B&K EX-442 Rehab

Not equal in both and that is why I was sort of thinking the the PS section might not be the trouble. Audibly louder in one than the other. I have not tested it since I did the last bit of work today but I will check those molex connectors as you said before I put it back together for the next test. I did replace 4 lytics on the PS board. There were only 6 total on that board (lytics). There were two non polar caps there and I did not have those in the shop so either way when I do my parts order they will be replaced. I do not really know how to measure the hum frequency. I will check about it increasing as power increases tomorrow as well. I am also going to try disconnecting a channel and seeing if that changes it. Thank you for the suggestions as I would really like to try and fix this one and use it while I restore the EX. I would like to get it repaired for another reason. It was damaged in shipping when a friend sent it to me and I would like to get back to him and let him know what the trouble was. That way we can finish our transaction. Thanks a million for all your help.
 
One thing I didn't realize before is that you looked at the signal with a scope. If you are hearing hum you should be able to visualize this on the scope. Sometimes if the hum is really low it can be tough to visualize if it doesn't trigger the scope. My knowledge of scopes is limited so I don't know too much on how to set it up for this situation.

Is the hum really audible? Is it there without a signal? If so you should be able to track down its origination if it is due to a faulty part. Ground issues will be tougher cause it can inject itself all over. A few times I've attached a wire to various ground points to help narrow down the problem.
 
OK, here we go again. Went out there and took it down some information. With both R and L RCA's plugged in it is very loud on the R channel and loud on the left.
With only the R RCA plugged in I get a soft buzz in both channels.
With only the L RCA plugged in I get a loud buzz in R and L speakers, the Right being the loudest of the two.
With both R and L RCA not plugged in the R has the buzz louder than the L. The L (when both RCA jacks are unplugged) is pretty quiet. Quiet enough that you have to put your ear up to the speaker to hear it.

So, that said I decided to probe the amp. This is what I did. I turned it on hooked to two old speakers and both RCA's plugged in from the B&K Pro 5 pre amp and took a bamboo chopstick (one of my favorite tools) and lightly moved around the wires and pushed on the boards. No changes whatsoever. I decided to try putting some pressure on the transformer so I used a hammer...No, not that end ! I used the fiberglass and rubber end and the hammer steel part in hand to press on it. I may have found some issue there. It seems to be the only place that I pressed that changed anything at all. I have not went much farther than that. I still do not see how this could be a PS issue but I am no expert. I suppose the transformer could well have been damaged when it was dropped.
 
I do not know enough off hand about testing toroidal transformers but I do recall several posts here on diyaudio that discuss it. I still think you have some sort of power supply and/or grounding issue. Make some RCA shorting plugs and see if that changes anything. I still think you should try disconnecting the molex connectors powering the individual channels to see how that changes the hum. Don't know if it will make much of a difference but worth a try.

I would also check you are getting a good ground on the star ground and any other point on the chassis. Might be another spot for you to "tap" to see if it changes anything. Another thing to try is putting the scope on the power supply.

Do you get any audio out of either channel?
 
I really respect these amps and am surprised the 442s haven't climbed in value compared to some other amps that are comparable in build quality and sound. While I'm downsizing my collection (hoard), and the 442s at 200+ wpc are very much overkill for my personal needs, I find it extremely hard to come to terms with parting with them.

I bought a Conrad Johnson MF2250 and while it is a nice amp, the B&Ks are hot on it's heels in sonics, and the B&Ks are more powerful.

Oh- and pretty straightforward intestines- no wild surprises in layout or design.

I have one of the later (early) Sonata versions, and two of the (late) original gold imprint versions. The Sonata is still working perfectly, the gold imprint versions are now due for a re-cap as they are both starting to get a bit noisy.

I'll be following your progress....
 
I really respect these amps and am surprised the 442s haven't climbed in value compared to some other amps that are comparable in build quality and sound. While I'm downsizing my collection (hoard), and the 442s at 200+ wpc are very much overkill for my personal needs, I find it extremely hard to come to terms with parting with them.

I bought a Conrad Johnson MF2250 and while it is a nice amp, the B&Ks are hot on it's heels in sonics, and the B&Ks are more powerful.

Oh- and pretty straightforward intestines- no wild surprises in layout or design.

