B&K ST-140 RECAP BOM
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- b&k bom / recap st-140
JR Harley
Active Member
The B&K ST-140 saga continues. Friday night efforts concluded with a minor setback. After replacing the bad transistors on the right board damaged by the over-current event, I set everything back up for a re-biasing. Bringing things up on the variac up to about 30 V AC seemed to be all good. I was inputting a 20 kHz signal to the inputs, each channel was connected to the 8 ohm dummy load, one meter monitoring the left, one meter monitoring the right. I had to tweak things of course as I advanced the voltage, but all seemed in line. At about 40 V AC, the LEFT board pot stopped having an effect on the current reading on the left side meter. But the current wasn't running away, and I could adjust the right board to be in balance. Advancing the voltage thru 75 V AC had me tweaking the right side down more than seemed to be right, and at 90V AC, something "broke loose". The limiter flashed bright and I immediately shut 'er down. Two steps forward, one step back....
Tonights efforts will be all about testing trannies. Q6 on the left board might have popped for some reason, I'm going to lift a leg and test it out of circuit. The base and collector on this particular transistor are jumped together on the schematic, so I'm not confidant with an in circuit test result that shows it's bad. Good thing I'm not working by the hour on this box! The fun just keeps on coming. Hope everyone is well.
jrh
Tonights efforts will be all about testing trannies. Q6 on the left board might have popped for some reason, I'm going to lift a leg and test it out of circuit. The base and collector on this particular transistor are jumped together on the schematic, so I'm not confidant with an in circuit test result that shows it's bad. Good thing I'm not working by the hour on this box! The fun just keeps on coming. Hope everyone is well.
jrh
I removed the extra capacitance after noticing that it might have lowered the frequency response(the AK member who advised to such, might be correct). More bass,though. The ESR on the can caps are about .5 and read nearly 20,000uf.
I also reflowed the solder on the bridge rectifier board.
I was looking to install a polarized plug since a three prong plug made my preamp's volume decrease substantially.
Keep plugin, you'll get there.
I also reflowed the solder on the bridge rectifier board.
I was looking to install a polarized plug since a three prong plug made my preamp's volume decrease substantially.
Keep plugin, you'll get there.
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JoseHH
Super Member
JR Harley
Active Member
Hey Greg, They are definately fragile. Compared to todays modern boards they seem cheap, but I think they were pretty standard back in the late 70's early 80's. As I mentioned before, if you work fast and fairly hot you should be good. Dwelling more than a few seconds risks lifting the traces for sure. Using a precise tip helps too. The best one I've found for these boards is a 1/16th" chisel point.How much soldering-unsoldering can these cheep B&K boards take? Solder points are so small and cheep that I wonder about a second life for these cheaply made boards.
jrh
JR Harley
Active Member
Hey Jose,
Q6 tested fine out of circuit. Output MOSFETS tested fine. I'm still going through things, but all the semi's on the Left board tested fine after the most recent event. And the pot seems to be functioning fine, even though it wouldn't adjust things when coming up on the variac after about 45 V AC.
Working through the Right board now, but I'm beginning to think I may have tweaked the pot into another current runaway by accident while I tried to get it to adjust. Maybe meter was set wrong? Not sure yet, but if the trannies all test out, I'm going to re-install and start again. There are no physical signs of damage on either board. I'll also double check the rail voltages to confirm the rectifier didn't pop.
jrh
JR Harley
Active Member
Hey ray,
Those certainly are pretty good numbers for the filter caps. I wonder why grounding the chassis to mains earth would reduce volume. Interesting.
Plug on I shall......
jrh
JoseHH
Super Member
Do you mind that I suggest a slightly different approach to test it?
May be you try setting the value of P2 to some 50 ohms or so before powering the unit again. Have two dvm unit, one set to volts n a 20 volts scale conected to the speaker terminals (without any load) and another one across the negative rail fuse socket (fuse removed) to monitor the current on a large scale (may be 200mA or even the 10A scale to begin ). Leave he positive fuse in of course. Also, please try to power it on without applying the sine wave signal. I am starting to think that shorting the input and applying no load may be a better starting test at least to see what the bias current and DC offsets do on power up, and if you can get the current and bias to adjust in case that the dim bulb dims after the PS capacitors get charged.
