What changed between the time when you took the photo and now?
Check your email in a few minutes. I have a question about an old 650.
Check your email in a few minutes. I have a question about an old 650.
The drawers i keep my parts in the 2A bjt's and A7 diodes are right next to each other. Somehow some A7's got mixed in with my 2A's and a A7 was installed in location Q119 when it should have been a 2A.
I have sorted through them and corrected the problem, note to self check the parts you are putting in to confirm they are what the drawer says they are.
Other than than that nothing. Still dont know why the protection transistors would cause distortion upon re-installing them (Q130 and Q126)
I have sorted through them and corrected the problem, note to self check the parts you are putting in to confirm they are what the drawer says they are.
Other than than that nothing. Still dont know why the protection transistors would cause distortion upon re-installing them (Q130 and Q126)
With Q130 installed the amp still produces clean audio, the first pic is before Q130 is installed and the second pic is after Q130 is installed. Looks the same to me but pic 2 I had a tad more volume so its a little more square.
Scope settings were 20 vol/div and 2ms. My probe is on 10x so I believe its 20 vol/div
Scope settings were 20 vol/div and 2ms. My probe is on 10x so I believe its 20 vol/div
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We'll have to troubleshoot that part of the circuit. Removing these two transistors did the same thing as removing the diodes Dx04, etc) in the later amps. It disabled the protection circuit. This allowed us to see if the problem was in the protection circuit or in the audio circuit.
Reinstall Q126 and remove Q120. Does the amp still produce clean audio?
Reinstall Q126 and remove Q120. Does the amp still produce clean audio?
Yes it does with a nice signal on the scope as well. Q126 is back in and Q120 is out.
I don't think the 2A will withstand the voltage in this circuit (driver voltage plus -rail is about 100v). Are you 100% certain that it's not leaking?
If so, the problem could be further back in the circuit.
If so, the problem could be further back in the circuit.
Well I just did a resistance check in ohms in all different probe configurations and it measures the same as a brand new one the only way I got any reading was black probe on collector and red probe on base and it was 17.54 M ohms.
I did replace 3 2L's in this channel with 2A's. I have several parts boards I can replace the 2A's back with 2L's maybe one of them failed?
The locations are Q135, Q136, and Q141.
Thoughts on those?
I did replace 3 2L's in this channel with 2A's. I have several parts boards I can replace the 2A's back with 2L's maybe one of them failed?
The locations are Q135, Q136, and Q141.
Thoughts on those?
I'd suspect 135 and 136. Reinstall Q120 and remove 135/136 to see if the amp will still produce clean audio.
Q120 installed Q135 and Q136 removed amp has clean audio and good signal on scope.
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Shall I replace 135 and 136 with 2L
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Shall I replace 135 and 136 with 2L
Replaced Q135 Q136 and Q141 with 2L's from parts board and amp produces clean audio both channels with good signal on scope.
When I try to set the bias the current draw starts going up as soon as you turn the pots at all. This is not normal, usually you get at least 1/4 to 1/2 turn out of them before it starts drawing current.
Thoughts?
When I try to set the bias the current draw starts going up as soon as you turn the pots at all. This is not normal, usually you get at least 1/4 to 1/2 turn out of them before it starts drawing current.
Thoughts?
Originally I replaced the MPS6521's which I believe are the biasing transistors when I replaced all the output fets. Is it possible I blew them with my other issues?
I've had a few do this. I think it's due to a lack of range of the bias control and the lower threshold of the newer outputs.
Replace Rx62 with a 300 ohm resistor (or something near). That should give the bias control a bit more range.
Replace Rx62 with a 300 ohm resistor (or something near). That should give the bias control a bit more range.
I had a couple do this as well just left the bias all the way down and never had an issue. I will replace them and post my result.
R162 and R262 with something around 300 ohm. I think I have those have to look. When you say something near what range can I choose from?
R162 and R262 with something around 300 ohm. I think I have those have to look. When you say something near what range can I choose from?
I really don't know. Lower will give more range but may stress the pot more due to more heat (but probably not enough to cause it to fail). Higher won't increase the range as much.
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