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I have the 1982 version 400RB and the schematic I downloaded here is pretty spot on for my unit. I started out diagnosing a problem in my power amp section. But like an idiot, I managed to mess up the power supply while doing some voltage checks. The problem is now this: While both 60V sections (+ and -) are working fine, both 16V sections are now blown. I managed to fry Q13, Q14, R36 & R37 by shorting some things out with a voltage probe. I've replaced the 4 bad parts, and all the others in this section seem to check out ok. Yet I can't get the + and - 16V to operate again. These 16V circuits seem pretty simple, but I'm stumped at this point. Any help as to where to start would be greatly appreciated. Thanks!
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What schematic, I don't see one.
let me see if I can find one in my files
OK, got something similar.
I see a pair of 56k resistors, they drop the 60v down to the 16v zener reference diodes. You got 16v on them each, or not? They set the reference voltage to tell the transistors what to put out.
If both 10 ohm resistors were damaged, then I would assume the transistors on both sides were damaged as well. The current that burnt up the resistor came through somewhere, and those transistors is it.Don't be cheap for a dollar's worth of transistors. 16v zeners are cheap too, so make sure you install zeners and not plain old rectifiers. And facing the correct way.
That assumes the power supply is at fault. Whatever you did, it could have damaged an IC, and an IC shorted power pin to power pin will also zero out your LV supplies.Education is what you're left with after you have forgotten what you have learned. -
Hi Mark,
Definitely replace ALL transistors again, both zeners, check those low ohm resistors...remember tolerances with the ohm values (5 or 10%), check those feedback capsComment
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...to continue, c8 thru c11 and then ....with no power on, check resistance from ground to + and - 15 volt point respectively. The readings should roughly match. If there is a low reading like say 3 to 6 ohms or so you have a shorted component down the line. Also check for burnt traces in the +- 15 volt supply as well. Hope this helps.
TronComment
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Wow thanks Enzo and Tron, some great info in there. So let me correct a mistake I made in my first posting- I fried Q14 & Q15, not 13 & 14 as I originally wrote. And I took them out on 2 different occasions. My 1st goof taking out the +15V side then later taking out the -15V side. So both TO-220 transistors survived, while both TO-3s did not (in fact one of them exploded).
Next, I didn't realize that the 1N966's were zener diodes. So now I have a much better idea of how this circuit works and where to start digging. While I'm procuring some new components, I would just ask if you guys know what this circuit is called, or the basic theory of it, so I can do some research on it. I'm going to try to upload my schematic here too. Thanks much!Comment
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Plain old vanilla zener regulated powr supply.
TO3 exploded? I think you might mean TO92. Q9 and Q12 are TO3, and they tend not to expldoe, at least I have never seen it.Education is what you're left with after you have forgotten what you have learned.Comment
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For further reading also check 'pass transistor regulator'.
In this case the 2 transistors form a darlington pair as described under 'linear power regulators' on this page: Darlington Pair Circuits | Radio-Electronics.ComOriginally posted by Enzo
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Thank you g1 and Enzo. I will study up on linear power regulators and Darlington Pair Circuits.
You're right Enzo, I meant TO-92. And fyi, I do have a degree in Electronics. But it's from 40 years ago. A bit rusty, but getting my chops back!Comment
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