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B&K ST-202 Plus :: hum
B&K ST-202 Plus :: hum
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Old 7th January 2019, 06:29 AM   #341
kenwood61 is offline kenwood61  United States
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Mark, you make a really good point. The original intention of this thread was to sort out a bad hum, which turns out was a loose fuse holder clip on the ps filter board. That was done, and it made sense to simply replace the four electrolytic caps and two trimmers on each output board. Some of the trimmers are out of spec.

If I knew for certain that I'd keep the amp a long time, or long enough I didn't care about resale value, I'd move forward with mods. I'll sleep on it, but at the very least, I'll order the caps and trimmers tomorrow. I appreciate you reeling it in for me, and of course to KC, and phase, your help has given me some things to think about.
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Old 7th January 2019, 12:11 PM   #342
phase is offline phase  United States
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If you really care only about resale value, you wouldn’t be spending a dime on parts at this point, especially with the possibility of selling in the near future. You could just put the parts back in and it should likely continue to operate for years to come.


I just took apart a 40 year old amplifier which has never missed a beat, and replaced nearly all of the electrolytic parts inside. All the old parts measured fine for advertised values. I did observe the circuit while doing this but it didn’t change anything I was planning. I have realized that after some 20 years of ownership that I will probably have this one around for a while and should make the move to improve it a bit so it is nice to listen to as a back up.
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Old 7th January 2019, 04:35 PM   #343
kenwood61 is offline kenwood61  United States
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Not necessarily. Three of the four trimmers I removed are way out of spec and need to be replaced. The electrolytic caps I pulled don't all measure in spec either. If I simply replace with new caps, same values ... from my experience, does not reduce resale value. I've sold amps and integrated amps that were recapped for more than if they were stock ... granted those buyers are people who don't want to do the work themselves, or know they can't do it.

Answering an earlier question about what kind of solder iron I have, it's a nice Hakko. I splurged on my solder iron once I knew I'd be needing one a few times a year. It's the best piece of repair equipment I have and just put new tip and sleeve on it. My soldering skills are above average and I've had a couple techs tell me they were impressed with my work. So assuming those veterans opinions matter, I'm comfortable with that aspect of repair. It's my knowledge of circuitry and function of components that is lacking. I rarely have projects like this, so what I learn during the project doesn't stick around in my 57 year old brain like it would have 20 years ago. Unfortunately, I find that unless I'm doing something I learn on a regular basis, I can't remember most of it. That's why I really appreciate forums like this with people who are knowledgeable and generous with their time!!

In stock form, I loved the sound. This is the only reason I decided to recap. At this point, it seems the best approach will be to replace parts at spec, with higher voltage values (as posted already), and keep it simple. I'll leave the ps caps for now and report back when all is together.

If I can't find an 8A 125V or 250v fuse for the AC holder on the back panel, is there any other values that you can suggest? The auto store has a 7.5 125/250v fuse, which I assume is close enough.

Last edited by kenwood61; 7th January 2019 at 04:42 PM.
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Old 7th January 2019, 08:14 PM   #344
kenwood61 is offline kenwood61  United States
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Ordered parts. Should be in a couple days and will post pics and update when done.
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Old 11th January 2019, 09:14 PM   #345
kenwood61 is offline kenwood61  United States
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Parts are here. Problem on trimmers.

The trimmer legs fit perfect into the board. I don't have to move them. However, the trimmers aren't as tall as the original Piher trimmers, which are Amphenol Piher now. One trimmer is okay and I can reach the trimmer tool to adjust. But the P2 (DC offset) trimmer adjustment screw butts into the BJT transistor, and that's without the heat sink on it.

So now I find myself needing taller trimmers and wondering if I should just buy the Piher style, instead of the Bourns??

The trimmers in the pics are just loose fit, not soldered.
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Old 11th January 2019, 10:25 PM   #346
kenwood61 is offline kenwood61  United States
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I think this may be the unit I need: Bourns 3386X 3/8" 500 ohm side adjust, stands about 10mm.

pin config: 321 3386X-1-501LF Bourns | Mouser

pin config: 123 3386H-1-501LF Bourns | Mouser

I assume that since the original Piher trimmers turned CW to increase value, that I need 123 pinout (3386H).

Last edited by kenwood61; 11th January 2019 at 10:48 PM.
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Old 11th January 2019, 11:22 PM   #347
huggygood is offline huggygood  France
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yes or bourns vertical monts
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Old 11th January 2019, 11:23 PM   #348
huggygood is offline huggygood  France
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Old 11th January 2019, 11:38 PM   #349
kenwood61 is offline kenwood61  United States
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I can't use vertical with my adjustment tool ... I'd have to get something else due to space to get tool down into. Thanks for confirming. I'll go order those so I can get this thing back together.
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Old 12th January 2019, 03:20 AM   #350
kenwood61 is offline kenwood61  United States
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I don't know what I was thinking ... the trimpots are on the solder side of the boards ... but there is still the fuse/fuse holder to contend with. I can use the trimpots I have, but they are too small and require positioning at an angle in order to get the adjustment tool in. I'm going to order the larger trimpots. They'll be at least equal to the Piher single turn that were in there, if not better. I'm guessing better based off my experience with Bourns.
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