Urgent help/advice needed

[maximumshow]

Hello all...

I really need some advice or help with a possible simple DIY project gone bad.

Today I opened up my B&K st-202 amp because I wanted to replace the binding posts and RCAs with newer ones of each. The old BPs could not accept 12 AWG wire and the rcas were corroded. I figured this would be a simple project (and it should be!).

I disconnected eveything from the amp, and then disconnected the internal power supply from the amps PCBs. I got everything soldered in and ready to go... I reassembled the amp and powered it on, but I got no sound. So I unplugged/disconnected everything from the amp again (except this time not the RCA ins) opened it back up, and noticed I mad a simple mistake in my excited haste. I forgot to plug the power supply back into the amp PCBs. So I waited for a while to let the caps discharge, and then went to plug the molex connector back in...

Zap! A little blue spark jumped as I plugged it in 8O. I guees I didn't let the caps drain enough . This only happened on the right channel.

Now 2 things happened.

1: The sound card I use as the source to the amp now refuse to ouput anything from the right channel. It is effectively dead ops:.

2: Using a different sound card as the source, the right channel on the amp seems to be a bit quieter, and the sound quality seems diminished. The bass especially seems to have been affected.

I know that nothing can be done to fix #1. But I am very worried about #2! I need some advice on how I can test the amp to see what is wrong, and if possible fix it. I would appreciate ANY advice!

This could not have happened at a worse time. I have been under a lot of stress lately, suffering from axiety and panic attacks, and after weeks of having no desire to partake in my hobbies I finally decided to do this little mod I had been intending on doing for quite some time. To make things 2x worse I haven't been working lately due to said stress, so I am very low on money and can't buy new gear or take it to a repair shop. Music has been keeping me relaxed too, but now Idon't even have that .

I know it's not a big deal in the grand scheme of things, but it really hit me kind of hard under the current situation. Sorry for the long winded explanation, but I would be grateful for ANY help! I feel so stupid, as it was supposed to be an easy little project.

Thank you ahead of time...

[Hdale85]

Well first off caps never drain on their own they can hold a charge damn near forever you have to use somthing to discharge them like a screw driver . Just thought id let you know.

[TacoD]

The amplifier was grounded through your computer, so I think it was some kind of ground loop. First some sanity checks,

1) switch left and right inputs, if the channel switches than this "new" soundcard is also defective.

2) if not, the right channel of your amp is not working properly. Check if you don't have any cold solderjoints on your new rca's.

3) and if that's not the case, check the voltages of the amplifier psu. Maybe the discharge destroyed some resistors other things. Also look if you can see some burned resistors.

4) Good luck.

[cjd]

Um... draining caps should be done with a decent wattage resistor... if they hold enough charge they could weld a screwdriver to the terminals!

C

[ThomasW]

Check any internal fuses....

[HMenke]

I would get a magnifying glass and carefully examine every component under good light to try to identify what if anything got fried. When you find the damaged part, replace it. Your sense of smell is also a good trouble-shooting tool. I wouldn't beat myself up over it - it's a learning experience.

[maximumshow]

Great advice guys, and I thank you.

Here is a few pics of the internals of the amp. I show the molex connectors that go to the PCBs. This is where it sparked when I reconnected it... Also I show how each PCB has what appears to be 2 variable resistors on the back. 3 of the 4 are "turned" to the same position, but 1 of the 2 on the right channel is on a different setting. I assume that this is done in the factory to balance any inconsistencies, but any info on this would be appreciated. I'm still learning !

First off, I thought that leaving the amp unplugged for a while would have been enough to discharge the caps. As I am now more aware that this is not true, I am more hesitant to just open the amp back up to check out the componentry. I don't want to make anything worse (or kill myself ops: ).

So before I check out all of the things you guys have mentioned, I want to drain the caps properly. The amp has 2 x 17,000 MFD 75VDC Mallory caps in it. What type of resistor should I use to drain these, and what is the best way to assure that they are drained?

[HMenke]

To drain them safely, use a 22kohm 1/2 watt resistor connected to some alligator leads. Leave it in place long enough to drain the voltage down to less than 5V (measured with volt meter across resistor).

There will be some power dissipated in the resistor:

[ (75V)*(75V) ] / 22,000 = 0.256 W

which will drop off over time as the voltage dissipates.

The time constant of the RC network for each capacitor is:

17,000uF * 22kohm = 3740 sec

Discharge time is 5 * 3740 sec = 5.2 hours

[maximumshow]

Ok I understand the math there... It has been a while siince I last took any electrical circuit classes.

Hmenke, thanks for the advice! I think you may have put an extra "0" in there . I double checked because 5.2 hours seemed like a loooong time.

Check HERE and tell me if my math is correct.

What does the x5 denote in the equation? At first I thought it was the time to get to 5V but that can't be right, because that would mean that it would be faster to get to 5V than say 10V .

[HMenke]

You may be right! I'm away from my beloved old HP 11C with Reverse Polish Notation and was using an accursed algebraic calculator program! I can't function! :B

R * C is the time constant of an RC circuit: "tau" (Greek letter). (5 * tau) is the nominal time to charge or discharge to a given voltage.