Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: WK3K on May 15, 2020, 02:13:42 PM
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Completed a new build and resistance checks. Resistance between 8U and 8U was 1.4ohm by my (cheap) meter, so I used this as my "0" value. All resistance values were within normal limits.
Moved on to check voltage and upon plug-in the resistor between 44L and 30L sparked and smoked. I immediately pulled the plug.
I just re-ran the resistance checks, and everything checks out with the exception of 26, 28, and 29 which are now measuring around 2.3-2.6 ohm (after the resistor blew). These were all WNL last night.
I've taken a look at the solder joints throughout the high voltage supply (and everywhere really) and don't see any glaring issues.
The only other thing I can think of is that I initially installed the MJE5731A's backward and had to pull them and reinstall. They seemed fine, but may have been damaged in the process?
Never had a snag like this during a build, so any advice on where to start trouble shooting appreciated.
Thanks!
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I am recalling from memory, but is that one of the 130 ohm resistors? In the PS, right? If you can’t get those ASAP I have a bunch of those. Let me know. I can send you several, but check for shorts. I blew up one of those when voltage checking my second SII build. It happens.
Jamie
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Thanks Jamie. Yes, the 130 ohm 2 watt resistors. I measured the rest of them in the power supply and they’re ok, but the one that blew is shot. I’ll got back through the circuit and double check for shorts. I didn’t see anything when I took a look yesterday, but I’ll pull out the magnifying glass. Thanks for the offer to send reinforcements, I shot you a PM.
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Please post some photos of your build. I suspect you either have a backwards power supply cap or you have the fat pins on one or both of the 2A3s plugged into the wrong holes in the 4 pin socket. You have to push pretty hard to get the tube to fit in like this, but it is ultimately possible to force the tube in, then you'll start blowing resistors.
-PB
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Hi PB,
I double checked the items you suggested.
- Caps are all oriented correctly.
- I checked the pins on the 2A3's multiple times as this point is emphasized quite well in the manual. I even checked them again when I removed them after the resistor popped and they were 100% oriented correctly (fat pins to the back). The tubes installed easily.
EDIT: Was able to get images uploaded. Originals were apparently too large.
Thanks,
Witold
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Witold.
Look from all sides for wires that are touching.
There must be something shorted (to ground) that causes the excess current draw.
Maybe redo resistance checks now, to see if anything has changed?
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Hi Tom,
Thanks for the advice. Just looked at everything again. The resistor to B7 *might* have been touching B8, but that's the only thing I saw. I decided just to reflow each terminal to be doubly sure there were no cold joints.
I redid resistance checks — this time 8U to 8U was exactly 1.0 so I'm using that as my "0" value (previously was 1.4 - I'm guessing this is just a meter thing). Either way, every terminal was within 1-2% of it's value with all "0" values right at 1.0.
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Witold,
Did you PM me? Where do I look for that? My personal email or somewhere on this forum? It may just be the perspective of the photo but the resistor (so possibly the lead) in question looks pretty close to the can(and possibly the leads) of that 100uF cap and possibly the leads of the preceding 220uF cap.
Jamie
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The image size thing is something we need to work on. Could you try uploading a top shot that isn't reduced by so much?
With the 130 ohm resistor that got hot, are its leads touching any metal that they wouldn't otherwise be connected to?
After an inspection of the overhead shot with more resolution, I can provide you with some steps to split the circuit in half to see which half is causing your issues.
-PB
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Image upload size has been set to unlimited for a long time. There a lots of 6MP and larger images being posted on a regular basis, as witnessed in people saying they are huge when they open them in the Chrome browser (requiring opening them in a new tab). I believe the actual issue may be trying to load multiple images into a single post, but there is no way to control that in the forum admin control panel. Anyway, it's a bug of the forum software.
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My computer takes a little time to load pics in the 3mb and higher res, but if someone links photos to imgur I can see the page but the pics are solid black. Rats.
