Yamaha CR-820 - guidance and possible help needed
- Thread starter JorgenSWE
- Start date
This is how they are mounted. As the others were that I took out. seen from front to back.
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The only way the screw can contact the heatsink is if the socket itself is not properly positioned. Or if the socket is damaged. Or if the socket is contaminated with something conductive. Like metal filings or copper wire strands.
Holes poked through mica sheets from metal filings or bits of splattered solder...Seen that before..
Holes poked through mica sheets from metal filings or bits of splattered solder...Seen that before..
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Well, non of the above seems plausable at this time. I cleaned all surfaces with alcohol before mounting. New micas. None of the sockets seems out of place or damaged. The original transistors had never been replaced. So very little disturbance had been made prior to swapping them out. But I agree that all the above mentioned can cause these issues.
But once I get the washers, I will recheck all the above, also measure collector - ground to make sure we don't have issues there. I will also check the solder side to make sure we have no joints or strands causing bridging where its not supposed to be.
Hopefully we can see some positive results. There are now only four diodes that i have not bulled out to test in the power supply, beside the four cheramic capacitors in the rectifier bridge.
Wish me luck!
Hi!
So I have now mounted the plastic washers. I have also soldered the rectifier bridge back in place, since they checked out ok.
When I measusure continuity between collector and chassie on the 1st (93G) and third transistor (93G) (counting front to back) they check out ok, no continuity. The other two still have full continuity. Have tried to reseat them and also checked for things that could be causing the continuity, such as faulty solder joints or strands of wires but can't find any.
Does this give evidence to faulty sockets?
I have measured all the transistors out of circuit and they are ok. Checks out fine when testing them in the component tester. Have also checked the solder side of the pcb, but can't see any obvoius signs of solder splatter or solder bridges. But then I'm not that familiar with the layout of the pcb to be fully confident it is not what is causing the problem.
The transistors can't be mounted any other way, since the e,b pins are not centered on back, so they only go in one way. You wrote about mounting them "upside down" could cause issues.
Suggestions?
So I have now mounted the plastic washers. I have also soldered the rectifier bridge back in place, since they checked out ok.
When I measusure continuity between collector and chassie on the 1st (93G) and third transistor (93G) (counting front to back) they check out ok, no continuity. The other two still have full continuity. Have tried to reseat them and also checked for things that could be causing the continuity, such as faulty solder joints or strands of wires but can't find any.
Does this give evidence to faulty sockets?
I have measured all the transistors out of circuit and they are ok. Checks out fine when testing them in the component tester. Have also checked the solder side of the pcb, but can't see any obvoius signs of solder splatter or solder bridges. But then I'm not that familiar with the layout of the pcb to be fully confident it is not what is causing the problem.
The transistors can't be mounted any other way, since the e,b pins are not centered on back, so they only go in one way. You wrote about mounting them "upside down" could cause issues.
Suggestions?
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Hi!
I have tested the transistors, they check out ok. But according to you, in this pisitions should be MJ21194G but what I have removed is the 93Gs. Could it be that the original were positioned wrong? That they should swap poitions. I was miticioulus when swapping out the old for the new, so that the correct one was placed in the same position. Repalaced them one at a time.
I have tested the transistors, they check out ok. But according to you, in this pisitions should be MJ21194G but what I have removed is the 93Gs. Could it be that the original were positioned wrong? That they should swap poitions. I was miticioulus when swapping out the old for the new, so that the correct one was placed in the same position. Repalaced them one at a time.
ecluser
Super Member
From the front of the receiver the correct position should be xx193-xx194-xx193-xx194. There should be no insulator on the mounting screws because the electrical contact to the collector is made by the screws. If the sockets are good and properly seated on the heat sink, the screws will be centered into the mounting holes and there will be no contact between the crews and the heat sink. The sockets have a small nylon sleeve to insure that the screws are centered into the holes. I didn't read all the thread but make sure you are using the good thermal paste. The one for computer is conductive and should never be used in amplifiers. Since you are using an analog meter, always check continuity in both direction because you might think that you have a short between the collector and chassis (ground) but in reality it might be conduction from the collector to ground via the main bridge rectifiers and the transformer, which has its center tap to ground.
