Hi, i have a boozehound phono pre I built a couple of years ago, been happy with it until yesterday when the left channel suddenly went quiet. I believe the problem is the jfet on the output stage.
My question is: can I replace just the bad jfet, or do I have to match it with either the input jfet or the output on the other channel?
Can’t recall what was in the original kit and Unfortunately boozhound seems to be out of business.
I believe the topology is similar or close to the la Pacific phono pre.
My question is: can I replace just the bad jfet, or do I have to match it with either the input jfet or the output on the other channel?
Can’t recall what was in the original kit and Unfortunately boozhound seems to be out of business.
I believe the topology is similar or close to the la Pacific phono pre.
Anyone have a clue of what is going on with my preamp? I changed the "bad jfet" but it didn't work out as expected. the reason I thought it was the transistor in the first place was when I measure Vdrop over R6 and R7 I get a lot more over R7 so it looks like the corresponding JFET is pulling a lot more current than it should. I get about 30mv over R6/R7 on the working channel and in the bad channel they are R6 30mv and R7 about 80mv so over twice as much.
I may also add that I changed all JFETS so IDSS is closely matched even the working channel, primarily because I didn't have any lsk170 so changed all to 2sk170.
I may also add that I changed all JFETS so IDSS is closely matched even the working channel, primarily because I didn't have any lsk170 so changed all to 2sk170.
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With other values as stated, this suggests R11 has 24V across it, and guessing a 240V supply (?) that makes about ZERO across Q2.
Is that what you got? Then Q2 is shorted; probably the wiring not the actual transistor. Otherwise you have a wiring or value error.
Measuring just one voltage drop is not much clue. I like to print the plan and use a fat crayon to mark most node voltages.
Attachments
Can’t find anything off with the resistor values, compared to the Working channel they’re the same. Solider joints also seems fine, nice and shiny.
Not sure what Vss is but if it’s something like voltage source, I run textbook lm317 set by fixed resistor.
Okay, things got a bit more interesting “found the problem” but haven’t solved it. Got tiered of the Psu/regulator board being in the way for proper space to work, so decided to remove it and just use a 15V Wall wart for the troubleshooting. To my surprise, Everything worked fine and I could not trigger the problem. I even tried playing music just to be sure and it works fine.
To keep it short: the problem reappears when going back to the “original” 24V PSU, I have tested this over and over and I’m certain it is the PSU that is making the problem.
Question now is: how is this possible? The 24V PSU seems to work fine, voltage is correct and steady, it should have no trouble providing enough current I think. (Running a Pass B1k on identical PSU)
To keep it short: the problem reappears when going back to the “original” 24V PSU, I have tested this over and over and I’m certain it is the PSU that is making the problem.
Question now is: how is this possible? The 24V PSU seems to work fine, voltage is correct and steady, it should have no trouble providing enough current I think. (Running a Pass B1k on identical PSU)
Disclaimer: I'm out of my league here, so what I'm about to say might not be feasible, but thought I'd share my thoughts in case it helps.
I'm noting your comments that the preamp works fine on the lower voltage but not on the higher. What components in the circuit are voltage-sensitive that could degrade, eg a regulator or a transistor?
I'm noting your comments that the preamp works fine on the lower voltage but not on the higher. What components in the circuit are voltage-sensitive that could degrade, eg a regulator or a transistor?
Or.... Russian paper in oil capacitors, just got it working properly again and it turned out to be the 0.1uf coupling cap between stages. The cap managed to block DC when voltage was low, but started conducting more and more with higher voltage. At 12V it blocked almost completely, but at 24V it was about a volt on JFET’s gate.
If I’m not mistaking, this makes the JFET turn on hard
Don’t know if I should credit the LSK170-2SK170 change or the bad PIO getting replaced with larger value PP(did both channels off course) but I believe the sound is a bit better than before and wonder if I should replace the rest of the Russian caps too..
If I’m not mistaking, this makes the JFET turn on hard
Don’t know if I should credit the LSK170-2SK170 change or the bad PIO getting replaced with larger value PP(did both channels off course) but I believe the sound is a bit better than before and wonder if I should replace the rest of the Russian caps too..