Troubleshooting CA-800

[warriorfan]

Looks like it is just the power pack. I tested the emitter resistor as Tom directed above. I will soon find out as I am ordering a power pack from flea Bay. Yay!

 

[Leafman60]

I wouldn't swap around the power packs, as you can now cause cascading problems on the good side. There is a guy on ebay who sells the powerpacks for ~ $12.00 shipping included "STK0080A / DARLINGTON POWER PACK (qzty)". You definitely have a bad Powerpack, and maybe a bad emitter resistor (2X.22ohm) The emitter resistor is the white porcelain rectangular component in front of the power pack. It should read 0.022 ohm from the center to each of the 2 sides. I expect that one side will show open, causing a high DC potential on the output. Thats what blew your speaker, and what caused R02 (big 2w) to start burning when you had headphones on. You could have other issues such bad transistors, diodes or other resistors, but it is possible the issue is just power pack and resistor, or just powerpack. That STK0080A produces 80W RMS and can really crank.
I would not connect ant speaker, or headphones until you get the problem fixed. I would suggest that you use 2A fuses for the troubleshooting, as well as a dim bulb tester. When power up the unit with a dim bulb tester, connect your voltmeter to the speaker out and make sure the voltage shows as close to "0" volts DC when the relay engages. If it is showing +55v or -55v you still have a problem, but I think the powerpack and emitter resistor (if it is bad) should fix it. When you are convinced that the problem is fixed, (you hear or see the relay engaging, and the DC volts is close to 0v on both outputs, then you can test with headphones/speakers. If it is now working good, you can put the 4a rated fuses back in.
Good luck
tom

I hope guys don't mind me reviving this thread.

My CA-800 (bought new in the late 70's, early 80's) recently conked out. First, the right speaker system went dead and, maybe a year later, the left speaker system died.

I opened it up and replaced the STK0080's, no problem. I turned it on and all speakers worked wonderfully - for a few minutes. Then, the right speakers started emitting some weird buzzing and popping and I smelled an odd burning smell and then the right speakers died.

I opened it up and found one of the STK's had boiled the heat sink compound behind it so I assumed it was burned out.

I then checked the emitter resistor by the STK's and the resistor by the burned STK read way off, showing a much higher number than the .22 ohm.

Okay, so now I have ordered 2 new resistors and a replacement STK0080 and plan to replace both resistors and the burned power IC.

Am I on the right track? Nothing on the board looks bad. The board looks brand new, not even any dust on it.

Thanks in advance.

 

[Tom B]

Be careful as the STK0080 and STK0080A are 2 different chips. Your unit uses STK0080A. If you used STK0080 that could be part of your problem.

If you do have STK0080A's in there, and it burnt again it could be shorted speaker although the protect circuit should have prevented that, or also look at the power supplied and make sure the plus and minus supplied are fine.

 

[Leafman60]

Be careful as the STK0080 and STK0080A are 2 different chips. Your unit uses STK0080A. If you used STK0080 that could be part of your problem.

If you do have STK0080A's in there, and it burnt again it could be shorted speaker although the protect circuit should have prevented that, or also look at the power supplied and make sure the plus and minus supplied are fine.

Well, the original parts were stamped with "STK0080" without the "A."

By the way, one of the 4 amp fuses blew out close to the burned STK.

 

[Tom B]

Here are the internal schematics of the 2 IC's
First The STK0080


My recommendation would be to use the original type STK0080A's and use a dim bulb tester when you bring it back on line



Next the STK0080A

 

[Bassblaster]

Here are the internal schematics of the 2 IC's
First The STK0080
View attachment 1330934

My recommendation would be to use the original type STK0080A's and use a dim bulb tester when you bring it back on line



Next the STK0080A
View attachment 1330933

Hmmm, i think ive seen those schematics before.

 

[Tom B]

Hmmm, i think ive seen those schematics before.

Sassblaster,
Yes somebody took a lot of time to put these really nice drawings together. seems like they used a CAD program. Really nice work. A lot better than the old PDF's found at other places. I found them by doing a google search looking for a picture of the 2. I also think I may have helped you out with a few STK's in the past.

 

[Bassblaster]

Sassblaster,
Yes somebody took a lot of time to put these really nice drawings together. seems like they used a CAD program. Really nice work. A lot better than the old PDF's found at other places. I found them by doing a google search looking for a picture of the 2. I also think I may have helped you out with a few STK's in the past.

you have helped me with some STK's in the past. but i actually made those 2 in the pics lol. i just use EasyEDA for all my schematics and PCB's. the reason i dont have the 505 after my name here on AK is the name was 1 character too long so i kept the numbers out.

 

[Leafman60]

Thanks for all the advice. However, what is the rationale for using a STK0800A in place of the original STK0080 ?? Just asking.

.

 

[BobGAL]

I have the CA-800, FM-600, CR-124, and a pair of DS-825's. This is my best sounding setup I have bar none. If I ever have any problems with my CA-800, I know who to go to. It all is in mint condition! I wished I could have found it with the rack and turntable. Now here comes the part I will brag about and how it was a deal of the century for me so far. I only paid $6 plus tax for it all at a resale store. My Wife works for the non-profit that owns it. The place is here in Michigan. The company that owns this resale store and a few other in Oakland County and the store I go to in Genesee County. The company works with physical and mentally disabled adults. I'm not going to name this company so I can keep it to myself haha. I do love my Fisher gear!