Putting caps between heatsinks like that is a really bad idea. Heat and age likely caused cap failure. You can test the caps if you pull them. Likely at least one on each phase given the browning is equal on both + and - rails. Good news is the preamp is likely fine.
I'd fab a new board. You can probably find a linear board on ebay and stick it in the same box. I'd get a new xformer if you do that as well. This one is cooked.
I'd fab a new board. You can probably find a linear board on ebay and stick it in the same box. I'd get a new xformer if you do that as well. This one is cooked.
You could du far better.
Leave out the toroid and the ruined PCB. That should get you ample space for a High Quality 100W dual, adjustable SMPS.
Adjust it to +/- 21volts and you´re homefree. 👍 😎
Second Nareshbrd suggestion, but stick with a linear supply. Look for a power-one HAA-24; or HBB or HCC (depending on your current necessities. Have bought many of these for projects - used on the Bay - for say $25. NEVER had a problem with one but replaced the caps anyway.
You can get
You can get
I might just to verify the pre itself has no issues hook the pre up to a variable bench supply and see if the current draw makes sense.
The power supply was actually still working before I started pulling components. Voltages were fine. I'm kicking myself for not checking the current draw at that time, and now I can't because I don't have a bench power supply. But seeing the preamp board's condition I doubt it was overheating and if it was, it probably hadn't been for long. Here are pics
Makes sense, sounds tempting... But I just can't bring myself to stuff a switching mode PS in a fine, single-ended Class A Krell preamp. I would be too afraid Audiophiles find out where I live 😂
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I am seriously considering this, but since realistically I am not going to etch my own PCB at home I've been looking at cloning the old PCB.
I have a hi rez flatbed scanner. I'm thinking of creating a new layout identical to the old one in software like Kicad / Sprint Layout / EasyEDA using scans of the old board as templates, generating Gerbers for copper and silkscreen and sending it out to be made.
Anyone have experience with this kind of process?
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Next on my list is to (a) carefully document the old PCB, then (b) depopulate it, (c) test all components out of circuit, and (d) determine the cause of overheating.
(which could be as simple as the previous owner leaving left it on for 15 years straight in the back of a cabinet with stuff stacked on top of it. Neither preamp nor PS have a power switch. They're meant to be left on 24/7)
Then, as I see it, I have the following options, in no particular order (feel free to add / edit):
1. Clone old PCB as mentioned in my previous post, rebuild with identical components, reinstall in old case
2. Get new linear PS from AliExpress / eBay and work it into the old case
3. Same as 2. but in new case
4. Assemble old components point to point per the old schematic (which I don't have) and toss the PCB. This would likely have to be housed in a new case.
5. JB-Weld ripped traces and eyelets and stuff back on to the old PCB, repopulate it and call it good!
I want to do the preamp justice and keep it as original as possible. Also, resale value. #1 and #5 win.
But I like easy, straightforward solutions too. #2 and #3 win (esp #3)
Finally #4 doesn't really have any value. I just like to mess with point-to-point 🙂
Anything I am missing?
Thank you all for the great feedback, please keep it coming!
(which could be as simple as the previous owner leaving left it on for 15 years straight in the back of a cabinet with stuff stacked on top of it. Neither preamp nor PS have a power switch. They're meant to be left on 24/7)
Then, as I see it, I have the following options, in no particular order (feel free to add / edit):
1. Clone old PCB as mentioned in my previous post, rebuild with identical components, reinstall in old case
2. Get new linear PS from AliExpress / eBay and work it into the old case
3. Same as 2. but in new case
4. Assemble old components point to point per the old schematic (which I don't have) and toss the PCB. This would likely have to be housed in a new case.
5. JB-Weld ripped traces and eyelets and stuff back on to the old PCB, repopulate it and call it good!
I want to do the preamp justice and keep it as original as possible. Also, resale value. #1 and #5 win.
But I like easy, straightforward solutions too. #2 and #3 win (esp #3)
Finally #4 doesn't really have any value. I just like to mess with point-to-point 🙂
Anything I am missing?
Thank you all for the great feedback, please keep it coming!
