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Yamaha CR-1020
- Thread starter Ray122
- Start date
Ray122
Member
Soldered the transistors back in place and tried re-setting the amp turning the variable resistor counterclockwise and back, but no change. My voltage between the test points is all the way down to 0.04 volts if the trimmer is turned all the way up. I double-checked my voltages on TR607 and 608. Weirdly enough, I do have different values from before. The new voltages are as follows:
TR607:
Emitter: 1.17V
Base: -.852V
Collector: .439V
TR608:
Emitter: -1.49V
Base -.852V
Collector: .692V
I passed some audio through it and I've still got some distortion. Checked the bias between the test points and no change.
I also just tested the trimmer out of the circuit and that's testing well as well with a solid 4.7K ohms.
TR607:
Emitter: 1.17V
Base: -.852V
Collector: .439V
TR608:
Emitter: -1.49V
Base -.852V
Collector: .692V
I passed some audio through it and I've still got some distortion. Checked the bias between the test points and no change.
I also just tested the trimmer out of the circuit and that's testing well as well with a solid 4.7K ohms.
Does it adjust ? From zero to 4.7K Ω ?
Ray122
Member
Yeah, adjusted exactly as it's supposed to according to my meter. Sorry, I know this is a tricky one.
Ray122
Member
Double checked that as well. And all the transistors are OEM. At the time I was taking my initial voltage measurements I was having a tough time identifying TR611 and TR612 until I took them off their heatsinks. Looks like we're good there as well.
I'm away from my bench again, so we'll have to pick this up in the new year. And on that note, have a great new year! I really appreciate all the help so far. Enjoy tonight, and looking forward to working with you on this next year!
I'm away from my bench again, so we'll have to pick this up in the new year. And on that note, have a great new year! I really appreciate all the help so far. Enjoy tonight, and looking forward to working with you on this next year!
Possibly an error with the schematic, TR607c(0.439V) should be the same as TR608e(-1.49V)
Check C613 orientation, solder joints and tracks, will think some more about it, go and enjoy NYE...
Check C613 orientation, solder joints and tracks, will think some more about it, go and enjoy NYE...
TR607 (2SA844) and TR608 (2SC458) are ecb when looking at bevel front face.
Maybe an error in measurement,
TR607:
Emitter: 1.17V....ok
Base: -.852V......probably 0.439v...Vbe 0.7V
Collector: .439V..probably -.852v
TR608:
Emitter: -1.49V... bit high but ballpark
Base -.852V
Collector: .692V
TR607c(-.852v) =TR608b(-.852v) possibly VR601 is open. Check the blue earth wire is secure.
Maybe an error in measurement,
TR607:
Emitter: 1.17V....ok
Base: -.852V......probably 0.439v...Vbe 0.7V
Collector: .439V..probably -.852v
TR608:
Emitter: -1.49V... bit high but ballpark
Base -.852V
Collector: .692V
TR607c(-.852v) =TR608b(-.852v) possibly VR601 is open. Check the blue earth wire is secure.
Ray122
Member
Hey everyone. Hope your new year is off to a great start!
Re-checked my voltages for TR607 and TR608 just now.
TR607:
Emitter: 1.156V
Base: 0.443V
Collector: -1.647V
TR608:
Emitter: -1.644V
Base: -1.007V
Collector: 0.443V
I think you're right that I may have mismeasured the collector and base last time.
VR601 still tests good, and I checked the continuity across the traces from the leads on VR601 its connecting components. Everything is looking good there.
The blue ground wire also has solid continuity to the chassis ground.
Re-checked my voltages for TR607 and TR608 just now.
TR607:
Emitter: 1.156V
Base: 0.443V
Collector: -1.647V
TR608:
Emitter: -1.644V
Base: -1.007V
Collector: 0.443V
I think you're right that I may have mismeasured the collector and base last time.
VR601 still tests good, and I checked the continuity across the traces from the leads on VR601 its connecting components. Everything is looking good there.
The blue ground wire also has solid continuity to the chassis ground.
Voltages suggest no outright failure, voltages just need a small tweak.
TR607b,e look fine. TR608b(-1V) is off spec (-0.57V) due to abnormal current
through R618(3.3k). Candidates are VR601, if you decrease the resistance (turn CCW)
TR608b voltage should rise closer to the -0.57V spec. Maybe TR606 is causing
a slightly higher current draw??? Not much help... TR607,8 basically look ok
TR607b,e look fine. TR608b(-1V) is off spec (-0.57V) due to abnormal current
through R618(3.3k). Candidates are VR601, if you decrease the resistance (turn CCW)
TR608b voltage should rise closer to the -0.57V spec. Maybe TR606 is causing
a slightly higher current draw??? Not much help... TR607,8 basically look ok
Ray122
Member
So after sitting on it for a couple of days, I decided to just clean the foil side of the PCB really well to see if I could find any cracked traces or poorly soldered components, and after staring at it for a few hours, everything is still looking OK to me. From there I re-measured all of the voltages for the transistors. I did have some variation from my original post. I've attached the updated voltages below and I highlighted the ones that I believe to be an issue.
