B&K EX-442 Disaster

[Ramatam]

Hi
I own an older version of the B&K EX-442 with the golden handles, no blue front panel lines and Toshiba outputs.
Recently I put it back into service after it having sat for 10 years. Fired right up with no issues. I carefully readjusted the DCOffset and Bias though it was difficult with the old pots. DC was around 60mV and Bias was around 78mA to start out of retirement.
I wrangled the DC offset to around 0 but it jumped around +5mV to -3mV and the Bias was set to around 175 mA then after a few days up to 200mV but then noticed that the values slowly crept up all the while the amp was sounding better than I had ever heard it....what's that old saying?? Amps sound best right before they die?Anyway I stopped using it as it approached 240mA. From what I've read on forums this particular amp should be set to 200mA, max.

An professional electronics friend offered to get new pots for me and then said we should re-cap it. Sounded reasonable. Note: he mainly works on tubed equipment and shys away from SS, Should have seen that red flag.

Anyway, long story short, he only ordered caps, rectifier bridges and the Bias pot pair and not the DC offset pair; why, I don't know? AND after me telling him to use extreme caution disconnecting the Mosfets from the heat sink, he broke the first one in the RIGHT channel. Not knowing that they have to be matched, (more importantly, probably counterfeit), I bought a replacement off eBay. He did the labor, replacing the Caps and and reset the Bias to 200mA. But while trying to re-set the DC Offset voltage, (he said was jumping all over the place), three other output transistors blew in the LEFT channel.

So now I'm stuck with 300+ bucks worth of parts and a dead amp.

Which brings me to my question:
Is there any hope now for this amp, seeing that obsolete transistor pairs must be matched, (according to B&K's schematic)? I have found some at a legitimate retailers but at $12.75 apiece, seems futile. Time to call it a day and sell the lot for parts???
Would appreciate any input. I am not an electronics person accept what I learned in college about DC circuits, and YouTube..... Sure hate to see this amp go....it was such an amazing sounding component.
Thanks for your time and input.

 

[gadget73]

personally I wouldn't replace the diff pair either if there was an adjustment pot and it could be set to 0 volts. A few mv of drift one way or the other could be noise, or could be the pot itself is flukey.

Unless you mean the pair of adjustment pots, I'd change those. Old amps seemed to use the most garbage adjustment trimmers available and they age poorly. I've had plenty of them just fall apart from trying to make an adjustment.

what are the original output transistors? Unless its something truly weird there is a fair bet subs can be made, though if genuine pieces can be had for under 13 a pop I'd probably go that route. Modern ones are not likely to be much less expensive, and it eliminates the question of how good of a sub was actually made. I've made bad subs before and ended up with amps that worked fine for a while until they randomly blew themselves up.

the other trouble with shorted outputs is they tend to take out other components. Make sure you check all the transistors for shorts, pretty common that at least the stage immediately before the drivers goes, and sometimes you'll get failures even further back. Torched emitter resistors are also a common thing, and depending on the resistor construction it may not be visibly burned. Have to check with the ohm meter.

 

[Ramatam]

I have inserted pictures below. One is the adjustable pots on the output boards and then a closeup, The upper left one is the bias adjust. The lower right one is the DC offset adjustable pot. When I adjusted, the slightest rotation caused the values to bounce around crazily. I was finally able to get them somewhat close to zero but they never fully settled.

The output transistors are circa 1989 Toshiba 2SK-405 and 2SJ-115's. They are considered obsolete though I did find a few at a US supply shop. They are importing them from Japan.
The $4 ones on eBay are considered dubious by many on this forum.

Again, I'm not a electronics tech. But it would be fun to learn and tinker with this amp if it comes to that. Recently picked up a Bryston 3BST for a good price to fill the void.

Wondering, does anyone know of a tech, who is well acquainted with this amp who might quote a price to repair?

Anyway, I do appreciate the feedback

 

[Ramatam]

Sorry for the double post of pics. Lost my txt and reattached pics with a txt re-do.

 

[Andy D]

I'd replace the pots with sealed ones. I had a similar issue with a Bryston 2B. I have a very similar setup in my B&K sonata video 5. It has the same output transistors and I'm trying to find a proper bias setting. I've seen 250mA, but I'm not too confident. I'd like to know what temperature the transistors should be at idle.

 

[Ramatam]

I think at this point the adjustable pots are moot until I can figure out the output transistor dilemma. So B&K says matched pair for output transistors, but others on the forum say it's not that critical. Wondering which it is. Would buying from the same batch be close enough? Probably not but I'm asking.

 

[mondialfan]

I think at this point the adjustable pots are moot until I can figure out the output transistor dilemma. So B&K says matched pair for output transistors, but others on the forum say it's not that critical. Wondering which it is. Would buying from the same batch be close enough? Probably not but I'm asking.

They're matched quads and not pairs in the 442. Yes they should be matched for best performance. These amps do not use degen resistors on the outputs of the mosfets so matching the quads to ensure that they are sharing the load is critical. I'm sure some have done it and gotten away with it but that doesn't mean it was correct or operating optimally. At this point finding enough of these long out of production mosfets to build up a pair of matched quads will be difficult and expensive. Not to mention you still have to worry about whether or not you're getting genuine parts. If you're really sold on this amp then buy a second one that is in bad cosmetic condition and use the parts from that amp to build one nice amp. Then you've got more spare parts for the future.

Its unfortunate because these old B&K amps are a very simple circuit and should be easy to repair except for when the outputs are damaged. Q1/Q2 can be sub'd with modern equivalents and need to be well matched. Q3/Q4 are the current mirror and also need to be very well matched and can be sub'd as well. If these two pairs are well matched and thermally bonded together you could remove R1 (bias pot) short across it and replace R4 with a 221r resistor to match R3 and you should have single digit DC offset. The bias pot was a crutch to prop up poorly matched pairs in the Q1/Q2 or Q3/Q4 location.