B&K EX-442 Rehab

Here is the latest. Replaced all the lytic caps on the main board still have a 60 Hz hum on one channel only. Does not seem as loud but still a nice hum. Hope someone is watching and can help. Here are images. Also I put in a new rectifier and new pads under the OPT. Oh, and a new power switch as well.
 

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Sorry lost track of this thread as work has been kicking my butt lately. Trying to read through it and play a little catch up.

The bias is set with an AC signal.

In regards to the hum, I would try switching the channels internally to see how things follow. By this I mean take the wires from the left channel power supply and move it to the right. May help you narrow down the problem. I would suspect its some sort of grounding issue. The molex adapters aren't the greatest way to connect internally. Might be one of the junctions is a little iffy.

If that doesn't change the hum location then something on the boards is not grounding correctly and I would suspect a cold solder joint or some sort of connection issue.
 
Good to see you are back. Sent you a text over at the bay to check in. Hope all is well with you there in Texas. As far as bias goes I went and set it ( both the 442 and the 202 btw) without any input whatsoever. I know in this thread I sound like a lunatic but I got all confused and now I think I am back on track. As far as I know the signal generator that I have is AC output but I will look that over tomorrow. I will look and see if I can move those PS wires over and switch them if I cant do it that way I am going to change the left output board/heat sink out with the right and see if the noise follows the output board.. I am sure as you say it is probably something simple like a solder joint or a wire/molex connector but I just cannot seem to hunt it down yet. That 202 board being mounted upside down is kind of a pain as I have to take everything apart every time I check something. Well either way that is what I will try then I will post back. Thanks Mj.
Sorry lost track of this thread as work has been kicking my butt lately. Trying to read through it and play a little catch up.

The bias is set with an AC signal.

In regards to the hum, I would try switching the channels internally to see how things follow. By this I mean take the wires from the left channel power supply and move it to the right. May help you narrow down the problem. I would suspect its some sort of grounding issue. The molex adapters aren't the greatest way to connect internally. Might be one of the junctions is a little iffy.

If that doesn't change the hum location then something on the boards is not grounding correctly and I would suspect a cold solder joint or some sort of connection issue.
 
OK, swapped the left PS input from the main board to the right output board and the right PS input from the main board to the left output board. Nothing, no sound buzz or hiss from either test speaker. Scratch head...Found that I forgot to hook up the inputs (rca) back to the mainboard. Plugged them back in. No change buzzing still loud in the right channel. So, that should prove the main board is good. Remembering that I had no sound at all without the inputs unhooked (rca) from the main board I decided to disconnect the molex plug and listen again. Suddenly static sounds from both right and left speaker and a fuse on the right side top of the main board blows. Cut it off, hook the molex plug back up replace the fuse. Bring it back up slow on the DBT/Variac and all I get now is a very quiet sound from the cd player on both channels as if there was no amplification at all. I can hear the cd player no problem but it seems like the amplifier is doing nothing but passing the cd player signal thru to the speakers. This is crazy. Its running me insane.
 
Now I go back out and find that the reason I was only getting a low volume sound from the amp. is that somehow it blew both woofers in my test HPM 40 speakers. Put it all completely back together and tested for DC on the outputs. No DC. Hooked another pair of test speakers up and it played just fine for hours but got too hot on the heat sinks. Especially the one on the left viewed from behind, 135 degrees . The right bank was cooler around 120 degrees best I recall. Went to check bias again and now one of the channels cannot be set correctly. The other can. Guess I scalded another part somehow. Oh BTW. The amp still has a hum/buzz one channel, however now its in the left channel. Crazy man, I am going crazy messing with this thing !
 
Okay need to do some retracing. Just taking pot shots...
You are sure the board swap results in hum to the other channel; granted..

Are you using this schematic; https://www.hifiengine.com/manual_library/bk-components/ex-442.shtml
I get the left right deal... I see a crap load of 1.0uf and 100uf caps to gnd in the schema's.

heat sink getting hot.. to is too high idle bias. You can drive the bias adjustment transistor to no effect. like the outputs are set in and thats it with mosfets / many fet types so if cold and dead pot works cut it back a few mv's.

