Salas my input and attenuator PCB is this one: relaixed passive and only the signal half of the pcb will be in the clean box. There are 2 IC's there a I2C 16bit port expander and a relay driver.
I'm playing around with layout for the case, do you think that I might get any interference on the input section of the FSP by having these 2 IC's near the 2sk369? About 4-5cm?
I can also rotate the pcb to have the IC's a bit more far away but at the expense of longer internal signal wiring?
Or am I being paranoid here?
I'm playing around with layout for the case, do you think that I might get any interference on the input section of the FSP by having these 2 IC's near the 2sk369? About 4-5cm?
I can also rotate the pcb to have the IC's a bit more far away but at the expense of longer internal signal wiring?
Or am I being paranoid here?
One board not working
A month ago, I posted a problem (#16262). It concerned, what I thought at the time was, a conflict between the FSP and a line-level, tube (6SN7) preamp that I was building. It appeared that some incompatibility between the two units resulted in damage to my mc cart. Salas advised that the conflict was unlikely. Long story short, I have now diagnosed the issue and could use some help. What I have found is that one of the two pcb’s has a corrupt signal output. There is no problem with the other pcb and no problem with the cart.
Background, I completed the build (mc configuration) in late December 2015. The assembly went smoothly; the unit came on line on the first attempt, and operated flawlessly until a month ago. As noted, it happened at the time I was experimenting with the new tube preamp; and perhaps there was some cause and effect. (There was a significant voltage regulator issue - now fixed.)
Now, one channel is working properly; the other is producing a corrupt signal output. I have checked all in/out connections and visually inspected all solder points. The DC voltage settings prescribed in the start-up instructions are correct. My theory is that there is a damaged component in the signal path. The problem is audible though speakers and visible on an oscilloscope. (I connected a 1khz output to the input jacks on the FSP and ran the output from both channels to the scope. The left channel showed a standard wave form; the right channel displays a corrupted wave form. I also used a probe which showed the same results around C-3; but it is difficult to follow the signal circuit with the probe.)
I wonder if my theory (damaged signal path component) is valid or needs further investigation. And, if it is valid, whether there is some method that I could use to find the problem – perhaps there is a set of expected voltage-ground readings (with or without a signal present).
Anyway, I would appreciate assistance on this.
A month ago, I posted a problem (#16262). It concerned, what I thought at the time was, a conflict between the FSP and a line-level, tube (6SN7) preamp that I was building. It appeared that some incompatibility between the two units resulted in damage to my mc cart. Salas advised that the conflict was unlikely. Long story short, I have now diagnosed the issue and could use some help. What I have found is that one of the two pcb’s has a corrupt signal output. There is no problem with the other pcb and no problem with the cart.
Background, I completed the build (mc configuration) in late December 2015. The assembly went smoothly; the unit came on line on the first attempt, and operated flawlessly until a month ago. As noted, it happened at the time I was experimenting with the new tube preamp; and perhaps there was some cause and effect. (There was a significant voltage regulator issue - now fixed.)
Now, one channel is working properly; the other is producing a corrupt signal output. I have checked all in/out connections and visually inspected all solder points. The DC voltage settings prescribed in the start-up instructions are correct. My theory is that there is a damaged component in the signal path. The problem is audible though speakers and visible on an oscilloscope. (I connected a 1khz output to the input jacks on the FSP and ran the output from both channels to the scope. The left channel showed a standard wave form; the right channel displays a corrupted wave form. I also used a probe which showed the same results around C-3; but it is difficult to follow the signal circuit with the probe.)
I wonder if my theory (damaged signal path component) is valid or needs further investigation. And, if it is valid, whether there is some method that I could use to find the problem – perhaps there is a set of expected voltage-ground readings (with or without a signal present).
Anyway, I would appreciate assistance on this.
First of all we are happy to know that the cart escaped damage. 
Who knows what may have happened between the not finished tube preamp at the time and the one channel FSP output, but its beside the point by now.
There could be a damaged signal path JFET with a leaky gate. Since you spotted trouble as early as around C3, the problem may be nesting in the first stage.
But don't forget to examine the DC rail also for any corruption. AC coupling on scope and 1x setting on probe. Say on 50mV vertical from slow to fast time scan.
If nothing appeared bad again, if in your shoes I would buy a second set of all semis for one channel. Then I would change Q1, Q2, Q3 for start. And so on and so forth.
If changing semis would not help I would see about testing with a new C3 also. Even a low quality but well working one would do for testing purposes.
