Dead Realistic STA-860

[malanga]

Hi. I just picked up a Realistic STA-860 at a local thrift recently. It was missing the fuse so I could not test it, but the price was right and condition physically very nice, so I gambled. Took it home and tried a fuse in it and the fuse popped immediately. Left the inside of the fuse slightly blackened, however the fuse was rated at 3 amps and the amp required a 5 amp fuse. So where should I look first? From what I have read here it seems that fuses blowing right away are usually the power supply or the output transistors?

 

[jblmar]

Get the correct fuse. You'll need to clean the fuse holder if black. Use Deoxit to remove the carbon. If the replaced fuse pops when powered up and is black, your problem is most likely in the power supply. Could be a bad diode.
Where are you located?

Ron

 

[malanga]

I will try the proper fuse tonight and see if this happens again. I live in Hammond, Indiana just outside of Chicago.

 

[jblmar]

Make sure that it's either slow or fast blow. I would think that a 5A fuse would be fast blow in that application. Slow would be a safety issue. Anyway, better to be sure.

Ron

 

[malanga]

Well, I found the right fuse and tried it again. Popped right away. Looked at the fuse and the waire was vaporized and left a brown residue on the glass. I also heard a brief hum when I turned it on. Bummer! :tears:

Any suggestions?

 

[TriodeGuy]

Popping fuses is one of the most aggravating symptoms. I just repaired a Realistic STA-780 that was blowing fuses. Don't know if this receiver is anything similar to yours though. Turned out in my case to be a shorted rectifier block.

Actually, I can't take full credit for troubleshooting the problem. I ran across this post in a forum while searching for a schematic of my receiver...

http://www.epanorama.net/phpBB2/viewtopic.php?p=69948&sid=1b76193dab2cf0d67cf5607d6c8c4c1e

In the last post, the guy mentions tracing the problem to the rectifier. Turned out to be the same thing in my case. Question is, what caused the rectifiers in these units to short out?. I've ordered some new electrolytic caps for the power supply in hopes that this prevents further problems.

Even if the problem doesn't turn out to be the same in your case, there are some good troubleshooting tips in that link.

 

[jblmar]

Where are you located? If you want to send it, I can repair it for you.

Ro

 

[kcollins4]

I would check the output transistors. There are several threads about how to do this, when you use the "search" button. Good luck, Malanga

 

[malanga]

Thank you for the offer JBLMAR, but I would like to attempt repairing this one myself. If I am unsuccessful I may take you up on your offer.

Kcolllins, thanks for that link. I cannot find a schematic anywhere for this receiver, but upon examining the unit I see that there is a recifier block early in the circuit. Has a heat sink attached to the top of it. I am going to check that first to see if that is the problem.

 

[malanga]

Okay, dumb question but...how does one test a bridge rectifier? I know how to test a diode, but I never tried a bridge rectifier. Any suggestions? :scratch2:

 

[seadzz]

This link may give you what you want.

sdz

 

[TriodeGuy]

You can think of the block as containing individual diodes. You should have four leads coming out of the block. Even though the connections for the bridge circuit are made internally (the four diodes are wired together as a bridge), you can treat them as individual diodes...

http://en.wikipedia.org/wiki/Diode_bridge

Place your test leads on any two legs that are "next" to one another on the block and walk your leads around until you've tested all the diodes.

Be sure to reverse your leads while your testing each one to test each diode in both directions.

 

[markthefixer]

You need to make a light bulb "variac", borrow a variac, or somethiong similar.
This will limit the current in to the ac, then take readings of the voltages, to see which circuit is proportionately lower than the others.

I have a procedure I follow with a real variac(variable autotransformer to be technical) and an inline ac ammeter, you crank up the voltage slowly until you have a measured current flow (i used 1 amp) and let it go for a FEW SECONDS... pull the power COMPLETELY and then look for what heated up, the actual short will be dissapating all that energy and will get hot very quickly.

I suspect a shorted output driver.....

What are the chances of getting a schematic for the unit? have you tried?

(I wish I'd seen this thread sooner....)

 

[malanga]

Well, I pulled out the bridge recifier and tested each side. 3 sides measure infinity, and then a voltage reading. The 4th side reads zero in both directions. I guess this means that one of the four is bad? Now to find a replacement bridge. It is marked "350" on one side, and the other side is marked "GI KBPC802" I assume 350 is the voltage, but I need to figure how many amps it is rated for?