I have one of the later (early) Sonata versions, and two of the (late) original gold imprint versions. The Sonata is still working perfectly, the gold imprint versions are now due for a re-cap as they are both starting to get a bit noisy.

I'll be following your progress....

Nice amps. I always liked the B&Ks since their early days. I'd like to see ATI bring back the brand.

To the OP: what changed when you pressed on the transformer? BTW, you're doing a nice job documenting your work. Good luck!
 
I do not know enough off hand about testing toroidal transformers but I do recall several posts here on diyaudio that discuss it. I still think you have some sort of power supply and/or grounding issue. Make some RCA shorting plugs and see if that changes anything. I still think you should try disconnecting the molex connectors powering the individual channels to see how that changes the hum. Don't know if it will make much of a difference but worth a try.

I would also check you are getting a good ground on the star ground and any other point on the chassis. Might be another spot for you to "tap" to see if it changes anything. Another thing to try is putting the scope on the power supply.

Do you get any audio out of either channel?
Gonna do both..
 
I really respect these amps and am surprised the 442s haven't climbed in value compared to some other amps that are comparable in build quality and sound. While I'm downsizing my collection (hoard), and the 442s at 200+ wpc are very much overkill for my personal needs, I find it extremely hard to come to terms with parting with them.

I bought a Conrad Johnson MF2250 and while it is a nice amp, the B&Ks are hot on it's heels in sonics, and the B&Ks are more powerful.

Oh- and pretty straightforward intestines- no wild surprises in layout or design.

I have one of the later (early) Sonata versions, and two of the (late) original gold imprint versions. The Sonata is still working perfectly, the gold imprint versions are now due for a re-cap as they are both starting to get a bit noisy.

I'll be following your progress....
These truly are some fine amps. As far as power goes these two SDA-SRS 2's could easily take double or I should as triple what the EX can deliver. Not that I run the volume up so high, they just love current, the more the better.
 
Nice amps. I always liked the B&Ks since their early days. I'd like to see ATI bring back the brand.

To the OP: what changed when you pressed on the transformer? BTW, you're doing a nice job documenting your work. Good luck!
All that seemed to change was the tone (and just a little bit) that comes thru the speakers. I could even be wrong about that. I am suspect of the transformer due to the shipping/dropping problem. I guess it could be the transformer but I do not understand why the hum at the speakers would be louder at one speaker than the other if it was the transformer. I thank you for your complement about documentation. I am going to upload some more images as I go along for both amplifiers as I work on them. I am not going to kid with you I feel lucky to have the B&K equipment.
 
I feel like insanity has finally taken place. When I checked bias and dc offset before above in post 6 I must have been puffin on crack. Seriously, If you take a look at the pictures I had my meters across the fuses on the power supply not on the output boards. Am I insane or was I right ? Either way I have been running the amp like this every since without one problem. I did notice when I did the adjustments that I never really ever turned the pots much at all. looking it over again when installing a better power cord today and taking pictures, making notes I see that (I think anyway) I should have removed the fuses on the amp boards and placed my meters across those fuse holders. OK, here is the next trouble...According to the documentation I have here (see below .pdf) I am to power up on a veriac. Monitor the rails on the amp boards with my current meters (with fuses removed). Connect two 8 ohm dummy loads. And supply a signal @ 100 mV RMS @ 20 Khz. Bring it up slowly and measure for excessive current draw (above 400 mA). Set it all to 250 mA. All well and good right ? Nope, as I bring it up I see no current draw across those fuse holders whatsoever. I took my true RMS meter and checked for 100 mV out of my function generator and it shows 100 mV. I was set at 20 Khz on the generator when I took the reading. I am not all that familiar with the function generator or the scope I have for that matter but this seems like it should be simple enough. I get no heat whatsoever at the output transistors, heat sinks or at the load resistors...Like I am getting no signal to the amp whatsoever..So you guys set me straight here please.
 
Here is and image and the .pdf I did not upload before. Of course this was with the amp put back together correctly. B&K EX 442 meter position -Bias adj..jpg
 

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Now you have me confused!! The pictures in post #6 are correct. The measurement for bias is across the B- fuse located on the power supply boards. In the picture your displays show ~0.250A on both channels which is spot on and probably why you didn't have to adjust much. At that setting your heatsinks should be getting warm.