Hi!
I'm using paste bought through a company dealing in electronics, so no computer paste.
Well, should the nylon flushers not be used then? Thought those were the problem, as mentionef in the earlier post. I have now removed them.
The transistors were in the correct position.
To be clear on the continuity test. In between what points should I measure? The transistor and what?
Please advice! Thank you!!
You wrote:
"If the sockets are good and properly seated on the heat sink, the screws will be centered into the mounting holes and there will be no contact between the crews and the heat sink."
Did some further testing. If I place a mounting screw in the hole (wthout the transistor) then turn it a few revolutions in order for it to make good contact with the threaded hole. When I measure wth the test leads between that screw and a screw that holds the outer heat sink, I have full continuity. The screw is NOT touching the heat sink at that moment.
I'm using paste bought through a company dealing in electronics, so no computer paste.
Well, should the nylon flushers not be used then? Thought those were the problem, as mentionef in the earlier post. I have now removed them.
The transistors were in the correct position.
To be clear on the continuity test. In between what points should I measure? The transistor and what?
Please advice! Thank you!!
You wrote:
"If the sockets are good and properly seated on the heat sink, the screws will be centered into the mounting holes and there will be no contact between the crews and the heat sink."
Did some further testing. If I place a mounting screw in the hole (wthout the transistor) then turn it a few revolutions in order for it to make good contact with the threaded hole. When I measure wth the test leads between that screw and a screw that holds the outer heat sink, I have full continuity. The screw is NOT touching the heat sink at that moment.
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ecluser
Super Member
There is a problem here. I see that your screw doesn't seem to touch the heat sink. The mounting flange of the socket makes a contact with the heat sink, or the supply line for that transistor is shorted to ground somewhere. Remove that screw and pull back the socket so it is not touching the heat sink. Check the continuity between the mounting flange of the socket (where the mounting screws should be inserted, the collector contact) and the chassis (ground). If you still have continuity, your short is probably not in the sockets itself but somewhere else on the board.
Hi!
This problem is valid for all screws. They are all in contact with the hest sink. Not just this one.
I did manage to get two of them to sit correct in the socket, but decided to investigate this further.
Just a quick question. Do you mean that it is ok to bend the "legs" backwords without damage the pcb? I do not want to cause any collatoral damage to the unit. Three brass legs goes from the socket and is soldered tonthe pcb.
This problem is valid for all screws. They are all in contact with the hest sink. Not just this one.
I did manage to get two of them to sit correct in the socket, but decided to investigate this further.
Just a quick question. Do you mean that it is ok to bend the "legs" backwords without damage the pcb? I do not want to cause any collatoral damage to the unit. Three brass legs goes from the socket and is soldered tonthe pcb.
ecluser
Super Member
Just another point. When you try to see continuity with your Ohm meter, between the supply lines (collector of power transistors) and ground, your meter is trying to charge the big 10,000uF capacitor. At first, there is a large current from the meter and your meter thinks there is a short. To have a better feeling of what is happening, take a 47uF capacitor and put your meter probes across the capacitor, with your meter in Ohm mode. You will see that your meter's needle will move to the right (showing 0 Ohm) for a brief moment, then the needle will move progressively to the left as the capacitor will be charged by your meter current source. It is the same thing for the 10,000uF capacitor but it takes 200 times more time. Since your meter's needle move very very slowly to the left you are thinking there is a short, but there is no short.
This is the reading I get when doing the same measurment, and it stays put. The DMT lits up when I fire up the unit and I get only a few volts on the secondary (red-red) where it should read 67V-ish. So something is off, as it shorts the rail voltage. Might not be tis, but still its not right.
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ecluser
Super Member
This amp can work without output transistors, with headphones alone. Pull out the output transistors. Do you still have only a few volts on the supply rails with your DMT? There are four small 1uF/50V capacitors on the supply rails. Replace them if you still have only a few volts.
Hi!