 

[markthefixer]

The General Iinstruments (GI) KBPC802 is a 200 volt 8 amp bridge rectifier. the 6 amp may pass muster, or if there are spade terminals, then rat shack is out of it as a parts source...


rat shack has:
4 amp 400 volt bridge 276-1173 at $2.49 (inline leads)
and
6 amp 200 volt bridge 276-1181 at 2.59 (4 corner square lead layout)

 

[malanga]

Thanks Mark! Once again you come to the rescue :thmbsp:

I guess I better bounce on over to Radioshack and pick up a new rectifier and give it a go. :yes:

 

[malanga]

Yep, that was it! I went to the local Ratshack and found the same rectifier. They actually were 200V 8A rather than six. The local ratshack is closing so I actually got the part at 60% off for $1.10
Popped it in, solder and working fine so far. My only concern now is what caused the old one to fail? Maybe I should replace the two large filter caps just to be on the safe side? Easiest fix to date! :thmbsp:

 

[TriodeGuy]

Feels good doesn't it? Congrats on getting her working again. Can't just be coincedence that these Realistic's are blowing rectifiers. I've been exhanging emails with the guy who repaired that STA-780 in that link above. He says he checked the filter caps in his unit and they tested good. I haven't gotten around to testing mine yet but hopefully I'll get to it sometime this weekend.

I'm also going to check the part # on my dead block when I get home. I'm curious to see if it's the same as yours.

-Chuck

 

[markthefixer]

Yep, that was it! I went to the local Ratshack and found the same rectifier. They actually were 200V 8A rather than six. The local ratshack is closing so I actually got the part at 60% off for $1.10
Popped it in, solder and working fine so far. My only concern now is what caused the old one to fail? Maybe I should replace the two large filter caps just to be on the safe side? Easiest fix to date! :thmbsp:

I've seen bridges go before.... it's usually the bridge.... something was on the light side when the bridge was made.... I would only be concerned if it did it again, then I would start looking, but otherwise I wouldn't give it another thought....

Edit: if it does, I have a few tricks up my sleeve.. including some sub 1 ohm power resistors that we could use to monitor (or eventually just limit inrush current) things... but if it dosen't pop the fuse, the inrush isn't out of hand...
With three exemplars, maybe GI had a bad (comparatively speaking qc wise) run of components.. in which case, it's definitely GONE....


I have seen bridges pop in older test equipment, where the stresses on the power supply are far smaller than those on an amp, that is FAR smaller capacitance, in the low hundreds of microfarads, not high thousands...

It could have been an external factor too, unlikely through the transformer, but possible.... don't sweat it, enjoy the sound and the sweet taste of victory.... I would...

 

[faber12]

I am bumping a 12 year old thread to say thank you to AK for coming through for me yet again.

I recently acquired a Realistic STA-860, same model as the OP.

I plugged it in, and hit the power switch. Nothing happened. Zip, zero nada.

Hit the power switch again, unplugged it, and started with the basics. Blown fuse. Violently blown, plating part of the glass inside with metal and blackness.

Break out the DBT time. Put a new fuse in, 5 amp, and screwed the 150watt bulb in to the DBT. Bright bulb immediately. Loud hum from the speakers. Tried the 200watt bulb. Same deal. Figured there was a short, as the original fuse blew for a reason.

Hopped on AK and found this thread.

Bad Bridge Rectifier. Ok, easy enough to check, even with my limited ability. I don't mind working on the bigger parts, harder to mess up. (Usually)

Pulled out the multimeter and loaded it for Ohms. Readings on the BR were as follows, same with either polarity: open, open, open, and 0.6 ohm. Now, I know how a rectifier works, and that was totally unacceptable.

Here is the culprit:

The screw threads in the aluminum heatsink had stripped, maybe due to heating and cooling? That allowed the heatsink to come loose, and probably hastened the death of the rectifier.

Now maybe some of you have these kind of things laying around all over, but I do not. I did however have 4 decent sized diodes and a soldering iron, so I made myself a bridge rectifier. Here it is installed:

Put the receiver back on the DBT with the 150 watt bulb and voila, the bulb flashed orange briefly and the receiver powered up with no hum.

Added a little Deoxit to the balance and volume pots, and this receiver is now restored to working order. My custom rectifier did not even warm up in testing, although I did not really put much volume out of it.

So, thank you AK, what would I do without you? You guys (and gals) are the best.