Hi all, so I have been around the Main B board and measured all the diodes and fusible resistors and transistors and MOSFETS and everything seems fine. I did notice that the 4 bridge rectifier diodes (D701/702/703/704) do not appear to show anything on my multimeter.
Also it has now popped the 1 ohm fusible resistor R653 again.
This is without the Main A board connected so there's nothing on there that is causing it.
I'm at a loss of where to go now.
Also it has now popped the 1 ohm fusible resistor R653 again.
This is without the Main A board connected so there's nothing on there that is causing it.
I'm at a loss of where to go now.
What does this mean? They should show something.
Are you using a dim-bulb tester? That might save you a few components while you narrow down your search.
Have you checked their voltage output? The EEVBlog first rule of troubleshooting: "Thou shalt check thy voltages." 🙂
If you´re measuring with DMM in OHM-setting, this is normal.
You need to use the DIODE function of the DMM.
You need to use the DIODE function of the DMM.
I am using the diode function. All the other diodes on the board measure a drop of about 0.5V as they should.
I'll check their voltages - are there any particular points around them that I should check this?
I'll check their voltages - are there any particular points around them that I should check this?
Some more observations:
IC601 - the protection chip - pin 6 has 53V across it when it should be 0.8.
Q466 and Q467 (2SK1529 MOSFETs) get extremely hot as soon as it is powered up
Q468 and Q469 (2SJ200 MOSFETs) get warm but not hot
All the MOSFETs on the other end of the board (Q418, Q419, Q416, Q416) stay cool
I don't have a dim bulb tester but the amp draws about 240W constantly according to my power meter.
Measuring the bias at the test points:
Right channel: ~250mV
Left channel: 0mV. Nothing.
IC601 - the protection chip - pin 6 has 53V across it when it should be 0.8.
Q466 and Q467 (2SK1529 MOSFETs) get extremely hot as soon as it is powered up
Q468 and Q469 (2SJ200 MOSFETs) get warm but not hot
All the MOSFETs on the other end of the board (Q418, Q419, Q416, Q416) stay cool
I don't have a dim bulb tester but the amp draws about 240W constantly according to my power meter.
Measuring the bias at the test points:
Right channel: ~250mV
Left channel: 0mV. Nothing.
240W is a bit too much. I would not switch it on anymore without the DBT. You really need it as the amp is known to be muerte (as in that there is no doubt) and it will for sure not improve with 240W drawn. Too far gone for you to repair I am afraid. It has a detective output stage it seems so many parts to check/replace.
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I have removed all the power MOSFETs and it still goes into protection with 57V across pin 6 of IC601 but with only 20-25W of power draw now.
The fact that pin 6 of IC601 goes high doesn't convey much information--- only that relay has opened, presumably because IC601 detected a fault in a power amp.
You noted Q466 and Q467 get extremely hot, but Q468 and Q469 do not. A big question is where was the excessive current flowing? Maybe the upper pair was hot because the there was a short in the lower pair? Any clues that might point the way? I assume you are testing with no load connected.
Given that you've removed all four, can you test them out of circuit?
Did you note the voltage at the amp output (eg. at the junction R485-R487, i.e. in front of protection relays)?
You noted Q466 and Q467 get extremely hot, but Q468 and Q469 do not. A big question is where was the excessive current flowing? Maybe the upper pair was hot because the there was a short in the lower pair? Any clues that might point the way? I assume you are testing with no load connected.
Given that you've removed all four, can you test them out of circuit?
With all output devices removed, you have opened the feedback loop. In that case all bets are off. If you want to test the amp without the output devices you have to improvise something to close the loop again. Two equal resistors in series between the drivers for instance, feedback connected to the center connection.
Did you check the DC offset adjustment after the recaping? You may have touched the variable resistors... 😳
See Service Manual, Section 4, Electrical Adjustments.
I would never recommend a complete recap. Nichicon capacitors, in particular, work perfectly, even if they're 50 years old. Also the caps of other manufacturers. Replacement is only necessary if there's a definite defect!
I've just been to test them all out of circuit and one of the 2SJ200s is shorted all the way through. The rest test fine. I have some spares so is it worth putting them all back in with a replacement J200 and seeing what happens?