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Anyone out there understand how the protection circuit works on the Yamaha G50 112 III amp? I don't have the schematic for the G50 112 III but I have the G100 112 III which is very similar. This amp has audio all the way up to the protection relay before the speaker but that's as far as I've been able to take it without a circuit description There is voltage at the Ry101 relay coil so apparently the IC111 is not grounding the coil. I'll attach the G100 112 III schematic that I got from other posts on this site
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Hi cablercdd1
I've attached my "edits" of scans Joem posted before the board lost attachments.
As PDF often don't seem to stick here I've put the little I have in a large jpg format.
Hope these are enough to help.
The protection circuit uses the Toshiba TA 7317p IC.
They are often found in Hi-Fi amps for protection.
The inputs to it pins 2 and 3 which in a stereo setup are normally connected to L & R outputs
(obviously before the relay contacts) ...here they are "commoned" and connected to the amp output.
Any dc here will not allow the IC to activate the relay.
I presume that is whats happening although it may pay to check all the components and replace some of those caps around IC 110.
If you lift one end of R210 (100K) if there is any DC there the relay should connect although I would check for DC first and definitely not have any speaker connected !
The chip itself can fail.Attached Files -
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Thanks OC, that gives me somewhere to start. I've always had problems with protection circuits. Just give me a blown component any day. I'll take another look at it tomorrow night.
Paul
Originally posted by oc disorder View PostComment
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It may be doing its job. Go on the amp side of the relay and see if there is DC offset on the output line.Education is what you're left with after you have forgotten what you have learned.Comment
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Not sure who did what...Education is what you're left with after you have forgotten what you have learned.Comment
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I have this in with little output. Discovered someone rewired the speaker to the headphone jack lol.
Now I'm discovering that there is no output with the speaker wired correctly. Guess they figured the output doesn't work but the headphone out does.
Both output Vbe are correct...and both rails are at +/-38.8v. I only get about 200mv across the relay coil.
R201 has 23v across it. Seems to me that IC110 is working and pulling one side of that resistor down. R201 measures correct at 390ohms. My guess is the relay is shot.
Looking for any input here. Thanks!Comment
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What kind of resistance do you measure across the relay coil?Originally posted by Enzo
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4ohms
I also bypassed the relay and had the amp on my bulb limiter. Worked. Plugged into wall and all is good. I'm also thinking that that coil should be more in the 100ohm range judging by this replacement.
https://www.mouser.com/Search/m_Prod...ey817-JV-5S-KTComment
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Yes, relay coils are usually 100 to 200 ohms.
You can pull them out and test them with a power supply if you have oneComment
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If you don't have a power supply, you can just use a battery..... or a couple of series batteries if you need more voltage.Originally posted by drewl View Post"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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Make sure that diode across the relay coil is not shorted.Originally posted by Enzo
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