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B&K ST-140 RECAP BOM

ChrisP2 said:
Hey guys....the notes Ray pointed out are critical to troubleshooting the ST140s.

1) Definitely work one channel at a time - Easy enough to unplug the Molex connector

2) Use the Variac to Start Up - This is how I resolved the MOSFET mounting issues

3) Replace the Transistors and the caps on the driver boards. I recall having an issue with the 0.1uf bypass caps, I replaced the electrolytic and the poly caps C4-C7.

The one I worked on (which the BOM came from) was dead shorted. MOSFETs were OK, Power Supply was OK, problem was on the boards, which got the full refresh and eveything came back up pretty normally. MOSFET mounts were a huge PITA, bad design. Variac start up is hugely important, Dim Bulb tester will not get it done.

Great Amp, hope you get it back on line JR !

Thx, Chris
Click to expand...
Thanks for weighing in Chris,
Given the COVID, I'm going to be really looking at this beast over the holiday weekend, since I should have the time. As if I haven't already looked hard at it, but I'm going to keep at it. I'll put together a Mouser order to include the transistors you showed on the BOM and see if they make a diff. They're cheap, and if I combine with some other needs, will be painless to acquire. I'll keep you guys posted on the lack of progress.

jrh
 
I used one of those cheap Chinese everything testers to check the components against the data sheets.
on mounting the MOSFETS, just put a new Thermo-pad on it and stick it in. Make sure the order is correct, though.
Have the heat sink mounted, otherwise, it is a PITA!
If your eye's aren't great, use a magnifying glass to prevent cross soldering issues. Clean it with a toothbrush and alcohol to remove micro solder pellets, etc.
The board is delicate and traces burn quickly if the iron is kept on long. I used a clasping scissor/ vice on the trannies to dissipate heat and while soldering them. Soldering paste protects the traces and aids in soldering.
I had trouble setting bias per manufacturer's recommendations- I kept getting a speaker buzz on either channel and as I raised the variac and gave up after numerous tries. Luckily, I had measured the old variable resistors when removed and set the new ones at the same ohms.
107-8 degrees Fahrenheit is what the heat sink emits at or near its center point.
I kept the side of my finger on the output transistor as I slowly raised the variac and adjusted the variable resistor, shutting it down when I thought it got too hot. I also used a digital thermometer and compared the temp to the temp in the data sheet.
The mosfets are available and relatively cheap.
When I initially purchased it, it just needed new capacitors and one resistor.
 
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raycarr00 said:
I used one of those cheap Chinese everything testers to check the components against the data sheets.
on mounting the MOSFETS, just put a new Thermo-pad on it and stick it in. Make sure the order is correct, though.
Have the heat sink mounted, otherwise, it is a PITA!
If your eye's aren't great, use a magnifying glass to prevent cross soldering issues. Clean it with a toothbrush and alcohol to remove micro solder pellets, etc.
The board is delicate and traces burn quickly if the iron is kept on long. I used a clasping scissor/ vice on the trannies to dissipate heat and while soldering them. Soldering paste protects the traces and aids in soldering.
I had trouble setting bias per manufacturer's recommendations- I kept getting a speaker buzz on either channel and as I raised the variac and gave up after numerous tries. Luckily, I had measured the old variable resistors when removed and set the new ones at the same ohms.
107-8 degrees Fahrenheit is what the heat sink emits at or near its center point.
I kept the side of my finger on the output transistor as I slowly raised the variac and adjusted the variable resistor, shutting it down when I thought it got too hot. I also used a digital thermometer and compared the temp to the temp in the data sheet.
The mosfets are available and relatively cheap.
When I initially purchased it, it just needed new capacitors and one resistor.
Click to expand...
Hey raycarr-
All good thoughts! I appreciate the advice.

