Need help to repair two phase 400 amps

laatsch55

laatsch55

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The 66456's are really RCA-410's. The bottom rows are "drivers" . In a quasi comp amp you'll have thoise. On a full comp amp they will be all the same in every other row. Frying speakers does not guarantee a shorted out, although that will be the failure mode 95% of the time....
Do you know how to test transistors??
 
LarryD

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laatsch55 said:
The 66456's are really RCA-410's. The bottom rows are "drivers" . In a quasi comp amp you'll have thoise. On a full comp amp they will be all the same in every other row. Frying speakers does not guarantee a shorted out, although that will be the failure mode 95% of the time....
Do you know how to test transistors??
Click to expand...
Yes Perry told me. I am getting ready to do just that. Just finished cleaning up the the first amp.! it's the first time is has ran in twenty years! What is the main difference between full comp and quasi?
 
laatsch55

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The output stage is Complementary usin NPN-PNP pairs instead of all NPN using a phase inversion on the pre-drivers for the negative half of the sinewave...
 
laatsch55

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Complementary is MUCH better.....that's why we do the White Oak goodies. When you get your outputs tested let me know how many ya need. I have a BUNCH, just cover the shipping...
 
LarryD

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laatsch55 said:
Complementary is MUCH better.....that's why we do the White Oak goodies. When you get your outputs tested let me know how many ya need. I have a BUNCH, just cover the shipping...
Click to expand...
Wow that is very nice of you to offer. Thank you so much. I have been off work now for three years. Machinist for 42 years. Now I am semi disabled. Can't stand for long periods. I am poor as heck! If and when I get my social and pension I will WOPL one of the 400's. I will get to checking the outputs In the morning. Thanks much again!
 
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Good Morning Everyone, On the second amp, I have inspected the board on both sides and It looks good. Nothing sticking out at me. I cleaned the amp well before starting to ohm out the output transistors. I am having a hell of a time getting a good reading. Cheap ass ohm meter is jumping all over the place! I checked the fuses and continued to look around. I decided to hook it up to the test set up. I turned it on, the light got bright and then dimmed. I checked the voltage at the speaker outputs , Left channel looked good the right channel was way out there at 55.6 V. I promptly shut it down. So at this point I could use some help. Maybe I am not measuring the outputs correctly. I have been reading the repair manual it tells you to check them but it really does not show you. Is there any pictured drawings around for rookies like me ? Thanks everyone-- In other words can someone please hold my hand cause I don't know shit !LOL!
 
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laatsch55

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If the DBT went dim you do not have a shorted output. A shorted output is a high current event and the bulb would not dim.
 
LarryD

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laatsch55 said:
The manual does tell you how to test them....
Click to expand...
Good morning, Yes it does, but like I said I need a pic because I am stuck on stupid this Morning! Can you give me a direction as to what to check next?


I have been reading the repair manual it (TELLS) you to check them but it really does not (show you).
 
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laatsch55

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You have to identify the collector bus bar and base bus bar, from testing transistors you should be able to figure that out. The other test lead should be on the common emitter resistor tie point...
 
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laatsch55 said:
You have to identify the collector bus bar and base bus bar, from testing transistors you should be able to figure that out. The other test lead should be on the common emitter resistor tie point...
Click to expand...
What if I told you that was the first time I have ever attempted to check a transistor on anything related to anything on a board? In other words I am as green as they come!
 
laatsch55

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I'm suggesting you check the outputs then move on. You could have an open output instead of shorted. You're gonna need a decent meter....
 
LarryD

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laatsch55 said:
We haven't done anything yet......
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I thought maybe you were done with me. I can imagine that it must be a bit frustrating when the person asking for help should understand the basics before asking for help. Maybe I should take a step back and get more educated before trying to take on such tasks? After this amp is up and running, I will hit the books and try to better my knowledge. When you say open output are you talking about a bad connection like a solder joint. ?
 
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