Hafler 220 screaming noise?

[tweedclassic]

I just picked up a Hafler 220 power amp it works great and i plan I making it better with this boards help eventually.

-The question I have is when I power it down the amp itself it makes a faint screeching noise?
1) is this something I should be concerned about?
2) any ideas as to what is causing it to make this noise?
3) on a different note any suggestions as to what modify first?

Thanks
I am new to amplifiers; now crossing over from speaker building. time to learn something new.

Cheers

Robert

[anatech]

Hi Robert,
Could be old caps. What is happening is that the amp is becoming unstable as the voltages fall.

It would be a very good idea to replace the electrolytic caps on the driver boards. Don't even bother to measure them. We already know at least a couple are bad.

-Chris

[unclejed613]

take the caps out and, believe it or not, shake them. if you hear what sounds like bits of sand rattling around inside them, replace them. the stuff rattling around inside is dried out electrolyte.

you can find the service manual here:
http://www.hafler.com/techsupport/pd...20_amp_man.pdf

[tweedclassic]

Are you talking about the 10,000 micro Farad caps in the power supply as well? Should I be looking for anything special as far as tolerances brand etc.. as far as the electrolytic caps go. Iv herd it doesn't make a whole lot of difference with electrolytic caps? so all the electrolytic caps should be changed in the circuit?

Cheers

thanks for the swift response.

Robert

[anatech]

Hi Robert,
I usually look at the ripple to decide on the main supply caps, and look at the seals. If the seals are ruptured, the cap is toast no matter how it measures or tests. I like the idea of shaking them. Morracas.

Yes, change all the electrolytics on the driver PCB's. It does matter that you buy new, first rate capacitors. Anything from Digikey, Mouser or Newark should be good. Stay away from the "deals" or anything that is too cheap.

-Chris

[bear]

Whoa! Hang on now!

Where is this noise coming from?
The amp chassis?
Or is it coming from a channel out of the speakers or BOTH channels out of the speakers?

Either way there is, iirc, a resistor that bridges between both channels, inspect that for cracks or overheating.

_-_-bear

[anatech]

Hi bear,

-The question I have is when I power it down the amp itself it makes a faint screeching noise?

I assume it's from both speakers as the amp shuts down. Have you seen anything different?

-Chris

[unclejed613]

yes i meant the 10,000uf caps. i have a cap tester i use for checking ESR on the smaller e-lytics, but even a dried out 10,000 uf will read low ESR most of the time.
here's the link to the cap tester i use. there are others, but this one uses a 100khz signal, and seems to be the most useful one i have used, except for a sencore (which is very expensive and takes about 10 seconds to calculate a reading). here is a link to the tester i use:
http://www.howardelectronics.com/ieinc/cwinfo.html

with any piece of equipment more than 10 years old, it is best to check all of the electrolytics for high ESR. checking capacitance is a good idea too, if the equipment has set idle for a long time, since inactive electrolytics will lose their oxide layer if not used for a long time.

if you find for instance a 100uf cap that reads 20uf, and it's ESR is still ok, you can re-form it by attaching it to a power supply (of equal voltage to the cap's voltage rating), in series with a 100k or higher resistor, and letting it sit like that for a few days. if you find a bunch of caps that need re-forming, that are for instance 50v caps, attach a 100k resistor to each and hook them up to a 50v power supply.
also this method is useful if you need for instance 250v caps for something, you can re-form a lower voltage cap to a higher voltage, but you will lower the capacitance proportionally, as well as increase the ESR. if you are re-forming caps at voltages higher than 75v, use 1M resistors instead of 100k

[anatech]

Hi unclejed613,
Today's capacitors are so much better than old designs, just change them. Reforming may get you out of a jam, but the cost of a new one makes it not worthwhile otherwise.

-Chris

[unclejed613]

i agree.... most of the time...... but when i'm stripping old chassis' for my junk box, i'll take the time and effort to rescue nichicon caps for example...... i'll overlook old elna's and such, but if i see the metallic shrink of a nichicon, i'll always keep it if it has decent ESR (they usually do, even if they're older than dirt).