I was able to get a nice B&K ST-202, I think is time for a restoration, I noticed the mods by musical-design and they are expensive.
Can you guys reverse engineering those upgrades by musical-design ?
I think the same upgrades are applicable to the Hafler series of amps
https://www.musicaldesign.com/MC_BK_Amp_Mods.html
https://www.musicaldesign.com/Demo_Used.html
Or if you guys have a list for updates to the ST-202 that will work to
Can you guys reverse engineering those upgrades by musical-design ?
I think the same upgrades are applicable to the Hafler series of amps
https://www.musicaldesign.com/MC_BK_Amp_Mods.html
https://www.musicaldesign.com/Demo_Used.html
Or if you guys have a list for updates to the ST-202 that will work to
First use it for a few weeks and make sure there are no surprises.
Do repairs if needed, before anything else.
Do repairs if needed, before anything else.
The amp is working great, I connected to a Harman Kardon ap2500 preamp.
Keep in mind the amp came in 1993, is going to need some work
Keep in mind the amp came in 1993, is going to need some work
I'm using a 1975 HK receiver that I converted into a power amp by bypassing the preamp.
It has all the original 1975 parts. It works and sounds great.
I know this post was months ago, but I modded my ST-202+ and had subjectively good results. First, you need to know what version you have. If early 90's, it likely is the version with the DC servo circuit, which is the same as mine. Look on https://www.hifiengine.com for the correct schematic.
Changes I did are as follows:
1. Bypass main caps with 10 uF PP caps. I drilled holes in CB and soldered them in. If the main cap ESR is high, they should be replaced. You can also add capacitance, but would not go crazy as it will put more stress on the diode bridge. The original main caps have a huge +75% tolerance. I would not go over 150% rated capacitance just to be safe (23,400 uF) and bypass with the PP cap.
2. Replace 100uF electrolytic caps on input to 150-220 uF. I used Panasonic FC. Use caps that have the same diameter/lead spacing to keep leads as short as possible and allow the cap to sit flat on the CB.
3. Replace 25uF non-polar electrolytics. I used Nichicon Muse ES.
4. Added 33.2k resistor per revised B&K schematic. Don't know the reason for the change, but my version did not have it, so I added.
The above should get your amp to better than new. I modded further just because.
5. I changed out all the polyester coupling caps to PP caps (Vishay), but I don't know if this was necessary. I did notice that the original designer Steve Kaizer used PP coupling caps in more recent designs, so why not. The Vishay caps were also a bit larger and fit the CB better resulting in shorter leads. The Vishay caps were also made in the US which is fringe benefit, at least to me.
6. I also changed all the carbon resistors to metal film. This probably does nothing to the sound, as the resistors in the signal path are already metal films, but it can't hurt reliability and it looks cool.
After the changes, I noticed a few things. A quieter noise floor. A bit more heft to the bass. A bit more clarify. A success in my book.
Later, I may upgrade the ceramic wirewound resistors to a better wirewound. Again, I don't know if there is benefit, but I hate the fact that the white ceramic resistors have developed mold on them when the amp was just sitting around for years.
Changes I did are as follows:
1. Bypass main caps with 10 uF PP caps. I drilled holes in CB and soldered them in. If the main cap ESR is high, they should be replaced. You can also add capacitance, but would not go crazy as it will put more stress on the diode bridge. The original main caps have a huge +75% tolerance. I would not go over 150% rated capacitance just to be safe (23,400 uF) and bypass with the PP cap.
2. Replace 100uF electrolytic caps on input to 150-220 uF. I used Panasonic FC. Use caps that have the same diameter/lead spacing to keep leads as short as possible and allow the cap to sit flat on the CB.
3. Replace 25uF non-polar electrolytics. I used Nichicon Muse ES.
4. Added 33.2k resistor per revised B&K schematic. Don't know the reason for the change, but my version did not have it, so I added.
The above should get your amp to better than new. I modded further just because.
5. I changed out all the polyester coupling caps to PP caps (Vishay), but I don't know if this was necessary. I did notice that the original designer Steve Kaizer used PP coupling caps in more recent designs, so why not. The Vishay caps were also a bit larger and fit the CB better resulting in shorter leads. The Vishay caps were also made in the US which is fringe benefit, at least to me.
6. I also changed all the carbon resistors to metal film. This probably does nothing to the sound, as the resistors in the signal path are already metal films, but it can't hurt reliability and it looks cool.
After the changes, I noticed a few things. A quieter noise floor. A bit more heft to the bass. A bit more clarify. A success in my book.
Later, I may upgrade the ceramic wirewound resistors to a better wirewound. Again, I don't know if there is benefit, but I hate the fact that the white ceramic resistors have developed mold on them when the amp was just sitting around for years.