[Mr Grumpy]

Hello phellow Philco phreaks,

It's been a while since I've posted anything, my 3 year old has been keeping me plenty busy.

I had a quick question about Isolation transformers...
Up until now, I've been replacing caps and resistors, testing tubes and getting rather lucky. I'd like to start doing some real testing on some of my problem sets but want to buy an Isolation transformer first.

I've notice the ones linked below but I'm not sure if they'd be right for radio repair. Does there have to be a minimum output from these? Anything in particular I should be looking for?
P.S. - I don't want to spend a fortune.

Thanks.

http://www.ebay.ca/itm/271405793357?ssPa...1423.l2649

http://www.ebay.ca/itm/180750352646?ssPa...1423.l2649

[TV MAN]

Mine is an older Isotap 400w, 4a transformer that I have powered about anything electronic you can immagine. I dont think a radio would even test the limits of either of those you are looking at. They are a pretty good idea, esp on those ac/dc sets where one side of the a/c line is connected to the chassis. In addition, you may want to also consider a variac transformer, but not as a substitute for an isolation transformer.

[morzh]

1.25A for the B&K (and those are good for smaller things to test) could become a limit with a large radio.
300VA is a good size.

I have this exact type

http://www.ebay.ca/itm/Dale-IT400-Medica...5401ff07d0

If you decide to by Dale/FLuke let me know, there is one thing you have to do to it first (an easy thing - disconnecting a jumper inside).

[klondike98]

$29 plus shipping
https://www.valuetronics.com/detail/Used...wuznIW9KHY

They have a few other models as well.

[tab10672]

I built my own with 2 radio transformers, you can wire both 6.3 (or what ever the filament voltage is as long as both trans. have the same fil. voltage) together. It takes 120v to 6.3v to 6.3v back up to 120v. Cheap easy isolation transformer that works just as well as something you buy.

[morzh]

This Valuetronics thing, is it used?
If it works, can't beat the price.

No picture though.

[skyscraper]

Be careful using two filament transformers. A larger Philco pulling 90 watts will put a more than 15 amp draw on the two back to back secondary windings. I doubt most are made for more than a few amps.

If you could get matched high voltage windings together, that would be better. I would suspect the voltage might not be as stable though.

[morzh]

Yes, I agree - the two transformers back-to-back at 6.3V winding might not be such a hot idea (well could become hot ) as at 6.3V the current draw even for a medium 60W radio could be too much.

[tab10672]

I built it after reading on the net to use the lowest voltage on the trans, something to due with the windings. I have used it for years. I have a 1 amp fuse on the line side and a 1.5 amp mini breaker on the load side, it has never blown either.

[skyscraper]

They wouldn't blow. 1amp x 120v is 120 watts. 120 watts going through at 6v works out to 20 amps. Stepped back up, that's an amp out. I don't know of many transformers that can handle 20 amps on a secondary winding, and I would imagine yours is either getting very hot or you haven't powered a large radio with it.

[tab10672]

So would the high volt winding be better?

[skyscraper]

I don't know a whole lot about high voltage transformers, but I do know Ohm's law. You can experiment, just make sure you have two transformers with matched windings.

I would guess the higher resistance would cause the voltage to fluctuate somewhat with load, but that's easy enough to test.

In the end, it's about the ratings of the transformer and Ohm's law.

[tab10672]

http://m.youtube.com/watch?v=QDSmv9E5VgQ
This is the trans. I built, I remember using the 5 volt 80 fil winding now that I watched it again.

[skyscraper]

That's even worse... That's 25 amps possible flowing through those wires with the fuse you have. The 80 tube uses 2 amps, for comparison.

Can anyone else confirm?

[tab10672]

I dont have a great understanding of electronic laws I just know it has ran many transformer-less radios for hours without getting hot or any other problems. Maybe I have just been lucky?