My set-up is pretty comprehensive: magnification handled, variac and isolation/current limiting handled, board cleaning post solder work, is SOP. At this point I am pursuing a strategy of selectively removing components, currently transistors, and seeing if I can isolate the short that way. It's tedious but systematic. I figure there aren't that many components fer 'chrisakes, I should be able to find the goof. Interestingly, I have so far removed Q2,Q7, and Q5, and the right side board, (only one in play with this effort), is still revealing a short, even with them out of the circuit. The hunt continues. - stay safe
jrh
 
ChrisP2 said:
Hey guys....the notes Ray pointed out are critical to troubleshooting the ST140s.

1) Definitely work one channel at a time - Easy enough to unplug the Molex connector

2) Use the Variac to Start Up - This is how I resolved the MOSFET mounting issues

3) Replace the Transistors and the caps on the driver boards. I recall having an issue with the 0.1uf bypass caps, I replaced the electrolytic and the poly caps C4-C7.

The one I worked on (which the BOM came from) was dead shorted. MOSFETs were OK, Power Supply was OK, problem was on the boards, which got the full refresh and eveything came back up pretty normally. MOSFET mounts were a huge PITA, bad design. Variac start up is hugely important, Dim Bulb tester will not get it done.

Great Amp, hope you get it back on line JR !

Thx, Chris
Click to expand...
Thanks for the encouragement Chris, and everyone else for that matter. Alas, I am still shorted, so I need it.

As an experiment, I'd like to try new output MOSFETs. The current ones were purchased on the Bay, and were sold as NOS. They certainly look authentic, and pass both a DMM check and an "all in one" tester check. Still........ they're off the Bay.

Trouble for me is, it seems like only NTE sells drop in replacements, and they're pricing each one of those things as if it was a boutique 6L6 or something. A set of four would cost well over a Franklin. Anyone know a better source? That's kinda' steep for just an experiment, but I may have no choice in order to get some sleep :)

Thanks again for the help all,

jrh
 
This post about the 140 will help me with the 202. I ask for help on adjustments a while back but no one chimed in.
 
ghamilton said:
This post about the 140 will help me with the 202. I ask for help on adjustments a while back but no one chimed in.
Click to expand...
Hey g,
I don't know if I can help, but I'm willing to try. "Adjustments" on a power amp would be limited to on board stuff, but let me know what you're looking for and I'll try to confuse you to the fullest extent possible!

jrh
 
I recall seeing the MOSFETS really cheap, somewhere. BUZ900 and 905 are the substitutes for the major output transistors. Newark has them for under 7 bucks each for 900; and 6 bucks for 905.
 
raycarr00 said:
I recall seeing the MOSFETS really cheap, somewhere. BUZ900 and 905 are the substitutes for the major output transistors. Newark has them for under 7 bucks each for 900; and 6 bucks for 905.
Click to expand...
Thanks raycarr00!! I will definitely check these out. That price is way more like it. Much appreciated.
jrh
 
Greetings all, hope you guys are safe and well. ST-140 update here.

So I tried new output mosfets on my dead shorted ST-140. Thanks to raycarr00, I was able to afford to score a set of replacements in order to confirm my originals weren't playing tricks on me.They weren't, same results. That's OK though, another possibility eliminated, and I've got a spare set for a reasonable price.

Just curious, does anyone know what the purpose of the "JN" and "JP" through holes on the circuit boards are for? They've never been populated with any components. Perhaps intended for a test point or something? The schematic seems to say they're tied thru a resistor to ground. There also are similar holes that seem like they might be test points for the rails, "JV+,JV-, JO". You could easily solder a test pin into those holes, but I'm not sure if that was the purpose.

At the moment, I'm reduced to replacing the poly caps C6,C7,C8. They're the only caps I haven't replaced. C1 & C2 I replaced with Nichicon "Muse" non polarized caps. I'm going to pull C8 and test it out of circuit. Whatever is causing this mystery short, it's applying to both boards independently. That's the thing that makes it seem less likely to be component based, but I can't find any hardware shorts. Bizarre.

jrh
 
definitely replace the caps on the boards. With a DMM and in diode mode(beep!), test all points on the resistors, traces on the boards for a break, short, etc... same with the cables that connect to the boards. I don't recall if the boards had fusistors. test the bridge rectifier, as well. Don't know about the JP, JN.... check/ replace all transistors, diodes.... how about the transformer, could it have a shorted winding? use a magnifying glass and light to rule out a cross solder. at best, you ruled out the MOSFET. could you post a close-up picture of the board(s)? if you're getting any sound out of a channel, try swapping boards and powering them up one at a time and from the working channel;check polarity on the board connectors. I'm able to get sound with the variac set at a very low voltage setting. test the bridge rectifier located under the board and between the large can caps.
 