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Just fab a board at JLPCB. If you excluding mounting the transformer on a board, that reduces the board size a lot.
Looks like more damaged devices on the board. The main board can be saved though.
Looks like more damaged devices on the board. The main board can be saved though.
The old PCB is 196 x 146 mm. I would lean towards keeping it the same size because (a) the transformer's side of the PCB supports a few components and related circuitry, and (b) if I'm going to go the clone route I would like the new PCB looking like the original in every way possible. My only change would be for a DB-3 output connector instead of the DB-9.
According to JLCPCB's online calculator, the minimum five-board order would cost $67 plus $31 shipping (2 oz copper and 2 mm thick FR-4).
So, $100 all in all. Not bad actually.
According to JLCPCB's online calculator, the minimum five-board order would cost $67 plus $31 shipping (2 oz copper and 2 mm thick FR-4).
So, $100 all in all. Not bad actually.
And bla bla bla 🤣 🤣
Long ago, one of my friends now living i Bruxelles bought himself a Linn LP12, a Linn Kairn Preamp, and I built him 2 monoblocks
to run his Martin Logan speakers. He has for Years been upgrading his LP-12,
and funny enough...... One of Linn´s first, big "upgrades to the Kairn was........ To replace the traditional PSU with a SMPS one.
I advised against it, but he did it anyway............ And guess what?? It actually improced the sound (at least acoording to him)
and he´s still happy about the upgrade.
Isn´t it??🤣😎😎
5: NO GO. As I told you earlier, the "blackened" fiberboard starts acting as a resistance thus adding to the heat.
Given time it will turn to zero ohms. Search for similar threads in here. Not long ago, someone was in the same "boat" as you.
Also Krell as far as I remember. PCB was glowing constantly, when the amp waa turned on, PCB acting as a 1-2 resistor
straight between supply voltages.😉
I was not actually considering doing #5, but it does help me in my process if I put every possibility on the table. Call it thinking out loud 🙂
A glowing PCB, that's kind of scary actually. Now I have to google pictures!
Forgot:
Try your DMM in auto-ohm settting, and poke the black part of the PCB with your probes. 20mm apart, 10mm apart, 5 mm apart.
If your DMM shows anything at all, you understand the NO GO.
Imagine putting 100K, 50K or 30Kohm between every component leg on a power supply, and guess what happens 😎
And then remember, that this is an accellerating process, with the resistance value going lower and lower over time.
Try your DMM in auto-ohm settting, and poke the black part of the PCB with your probes. 20mm apart, 10mm apart, 5 mm apart.
If your DMM shows anything at all, you understand the NO GO.
Imagine putting 100K, 50K or 30Kohm between every component leg on a power supply, and guess what happens 😎
And then remember, that this is an accellerating process, with the resistance value going lower and lower over time.
The pre amp main PCB also has blackened areas, could be from the photo angle, but needs inspection.
If that is damaged, I would have to be taking a big decision about repairing the PSU, or tossing the unit.
If that is damaged, I would have to be taking a big decision about repairing the PSU, or tossing the unit.
I had done this before, but I did it again more thoroughly. No PCB conduction is present, even poking pretty hard into the worst-looking areas of the PCB, even with probes spaced 1 mm apart.
I did the same test on the preamp board and found no PCB conductivity either.
You are correct, the preamp's main PCB does have areas of discoloration.
These discolored areas are all located under power transistor heatsinks. They appear to be the product of prolonged heat exposure sustained over years of normal operation rather than acute electrical / component failure.
After the power supply works again, I will be monitoring component temperatures on the main preamp board and troubleshooting / repairing any issues found, something I intended to do regardless (I think I've just half-talked myself into getting a thermal imaging camera 🙂)
To put things in perspective, this unit was sold new in 1991 or thereabouts I reckon. It is, as mentioned above, a single-ended Class A device which runs full throttle the instant it's plugged into AC power. It does not have a power switch. Whether you're playing music loud, soft, or not at all, it runs at the same temperature. Conceivably, I say conceivably, this unit could have been plugged in in 1991 and running full bore nonstop for 34 years, until I recently acquired it.
