1176 and Power Transformer

I got my 2 1176's all stuffed and wired, fired it up and had no smoke.

So far so good.

I went to check voltages and found that I only had about 20volts at the first filter cap. I then checked the AC coming from the PT and measured about 15volts.

I ordered the PT that is listed on the North American Part list: POWER SUPPLY TRANSFORMER 2X24V/500mA TE62052-ND

I think that either the part number is incorrect, has changed, or I am hooking up the secondaries incorrectly. I hooked the secondaries up in series as per the data sheet, and took the output across the outer legs. I also made a center tap from the middle legs of the secondaries and connected that to ground.

Has anyone used this transformer? Any ideas where I am going wrong?


Thanks.

> I ordered the PT that is listed on the North American Part list: POWER SUPPLY TRANSFORMER 2X24V/500mA TE62052-ND ...from DigiKey, right?

"[email protected]" It is a 2*12V transformer. With light load I would expect 20% higher, 14.4VAC. With rectifier and filter, I would expect 19VDC-20VDC on the first cap.

I don't have a 1176 schematic up my sleeve. Ah... this is the one with the +24V/-10V supply. OK, you should have bought (been told) a number for a 2*24V transformer, not a 2*12V tranny.

Here is a fix that will get you through the night, without a new tranny:


Re-wire the two 12V windings as one 24V winding. I've shown colors for the TE62052-ND, according to the documentation from DigiKey and Amveco. Ignore connection to CR8 CR10 (you can leave them on the board, just don't connect to their junction on their transformer side). One end of the 24VAC goes to ground (common between the two 1,000uFd caps) and the other end feeds the junction of CR7 CR9.

The drawback of this is more buzz, and not particularly good transformer utilization. The buzz should not be a big problem. If DS1 DS2 lamps are just for the meter illumination, leave them out for now to ease the load on the tranny. If the buzz is truly inaudible and the tranny runs just-warm, put the lamps in and enjoy. If there is a faint buzz or a hot transformer, you'll need to get the 2*24V transformer (Amveco 62055 2*22V) and yell at whoever posted that parts-list. (Yell gently; it was surely an understandable mix-up.)

Thanks for all the info. I was searching the site like mad and in between my post and reading your post I found that someone mentioned that this is the wrong transformer for the job. Of course it was smack dab in the middle of that HUMUNGOUS 1176 discussion.

I actually thought about ditching the center tap, but I have read warning after warning to connect it.

Since I have been working on this for the past 3 months, I think I will just order the correct transformer (thanks for the part number) and wait it out.

I am just thankful that I had no smoke on fire up and that there is such a cool forum such as this with a bunch of really helpful people on it.

I will make a note on the North American Parts list about the error so that future builders will not make the same mistake.

Thanks a bunch!

Okay, now I feel really dumb. Had I simply clicked the second page of the 1176 North American Parts list, there is a great little discussion about the correct transformer.

Sorry for wasting your time, but I guess in the end I learned something.

> thought about ditching the center tap, but I have read warning

Using the original plan, if you don't have that centertap, the unbalanced loads on the two sides will lead to wildly unbalanced voltages. With the "right" transformer, at start-up, VR1 might see 60VDC and pop instantly.

The hack I showed will give a solid 35V each side. The voltage is balanced. But the ripple and sag are twice the intended values. The ripple and buzz are well filtered, should not be a big problem. Sag won't be a problem except when the utility voltage is sagging too.

And with a large stacked EI core, the fact that the load is heavier on one side of the AC wave than the other is not a huge problem. But with a small torroid, it might be. This is equivalent to running DC through a winding. Small DC though a conventional core is a small problem. But torroids are more perfect AC devices, and also lousy DC devices. So I was concerned that the half-wave load could semi-saturate the core, leading to excess heat.

Okay, now that I look again at your "hack" schematic, I see that you aren't simply dropping the center tap.

So this solution uses the transformer and rectifiers in a half-wave rectifier fashion rather than the full-wave rectifier circuit in the G1176.

I always say that I like to have problems with my builds because I always learn something new. I remember when I was building my 18watt Plexi-ish amp and I learned the difference between Negative Feedback and Positive Feeback. Boy was that scary! But...I'll never get that wrong again.

Thanks PRR!