Not necessarily, I have seen some amps which had blown one or two rectifier diodes without anything broken downstream.
Replace them, raise the fuses at the secondary output to isolate the power PCBs, apply power to see if you have normal DC voltages
Rectifier diodes, yes. They can go bad due to repetitive surge current. But both potential paths to ground or opposite rail are limited by either R12 or R14 here, even in the transistor is open or shorted. Somebody would have to drop something on the PCB or have R14 low by an order of magnitude or two to get enough fault current here to hurt healthy diodes. The only other possibility is a manufacturing defect.
If the diodes are open, and nothing else wrong, the front end diff pair will run at way too much current. That will upset things, at a minimum cause the amp to run out of common mode range early. Result is distortion at significant signal levels, which matches the symptoms. The diff pair current is strictly limited by R12 and probably survived (if it didn’t you’d get DC offset and loss of feedback, forcing the output to stick to one rail). But check those for signs of stress and replace if necessary too.
If the diodes are open, and nothing else wrong, the front end diff pair will run at way too much current. That will upset things, at a minimum cause the amp to run out of common mode range early. Result is distortion at significant signal levels, which matches the symptoms. The diff pair current is strictly limited by R12 and probably survived (if it didn’t you’d get DC offset and loss of feedback, forcing the output to stick to one rail). But check those for signs of stress and replace if necessary too.
Awesome. I Actually pulled several of nearby transistors/resistors and all were fine!. I will replace the diodes. and bring up slowly on variac! Thanks for all your help!!
Jim
Jim
I replaced the diodes on the bad channel (D1/D2). Signal on that channel still distorted. When I checked the diodes in the good channel in the unpowered state with my diode meter i get 0.539v (forward bias)/ and reverse bias is 2.399 which is the same for bad channel. I pulled Q3A and appears to test ok? R12 test ok. Any suggestion where to go from here.
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I still suggest the 2nd sentence of post # 9. You need to see the distortion with the scope to find what stage it is occurring at. Good channel input of Q1a should be sine wave on the good channel, is it different on the distorted channel? If not, move back to collector of Q7a the VAS. Different there between the channels?
This amp may be full of 25 year old electrolytic capacitors. Rubber sealed aluminum bottles full of slime. No more reliable than 25 year old tires. Before swapping parts find out where the problem is. Since you have a signal generator and a scope, use them.
I certainly do not suggest pulling transistors and putting them on a tester. Testers use 2 v or less test voltage. Damaged transistors can work fine at that voltage but not at 15-35 v actual operating condition. Plus amateurs make bad solder joints. I certainly do. Plus I have pulled a land off the circuit board while removing a dead transistor. Too much risk, not enough reward. If you have to go to the component level, connect the negative of a dvm to power supply center connection of C7+C8 etc with an aligator clip lead, and look at no signal DC voltages in the stack from plus to minus supply. That is a better test of transistors. Write down the transistor & diode numbers plus to minus in a stack, with e b c suffixes, diode anode & cathode, measure and write down voltage from up to down. Algebraically subtract voltages to find Vbe is ~0.6 v Vce 1.5v or greater, diodes ~0.6 v from c to a.
This amp may be full of 25 year old electrolytic capacitors. Rubber sealed aluminum bottles full of slime. No more reliable than 25 year old tires. Before swapping parts find out where the problem is. Since you have a signal generator and a scope, use them.
I certainly do not suggest pulling transistors and putting them on a tester. Testers use 2 v or less test voltage. Damaged transistors can work fine at that voltage but not at 15-35 v actual operating condition. Plus amateurs make bad solder joints. I certainly do. Plus I have pulled a land off the circuit board while removing a dead transistor. Too much risk, not enough reward. If you have to go to the component level, connect the negative of a dvm to power supply center connection of C7+C8 etc with an aligator clip lead, and look at no signal DC voltages in the stack from plus to minus supply. That is a better test of transistors. Write down the transistor & diode numbers plus to minus in a stack, with e b c suffixes, diode anode & cathode, measure and write down voltage from up to down. Algebraically subtract voltages to find Vbe is ~0.6 v Vce 1.5v or greater, diodes ~0.6 v from c to a.
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Start at the beginning….. how much voltage across R12? That will verify operation of the master controlling current source.
Using two probes in two hands to answer that question is a good way to kill yourself fixing a 200 watt amp. Using 2 hands in a powered up amp is a bad habit. The proper way to answer that question, and to save a lot of time, is to put the black probe of the DVM to the analog ground, say connection of c7 minus & c8 plus. Or some other convenient ground place say bottom of r14a. Then, writing down the numbers, measure dc voltage unline of d1a (plus voltage rail) to unline of d2a to base of q3a to collector of q3a (top of r12) to connection of r12a with r10a to emitter of q1a to base of q1a to to collector of q1a to line of d3a to connection of r15 with r16 which is the negative power supply. Voltage across R12 is now subtraction of connection of r12a with r10a from collector of q3a. Also answers question, are power supplies symmetric, also can show up many other problems in that stack. Only one hand used, the other operates a pen to write down the numbers. No current can flow from one hand to the other.
I still think oxidation of connectors of low voltage path from the input jack to base of Q1, or deterioration c7a, are significant risk at 25 years of age. The test with the signal generator and scope in the second sentence of post 9 would answer that question.
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The proper way is to put a clip lead on each end of the damn resistor, run them to the meter and throw the switch from a distance. No one said to stick both hands in the chassis. Using probe tips at all is a good way to short something on the PCB when it slips and cause a bunch of damage.