Entertainment Center: TVs, Stereos, VCRs and DVDs - RCA STAV-3970 (cat#31-5011) problem

Hi,

I hope you can help. I'm trying to fix an RCA model STAV-3970 for a friend of mine. I did a search of the forums and found nothing so far, so I figured I'd ask. Here's what's up (as far as I know):

He had it up and running until one of his kids tripped over something that pulled on the right channel speaker cable. One side of the cable pulled free and contacted the other polarity, at which time the unit turned off. When he powers it up, it will come up with the last device selected (in this case, CD), then flashes the word OVERLOAD 5 times and powers off.

He said he found out that this error code either means that the output transistor or some other component is blown - not being an electrono-phile, he didn't recognize the name or type of the other component. I'm thinking the output transistor is most likely, as this tends to be the weak link in these types of failures according to my limited experience with amplifiers.

I can't find any reference to the error code in any of my searches. He said he found the information on a DIY site, and even had pictures of the circuit board and likely components, but can't remember the site.

I found the output transistor for the right channel, removed it, and found just about 3.8M ohms from collector to emitter and open from emitter to base in both directions leading me to believe it is that the output transistor has gone bad. I found the part number for the transistor in the part's listing on Radioshack.com's Legacy support site (http://support.radioshack.com/productinfo/DocumentResults.asp?sku_id=31-5011&Name=Amplifiers%20and%20Receivers&Reuse=N)- a 2SC2440 NPN low-noise transistor in a TO-92 case. Radio Shack apparently doesn't carry this exact part any more. Of course, the user manual on the site is useless for this problem.

Unfortunately I have no money at all for parts and we live in a little tiny burg where our only remaining electronics outlet within about 50 miles is, in fact, Radio Shack. My friend picked up an example of every NPN TO-92 transistor they have and I tried the most likely example I could find (an MPS2222A), but still have the exact same problem so the transistor may not have been a close enough match (no smoke came out when plugged in and turned on, but...). I'm trying to not just put in part after part until he's spent more than the unit is worth, and approach it with some intelligence.

Just at the output of each channel on the circuit board is a small blue "box" that I don't recognize specifically - I believe it may be either a resistor pack, an SCR, or a relay, but I'm not sure. Unfortunately I wasn't smart enough to write down the specifics written on it, thinking I could likely track down the DIY information he found.

All I have is a Fluke-77 multimeter and some general electronic tools, my general electronic repair experience (which is pretty good), and my amplifier-specific repair experience - which has never been extensive and is quite well dated (meaning of course that half the stuff I knew, I've forgotten) - no o-scope, repair manuals, data sheets, etc.

If anyone could help me narrow it down or find a site that can explain the error code in more detail, we'd be greatly obliged.

Thanks in advance.

Power transistors are not interchangeable, and "close" part number's specs can vary considerably. The power transistor's driver could also be toast. The little blue box is a relay that clamps the output when a fault is detected. This is also the delayed "click" you hear when a working unit is first turned on. If it's clicking, it's probably good.

Also check the connection where the speaker terminal is soldered to the PC board. It could appear to be a solid connection, but the kid may have done physical damage.

If your friend visited a web site in the last couple of weeks it might still be in the History folder in his web browser.

Thanks, Rick.

I'll see if I can find out what's on his computer, check the connection, and see where I can find a direct replacement for the transistor. You don't know where I can find free data sheets or cross reference tables, do you?

It's been far too long since I did any real amp repair - any tests you can recommend with basic tools to test the driver circuit?

Make sure you have bias voltage, and maybe pull the driver transistor(s) to check 'em for shorts. Amp tech hasn't changed all that much in the last 30 years. It is, however, driven by logic nowadays. Once the logic says it's bad, the only recourse may be a service center.

Cross reference tables are all over the 'net. Problem is, some manufacturers still play it close to the vest and literally scratch the original manufacturer's number off the component.