10-10-2016, 07:07 AM
Right chaps, to clear things up the Marantz in question led me a merry dance by presenting odd issues every time I turned it on, after a fault finding/testing spree, like buzzing and a bright bulb on the lamp limiter, OP trannies shorted? No, they're fine. This fault disappeared after I unsoldered the OP trannies for testing. Then I had oscillation of the OP, IE instability and a VHF signal sitting on top, had the top of the sine chopped off and other intermittent faults. All I found was two or three off spec resistors and dry joint's.
I tested the PSU, which for the OP section is a bridge and two big caps, it does have quite a big ripple current 300mV P-P. The caps are fine as are the individual diodes that make up the bridge, that is they test ok on a diode checker, but a leaky diode could be possible.
There is a regulated PSU for the graphic EQ and other sections, tests ok.
So eventually I got two good sinewaves out. This is with a 1khz sine at the voltage as stated in the manual, 8 ohm dummy load. Music in, all well, power meters indicate a power near to the stated OP. Sign back in to test for rated power OP, IE check RMS voltage at OP, squared divided by 8. the meter briefly reads around 25v, which is about right for a stated power of 70w, then it crashes to 2v ish and won't go back up. Music back on, all fine. WTF???
You make two good points Mark, IE look at the bootstapped Q's and their caps and look at the bridge. I've pulled and tested all the trannies (Q's)and in some cases temporarally popped in a new substitute. Found no issues, all Q's test ok.
The Sony amp is very simple, the power amp consists of a LTP/differential input which is a Sony 2SA884 DIL package ( two PNP Q's), a class A stage, and an OP stage. Sine in, all ok up to power amp, then the sine shows X over distortion. All ok up to the power Q's, but now were getting a voltage across the right hand side of the combined E resistor, non to speak of on the left. The e R is one of those 3 legged jobbies. Bias pot does nothing to speak of. One possible suspect, there is a diode that is mounted on the HS, for thermal coupling that sits between two Q's, but this tests ok.
These old amp's arn't well designed, because the input signal gets sent from PCB to PCB back and forth all over the shop, through switches, etc before it gets to the power amplifier.
So, hence my question, when you've tested and affirmed as good all resistors, caps, diodes etc your left with the transistors. I'd read that some Q's in certain amps go "bad" and need replacing, probably from moisture ingress so wondered if there were other ways a Q can fail apart from the usual.
I wanted to build a semiconductor tester for three reasons, A) I hate buying new stuff especially from China, yes I know everything is made in China, but I try and limit the ammount i buy. B) As you know I'm not that experienced, havn't been at it as long as most of you, so I learn by building stuff, and C) Its more fun building than buying.
Andy.
I tested the PSU, which for the OP section is a bridge and two big caps, it does have quite a big ripple current 300mV P-P. The caps are fine as are the individual diodes that make up the bridge, that is they test ok on a diode checker, but a leaky diode could be possible.
There is a regulated PSU for the graphic EQ and other sections, tests ok.
So eventually I got two good sinewaves out. This is with a 1khz sine at the voltage as stated in the manual, 8 ohm dummy load. Music in, all well, power meters indicate a power near to the stated OP. Sign back in to test for rated power OP, IE check RMS voltage at OP, squared divided by 8. the meter briefly reads around 25v, which is about right for a stated power of 70w, then it crashes to 2v ish and won't go back up. Music back on, all fine. WTF???
You make two good points Mark, IE look at the bootstapped Q's and their caps and look at the bridge. I've pulled and tested all the trannies (Q's)and in some cases temporarally popped in a new substitute. Found no issues, all Q's test ok.
The Sony amp is very simple, the power amp consists of a LTP/differential input which is a Sony 2SA884 DIL package ( two PNP Q's), a class A stage, and an OP stage. Sine in, all ok up to power amp, then the sine shows X over distortion. All ok up to the power Q's, but now were getting a voltage across the right hand side of the combined E resistor, non to speak of on the left. The e R is one of those 3 legged jobbies. Bias pot does nothing to speak of. One possible suspect, there is a diode that is mounted on the HS, for thermal coupling that sits between two Q's, but this tests ok.
These old amp's arn't well designed, because the input signal gets sent from PCB to PCB back and forth all over the shop, through switches, etc before it gets to the power amplifier.
So, hence my question, when you've tested and affirmed as good all resistors, caps, diodes etc your left with the transistors. I'd read that some Q's in certain amps go "bad" and need replacing, probably from moisture ingress so wondered if there were other ways a Q can fail apart from the usual.
I wanted to build a semiconductor tester for three reasons, A) I hate buying new stuff especially from China, yes I know everything is made in China, but I try and limit the ammount i buy. B) As you know I'm not that experienced, havn't been at it as long as most of you, so I learn by building stuff, and C) Its more fun building than buying.
Andy.
