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Thread resurrection! Is that cliché?
Welp, I’m back, and so is this amplifier I worked on 8-9 years ago...
It’s my uncles amp, and I took it upon myself to repair it, because he surely was going to let it sit. I have gone through and thoroughly restudied this thread and had a question.
To summarize what happened last time was basically I ended up replacing q2 (shorted), r26 (as a result of q2 shorting), and vr2 (because had no -15v rail). Also replaced 3 aftermarket nte transistors with pv oem ones.
Question is, is it possible that q2 shorted because I had no -15 v rail from vr2, and I had failed to disconnect the autoformer from the board when I applied power that first time?
I am making an earnest effort to really understand how all this works, and I will be reading as many threads on this series amp as I can. Really want to master this guy.
Anyway, now to the current issue at hand. Got amp back from uncle a couple days ago. Blown fuse. Found output transistor q13 shorted. So, I just took it out of the circuit, replaced fuse, and tried amp. Everything was stable, so started taking readings to be sure I wasn’t missing anything. I was trying really hard to keep a steady hand, but alas my probe slipped and shorted the emitter and collector together on q12. R61 lit up and smoked. After a few choice words and a walk around the neighborhood, I came back and checked q12. Yep, shorted emitter-collector. So now instead of possibly just needing an output transistor, I’m also out q12 and r61 at least. Don’t know what other damage I might have caused, but I want to try to get all the parts I need in one go from pv.
Could I have avoided damaging things if I had just had the autoformer and output transistors isolated from the board? Would it still have damaged q12 regardless of whether or not I had unplugged that stuff?
As far as the output transistors, what I did last time is replace outputs q4,5,6 with the new transistors I got from pv. The rest are original to the unit. As said before, this time, before any of this other stuff happened, I found q13 shorted (no resistance between emitter and collector). Believe q13 is wired like a driver on this bank.
Do these transistors just fail at random sometimes, or is there usually a cause? I noticed all the black plastic covers on this bank were slightly deformed and slightly harder to remove, like they got somewhat warm. Maybe it could have been the way I put it together last time I worked on it, or due to the fact that these transistors are shaped differently from the newer ones, or maybe they actually did run warmer on that bank. There is no heat-conductive paste on any of these outputs, and looks like never has been. There is a vinyl-like light blue insulator piece between heat sink and output that looks in good shape. Do you think it’s necessary to put some heat sinking paste on these output transistors? Is there any particular way I should arrange these transistors between the two banks (I have 4 SJ6357’s and now will have 4 70484140’s) so I can balance out the load on them? I’ve noticed it always seems to be the old ones that fail (go figure). Last time I worked on the amp, I bought enough to replace all of them. I hate to replace parts that are still working, but will that actually be the best thing to do for this amp?
I’m sure I have other questions but this is probably enough for right now. Thanks for taking the time to read and follow.
Welp, I’m back, and so is this amplifier I worked on 8-9 years ago...
It’s my uncles amp, and I took it upon myself to repair it, because he surely was going to let it sit. I have gone through and thoroughly restudied this thread and had a question.
To summarize what happened last time was basically I ended up replacing q2 (shorted), r26 (as a result of q2 shorting), and vr2 (because had no -15v rail). Also replaced 3 aftermarket nte transistors with pv oem ones.
Question is, is it possible that q2 shorted because I had no -15 v rail from vr2, and I had failed to disconnect the autoformer from the board when I applied power that first time?
I am making an earnest effort to really understand how all this works, and I will be reading as many threads on this series amp as I can. Really want to master this guy.
Anyway, now to the current issue at hand. Got amp back from uncle a couple days ago. Blown fuse. Found output transistor q13 shorted. So, I just took it out of the circuit, replaced fuse, and tried amp. Everything was stable, so started taking readings to be sure I wasn’t missing anything. I was trying really hard to keep a steady hand, but alas my probe slipped and shorted the emitter and collector together on q12. R61 lit up and smoked. After a few choice words and a walk around the neighborhood, I came back and checked q12. Yep, shorted emitter-collector. So now instead of possibly just needing an output transistor, I’m also out q12 and r61 at least. Don’t know what other damage I might have caused, but I want to try to get all the parts I need in one go from pv.
Could I have avoided damaging things if I had just had the autoformer and output transistors isolated from the board? Would it still have damaged q12 regardless of whether or not I had unplugged that stuff?
As far as the output transistors, what I did last time is replace outputs q4,5,6 with the new transistors I got from pv. The rest are original to the unit. As said before, this time, before any of this other stuff happened, I found q13 shorted (no resistance between emitter and collector). Believe q13 is wired like a driver on this bank.
Do these transistors just fail at random sometimes, or is there usually a cause? I noticed all the black plastic covers on this bank were slightly deformed and slightly harder to remove, like they got somewhat warm. Maybe it could have been the way I put it together last time I worked on it, or due to the fact that these transistors are shaped differently from the newer ones, or maybe they actually did run warmer on that bank. There is no heat-conductive paste on any of these outputs, and looks like never has been. There is a vinyl-like light blue insulator piece between heat sink and output that looks in good shape. Do you think it’s necessary to put some heat sinking paste on these output transistors? Is there any particular way I should arrange these transistors between the two banks (I have 4 SJ6357’s and now will have 4 70484140’s) so I can balance out the load on them? I’ve noticed it always seems to be the old ones that fail (go figure). Last time I worked on the amp, I bought enough to replace all of them. I hate to replace parts that are still working, but will that actually be the best thing to do for this amp?
I’m sure I have other questions but this is probably enough for right now. Thanks for taking the time to read and follow.
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