41Hz Amp1B build (and problems)...

[chris661]

Me again, this is spilt from the other thread I made about the problems I'd had getting hold of the documentation for the build. Here... 41Hz Amp1B documentation...

So (my first time soldering such tiny SMDs), I built the amp and power supply.
PSU is a pair of 26-0-26 transformers, which gives +/-~34v rails (doesn't pass 35v), with 2x10,000uF caps per rail.
The output relay still won't click on. There's 2.8v on the output pins of the chip, no matter if it's muted or "awake". Explains why the relay won't click, but still not sure where the volts are comming from...
When awake is selected, the chip gets slowly hotter. It remains cool on mute.

I can supply the DC voltages on each of the pins if needed, but any pointers would be fantastic.

Cheers
Chris

[Boscoe]

Put a 50Hz sine wave into the input and measure any ac voltage out on the relay pins before it's switched. This will show that the amp is working you just have a problem with the relay circuitry and gubbins. If that doesn't work then theres something bigger wrong then you need to go over all the components and look for any shorts or mistakes.

EDIT - Do you have the documentation yet? Is there a schematic?

[chris661]

Thanks Boscoe, the output is modulating with a 50Hz input (seen on scope), so it looks like it's working... but, there is a "DC" offset of -1.5V (sure it was 2.8 before) (measured on meter) on both outputs. The chip gets warm, but the pins get quite hot, so something's not happy.

[Boscoe]

Hmm yeh something else is going on. Sorry I can't help anymore!

[brig001]

Check if the voltages on pins 30 and 31 are equal. They should be around 2.5 volts. If these are equal but you still have an offset, then check the feedback resistors around those pins. Try running the whole thing on a lower voltage while checking too.

[djQUAN]

did you clean the PCB? I had problems before with T amps when there is flux under SMD resistors. After cleaning, everything worked fine upon power up.

[chris661]

I plan to have a look and hopefully get things sorted tomorrow. I'll run around and heat up all the SMDs again to be sure, thanks for the tip

[chris661]

Right, heated up all the SMDs (that should've burnt off the flux, right?). Nothing's changed.

Removed the relay, soldered a speaker directly to the amp's output.

When switching between mute and alive, there's a small click on the output, but the worrying part is the negative rail of the power supply sags by a few volts, and the positive rail moves up by a few volts. They recover quickly enough from it, but it's not something I've heard of.
Sometimes, music comes out for a little while. I got maybe 10 seconds of U2 from it before the circuitry decided it wasn't happy and cut out again.
Putting ~50ohm across each rail to neutral meant the pulses on the power supply are smaller, but still present. Doesn't make the chip happy though. The resistors got nice and warm, so they won't be a permanent feature.

Playing around on a large (2kVA) variac, no voltage I tried (from 20 to 32 volt rails) made the chip happy either.

I suppose the best thing to do would be to wait until my 41Hz forum membership works properly, so I can post a thread there. Currently, permissions don't allow it.

Chris

[MatchASM]

Originally Posted by chris661

Right, heated up all the SMDs (that should've burnt off the flux, right?). Nothing's changed.

The common way to clean off excess flux is to use alcohol (contact spray might also work, I'm not sure).

So maybe alcohol and a good wipe with a cloth gives you better results.

--
Greetz,
MatchASM

[Boscoe]

I have just managed to create a forum account with no problems but for some reason I can't access the info you need due to insufficient forum privileges. I'll see what I can do.