Stop using the variac! just the dbt.. and if the ps tests okay with the dbt don't need that unless in the ps has new components.
If warm up and still no bias idle adjustment? hmmm perhaps some bournes. might go ahead and replace those.

I don't have high batting avg.. admitted but I've hunted the 'wiley wabbit' many times and 'bugs' fudged me many times. :D
 
Okay need to do some retracing. Just taking pot shots...
You are sure the board swap results in hum to the other channel; granted..

Are you using this schematic; https://www.hifiengine.com/manual_library/bk-components/ex-442.shtml
I get the left right deal... I see a crap load of 1.0uf and 100uf caps to gnd in the schema's.

heat sink getting hot.. to is too high idle bias. You can drive the bias adjustment transistor to no effect. like the outputs are set in and thats it with mosfets / many fet types so if cold and dead pot works cut it back a few mv's.

Stop using the variac! just the dbt.. and if the ps tests okay with the dbt don't need that unless in the ps has new components.
If warm up and still no bias idle adjustment? hmmm perhaps some bournes. might go ahead and replace those.

I don't have high batting avg.. admitted but I've hunted the 'wiley wabbit' many times and 'bugs' fudged me many times. :D
"You are sure the board swap results in hum to the other channel" I cant even remember anymore. I am going to start again from jump street.
"Are you using this schematic; https://www.hifiengine.com/manual_library/bk-components/ex-442.shtml"
Yes, that is the one and I see no info about the PS whatsoever. Even the fuse rating on the 202+ is not seen.
"heat sink getting hot.. to is too high idle bias" No doubt. I was able to adjust that without an issue however now it seems I cannot adjust the other channel up high enough. May well be a dead pot but it worked without a hitch before the speaker cooking party.
"You can drive the bias adjustment transistor to no effect. like the outputs are set in and thats it with mosfets / many fet types so if cold and dead pot works cut it back a few mv's.' Sorry, I do not understand what any of that means.
"Stop using the variac! just the dbt.. and if the ps tests okay with the dbt don't need that unless in the ps has new components. " Absolutely, done. That thing was being a PIA anyway.
"If warm up and still no bias idle adjustment? hmmm perhaps some bournes. might go ahead and replace those." No reason I cannot get some on the next part order.
Man I really do thank you Binkman !
 
Hope that bournes helps out.. self admitted I'm usually batting 50/50 but bias pot replacement is common.

'cutting back mv's" was referring to bias/idle. usually sms allow for a +/- range adjustment and I'll usually take the min.
 
Way I see it even if the new Bourne's do not fix it they are needed to do the job the right way especially being I am keeping both amplifiers, the 202 + and the 442. The 202+ I was fixing up to use as a backup while I took the 442 I am using now down for a total restore/rehab.
 
Have to say I am a little bit confused about the course of events. Maybe you can post some pictures to help clarify what you did and where exactly the fuse blew. Not sure the bias pots will help but I agree it is worth replacing.

Hopefully no output transistors were fried. They are a pretty penny to replace. I think you need to start over or give it a few days before diving back in to look at it with fresh eyes.

I would unhook both power supply moles connectors to each channel. Confirm you have the appropriate voltage. Then hook up one channel and focus on making that one work before dealing with the other.

Don't know if this has been said or if you know, but you should never set the bias with the DBT in place. Just want to eliminate any unsaid variables.
 
Can and will do. I know some of the problems that that have plagued me and will need to figure them out before I can go much further. 1. The driver / PS board is upside down screwed into the filter caps so in order to check anything on that board I have to disconnect it every time do the work then put it all back. This includes connecting and disconnecting the molex connectors. 2. My meters are very old, Simpson DVM's 467's x 2 of them. I think, in fact I am almost 100% certain one or both are giving me trouble. 3. In order to swap amp board for amp board (or left output board to right and right to left) I had two options. Remove each one from the back panel of the chassis, de solder the wires and swap sides or turn the whole back panel upside down and leave it loose from the chassis to swap them as there is not enough wire length to leave them in place and do so. The last time (when I blew the fuse) I had the whole back board upside down and I think something touched causing whatever the problem is now with the bias adjustment. I will follow your directions and post some pictures then wait for farther instructions before moving on anything else. I doubt there is any trouble with the OPT as I have played the amp after all of this was done and it sounds fine on both channels with everything hooked back up. Just cannot get the left channel to bias over 198 mA. The right will bias just as before at 250 mA no problem. Oh, and I do know not to use the DBT when setting bias thanks for the reminder. I do not want to sound pitiful with my next statement but this is the main problem. I am disabled and have trouble sorting things out in my mind due to it. I get confused very easily and I forget things as well. So I think due to the above this has caused me to be where I am. I want yall to know that I really appreciate all the help.
 
