Completely toasted Farfisa Compact 23 mains Transformer - Replacement?

[FPALB]

Took a house call for a studio here in LA after they called about a Farfisa compact releasing the ghost. After measuring nothing on the secondary side we popped open the mains transformer and WOW. It definitely would have started a huge fire if they didn't power the unit down when they did. This thing was toasted! Found a 20Amp fuse was put in instead of a......... .5Amp fuse.

Problem is I can't find a replacement and ive never tried to measure for a new one - finding secondary currents and such.

Does anybody have any ideas for what I could use as a replacement or help me out to measure out what the secondary currents need to be?

service manual attached.

thanks in advance.

 

[C.B. - Boudio]

You could look for a normal Hammond transformer, with a 5V tap to put in series with the 6,3V (as to get 11V in replacement of the 9V; a resistor will drop those 2V), and use a standard 24V transformer for the 22V tap.
You could start looking up the current draw of the heaters and plates from the tubes. As long as there are no heavy power amplifiers that draw much current, you'll get away with quite a lot I think.

But have you found the fault that caused the transformer to burn out in the first place?

 

[FPALB]

You could look for a normal Hammond transformer, with a 5V tap to put in series with the 6,3V (as to get 11V in replacement of the 9V; a resistor will drop those 2V), and use a standard 24V transformer for the 22V tap.
You could start looking up the current draw of the heaters and plates from the tubes. As long as there are no heavy power amplifiers that draw much current, you'll get away with quite a lot I think.

But have you found the fault that caused the transformer to burn out in the first place?

Extremely helpful! Thank you. Ill take a look at some Hammonds

I didn't, usually I find that kinda thing with the unit on a variac.

 

[FPALB]

Finally got back to this Compact and decided to look at all the components right after the PT secondaries to size for current. I could use anybodies thoughts about these currents.

22V Rail:
A selenium B30C400 - 30V 400mA - I doubled 400mA to 800mA and thought that would be good enough for a secondary

210V Rail:
Selenium E250C50 - 250V 50mA -doubled that to 100mA and thought that would be enough for a secondary

6.3V Rail:
2 12ax7 with heaters in parallel ( 300mA per tube) = 600mA. bumped that to 1A and thought that should be good for a secondary.

9V Rail:
I am unsure on this one as the schematic doesnt' go into its uses.

The owner is interested in having a custom transformer made - unsure what that sorta thing costs, but price isn't a huge factory here.

 

[Walrus]

A quick bit of Ohms Law says that the 22V rail is drawing no more than 200mA, and the 210 V no more than 2.5 mA (allowing for leakage).

 

[FPALB]

A quick bit of Ohms Law says that the 22V rail is drawing no more than 200mA, and the 210 V no more than 2.5 mA (allowing for leakage).

Thanks Kevin, I attached what I could find of the service manual to my first post - can you get any idea of what the 9v draw would be?

 

[Walrus]

Sorry, no. Would need the pre-amplifier schematic.

 

[Disco Volante]

If the customer really wants the correct PT, and there's a budget for it, unwinding the old one would reveal the specs...
I tend to rplace those old metal-oxide rectofyers, they're generally just waiting to fail after all these years. Same goes for electrlytics and tants. You may need to add some series resistance to get the correct voltages. YMMV.
Good luck

 

[FPALB]

If the customer really wants the correct PT, and there's a budget for it, unwinding the old one would reveal the specs...
I tend to rplace those old metal-oxide rectofyers, they're generally just waiting to fail after all these years. Same goes for electrlytics and tants. You may need to add some series resistance to get the correct voltages. YMMV.
Good luck

I have it, but its been burnt down to a crisp.

 

[CJ]

measure the core,

 

[FPALB]

Hi CJ,

I was thinking more and more about this - I think I would like to send it to someone who knows more about this and hopefully have them make me another one - Do you have any good contacts for this? Im in the Los Angeles area. Would love to learn more about transformers myself, but got a bit of a repair back log and would love to move forward with a specialist.

best,

Jon

 

[CJ]

Cinemag would probably reverse and build you one,

Or I could take a stab at it,

 

[FPALB]

Thats extremely kind of you to offer, im not to far from cinemag, so I sent them a message - let me hear what they have to say and ill get back to you. Thanks CJ.