Calculating resistor values in parallel with Caps

[Crystaleyes]

Hi all.

I recently picked up a dead car amplifier from the flea market, with the intention of learning some more in the area of audio repair.

It has a stereo output with an added mono woofer output. The stereo channels are on one board, whilst the woofer section is on a separate board. There are also three large (ish) transformers.

Apart from the fact that it was seriously dirty inside, it seems to be essentially complete with the only clearly damaged component being a toasted 1/4 watt resistor on the woofer board and it is this which I am wondering how to replace?

I'll post some pictuires if requested, however the resistor in question is in parallel with a 47uF 63v electrolytic.

I won't lie that I'm not familiar with the reason behind using resistors in parallel with caps, but surely, for the cap to have any effect, then the resistance cannot be too low?

That said, is there some kind of calculation that can be used to discover an appropriate value for the resistor, considering we know the value of the capacitor?

Everyday is a school day...

[tester272001]

Re: Calculating resistor values in parallel with Caps

I'd suggest you post a picture. Best to try to see if you can read the burnt resistor value. One use of a resistor across a cap is for bleed off the voltage. It would normally be a higher ohmage value. Seeing the component and what is NEAR might help too.

[Crystaleyes]

Re: Calculating resistor values in parallel with Caps

Thanks for the response.

The large thing at the bottom is a transformer, but not the mains transformer...

[Loading666]

Re: Calculating resistor values in parallel with Caps

Cant u get with a multimeter?

[Crystaleyes]

Re: Calculating resistor values in parallel with Caps

Yes, and it reads less than an ohm, but I can't rely on any value given by a toasted resistor.

In orde for the capacitor to actually have any useful effect, I would have imagined that the resistor would have been much higher, but I honestly don't know... Hence my asking here.

[Per Hansson]

Re: Calculating resistor values in parallel with Caps

It does not look burnt, it only looks like it desperately needs a bath in some warm water with dish soap in it!

[faraday28]

Re: Calculating resistor values in parallel with Caps

Thanks for the response.

The large thing at the bottom is a transformer, but not the mains transformer...

It looks like snubber circuit, diode + cap + resistor but snubber usually don't use polar capacitor and the resistor usually in series with cap in PSU design.

There are ways to determined the resistor but it does involving removing the parts and not burnt.

Unsolder the resistor in question and check if it really dead or not with multimeter.

[stj]

Re: Calculating resistor values in parallel with Caps

unusual visuals there.
remove the electrolytic and check it didnt leak.
and show us the trackside of the board.

if thats the sub, it may be a lowpass-filter to make sure the center-drive only amplifies the bass.

[stj]

Re: Calculating resistor values in parallel with Caps

incidentally, watch where you put your fingers while testing it later.
the reason for so many transformers is because they usually have a boost convertor that steps the 12v upto around 40v to feed the amps.

[Crystaleyes]

Re: Calculating resistor values in parallel with Caps

Thanks for all the input fellas.

The cap in place is a replacement and the original hadn't leaked.

I've emailed the manufacturer a couple of times now but no joy with any response, but I will pull the resistor and see how it reads...

I'll be back

[Crystaleyes]

Re: Calculating resistor values in parallel with Caps

So...

Came across an article which suggested removing the burnt coating from the resistor in order to see how many spiral stripes there were.
The less stripes, the lower the resistance value. Makes sense...

As can be seen in photo #1, the original resistor appears to have roughly two stripes, which (according to the article) would indicate a value of between 10 to 100Ω.

Measuring the burnt resistor however reads just over 150Ω.

Considering that resistances tend to increase once a resistor has been overheated, I'm thinking of just sticking a 100Ω resistor in its place and see what happens.

The hope is, that the damage was caused by the output wires being shorted and not some other fault on the pcb.

If anyone has any other opinions then I'm all ears.

EDIT:

After stripping down a 62R resistor (pic #2), it looks like that wouldn't be such a bad replacement, especially when compared to the 100R in pic #3.

It should perhaps be an 82R but I don't have one here

[Crystaleyes]

Re: Calculating resistor values in parallel with Caps

and show us the trackside of the board.

The two empty solder holes are where the resistor came from...

[Crystaleyes]

Re: Calculating resistor values in parallel with Caps

In the end I found a single 75R (which measured 79Ω) 1W resistor in my supplies and installed that.

So, I reckon this thread is closed.

I needed to work out what the burnt resistor value had been and am 90+% sure that it was +/- 82Ω.


Thanks for all the input as I have learned a valuable lesson here.