Looks like you blew the negative rail fuse to the driver stage. Tough to tell 100%. The other fuse looks like it may go to the output stage but you should confirm this by looking undAt this point I would disconnect both output stages (the two boards on the back wall. Meaning disconnect the moles connectors going to them.

I would then remove the main board and check/compare every component using your multimeter. Use resistance for resistors and a diode check for any transistors. Sounds like one half of the main board is good so you have something to compare. If anything is off, replace it. Gonna be a little tough but I think you can check some basic voltages when the board is installed. The only you won't be able to do with the back boards disconnected is set bias.

See how that goes
 
Looking at the previous pics I would be tempted to figure out a way to flips that main board over and remount that bias pot
 
God I know. Perhaps I could. I will look it over and see. I am goin to try and get back on it this week.
 
I know you probably think I have abandoned ship but oh no. You know how things happen in life but now I am back. I pulled the main board and measured everything on it, transistors, resistors, diodes, pots and I cannot find one single difference between the two channels on the board. The only thing I found was a ground wire that was not attached to the board as well as it should have been but even that was not loose. Sorry, I do not get this thing. I get no bias reading across the left rail fuse holder. Right rail reading is fine. There is no way to mount this board upside down short of moving the board and re mounting it above the transformer either. Even doing that would be a huge job. This thing has been the bane of my existence for a while now. I think I should ship it to you
Looks like you blew the negative rail fuse to the driver stage. Tough to tell 100%. The other fuse looks like it may go to the output stage but you should confirm this by looking undAt this point I would disconnect both output stages (the two boards on the back wall. Meaning disconnect the moles connectors going to them.

I would then remove the main board and check/compare every component using your multimeter. Use resistance for resistors and a diode check for any transistors. Sounds like one half of the main board is good so you have something to compare. If anything is off, replace it. Gonna be a little tough but I think you can check some basic voltages when the board is installed. The only you won't be able to do with the back boards disconnected is set bias.

See how that goes
 
As far as losing track of things.. get a decent note pad to make notes of what you just did. I do. On bias and the hum... remove the trouble some pot and test it. or go to bournes. (btw I've never heard of measuring bias from a fuse as a test point.)

Looking at the previous pics I would be tempted to figure out a way to flips that main board over and remount that bias pot

Replace bias pots to opposite side: Good idea if bias pot is not accessible per assembly, just flip the mounting,
same leads in same holes same cw and ccw
 
Checked both pots and they both adjust out exactly the same so I guess they are fine. They are TOCOS brand pots. As far as checking bias at the fuse points what the B&K manual calls for is removal of the two negative rail fuses and placing a current meter (mA) across each while adjusting the pot to 250 mA. I have not heard of such either till I ran into these Amps.
 
Anything going on with this thread? I've studied it closely as I am working on a 442 Sonata with Toshiba devices. My main board is similar to the pics posted here but has some differences, mainly in where the rail fuses are located. Also, I have a main rectifier for each channel. I can share pics/details later but based on the above discussion I should mention that on my main board each channel has a dual op amp (U1) and only 1 trimpot per channel. I think in my design U1 implements a bias servo function that keeps DC offset low, and that is why there is only 1 trim pot; that trimpot is for DC bias (current) not DC offset (output voltage). Note in the schematics that one circuit has U1 and one doesn't. Note also that the pdf posted earlier mentions two trimpots (P1 and P2). So if you only have 1 trimpot per channel and U1, you have a design with a servo bias loop and the pdf adjustment procedure posted earlier clearly is inappropriate.

With the servo design I don't think you need the 20kHz input during the adjustment, and that is where you had success so I think you have a servo design. Also, because of the servo feedback design be aware that you cannot get useful transistor voltage measurements (e.g. checking base-emitter drops) with the FET boards disconnected.
 
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