Acoustic 140 question

[pontiacpete]

Pulled the transformer secondaries out and it is not shorted to ground or case. Between each of the windings there's
very low ohms 3.4 and 2.2.

[Enzo]

If you have both ends of each winding disconnected and there is still continuity between them, that is not good.


That also would solve the mystery. I have been going nuts looking for where a 15 ohm path to ground could come from. Well, if we know the windings are shorted together, we also know the drive winding would be connected to ground.

[pontiacpete]

This doesn't look good. There is continuity between the 2 windings of the secondary.

[pontiacpete]

Before I pack it in I have to ask,, Is this amp a loss until I find a replacement for this transformer?
Or is there a modification that can be done to a get around this problem?

[J M Fahey]

*IF* the driver transformer is dead I would junk that power amp and replace it with a new one.
The easiest option (although not the best) would be to replace it with some TDA7294 kit, with a couple mods so it can work from a single supply.
Best would be to make a discrete one, even to clone a later Acoustic power amp which was *designed* to replace the one you have, but that would mean making a custom PCB.
If you think it´s worth it .....

[pontiacpete]

"Best would be to make a discrete one, even to clone a later Acoustic power amp which was *designed* to replace the one you have, but that would mean making a custom PCB."

Thanks JM, nice idea, I like the idea of etching a new pcb. I've only done it a handful of times and only twice for an amp, a polytone and a fender rhodes, but I had the original to copy the layout.
I'll have a look through the acoustic schematic and see which ones you are referring to.

[pontiacpete]

Does this look like something you have in mind?

[olddawg]

Pulled the transformer secondaries out and it is not shorted to ground or case. Between each of the windings there's
very low ohms 3.4 and 2.2.

Across each winding or between the winding? If the primary is shorted to the secondary that is bad. If it is simply the DC resistance of each winding by itself it is probably fine.

[pontiacpete]

when I measure between the 2 different secondary windings they all seem to have continuity. That is green to green/blk, green to blk/ylw and so on. Is this not good?

[Jazz P Bass]

Philcoradio.com - Tech Section - Audio Transformer Issues
ANK Audiokits - Interstage Coupled Amps

[Enzo]

Your transformer has three windings.

One uses red and blue wires and is connected to Q302. That is the primary winding

There are two more windings that are secondaries. One of them uses the green and yellow wires, and the other uses the green/black and the yellow/black wires. There is one for each pair of output transistors.

Each winding ought to have continuity through itself - that is to say some relatively low resistance from red to blue or from green to yellow.

But they are still separate windings. So with them all disconnected, that means all six wires are not hooked up, there should be NO continjuity BETWEEN them. So from Red to Green, it should show OPEN, and from green to green/black it should show open.


Looking at the schematic, it seems green to green/black ought to measure about 406 ohms.

[pontiacpete]

There's nothing over 6 ohms between the secondary windings.
Here are the measurements all of them disconnect from the board:
green to green/black = 2.4 ohms
green to yellow/black = 5.5 ohms
green/black to yellow = .4 ohms
yellow to yellow/black = 3.5 ohms

The secondary and primary there is no continuity.

[gbono]

I believe the transformers are needed to get the correct phase change between driver stage and output transistors....... Never had a 150/140/etc with a bad transformer (actually never found a bad inter-stage xformer in a Sunn concert series amp either).

I would convert to a "chip" output like JM suggests. It's too bad guitar center "reissued" this brand - it has damaged the prices of "vintage" ACC gear.