B&K EX-442 Rehab

[vsat88]

Starting a new thread for an EX-442 B&K components amplifier here. Still not done with the PL 400 but I have to take a break from that one because it is getting on my nerves. I have learned that I can set something aside over the years and come back to it when I feel like it. This rehab will take a while to complete as to start I will only ask for recommendations. After the recommendations come in I will make a parts order and start the rebab as well as include some HD photos of the amp as I go along : ) This amp is in great order right now. The Bias and DC offset have been set and she plays beautify..I just want to keep it that way for a long long time as this is one that I will keep from now on, just like my ST 70 these two are going nowhere.. I do not want to try and improve or "hot rod" this amp. I want it to sound basically the way it came from the factory and to be reliable and thats all. Oh, I have lucky enough to have had had Mstryker (hope I got that right) give me a few pointers already..1. Replace bridge rectifier 2. Replace all the thermal pads. 3. Go ahead and do this while replacing all the electrolytic capacitors. Looking forward to reply's, thanks.

 

[mjstriker5]

Received your message from the auction site. Here to lend a hand.

I source almost all my parts from Mouser. I think the best thing to start with is the schematic and take some pics of the inside of your unit and let us know the capacitor values you have:

https://www.hifiengine.com/manual_library/bk-components/ex-442.shtml

I know some of the output mosfets differ in type so it would be worth making sure before ordering the silicone thermal pads. Mouser doesn't give good pictures so I actually search other websites for the part number based on the output case type then put the number into Mouser. I prefer the Bergquist Silpad 900s series (pink in color).

I would get a 35A 1KV bridge rectifier. It will look just like the one that is in their but better in ratings.

For the electrolytics:

- The large "can" lytics are probably fine but if you want to do everything just replace with the same exact value. The most important thing is the voltage. Never get underrated voltages. You can go a little higher in capacitance but it won't make much of a difference. I usually just measure the dimensions and make sure they match up so you don't run into any issues with fitting them into the amp. If you let me know the dimensions and values I can give you some options.

- The board lytics are mostly going to be the polar ones. I use either Elna Silmic II of Nichicon FG. Just replace with exact values. For the small value ones (less than 1uf) I would use film (i.e. polypropylene). The 0.1uF caps can be replaced with WIMA caps. The real small ones (i.e. picofarad) can be replaced with silver Mica or Multi-layer ceramics depending on budget/preference.

Out of curiosity what is going on with the PL 400? Can't remember if we talked about this amp but I have a soft spot for it as it was my first amp and have repaired quite a few.

Good luck

 

[vsat88]

Let me upload a helper piece that was shared with me. This was a hard to find .PDF but was really cool to have. A spec list from one of the guys over at Club Polk. I will get the info above posted as soon as I can bring myself to pull the EX apart. Oh, I am glad to know its mjstriker (5) now, not mstryker !

 

[mjstriker5]

Cool find! If you notice the EX-442 comes in two different output versions- Hitachi and Toshiba. That's the question we need to answer to determine which thermal pads you need.

 

[vsat88]

Mine is the Hitachi as far as I know. It has the gold writing without any blue and red stripes. It does not say Sonata on the amp.

 

[vsat88]

Cool find! If you notice the EX-442 comes in two different output versions- Hitachi and Toshiba. That's the question we need to answer to determine which thermal pads you need.

These are the images I took while setting Bias and DC offset..Got the Pro 5 B&K pre-amp with it as well. That is a totally different story. I am not impressed with the sound of it even though it seems to have much more gain than the C2 Kenwood I am running. The C2 runs all over the Pro 5 sound quality wise..At least to my ear..The one I have has the upgraded gold plated RCA board on the back for inputs/outputs. Still a very nice pre amp.

 

[vsat88]

Somehow I quoted myself...I must like to hear myself talk..One more thing though. I have not noticed very many threads on here or elsewhere for that matter about B&K. They must last so long and sound so good no one wants anyone else to find out the secret. OK, I will keep it that way...Terrible stuff, move on right ?

 

[mjstriker5]

Ah hh the infamous kenwood C2. Don't talk too much about this or the B&K. I actually fixed up a C2 for my father. Very nice sounding pre.

Anyways, back to the B&K. It actually looks like you have toshiba outputs. Classic mosfets. Unfortunately difficult to come by. You need to take special care of these thus the reason to replace the thermal pads. Plus you have to remove them from the heatsink anyways to replace the electrolytics on the board.

Part number for the new pads: 951-SP900S-009-00104

Boards look just like the ST-202+ except the extra outputs, the power supply is different. Good thing is that working on this will get you ready to fix your other amp.

 

[mjstriker5]

I think the bridge rectifier is soldered in and this is the part I used: 512-GBPC3510W

For the board I would try and get a picture of it after it has been removed so you can determine all the capacitors that need to be replaced. I know a few of the bipolars are in the signal path and I used nichicon muse. One thing I would recommend is being careful removing the mosfets from the heatsink. They can be prett stuck to the heatsink and you don't want to bend the leads too much. Just take your time.

