Thank you everyone for the great posts!
My next step is to properly test and audition the preamp proper, so as to make sure it does not have major problems. I don't think it does, but I'd rather be sure. Think of it as the go / no go checkpoint.
I've got a new bench PS coming that can deliver +/-21V.
Assuming no horrors are found and the main preamp board checks out, my next step will be to download Kicad and draw the schematic for the PS, which has only 41 components including connectors etc in it, so hopefully I'll be able to manage 🙂
My next step is to properly test and audition the preamp proper, so as to make sure it does not have major problems. I don't think it does, but I'd rather be sure. Think of it as the go / no go checkpoint.
I've got a new bench PS coming that can deliver +/-21V.
Assuming no horrors are found and the main preamp board checks out, my next step will be to download Kicad and draw the schematic for the PS, which has only 41 components including connectors etc in it, so hopefully I'll be able to manage 🙂
Once that bench supply is up and running please post the current draw for the rails.
Also, looking at the main board pix it appears those heatsinks were quite hot, too. Are you able to have a look at the semis on those heatsinks and post what they are? I’m curious about what’s a reg vs. signal/preamp semi and part numbers.
Also, looking at the main board pix it appears those heatsinks were quite hot, too. Are you able to have a look at the semis on those heatsinks and post what they are? I’m curious about what’s a reg vs. signal/preamp semi and part numbers.
I did a nut and bolt rebuild of a ksp-7b and the ksa 200 mk1 about ten years ago. The psu on the pre runs hot as standard, much more so when the caps are 20 years plus old and those ROE phenolic caps were all measured at a couple of ohms esr. Looking at yours I'd just lay out a new pcb. The material used on those boards is just crap. It goes like cardboard.
https://www.flickr.com/photos/59684834@N08/albums/72157629656297809/
I recapped the entire psu and all the lytics on the pre amp board. Added extra ventilation under the psu case and drilled additional holes through the pcb around the regs as otherwise the air just flows around the pcb and out the vents on the case top.
The ksa 200 needed a few new output devices, couple of driver boards needed rebuilding and I recapped the whole amp. About £500 in parts, ten years back.
I ended up remounting every output device, new paste and pads, new bridge rectifier, all new wires and replaced every part on the soft start board.
https://www.flickr.com/photos/59684834@N08/albums/72157634946321928/with/9598289153
Good luck, the pre is worth the effort.
https://www.flickr.com/photos/59684834@N08/albums/72157629656297809/
I recapped the entire psu and all the lytics on the pre amp board. Added extra ventilation under the psu case and drilled additional holes through the pcb around the regs as otherwise the air just flows around the pcb and out the vents on the case top.
The ksa 200 needed a few new output devices, couple of driver boards needed rebuilding and I recapped the whole amp. About £500 in parts, ten years back.
I ended up remounting every output device, new paste and pads, new bridge rectifier, all new wires and replaced every part on the soft start board.
https://www.flickr.com/photos/59684834@N08/albums/72157634946321928/with/9598289153
Good luck, the pre is worth the effort.
Every heatsink-mounted transistor on both boards (main and power supply) is either a MJF15030 or MJF15031.
DigiKey stocks them as MJF15030G and MJF15031G. G variants are lead-free.
About transistors, It was interesting (at least to me) that the 8 transistors in the power supply were bolted dry to their heatsinks - no thermal grease or pad.
MJF15030/31 are plastic-bodied, electrically-isolated TO221D packages. I thought maybe that's why no grease is necessary, although that doesn't really make logical sense.
The data sheet says nothing about grease. It refers the reader to "APPLICATION NOTE AN1040/D - Mounting Considerations For Power Semiconductors" (attached below), which does discuss plastic packages mounting starting Pg 13. It states:
On Pg 15, illustrations depict the various mounting methods. Neither Fig 26 (screw-mounted) nor Fig 27 (clip-mounted) show any thermal grease or pad being used, despite the tact that the text quoted above explicitly requires it at least in the case of clip-mounting.
So, should or shouldn't thermal grease be used with those plastic packages? I'm totally confused.
MJF15030/31 are plastic-bodied, electrically-isolated TO221D packages. I thought maybe that's why no grease is necessary, although that doesn't really make logical sense.
The data sheet says nothing about grease. It refers the reader to "APPLICATION NOTE AN1040/D - Mounting Considerations For Power Semiconductors" (attached below), which does discuss plastic packages mounting starting Pg 13. It states:
Does this mean grease is required when clip-mounting but not when screw-mounting...?
