Hello again, everyone. I need the help of some experienced or semi experienced amp techs or even tinkerer’s in helping me find a suitable Hexfred upgrade from basic cheap silicone diodes found in my B&K ST-140 (the very first production run with the massive EI core transformer- very rare at this point, only one I’ve seen without the toroidal transformer).
Interestingly, I was also under the impression that the Hitachi lateral MOSFETs that were supplied in these, before they switched to Toshiba MOSFETs later on in the early 90s, were the 2SJ50 & 2SK135 complimentary outputs. However, this particular ST-140 has the 2SJ56 & 2SK176 complimentary pair in each channel, which I’ve never once seen mentioned on any audio forum.
I’ll post the schematic that ATI who bought the rights to B&K Components provided to me, but keep in mind that what you see is from a 1989 model with some minor differences (& it oddly forgot to include the feedback loop 47uF caps), however there shouldn’t be any problems for the purpose of this particular inquiry. Ill go ahead and include a picture of one of the channels board.
Thanks for any recommendations, in advance. It means a lot to a newbie like me trying to soak up as much circuitry knowledge and philosophy as I read, study & learn from articles, forums, and helpful techs with real experience like many of you! 
Do you have any specifications for the diodes you want? I have asked all of our techs internally to take a look at your post in the meantime to see if one of them has an answer for you.
No specifications. Hopefully they can take a look at that schematic and be able to identify what the equivalent to the listed silicone diodes are as far as required specs & product attributes needed.
I will say, however, that I did read a post on an audio forum where someone else recommended (to a different member) 8A Hexfred’s and I believe they mentioned 4 per channel was the number of them needed, but I’m hoping someone can verify these numbers or make a better suggestion if they feel that’s not correct on what the circuit calls for.
Other than that, my only other preference is just make sure they’re the best ones you guys carry in stock;) the ultrafast-est softest recovery ones that’ll sound the smoothest in a stereo power amplifier haha…I’ve heard the Vishay ones are good? You guys tell me!! :))
I really appreciate the prompt response too btw. You guys are awesome. Whatever we all can decide on I’m gonna order them with a few Elna Silmic 100uF 100V caps I need to grab & maybe a few new trim pots for the boards as well.
Thanks again!
- Clay
I see only one diode in the posted schematic, and it’s described only as an LED. Those exhibit significant variation device-to-device, with temperature, forward current, and probably also on how one’s paternal great grandmother preferred their coffee… That the design uses a single such device as a shared reference for both 1st and 2nd stage current sources in about the most simplistic manner possible suggests that this design was very much about saving pennies.
As such, it seems likely that the cheapest available rectifiers were chosen for the not-shown power supply section; perhaps they splurged on a fast-recovery type, but I’d not be surprised if standard were used as well. It has not been stated that these are demonstrably broken; if they are not, why replace them?
Be advised that audio takes a close second to free energy/perpetual motion in terms of being an interest niche where the quality of information in circulation is (ahem) variable.
I’ll include some more photos that were taken. The reason for the change is Hexfred’s always subjectively equate to to a smoother sound as far as causing less fatigue when played at higher levels, especially. Now as far as objective reasoning behind that desired result, supposedly it’s a result of them being much faster than the cheap silicones used back in these times of production. However, I can’t give you an answer I can claim as fact as to why they subjectively sound better.
All I can tell you is that it was mentioned by several people they had great results upgrading to Hexfred’s in this particular amp model. One of those people shared that they used 8A Hexfred’s & there were 4 per channel used. That’s where I’m wanting help & verification in choosing the appropriate parts. Whether or not you recommend me some Hexfred’s that will be just suitable vs some that are great performance but may be slightly overkill for this application is entirely up to you. (I suppose if anything, include both)
Thank you, again, for your patience in putting up with my novice skill set!
- Clay
For some of the items/ images
Would you be able to mark the images it indicate the items you are looking at in the image?
Or close up images of the specific item with the marking. As some of the transistors markings are blured or not shown.
Anything involved with the bridge rectifier & switching to Hexfred’s instead of the silicone parts that make up the rectifier. This is the left channel board. The only other board in the amp besides the right channel is the fuse board that mounts on top of the two old 15,600uF (being upgraded in capacitance) main filter caps
Looking over a couple of the items shown (example FPN4889).
These are transistors (ie PNP or NPN) and from what I can find HEXFRED were either IGBTs or Diodes.
IGBTs | Transistors | Electronic Components Distributor DigiKey
For the J56 and K176 do you happen to have the data-sheet for it?
If not would you know the electrical specs for them?
It would be for the little black silicone diodes I believe. I’ll post the data sheet for j56/k176 in another reply
2SJ56.pdf (515.2 KB)
2SK176.pdf (387.0 KB)
Hello clayjwright94,
I’m sorry, but we do not have direct crosses for the J56 or the K176 Mosfets.
I apologize. I thought this picture above had been included with the first few photos I posted. Apparently it didn’t upload. I suppose that might be helpful to see in order to give me the answer I’m seeking lol whoops
Hi clayjwright94 ,
This looks like a bridge rectifier. Please review datasheet for dimensions and specifications. You may want to select higher current and voltage ratings to be on the safe side if you are unsure what they are.
The datasheet for FPI4020W is pretty easy to find. The important specs here:
The trouble is going to be finding a bridge rectified that has an offset pin like your example.
Looks like Digi-Key PN: MP502W-BPMS-ND has similar specifications/dimensions.
No need to worry about any offset pins or really much at all as far as it’s physical package is concerned. It’s no longer going to be mounted on a pcb board. I’m only utilizing the center screw hole for mounting it + direct wire-to-pin soldered. I’m just interested in finding a hexfred of some sort that I can use and mount in a similar fashion.
Thank you for the replies thus far!
-Clay
HEXFRED® branded diodes can be found here. You’ll need 4. None are going to fit particularly well because they’re intended for an entirely different sort of use.
Also, since their Vf is roughly double that of those typically used for this application, supplemental heat sinking may be necessary.
AIUI, It is claimed by some that HEXFRED®’s in a power supply impart sonic qualities that are not measurable but are greatly desired by many audiophiles.
I’m a, if it ain’t measurable it don’t exist, type electronics engineer myself.