Really like the mosfet sound. Have a hafler p-500 that I picked up for $50. Unfortunately some of the mosfets are bad on one channel and they are like gold. Debating on getting a whole new set of Exicons for the amp. After hearing the B&K I am on a little bit of a mosfet binge,

 

[mjstriker5]

Watching some rugby and now I am rambling. One thing nice about these amps is they are very straightforward as easy to work on. I just finished repairing a harman kardon citation 7.1 and it is leaps and bounds more complicated with safet trips, negative feedback, and DC offset regulation. Quite the headache to find one resistor that was bad.

 

[vsat88]

Guess the same here "now I am rambling". I have to agree with them being easy to work on and straight forward. I was sitting here watching Coriolanus (again) thinking about the ST and what might be wrong there. I have done a good bit of troubleshooting so far. It was damaged in shipping I really did not know what to expect. I still have not found what needs fixed to make that thing work properly. Has a nice loud hum in one channel and a less loud but still clearly audible hum in the other. When I got it the case was bent in by the weight of the transformer due to a drop I suppose. I looked it over very closely and found no broken boards. Re did a few ground connections that were loose, one was very loose from the transformer to the star point ground. I found a few small value caps that did not test so hot and replaced them with known good stock (100V 100uF). I will never be able to bend the metal at the bottom of the amp back completely straight, too much damage and that makes the filter cans sit funny tweaking the main amp board. The damn thing is beautiful though. Very nice condition and as far as I can tell it has the Toshiba OPT as well...This **** haunts me sometimes...Guess I am just an Addict. Man, it is 2:30 AM here and it is taking all I can do not to head outside in this thunderstorm to start working with one of these amps.

 

[vsat88]

These images are of the ST 202 + not the EX-442 for anyone who may follow this thread....Check out the tilt in that transformer where the amp was dropped and bent...

 

[mjstriker5]

Too bad about the ST. Yours is different than mine. The main power supply board is much more extensive. Mine was literally just a bridge rectifier attached to the main caps. Looks like your's has some sort of protection built in or something but tough to tell.

One thing is it is a pain to troubleshoot as the board is upside down. Have to rig it "upside down" to check the parts or build a diagram so you know what is what when checking solder pads. Have you powered it up? I wouldn't or at least I would disconnect the output boards don't want to risk damaging those mosfets.

 

[vsat88]

Too bad about the ST. Yours is different than mine. The main power supply board is much more extensive. Mine was literally just a bridge rectifier attached to the main caps. Looks like your's has some sort of protection built in or something but tough to tell.

One thing is it is a pain to troubleshoot as the board is upside down. Have to rig it "upside down" to check the parts or build a diagram so you know what is what when checking solder pads. Have you powered it up? I wouldn't or at least I would disconnect the output boards don't want to risk damaging those mosfets.

Yes, I have fired it up. I had no reason to believe that there was any problem when I received it so best I remember I checked DC offset and turned her on. She played just fine but almost immediately I heard the noise in both channels. Put it aside for at least a month while trying to rectify the shipping deal after noticing the bent parts on the case. After that I wnt in and straightened the case as well as I could and that is when I checked everything I could without spending hours to check every detail however I was quite sure to check all the major parts such as broken boards. Loose connections.

 

[mjstriker5]

Well when you are ready to tackle that one let us know. I would invest in building a dim bulb tester. Mine has saved me countless headaches and most likely a lot of money.

 

[vsat88]

Well when you are ready to tackle that one let us know. I would invest in building a dim bulb tester. Mine has saved me countless headaches and most likely a lot of money.

Yep, built one a long time ago. I have basic stuff. An o scope , DBT, soldering station, a veriac, and a new signal generator. Two identical true rms meters that I love. Bench top models. Most of my equipment is pretty old but in good condition.

 

[mjstriker5]

Hope I didn't offend you. Hard to tell what people know over the internet. You have way more test stuff than I do and probably could teach me quite a few things. I get pretty excited thinking about fixing up old gear that has had some rough days. Looking forward to the journey with these awesome amps.

 

[vsat88]

Absolutely no offense at all. Fact is having the equipment and knowing how to use it are two completely different things. I can use most of the equipment but I do not have anywhere near the knowledge that I could have. I am definitely a hobbyist.

 

[vsat88]

Still kinda concentrating on the ST. I really want to know what is making that amp hum on each channel. I looked at it on the scope and I just do not see anything that looks bad by my eye . Ran a signal thru it on the generator at around 700 m to 1khz with two dummy loads. Compared the channes and looked individually. Just did not see anything out of the ordinary. I pulled it back apart and resoldered both grounds on the phone/RCA jack as they looked a bit crusty and found a few solder joints I did not like the looks of one the amp boards and re flowed them as well. I am going to need another amp to use while I tear down the EX. I do have a Hafler 220 and of course the Dynaco I could use for while while tear down the EX but I want this ST off the bench.

 

[mjstriker5]

Hum can be a real pain. If it is equal in both channels then I would focus on power supply and grounding issues. Have you replaced any of the electrolytics in the power supply? Double check the grounding on the power supply board and make sure the molex connectors are making a good connection.

Do you know the hum frequency? Does the hum change with increased volume?

I doubt it would matter but maybe try disconnecting one channel and see if it changes the hum.