On Pg 15, illustrations depict the various mounting methods. Neither Fig 26 (screw-mounted) nor Fig 27 (clip-mounted) show any thermal grease or pad being used, despite the tact that the text quoted above explicitly requires it at least in the case of clip-mounting.
So, should or shouldn't thermal grease be used with those plastic packages? I'm totally confused.
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Power supply in a bowl
Just wanted to give the Hakko desoldering gun a shoutout here. Sure helped make short work of things! Just clean it often though, or be prepared to bust out the propane torch 🙂
Just wanted to give the Hakko desoldering gun a shoutout here. Sure helped make short work of things! Just clean it often though, or be prepared to bust out the propane torch 🙂
Thermal grease is always required to fill the voids and reduce the thermal resistance.
The heat sink, plastic package, and metal tab package are not machined and will have gaps.
There are numerous PS gods 😉 that frequent these forums. Mark Johnson, Elvee, and Trileru, just to name a few. If you could find your way into their good graces you might end up with a power supply that is an improvement over the original.
Regards,
Dan
Regards,
Dan
I would check the main board thoroughly, before making the effort to build a new supply.
A slightly lower rail voltage, +/- 18V instead of +/- 21V may reduce the heat build up.
Is it worth checking / salvaging the transformer after removal from the rest of the circuit?
Might be useful during testing, with a simple bridge, smoothing caps and a zener regulator?
A slightly lower rail voltage, +/- 18V instead of +/- 21V may reduce the heat build up.
Is it worth checking / salvaging the transformer after removal from the rest of the circuit?
Might be useful during testing, with a simple bridge, smoothing caps and a zener regulator?
Just sell it, it is cooked and EOL and had its useful life. You can do without it or go the simple setup of relay based input detection and relay based volume control with a simple neutral buffer. If you still use phono things get more complicated.
That PCB can be redrawn in an evening, the old one can not be reused. The mainboard also looks like it has cooked. But then the toroid and all electrolytic caps (certainly the notorious ROE ones) need replacement because of the heat and the age. Likely even the switches and semis have suffered from the apparently extreme heat (for whatever reasons/defects). In stuff becoming so absurdly hot I usually disable the phono stage to have less heat. It works.
If you master KiCad it is a nice challenge but please look at the costs and if it is worth it. The bones is that the heat transfer to the environment can be improved. Like adding many 6 mm ventilation holes in the PSU PCB, use larger heatsinks etc. If you, for instance, today only have an audio player and a CD player you may simplify your setup by omitting a preamp as you may not need much gain today. Don't stare too much at the brand name, everything turns to dust in time.
That PCB can be redrawn in an evening, the old one can not be reused. The mainboard also looks like it has cooked. But then the toroid and all electrolytic caps (certainly the notorious ROE ones) need replacement because of the heat and the age. Likely even the switches and semis have suffered from the apparently extreme heat (for whatever reasons/defects). In stuff becoming so absurdly hot I usually disable the phono stage to have less heat. It works.
If you master KiCad it is a nice challenge but please look at the costs and if it is worth it. The bones is that the heat transfer to the environment can be improved. Like adding many 6 mm ventilation holes in the PSU PCB, use larger heatsinks etc. If you, for instance, today only have an audio player and a CD player you may simplify your setup by omitting a preamp as you may not need much gain today. Don't stare too much at the brand name, everything turns to dust in time.
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Excellent. Please continue the adventure. Sometimes the cost is not entirely a determining factor. The learning curve can be enjoyable and almost fun. I guess the correct word for DIY is that such projects are, or could be, "rewarding". Most of the projects that are for me are NOT financially balanced. The biggest determining factor for me to build/rebuild something, is if I like it. It is good to be aware of time investment and money costs and diminishing returns. It is my opinion that some of the best equipment threads on this site are where people go over the top to build or repair a piece. Some/many of the people here are remarkable and outstanding. It is a privilege to share the sandbox with them. I adore the craftsmanship and I covet the pieces. Sorry for the ramble...
If you persevere in repairing it you definitely want a better and a lower losses PSU. You don’t need any PSU God, you also don’t need old LM317/LM337 as these have too high drop-out voltage translating to unnecessary heat, you need uLDO regulators that combine both factors and a lower voltage transformer. The path to less dissipation by the PSU.
With careful selection of transformer you probably can use a 2 x 18V one (a 2 x 20V type will work for sure), Schottky diodes and larger filter caps and uLDO ultra low noise regulators. Also many ventilation holes.
Less noise, less heat generated by the PSU. Win-win.
BTW I have done such repairs in the past but have never kept the device as it still was … an old device becoming hotter than my power amplifiers. No need for gain, no need for heat so a new device based on relays may be both superior and cheaper.
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