In any case extra semis are not a bad investment since they are rare NOS types and they can prove invaluable for future repair security even if the problem proves to be something else at this time.
Good luck and let us know.
Who knows what may have happened between the not finished tube preamp at the time and the one channel FSP output, but its beside the point by now.
There could be a damaged signal path JFET with a leaky gate. Since you spotted trouble as early as around C3, the problem may be nesting in the first stage.
But don't forget to examine the DC rail also for any corruption. AC coupling on scope and 1x setting on probe. Say on 50mV vertical from slow to fast time scan.
If nothing appeared bad again, if in your shoes I would buy a second set of all semis for one channel. Then I would change Q1, Q2, Q3 for start. And so on and so forth.
If changing semis would not help I would see about testing with a new C3 also. Even a low quality but well working one would do for testing purposes.
In any case extra semis are not a bad investment since they are rare NOS types and they can prove invaluable for future repair security even if the problem proves to be something else at this time.
Good luck and let us know.
First of all we are happy to know that the cart escaped damage.
Who knows what may have happened between the not finished tube preamp at the time and the one channel FSP output, but its beside the point by now.
There could be a damaged signal path JFET with a leaky gate. Since you spotted trouble as early as around C3, the problem may be nesting in the first stage.
But don't forget to examine the DC rail also for any corruption. AC coupling on scope and 1x setting on probe. Say on 50mV vertical from slow to fast time scan.
If nothing appeared bad again, if in your shoes I would buy a second set of all semis for one channel. Then I would change Q1, Q2, Q3 for start. And so on and so forth.
If changing semis would not help I would see about testing with a new C3 also. Even a low quality but well working one would do for testing purposes.
In any case extra semis are not a bad investment since they are rare NOS types and they can prove invaluable for future repair security even if the problem proves to be something else at this time.
Good luck and let us know.
OK, thanks for the guidance and advice. I'll keep you posted.
Using the configuration discussed above, AC voltage at the Rail+ test point, on both boards (good & bad) appears exactly the same: a straight, clean horizontal line.
Could not find a distinct DC voltage image from either board.
When a PSU located disturbance seems excluded, the damaged signal path semi(s) scenario is even more likely. Also see if there is any little DC across R7 by any chance. That would indicate a leaky gate on Q4 for instance. If eventually you will start changing semis, do Q3 first which is an easy to find BJT that does not give IDSS selection headaches.
I'll check r-7. Tea-Bag has some match pairs 2sk369bl. I know the idss readings of q1&q2 that I installed as well as the installed r2 & r3 values. If he can get a close matched set, changing them should be straightforward. Pretty sure I've got a q-3 around. If so, install it let you know. Thanks
Alright. For easy and clean rework consider a desoldering tool if you have not mastered any other technique. Such tools always come handy for other projects as well. If Hakko FR300 portable gun does not seem a good investment there is the cheaper Aoyue 474A++ station to consider. Amazon has both.
I'll take a look at those; but I've had lots of practice fixing my many, many mistakes. I usually cut cut up the old component until I can grab a single leg with forceps. Then heat and pull each leg, then remove most of the old solder with braid, then put a probe into the hole and let the solder set, then solder-in the new component. But I've heard very experienced people recommend desoldering tools too. Thanks for the tip.
With no signal to the circuits, there is no dc voltage across r-7 in either board. With a 1k hz signal, both boards show the same: about 0.5 vdc and 0.5 vac.
Your comments about de soldering tools are beginning to take root. The Hakko seems much the better choice. I always regret buying inferior tools as I always have to replace them later.
Anyway, is the next step to replace Q-3?
Your comments about de soldering tools are beginning to take root. The Hakko seems much the better choice. I always regret buying inferior tools as I always have to replace them later.
Anyway, is the next step to replace Q-3?
Reading about 0.2 mvdc on both boards.
Here's a question: given what we know, is there much chance that replacing Q-3 will be the fix? Here's why I ask that: I've got a replacement Q-3 on hand and I could install that in less than an hour. It's going to take a week to get new Q1 and Q2 replacements; and, during that week I'll probably spring for the Hakko de soldering tool. If the odds are that I'll have to replace Q& Q2, I'll probably just wait a week and do everything at once.
Here's a question: given what we know, is there much chance that replacing Q-3 will be the fix? Here's why I ask that: I've got a replacement Q-3 on hand and I could install that in less than an hour. It's going to take a week to get new Q1 and Q2 replacements; and, during that week I'll probably spring for the Hakko de soldering tool. If the odds are that I'll have to replace Q& Q2, I'll probably just wait a week and do everything at once.