What I'm finding most concerning is the emitter and base voltages for TR609 and TR610. According to the schematic, the emitter voltage on both of those should be -0.01V but I'm getting a value of +0.017.
I'm going to do some more research to see if I can make sense of the design of this amplifier for myself but at the same time, my instinct is telling me to check for leaky diodes next. Maybe D606, 605, and 603 are potential culprits.
What I'm finding most concerning is the emitter and base voltages for TR609 and TR610. According to the schematic, the emitter voltage on both of those should be -0.01V but I'm getting a value of +0.017.
I'm going to do some more research to see if I can make sense of the design of this amplifier for myself but at the same time, my instinct is telling me to check for leaky diodes next. Maybe D606, 605, and 603 are potential culprits.
To me, TR609, 610 are part of the overload protection, as you wind up the volume the signal at
TR613,614 emitters increases, when threshold is exceeded TR609 turns on and signal is channeled
via D605. I don't think the emitter -0.01V or +0.017V is significant, this is just the dc offset. D605,6
limit the base voltage of TR609,10 to 0.65V(max).
Still think your problem is in the area of TR605,6,7,8. With TR607emitter being ok why is TR608e
wrong. I would monitor TR608e with adjusting VR601, verify it responds to bias adjustment, wanting
-1.15V.
TR613,614 emitters increases, when threshold is exceeded TR609 turns on and signal is channeled
via D605. I don't think the emitter -0.01V or +0.017V is significant, this is just the dc offset. D605,6
limit the base voltage of TR609,10 to 0.65V(max).
Still think your problem is in the area of TR605,6,7,8. With TR607emitter being ok why is TR608e
wrong. I would monitor TR608e with adjusting VR601, verify it responds to bias adjustment, wanting
-1.15V.
Ray122
Member
Alright, back at it again today. I read up on amplifier design to try to at least get a better sense of what components do what so I can have a better idea of what might be the trouble spot.
With that, I thought it would be worth while to hook the output of the amp to an oscilloscope to check if I could see what I thought was the crossover distortion, or if maybe its distortion created from the VAS. Either way, still thinking it's likely some kind of bias issue. To my surprise, I wasn't seeing any distortion at all, but when I switched back to the speakers the distortion was still pretty evident. So Natural next step was to look at the output under the load, and long behold, with the speakers hooked up, I could see the distortion clipping the negative swing of the sine wave. Evidently, not crossover distortion. Moving over to my dummy load showed an even sharper clip of the sine wave, and if I brought the volume up, it would even throw the unit into protection mode.
With that my next step was to re-measure my voltages under this condition. I've attached an image of the clipping on the oscilloscope, as well as my updated voltages, running an 8-ohm dummy load with a 100Hz sin wave at 1.01V into the aux input. Now things are really out of hand. Looks to me like the -90 volt rail is loading up pretty severely, throwing everything else off. As I bring the volume up, the negative voltage on that rail continues to drop. The output stage I'm guessing is actually solid, and it's either the input or voltage amplification stage.
I'm thinking my next step is still to just start pulling components and testing each individually, but I'm interested to hear what everyone else is thinking with this new development... Also not sure if it was a rookie move to be testing without a load from the start, but hey, you live and learn.
With that, I thought it would be worth while to hook the output of the amp to an oscilloscope to check if I could see what I thought was the crossover distortion, or if maybe its distortion created from the VAS. Either way, still thinking it's likely some kind of bias issue. To my surprise, I wasn't seeing any distortion at all, but when I switched back to the speakers the distortion was still pretty evident. So Natural next step was to look at the output under the load, and long behold, with the speakers hooked up, I could see the distortion clipping the negative swing of the sine wave. Evidently, not crossover distortion. Moving over to my dummy load showed an even sharper clip of the sine wave, and if I brought the volume up, it would even throw the unit into protection mode.
With that my next step was to re-measure my voltages under this condition. I've attached an image of the clipping on the oscilloscope, as well as my updated voltages, running an 8-ohm dummy load with a 100Hz sin wave at 1.01V into the aux input. Now things are really out of hand. Looks to me like the -90 volt rail is loading up pretty severely, throwing everything else off. As I bring the volume up, the negative voltage on that rail continues to drop. The output stage I'm guessing is actually solid, and it's either the input or voltage amplification stage.
I'm thinking my next step is still to just start pulling components and testing each individually, but I'm interested to hear what everyone else is thinking with this new development... Also not sure if it was a rookie move to be testing without a load from the start, but hey, you live and learn.