Now if you did this all over again using the fuse on the driver board then things are messed up. You will have to repeat what you did the first time in post #6.

I did also notice that it appears your offset is fairly high in post #6 (120mV). This is indeed adjusted by placing the leads across the speaker outputs and measuring DC volts. You should be able to adjust this down to 0mV or very close to that.

Good luck!!
 
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Alrighty then ! I was doing it right the first time. Negative rail should be marked underneath the PS boards right ? If not I suppose I could trace the boards out to the negative side of the filter caps somehow ? The heatsinks were getting warm when I did this the first time in post 6 however I did not examine the boards to find the negative rail. The offset was all good around 0 mV when I was done there best I recall now. I will be right back in there this AM and I am going to remember this lesson. One of the reasons I went in to start with was to determine for sure what capacitors to buy. I do have a list now of what I should buy...I think. What about the 330 pF capacitor, think its a mylar did you replace that as well ?
 
Just use the DMM and measure DC voltage on the fuse in relation to ground. One will be positive and one will be negative. Remove the negative one for testing. You may need to have the amp warm up for a bit to get a stable adjustment. If each channel is biased to the same value, the heatsinks should "feel" the same temp.

The small pF caps can be replaced with pretty much anything like a Wima polypropelene cap or silver mica as long as the voltage is sufficient. Not terrible critical but if you want to be thorough.
 
Things have been pretty crazy round here of late and have had almost zero time to get in the shop however I did get time to set the bias and DC offset. This is one nice amp and I am glad I did not screw it up. I was going to post all the things I went thru trying to get this one right but so many things happened that I cant recall every one. I have that new signal generator and do not have a grasp on it yet and that caused me many a headache when doing this. I also have the old Simpson meters and I could not figure out what I could not get a mA reading across those fuse holders while I was trying to do this. Well, I forgot to change the leads from the voltage setting to the mA setting and that almost drove me insane. Of course I blew the 3A fuse in the meters when I did not do as I should have and that drove me even farther towards insanity...All is well that ends well right ? I have not been in the position to make a parts order for both amps but I do have a list somewhat put together. Hopefully soon I will be able. Hot in the South (as you know) and the AC went down hard in the house so....gotta go. Take car and thanks for checking in on me
 
Mj got a chance to get out to the shop for a while this PM and had a question for you. In the B&K Bias and DC offset specs manual I have it says to inject a 20 Khz signal at 100mV. Is it 100 mV DC or AC that I should inject. I would think it would be AC but I wanted to run it past you because I am still not confident in my skills using the equipment I have. Thanks.
 
OK, should have taken the simple way out here and set the bias by removing the two negative rail fuses, putting my meter across them ( in DC mA range) turning it on an letting it roast for 20 min or so then set to 250 mA. None of the rest. No 20 Khz sine wave 100 mV signal injected and no dummy loads. This worked for me. I had so many issues trying to do it by that B&K service bulletin I posted in post 31 that I could not write it all here if I tried. I chased my tail big time on this one. I second guessed myself so many times its not even funny. No kidding, I would challenge a B&K factory tech to do it by that service sheet. If anyone has the same issues or questions about setting bias one of these amps let me know and I will help to the best of my ability. As far as DC offset that was a bear as well but very straight forward. Fuses back in, meter set to DC mV set to as close to 0 mV -/+ as I could get.
 
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Anybody out there ? Sure hope so ! Pulled the amp boards on the ST 220+ . After taking them down far enough to take a reading of all the resistors I found a suspect at R 22. From what I read R 22 should be 4.7K ohms 2 watt. From the meter reads 100 ohms and the color bands read 130 ohm. What is going on here I am I drunk ? This is the same on every other resistor of like color bands on both amp channels.
 
I am going to continue documentation here for the ST 202 + and the EX 442 as I go on. It looks like everyone else has hopped off the thread but I will continue to try and document anyway.. Took the ST down again and used the schematic posted in this thread to try and get some values as in the above post of resistors, capacitors and such. Seems like the parts used in the ST are the same as in the 442. I will be back as soon as I can to let you know more and upload some new pics.
 
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