JR Harley
Active Member
Appreciate that jose!, Interesting thoughts all. I'm not sure I'm tracking you 100%, but I'll ponder your suggestion for a bit before trying to do the bias testing as B&K suggests. Not sure what you mean by "shorting the input". That sounds like something you would not want to do. Thanks for studying this!
jrh
JR Harley
Active Member
A polarized plug makes sense to me ray. I may do the same when it comes to it. Batteries checked on my DMMs. We'll get it! Stay tuned.
jrh
JoseHH
Super Member
You can always short the input of any amplifier. It is in fact a good way to ensure that there is no signal applied to the input since you are applying zero volts. There is no stress on the amplifier because of this. It is commonly done to determine if hum or hiss noise arises from the power amplifier stage, and there is no harm on shorting the input of the power amplifier to set the DC offset. You may do this with a shorted RCA connector plugged at the input of your amplifier. In fact some amplifiers (Au-919 and others) came with factory installed shorting RCA connector at their input in such a way that when no cable is plugged on that input, they automatically get a shorted input. This is specially true on high gain inputs like the phono inputs.
.01uf cap across AC leads on bridge rectifier.
"...to shunt switching noise. If the capacitors are of the wrong type, they can create more noise than they attempt to remove. The size of the cap vs the expected 'load' is key to their effectiveness. ...
Standard bridge: high grade cap - on the pins if at all possible.
Soft recovery: not needed.
Ultra fast: High grade cap - On the pins if at all possible.
This is with regard to bridge rectification of PS rails, only.
The values are low as it is peak switching noise shunting here, that's all. kbk"
square bridge rectifier(BR) not screwed into chassis -on board ... temp after 1hr 80 -87 deg. fah., at 120v.
attaching BR to chassis (heat grease) dissipate heat, extend life. other benefits not researched.
low sound after installing 3- prong plug.. preamp inadvertently switched to tape, not AUX. oops!
"...to shunt switching noise. If the capacitors are of the wrong type, they can create more noise than they attempt to remove. The size of the cap vs the expected 'load' is key to their effectiveness. ...
Standard bridge: high grade cap - on the pins if at all possible.
Soft recovery: not needed.
Ultra fast: High grade cap - On the pins if at all possible.
This is with regard to bridge rectification of PS rails, only.
The values are low as it is peak switching noise shunting here, that's all. kbk"
square bridge rectifier(BR) not screwed into chassis -on board ... temp after 1hr 80 -87 deg. fah., at 120v.
attaching BR to chassis (heat grease) dissipate heat, extend life. other benefits not researched.
low sound after installing 3- prong plug.. preamp inadvertently switched to tape, not AUX. oops!
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JR Harley
Active Member
OK, ST-140 update here. The boards checked out fine all around. I couldn't find anything wrong at the component level. Power rails checked out fine at just under +/- 62 V DC each side. Everything is put back together and I'm pondering the next move. I'm intrigued by jose's suggestion to short the inputs and just adjust DC offset, along with monitoring current. Makes some sense, but I think I need to remove the right board from its connection to power. I don't want that guy to fry while I'm focused on the left board DC offset. Each side is interrelated, which is why I think the B&K procedure is written as it is, but I'm so far out of whack with this thing that it does make some sense to isolate each side for a basic setting like DC offset. Current thinking is that I'll adjust the DC offset first as jose describes, then go from there. We'll see how it goes....
jrh
jrh
I'll reposition bridge rectifier to case; add thermal grease, shunt cap. add new can caps.
JoseHH
Super Member
JR,
The channels are not interrelated. You can safely remove all 3 fuses from the channel that you said it works fine so it will not be harmed by work done on the problematic channel.
Then, you may try to power the bad channel, in which you will only need to remove the V- fuse to monitor the current there with one dvm. Then , a second dvm on the speaker terminals of the same bad channel, to observe. the dc offset.
If one channel works, then no need to power that channel on right now. You can focus on the bad channel.
Try to see if on a 2V or 20V scale there is a large DC offset at power on. Write down the value that you see.
Similarly, see what current you read at the negative fuse socket, on a scale that you can read it (use 200mA or even a 10 A scale if necessary} . Write it down, and power it off and let us know what you got.
The channels are not interrelated. You can safely remove all 3 fuses from the channel that you said it works fine so it will not be harmed by work done on the problematic channel.
Then, you may try to power the bad channel, in which you will only need to remove the V- fuse to monitor the current there with one dvm. Then , a second dvm on the speaker terminals of the same bad channel, to observe. the dc offset.
If one channel works, then no need to power that channel on right now. You can focus on the bad channel.
Try to see if on a 2V or 20V scale there is a large DC offset at power on. Write down the value that you see.
Similarly, see what current you read at the negative fuse socket, on a scale that you can read it (use 200mA or even a 10 A scale if necessary} . Write it down, and power it off and let us know what you got.
ghamilton
Super Member
I need a LED for my B&K board because I poped it.
Will this work?
https://www.amazon.com/dp/B08HYN44JM/ref=cm_sw_r_cp_apa_i_Y-1CFbPAM64VN
Will this work?
https://www.amazon.com/dp/B08HYN44JM/ref=cm_sw_r_cp_apa_i_Y-1CFbPAM64VN