I suggest tracing it out on the schematic. Follow the 37 V line until you find the regulator for it. If the line terminates on a connector, trace the connection on the top level diagram on one of the first pages of the PDF.
Tom
Yes, but what voltages did the Zener show In Circuit and powered up?
By my experience, old Zeners fail Open more often than Shorted. In this circuit, an Open D119 is going to subject everything downstream to overvoltage. The way the circuit is designed, one might think Zeners were terribly expensive. The regulated 12V supplied by Q159 and Q160 will see overvoltage, too.
Cheers
edit: Also, as a general principle, I recommend trying to avoid the temptation to replace R245 and others like it, with higher-wattage parts, even if they have failed AND run hot. Most resistors in the Collector supply of a regulator transistor are intended to sacrifice themselves if the load is excessive, reducing damage to devices downstream.
By my experience, old Zeners fail Open more often than Shorted. In this circuit, an Open D119 is going to subject everything downstream to overvoltage. The way the circuit is designed, one might think Zeners were terribly expensive. The regulated 12V supplied by Q159 and Q160 will see overvoltage, too.
Cheers
edit: Also, as a general principle, I recommend trying to avoid the temptation to replace R245 and others like it, with higher-wattage parts, even if they have failed AND run hot. Most resistors in the Collector supply of a regulator transistor are intended to sacrifice themselves if the load is excessive, reducing damage to devices downstream.
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OK, then R245 has opened again, or Q155 has failed. Q155 might seem OK at a glance because its Base-Emitter junction will still be forward biased by the string of three 680r resistors. But the rail will lay down without the transistor's current gain.
If you don't have a bench supply you can use to substitute for the +18V rail, you will need to start disconnecting stuff from both of the ±18V rails. Pick someplace easy at first -- a plug or other connector going off the board. Both of the regulators should function somewhat, with little or no load on them. Something on that +18V rail is soaking up too much current.
Regards
If you don't have a bench supply you can use to substitute for the +18V rail, you will need to start disconnecting stuff from both of the ±18V rails. Pick someplace easy at first -- a plug or other connector going off the board. Both of the regulators should function somewhat, with little or no load on them. Something on that +18V rail is soaking up too much current.
Regards
I just noticed 
o) that the motor drive circuit uses the +18V -- start by disconnecting that.
There are enough other places that +18V gets used, but motor drives are especially failure-prone. And this one doesn't have flyback diodes. Unsolder the Emitters of Q303 and Q304; replace R245 and see if the ±18 rails come up and regulate.
The 18V problem must be solved before the power amps and protection circuits will work right. (Note that the power amp input diamond buffers use ±18V to bias their cascode transistors.)
Cheers
o) that the motor drive circuit uses the +18V -- start by disconnecting that.There are enough other places that +18V gets used, but motor drives are especially failure-prone. And this one doesn't have flyback diodes. Unsolder the Emitters of Q303 and Q304; replace R245 and see if the ±18 rails come up and regulate.
The 18V problem must be solved before the power amps and protection circuits will work right. (Note that the power amp input diamond buffers use ±18V to bias their cascode transistors.)
Cheers
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OK, if it takes more studying the schematic, I'm in.
Might be worth trying to locate the missing section of the schematic -- the 8 unlabeled traces heading off to the right, bottom of page 6, from the ±18V and 12V supplies we were working on.
But so far it seems you have a bundle of contradictions -- and I'm still trying to figure out how in 2 hours and 13 minutes, you managed to remove, test, and reinstall every component in a very complex power amp! .. and not induce ANY new problems!
Regards
Might be worth trying to locate the missing section of the schematic -- the 8 unlabeled traces heading off to the right, bottom of page 6, from the ±18V and 12V supplies we were working on.
But so far it seems you have a bundle of contradictions -- and I'm still trying to figure out how in 2 hours and 13 minutes, you managed to remove, test, and reinstall every component in a very complex power amp! .. and not induce ANY new problems!
Regards
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