ACA Problem

[vdi_nenna]

I'm having trouble with the ACA. So much for simple.🙄

Using schematic from ACA on diyaudio articles page.

In one channel, R1 and R2 over heat while bringing up the amp with a variac. At around 90v AC and 5v bias at Q1, the resistors begin to smoke. With a laser temp probe, the resistors hit over 200 degrees F.

At full AC voltage, the other channel will not go past 2v bias.
18v DC with load.

Only mods made are Vishay 1uF metal film caps on input and output electrolytic caps. Also used 4700 uF caps instead of 3300 uF caps. Used IRFP9140 mosfets.

One other odd thing could be the transformer has four 15v outputs and is 287 VA. I paralleled the outputs. Prior to installing it, I checked the polarity of the secondaries with a variac. Power supply (CRC) read 21.86v and 21.84v per side with no load. Transformer doesn't get warm to touch.

The Amveco transformers were custom made and we passed on to me. They have no indication of polarity or color code.

Can test each channel with variable 10A lab power supply.

Other than starting over, any suggestions or ideas what could be wrong?

thanks,

Vince

 

[Dennis Hui]

Used IRFP9140 mosfets.

Hi Vince,

Is that a typo? Did you mean IRFP240 or IRFP140? (N-channel mosfet)

Thanks,
Dennis

 

[vdi_nenna]

That's it! I used a few leftovers. One bag must have been irfp9140! I assumed...
Installed a irfp9140 on Q2 by mistake.

 

[Dennis Hui]

Yeah that would do it. Perhaps you may consider replacing R1 and R2 if they
were smoking.

Cheers,
Dennis

 

[vdi_nenna]

You wouldn't belive how many times I tried to look at the mosfets in the sunlight to be sure.

Thanks. You made my day!

 

[vdi_nenna]

Installing the wrong ch type mosfet resulted in having both mosfets needing to be replaced.

It's up and running.

Thanks again!

Vince