I am not worried about the main preamp PCB. I do intend to monitor operating temperatures though, and act accordingly as described above.
I take it that by "damaged" you mean electrically compromised / degraded, and / or unsafe.
You bring up an interesting philosophical question though. When should repair efforts end?
I'm asking because I'm kind of surprised to see tossing a fine piece of vintage equipment in the trash being casually mentioned on a forum whose name is, after all, diyaudio.
Unless I'm misunderstanding something, and if so I apologize, it seems to me that the focus here should be on keeping gear alive and going and in good nick for the next 10 or 20 or 30 years. Isn't that why most folks come here?
Hope this is not coming across as confrontational. I am curious is all.
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I kind of like the "philosophical" approach, the thinking about the way one should handle such equipment. I've been yet through this repeatedly. One factor to consider is that, even decent manufacturer wouldn't survive without cutting a corner here or there. It takes good deal of R&D engineering to make things functional and keep the cost well within limit. That being said, I don't necessarily see an attempt to upgrade existing equipment as a sacrilege.
Ouch. a buddy of mine had one of his 2 space heaters go down. I had not worked on a Krell before, but as an old designer and builder, I must say they really over-killed the design, regardless of the performance. I am curious as to what they did to offset the tremendous amount of noise from all the thermal generation in the massively parallel circuitry. It took many hours to track down the failures (multiple). I told him how much I would have to charge to fix it. He said just keep it... Makes a nice anchor for a sun umbrella, though. The best amplifiers I ever designed put out about 250W at 8 Ohms with about 0.05%THD 20-20kHz, using quad-parallel output power transistor sets with individually tuned base bias. But, they did not put out a lot of heat in operation.
It is a practical decision, my gas stove was bought new in 1970, most of my amps are pre 2000 🙂
I have reported on the repair thread here about the non-audio items that have been repaired by me or at my instance, India does not have the Western throw away culture.
The power supply had a catastrophic failure, and it is stupid to put caps next to heat sinks. That makes me question the quality of the overall build, maybe it was not kept in a ventilated place.
Unless you find what caused the transformer to fail, it is speculation.
If you do find the fault, the parts needed may be difficult to find.
And that eyelet on PCB construction is unusual.
So I am looking at a cranky design from a maker that did not sell in large volumes, who knows what design they used (Sony was famous for that in TVs).
So finding schematics, checking each part, replacing if needed, end result is a long and tedious restoration.
From the PCB, it has run hot, and thermally degraded parts can cause all sorts of circuit problems, it frequently happens that the PSU levels go high or low, and the current will obviously go the other way.
So, bear this in mind when you think about repair, the parts on the main PCB are degraded as well.
And in the end, the audio quality after repair is to be considered.
Personally, I think keeping equipment on when not in use is wasteful, and inviting damage from voltage spikes, which do occur here in India.
A waste of limited energy from fossil fuels, they will run out one day, faster if you waste energy like that.
I have reported on the repair thread here about the non-audio items that have been repaired by me or at my instance, India does not have the Western throw away culture.
The power supply had a catastrophic failure, and it is stupid to put caps next to heat sinks. That makes me question the quality of the overall build, maybe it was not kept in a ventilated place.
Unless you find what caused the transformer to fail, it is speculation.
If you do find the fault, the parts needed may be difficult to find.
And that eyelet on PCB construction is unusual.
So I am looking at a cranky design from a maker that did not sell in large volumes, who knows what design they used (Sony was famous for that in TVs).
So finding schematics, checking each part, replacing if needed, end result is a long and tedious restoration.
From the PCB, it has run hot, and thermally degraded parts can cause all sorts of circuit problems, it frequently happens that the PSU levels go high or low, and the current will obviously go the other way.
So, bear this in mind when you think about repair, the parts on the main PCB are degraded as well.
And in the end, the audio quality after repair is to be considered.
Personally, I think keeping equipment on when not in use is wasteful, and inviting damage from voltage spikes, which do occur here in India.
A waste of limited energy from fossil fuels, they will run out one day, faster if you waste energy like that.