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Hey all. First off, thanks for everyone jumping in and helping on a Saturday.
Jamie: I think the PM should go to the email that you have associated with your forum account. If you click "Profile" you'll see a little envelope where members can send you a note. Same thing if you click another member's profile. I'm pretty sure it's a perspective thing in the photo as the resistor goes down to the lower terminals and the caps are perched on the top of the upper terminals. There's a good gap there IRL.
PJ: I see no contacts on the blown resistor other than 270K resistor that shares 44L, the 130 resistor that shares 30L, and the 130 resistor that goes from 44L to 31L. The 130 ohm resistor that shares 44L and goes to 31L is just fine, so that might provide a clue?
Doc: Understand the limitations. I'll give it another go with images. I tried some single photos at full size and they produced the same error. Maybe it was just a timeout? I'll give it another try. [EDIT: worked to post one at a time by posting the text first and then "modifying" the post and adding attachments 1 by 1]
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Could have been a timeout. Our webhost has been staving off DDoS attacks the past week and there have been some short service interruptions.
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Witold,
I did not get your email. Try emailing me from your email providers webpage. I emailed you this AM from the forum's member page, did you get that? My correct email is the one that appears on the members page in this forum. If that doesn't work we will figure something else out to get you those resistors. Don't post your personal info directly in this thread! You probably know that already though.
Also, when you remove the bad resistor consider clipping the leads with plenty of length so you can create a hook on the lead to attach the new resistor to. This will leave the solder joint at the terminal strip undisturbed and anything else that is connected to it. I think the bleeders are connected there, I don't remember exactly.Also, clear this with PB. I did this when i replaced my blown 130/2W and it worked out well, but maybe he feels differently about this. I attached a photo of my repaired
130/2W.
Jamie
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Hey Jamie,
I didn't get your email either — even checked spam, made sure my forum account was allowing mail, etc.... I tried to resend again (I discovered the actual email isn't displayed to other members, just the link to the form), so fingers crossed something gets through. I'll give it 24 hours and see if it shakes its way through the internet. If not, I'll try to send a note by raven or owl or smoke signals.
I like the idea on the hook for the resistor — it will be tough to work under all those caps and 44L is a little crowded with that 270K resistor in addition to the 130's. I'll let PB add his two cents, but that will certainly make it easier if (heaven forbid) it blows again.
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I did this on both ends,actually, because the other side connects to the second 130/2W which has the 90 degree bent leads. Just wanted to be clear about that. I found that this approach reduced the complexity of the repair for me. PB does this stuff all the time so his skill set and confidence level may take him a different direction but, yes, get his approval before you do it.
Also, if you connect your components with solid mechanical connection, as Doc advises, this hook approach makes it a lot easier as unsoldering those locked on bent leads is almost impossible, at least for me.
Jamie
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Our webhost has been fending off DDoS attacks and there has been some discontinuity of service here and there for a few minutes. That may have something to do with messages not getting through.
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OK, two connections leave terminal 27 to use the high voltage B+ feed. One goes to the plate choke, the other goes to the C4S board.
I would start by popping the red wire off terminal 27 that goes to the C4S board, then run the amp with no 2A3. You can monitor the voltage at terminal 27 as you power the amp up, it should be about 390V and just kind of sit there.
If you blew that 130 ohm resistor the point where it's open and no longer a resistor, you can temporarily install a piece of wire in its place.
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PB,
I removed the red wire at 27L and replaced the blown resistor with a piece of wire.
Without the 2A3 tubes installed, I powered the amp on and got 0.0v at 27. Half of each of the 12AT7's lit up (the side closest to the center of the amp).
Thanks, Witold
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PB - looks like the resistor from 30L to 27L is blown too. No resistance or conductivity across it.
[EDIT] That resistor is sitting very close to the terminal strip and it looks like it could have possibly contacted the ground buss at 29L. I don't see contact, but it is tight in there and is the closest place where I could see accidental contact occurring.