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Thanks raycarr00, I always appreciate the feedback.
It's hard for me to see from the schematic how the capacitors remaining could cause what I'm seeing, but I'm gonna' replace 'em anyway. The ones I've pulled have tested good, but I might as well, I've rebuilt the entire boards. After I do, I'll post some close ups of the right side board, front and back. C8 appears to be a little different, kinda' looks like a mica cap, 200 pF. I pulled it and tested it for value and Leakage, on a nice old Heathkit C-3 testor and it tested good. Mica caps, if it is one, would be chosen for temperature stability among other things, so that may be a clue. Still, the troubleshooting seems to say it's a basic hardware related short, since it affects both boards independantly, but I've never changed anything on that front except my quick disconnects from the input to the boards. And I've triple checked polarity on the connectors. And I've replaced the rectifier.

I'm starting to wonder if my dim-bulb setup is giving me a false indication for some reason, which is not a secure place to be from a troubleshooting perspective. I've used it for many years now and it's worked great. I sure wish B&K had had the wherewithal to make good information available to us hobbyist Shmucks for posterity. In any case, I'll report back on the lack of progress as I make it. Be safe.

jrh
 
I use a dim bulb tester and this is what I get with a flood bulb (120w / 130v) at 120v set on the variac. it's not as bright as it looks on the pics. Are you getting any sound? IMG_20200824_181114.jpg
 

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Thanks for getting back raycarr00,
Interesting. So you get a light when the amp is just idling. That IS different than anything I've ever worked on. Normally, when things are good, the light comes up quickly, then fades to nothing or virtually nothing. So far, I just get an increasingly intense light as I increase voltage. The filter caps are big, but not so big that they take more than a few tens of seconds to charge. I use different bulbs, but my typical bulb is a 300 W unit.

I'm not even trying to push a signal thru her at this point, as it seems like my setup is telling me I have a dead short. Very curious! I'll keep you posted.

jrh
 
I have been jamming at my current set-up for some time. The bulb will shine. Different shines at different bulb wattage. Try putting a signal through it and see what the sound is like. Don't fixate on the bulb; it's a simple set-up. if it produces good sound with no significant problems, enjoy it!
 
raycarr00 said:
I have been jamming at my current set-up for some time. The bulb will shine. Different shines at different bulb wattage. Try putting a signal through it and see what the sound is like. Don't fixate on the bulb; it's a simple set-up. if it produces good sound with no significant problems, enjoy it!
Click to expand...
Hey raycarr00, thanks for the dialogue.
I hear you, but I've just got too much current surging through this thing to even think about asking it to amp some sound. At least that's how it seems. The last time I doubted the limiter, I blew the whole shootin' match up. I'll replace C6 and C7 and re-test with current monitoring. I'll report back soon, and I really appreciate the input.

jrh
 
Do what you think is best. I was able today, to get sound out of the st 140 on 30 volts using the variac, alone. My point was/is, that the bulb will light regardless if you have an issue or not. Take care.
 
raycarr00 said:
Do what you think is best. I was able today, to get sound out of the st 140 on 30 volts using the variac, alone. My point was/is, that the bulb will light regardless if you have an issue or not. Take care.
Click to expand...
Thanks!, I'll be seeing if the current is behaving on the next hook up. Good to know that your limiter stays lit when powered up under normal circumstances, that bit of info helps a lot. If I hook it up and don't see more than .25A or so, I'll feel better about things. I should be able to post a pic of the testing. Stand by.

jrh
 
I was able to produce sound at about 2.7 volts(distorted). Yes, two point seven volts, on a variac and with no dimbulb.
 
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