Ok. I have recapped this whole board, so all elyt-caps are new. Have also taken out the rectifier bridge (twin diods) and it also is ok.
When mesuring with transistors out, I get 66,4 V. No sound from the headphone jacks though. Tried both the tuner and the aux.
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ecluser
Super Member
On your post #111
With all screws removed, the collector of the power transistors should be disconnected. Inspect the mounting flange of these transistors. Look for sharp edge or small protuberance that could puncture the mica insulator.
Well, decided to review some of my previous work.When measuring between chassie and collector when all screws are in place, gives full continuity on all four, collector to chassie ground. But when all screws are out, only held in place by the paste, I get continuity only on the 4G (second and last seen from front to back)
I mounted them with new mica insulators and new paste. Made sure they seated correct in the socket. Screwed them gently to the chassie. Am I missing something here?
Hmmmm... Just relized that there are no nylon washers between the chassie and the collector to isolate the screws (ie chassie ground) from the collector. The odd thong is that there were no washers there in the first place.
With all screws removed, the collector of the power transistors should be disconnected. Inspect the mounting flange of these transistors. Look for sharp edge or small protuberance that could puncture the mica insulator.
When mounting the new transistors I cleaned all surfaces with alcohol and used new micas. Nothing seemed out of the ordinary. But I will clean the surfaces again and inspect the micas for any anomilies, but tomorrow. It is getting late here (Sweden) and my brain fails me at this hour.
Also tested the dc voltage,
27.7
-4.9
12.5
12.5 (M)
Nomspund is coming out of the unit, using the headphone jack.
Hi!
Had some time today to look into the issues with the end transistors and the shorting of the unit to ground.
I installed one transistor at a time, and checked in between. it is the second transistor from the front that is causing the shorting. I have tried to clean the socket using a small tooth pick (those with a tiny brush) but it does not solve the problem.
I also checked solder joints for unwanted bridges or other anomilies, but I seem to not find any.
I have attached a photo of the solder side, where this transistor is located. If someone with a trained eye could take a look to see of you find anything, I would appriciate it.
It could of course also be a faulty socket, but let's rule out any other possiblilities first.
Again, thank you!
Had some time today to look into the issues with the end transistors and the shorting of the unit to ground.
I installed one transistor at a time, and checked in between. it is the second transistor from the front that is causing the shorting. I have tried to clean the socket using a small tooth pick (those with a tiny brush) but it does not solve the problem.
I also checked solder joints for unwanted bridges or other anomilies, but I seem to not find any.
I have attached a photo of the solder side, where this transistor is located. If someone with a trained eye could take a look to see of you find anything, I would appriciate it.
It could of course also be a faulty socket, but let's rule out any other possiblilities first.
Again, thank you!
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Hi!
I removed the TO3 socket and cleaned it, resolder and put a new mica, now things seems to run ok.
TP 1 = 24.7 V
TP 2 = 24.5 V
TP 3 = - 31.8 V
TP 4 = - 31.7 V
Measured the voltage on the pins.
25.7 V
- 0.09 V
11.6 V
11.6 V (M)
The unit seems to have lost the negative 12 V along the way. The voltage on the test points also seems a bit low compared to the initial values. What could have caused the drop in voltage? Where should I look for the lost - 12V?
No sound coming out of the headphone jacks, but the meters move so they register something. All diods on front panel lights up and the FM-meters registers that there are stations.
Again, thank you for helping out!
I removed the TO3 socket and cleaned it, resolder and put a new mica, now things seems to run ok.
TP 1 = 24.7 V
TP 2 = 24.5 V
TP 3 = - 31.8 V
TP 4 = - 31.7 V
Measured the voltage on the pins.
25.7 V
- 0.09 V
11.6 V
11.6 V (M)
The unit seems to have lost the negative 12 V along the way. The voltage on the test points also seems a bit low compared to the initial values. What could have caused the drop in voltage? Where should I look for the lost - 12V?
No sound coming out of the headphone jacks, but the meters move so they register something. All diods on front panel lights up and the FM-meters registers that there are stations.
Again, thank you for helping out!