W
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Either something is touching that shouldn't be or that second 130 ohm resistor is blown and open.
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Hey PB. Yes, the second resistor is definitely blown.
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I don't suppose you have any other resistors sitting around? Ultimately you'll want to replace the 130 ohm resistors and be sure that no part of those resistors touches a lug where the striped end of a power supply cap mounts, as that will just blow those resistors up immediately.
With fresh resistors, you'll want to power up the amp with no 2A3 in that channel and that red wire pulled to see if those 130 ohm resistors hold, then we can start adding stuff back.
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Sounds good. I don't have any extra, but it sounds like @Jamien is going to send some over. If that doesn't pan out, I'll track some down.
Thanks again for your help. Now that I've pulled things apart I'm suspicious of that resistor between 30 and 27 touching the terminal strip or the black wire from the ground buss going into 29. I'll report back when I can replace the 130ohm resistors and then measure 27 again.
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Witold,
Just got home after being gone all day dealing with work stuff. I got your email w/ address and I will send you 6, 130/2W
Tomorrow. I live in South Orange County, CA, so they should get there pretty soon.
Jamie
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Hey Jamie, can’t thank you enough! ;D
This community is the best.
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Witold,
Your resistors hit the mail about an hour ago. I emailed you the tracking info, etc.
Let me know when you get them.
Jamie
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Hi PB,
I replaced the 130ohm resistors, re-did all the resistance checks (all WNL), removed the red wire at 27U, removed the 2A3's, and powered the amp on with the black lead of my voltage meter to 8U and the red clipped to 27U. I'm getting 460v.
I guess the good news is that nothing popped or smoked this time... I should mention all my power transformer secondary tests were totally normal too.
Will wait further instructions from you before proceeding.
@Jamier — thanks again for the resistors! They got here quick as a wink.
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Go ahead and plug in the 2A3, then measure the DC voltages on that 4 pin socket.
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Thanks PB.
A1: 63
A2: 375
A3: 0
A4: 63
D1: 63
D2: 375
D3: 0
D4: 63
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Hey, that's great. You have both HV branches operating with both 2A3s.
Next you will want to attach the red wire back where it went, fire up the amp, then keep an eye on the voltage at pin 1 on the 4 pin socket on the dead side. If it settles to about 375 again and just sits there, then you can measure the output voltage of each side of the C4S board (easiest to just probe the metal tab on each MJE5731A). If they are both around 220V, then your amp is ready to listen to.
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Gents. Sitting in the living room listening to music!
Ran all the voltage checks and everything was right where it should be. Time to sit back and enjoy.
My best guess on the original issue is that the 130ohm resistor between 27L and 30L was too close to the terminal strip and one of the leads shorted against something wired into 28 or 29. This blew both the 130's on that side. Replacement resistors more carefully wired seemed to do the trick.
Thanks PB for all your help and to Jamie for the spare 130's that brought this baby to life. I'll post pictures and impressions once I have put in some listening over the long weekend!
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Witold,
I’m happy to hear that it all worked out.
Jamie
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I learned an important lesson about parts availability when I had my mishap with the 130/2W resistors. When I blew mine up, I ordered those resistors from Mouser, but they said they were out and the lead time was 12 weeks. I thought, surely, it couldn’t possibly be 12 weeks, right? So I placed the order and then called Eileen to see if she had any that weren’t already allocated to kits that were in process. She did have some that were available and she sent them to me. I got them in a couple days, put the amp back together and lived happily ever after (Eileen is, often, the best friend you’ll ever have). Anyway, having forgotten about the Mouser order, 12 weeks later those resistors showed up in my PO Box. So, the moral of this story is; when Mouser says it will take 12, or whatever number of weeks to get something, they aren’t kiddin’. That event also showed me the wisdom of using clip leads, instead of probes, when testing in tight spots. When that resistor blew it was like a firecracker goin’ off.
Jamie