White Series 4000 EQ The strangest PSU design EVER.

[GLORY_MORRIS]

So, I'm repairing a White 4001 EQ and I've come across something I've never seen before. May not shock anyone else.....

In the PSU of this EQ, the designer used TWO positive rails with positive 18v regulators to realize his +18 and -18 Rails. These halves of the circuit are identical, save the configuration of the LM342p-18 used to provide the negative rail. The pins are simply configured differently, which apparently causes the regulator to invert it's +35v input and regulate it to -18. I've never seen anything like it, and what's more, I have no idea why this was his approach. The 342 datasheet doesn't mention anything about any such configuration.

Needless to say, the negative rail is where my problem lies and I can't find an LM342 of ANY variety anywhere. I've checked with White Instruments, and apparently they don't have any. I've entertained building a sub-board using 7818/7918 regulators, but seriously, I'd rather just find an LM342.

Anybody have a solution?

[ph_christensen]

Do you have a schematic, Are you sure that the LM324 is a voltage regulator? Is it possibly an op amp?

Edit ~ nevermind I misread what you wrote 342 not 324.

[dmills]

Sounds like a trick calrec used to use, two positive regulators, one at +17.5, one at +35, from a floating transformer/rectifier/cap, ground the +17.5V rail and you have +- 17.5V for the rest of the circuit.

They did it with 2N3055 and transistors, but you can do it with almost any regulator.

Regards, Dan.

[GLORY_MORRIS]

s'ok. It's definitely the oddest configuration I've ever seen. I'll have to scan the schematic to get an image.

[paulb]

Originally posted by GLORY_MORRIS
...the LM342p-18 used to provide the negative rail. The pins are simply configured differently, which apparently causes the regulator to invert it's +35v input and regulate it to -18. I've never seen anything like it, and what's more, I have no idea why this was his approach. The 342 datasheet doesn't mention anything about any such configuration.

I can't find a National LM342 in either the 1980 or 1995 databooks, nor on the website, but I can find references to it on the web. It does sound like just a fixed output regulator.
We'll need that Schematic.

[GLORY_MORRIS]

so its likely I could do this with a 7818?

[GLORY_MORRIS]

Here's a link to the datasheet:
http://www.alldatasheet.com/datashee...NSC/LM342.html

Here's a blow-up of the PSU:

[paulb]

That's what dmills suggested it was. 2 independent +ve supplies, connected in series.
Yup, 7818 should work. LM340 (18V version) too.

[GLORY_MORRIS]

Thanks fellas. I'll get right on it!

If you feel like it, I'd love an explanation on why this works.

[ph_christensen]

The ps is using a virtual ground. The ground from the output is not connected to the earth ground. If you measure the +18v output with reference to an earth ground it will actuly measure +36 volts. If you do the same from your ground point with reference to earth you should measure +18 volts. and from The -18v to earth will actuly measure 0 volts.