1176 about complete, couple things though....

[nicksense]

Hello all,

After months of working on it on and off, I powered my stereo 1176 up yesterday with no incident. Both sides pass signal and compress fine, though one side had a low output which I think I fixed (a resistor with the wrong value and a cap backwards, opps.)

There is a little weirdness with the power transformer though, when I measure it, a 24-0-24 part# TE62072-ND from Digikey, I am getting about 29 volts. The leg on the 7824 that is supposed to read 30 volts reads almost 38 volts.

Does this seem acceptable to you guys or should I return the transformer?

Ill post some pics soon, thanks for the help.

Nick

 

[smilinfu]

It looks like 62072 is 12-0-12 or 24 Volts in series, not 24-0-24.

If you have it running in series I don?t know how you managed to hook it up to the board. If you wired it in parallel (what I think is correct), 29volts is a lot to be getting out of what should be 12volts. If it is 24volts, then from my little experience 29volts could easily be coming out if it is under loaded.

Another way: if it is a 24-0-24, it may still be to big of a transformer, if the VA rating (is that the right language to use?) is to high. If there is to small a load on the transformer it can put out more than what it is supposed to, and may be going over what the 7824 can deal with.

If the transformer is built for a give you 2.5 amps and you are only drawing .5amps, you can get excess voltage.

I may be completely off in this, but I have had this issue myself and resolved it by using the same voltage transformer with a lower VA rating. I barely understand this myself, and have been wanting to bring it up, as most people around here seam to recommend using the biggest power transformer possible.

 

[bcarso]

A higher VA rating generally means better load regulation, but will often be a slightly higher lightly-loaded output voltage than a lower rating.

If you are counting on loading down a transformer to get a lower voltage you may be getting more internal dissipation than the transformer likes.

The standards for transformers are not well-defined, other than that they are supposed to not start fires. So they are supposed to have thermal cutouts in the primary buried in the winding somewhere. Cheap lamination material, high impurity copper (coming in from the Far East typically) will conspire to give poor regulation especially under the typical pulsed currents of a cap-input solid-state-rectified load. In principle transformers are rated for a resistive load for their VA rating, even though virtually no one uses them this way.

It's instructive to pick up a d.c. output wallwart and look at the rating vs. the heft. Usually the size of the transformer is overkill for many apps where the power is not at the constant max. draw, such as power amps in powered cheap speakers.