posted by Electronic M

Most Dumonts from the end of WWII to the Emerson merger used a 72 Ohm (not 75 or 300) coaxial antenna hookup and had an RCA jack as the factory antenna connection. Every RA-103 I've seen and my manchu are setup in that manner.

OK, so the RCA Jack is actually original and the 300 Ohm Twin-lead wire in the back was added later on in an attempt to make it work with a "modern" 300 ohm twin-lead antenna hook-up if I'm understanding this correctly.

Is there any way to make this 72 ohm RCA antenna setup work with a more modern 75 ohm antenna setup like what is used on a digital converter box or a VCR or a RF Modulator?

[dtvmcdonald]

For the antenna all you need do is get it somehow connected. Both are unbalanced
coax and the impedance difference is immaterial. Adapters are readily available, but in my experience not exactly long run reliable. Just disconnect any 300 ohm stuff.

[Electronic M]

They make RCA to F-type adapters...one should be all you need. Or if you have the parts laying around just solder an RCA plug onto an end of a 75 ohm TV coax cable and call it good.

[vortalexfan]

Originally Posted by dtvmcdonald

For the antenna all you need do is get it somehow connected. Both are unbalanced
coax and the impedance difference is immaterial. Adapters are readily available, but in my experience not exactly long run reliable. Just disconnect any 300 ohm stuff.

Well All I would be doing would be just hooking a VCR up to the TV to use it to watch old Black and White Movies on, or a converter box so I could watch old TV shows on it. So I think just using an adaptor of some sort should be plenty enough to get it to work.

[vortalexfan]

Originally Posted by Electronic M

They make RCA to F-type adapters...one should be all you need. Or if you have the parts laying around just solder an RCA plug onto an end of a 75 ohm TV coax cable and call it good.

Sounds like a plan to me.