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B&K ST-140 RECAP BOM

Very interesting ray, it sounds like you have improved a number of things on your amp. You've got some cojones' man, increasing the capacitance some 45%. I don't really understand the thinking behind the filter cap arrangement in this amp yet, but I'm still pondering it, and it's interesting to hear about the improved sound you've experienced. I'm curious as to whether you feel the original design had inadequate capacitance to begin with, or whether the caps in your amp are getting weak. If they are getting weak, installing a cap in parallel is risky stuff in the long haul. My understanding is that electrolytic filter caps often fail in a shorted condition. That would be no bueno for the nice toroidal transformer that these amps are fitted with, but who knows how these things go, right?. This is a delightfully complex hobby.

Shielded cables from the inputs is a nice touch. Makes sense. For what it's worth, I've learned that you generally don't want to ground the shield at both ends when using shielded hook ups. That tip has been a good one for me over the years.

Good listening!

jrh
 
You might be correct on the life of the filter caps, considering their age, they must need replacing. I initially noticed that when the power is cut off, they drain rather quickly. Now they don't with the added capacitance. The sound is improved with male voice sounding deeper. Even Bob Marley's music sounds better. I also have some Nichicon 80V 15000 uF, NR Series coming in the mail.
This is what an AK member says(yes, I cherry a few comments since opinions vary): "Diodes are cheap compared to capacitors, so if you are worried about initial inrush current, replace the rectifier diodes as well with devices that are clearly able to handle the expected current. Transformers are unlikely to blow due to an initial surge increase to charge larger capacitors unless they were quite marginal to begin with."

I had replaced the bridge rectifier with: 1000V 50A Metal Case Single Phase Diode Bridge Rectifier KBPC5010 USA
#: 610-2N5210.
The original one was 600v 40a.

Another member says:"Here is a fairly detailed article on power supply design http://sound.westhost.com/power-supplies.htm
Rod Elliott is a "no BS" engineer and (one of the few) folks on the Internet who doesn't spout a lot of malarkey. He gives some very good guidelines for capacitor sizing, along with real numbers on ripple voltage and inrush current. He recommends a minimum value of 2000uF per amp of DC current."
The web link's not working.

On the shielded cables, I did initially considered your point, so I cut the shielding short at both ends to expose the center cable and avoid the shielding making contact with anything.
Once I get the new caps in, I'll see if the sound remains the same or improves.
 
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Another member says:"
Upping capacitance can (not always) mess with frequency response of the intended design. If you want more "oomph", another approach is to keep capacitance the same and increase voltage 2-3 ratings over stock which will increase available ripple current at both low and high frequencies which will result in more impactful sound. Ripple figures give an indication of how much "energy" the caps present."
 
Hey ray, I'm now realizing you are in an active mode with your ST-140. I didn't get that initially.

From my perspective, Mr. Marley's music sounds good on a portable transistor radio. :)

I've hit a bit of a snag on my ST-140 this evening. DC offset won't adjust on the right side, and it's not the pot. I'll keep you posted.

jrh
 
Just to clarify - Using shielded hook up cable is fine, but only ground the shield at ONE end, and usually best to do so nearest to chassis ground. Hope that makes sense.

jrh
 
The pin-out for the PN4889 should be as shown here:...https://www.web-bcs.com/transistor/tc/pa/PN4889.php?lan=en&cl=1

Now the pin-out for the replacement is as you see in below photo. Does this mean that I should put the new one in backwards?
index.php
 
just make sure the pin placement is correct cbe ebc, regardless in what direction the transistor points to. See on AK:
pn4889 transisitor
 
I now realize that B&K must stand for "Back up & Knock head"
Thanks for the info ray, I'll check it out.
Something is keeping me from adjusting the DC offset on the right side. It just hovers at a blistering 51 V! no matter how I turn the pot, and I've verified the pot is good. So I'm tracing components and comparing measurements to the same on the left board, which is working fine. I've focused on the input portion of the circuit since everything is so direct on the schematic, and R5 has jumped out as a suspect. I think I'll lift a leg and test it out of circuit, as it's measuring super high,much more than its sister on the left board.(3.6M vs 24K). We'll keep after it.

jrh
 
you can measure resistors with a dmm without having to unsolder them from the board. R5 is in between a transistor and a capacitor, which is what was discussed in AK: "How to fix DC offset with no trim pots available?"

By the way, R5 is listed as being 1K on the schematic dated 2/20/90.
 
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+51v means your nchannel is conducting, or shorted. You need to check it. Also which rwsistor went high? Was ir R5? The value you menrion is for R6. That is your feedback resistor. If it goes open, then it could cause the output to rail. Did you check Q8, Q9 and Q10?.
 
Hey ray AK dummy here. I can't seem to access the discussion you reference "How to fix DC offset with no trim pots available"
What's the best way to do that?
 
Hey jose,
I haven't re-checked Q8,9, and 10 yet, but they're on the list of suspects if R5 turns out to be good. The point I take away from my testing is not the absolute value of the resistors, but more that R5 on the right side, is testing so much higher than R5 on the left board. Seems to be a clue worth looking at. Thanks for looking at this with me, I appreciate your insight.

jrh
 
highlight "How to fix DC offset with no trim pots available", cut and paste it in google. or you can highlight, right click mouse and select search google.
R5 is 1K per the schematic.
check them all while your in there.
same and only resistor that was blown when I first purchased my amp.
 
I must be missing something. It appears to me that it would very hard to blow R5, considering that it is in series with a 24K resistor which should limit the current. Where would the current to blow it come from?
 
in harley's case, probably like my own, who knows. maybe a chain reaction of out of spec parts and settings. In my case, R5 was partially burned and out of spec. or essentially blown. finito. the caps in my case read in the pF range not uF. it's a 30 year old amp; pretty good.
R5 or 1k ,per schematic, not "...3.6M vs 24K...".
 
Update for B&K Fan Boys:
To revisit the situation - During the biasing process, I subjected the right board of the ST-140 to an over current condition that burned out resistor R10. Fortunately, I was only testing the right board. This resistor is located between the collector of Q5 and the base of Q8. Also in that chain is Q6. All are NPN signal transistors, and all are connected to R10 one way or another. Focusing on those 3 trannys, they tested "hinky" in circuit. Removing them and testing with the little "all in one" testor revealed the following: Q5 and Q6 tested as "resistor". Q8 tested as "2 diodes". Yeah, they're toast. I replaced 'em. We'll see if that allows the DC offset to be set. First, a board cleaning. Updates will be posted soon.

jrh
 
There is no R10 listed in the b k schematic dated 2/20/90. There is no resistor R10 listed on the schematic, let alone between Q5 and 8. Yes, replace the trannies. You're welcome to the one's I have. Message me.
 
Hey ray, do you actually read these posts friend? :)

Yeah, R10 is not shown on the schematic. Trannies were replaced. But R10 does physically exist, at least on my amp. That's been an interesting discovery. I'm curious if you've found it on your amp boards? It's pretty easy to spot. I've attached my working schematic where I've drawn it in. Thanks again for the parts offer, but I'm good for now. The way this thing has been going though, I may run out!

Anyway, tonights work will involve removing Q9, another NPN in the chain, albeit a different type. It's also testing suspiciously in circuit. I'll take it out and test it. If it's toast, I'll replace it. I'm pretty sure that's as far as the damage is likely to have gone. I should be ready to re-set the bias and the DC offset this weekend with a little luck.

Thanks to your help, and the help of all the others following this thread and weighing in, I'm confident I'll get this 'ol Gal back up and running! Much obliged.

Another posting soon.

jrh
 

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