Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: Neuronal on May 18, 2013, 05:59:50 PM
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I was replacing the Shottky diodes on my SEX amp (which, I kid you not, blew out while playing R. Kelly in my lab at like volume 9) and since I am a total repair newbie I think I accidentally touched the side of the 1 uF/630 V cap as shown below and melted the outside some. What to do? I could ignore it, cover with electrical tape or call the Queen to replace the cap (and potentially melt some other part during that replacement install :) ) Any advice is truly appreciated!
best to all - Bob D.
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Replace the cap. There is a significant possibility that it is now internally shorted, or that it will soon be shorted.
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Thanks Paul!
I was planning in the future to upgrade to new iron with the C4S board and new caps. I already have bought the cap upgrades - for the coupling cap (which is the one I think I melted) the ones I have in hand are Auricap .2uF/600V. Four easy questions: Can I just replace the coupling caps now without making any other changes? Does orientation matter? Because of the size of these new caps, I am also worried about space - can I leave the leads long and tape the caps into place? Will doing this now get in the way of the iron/C4S upgrade?
Thanks again for the help - I would love it if I could just drop these in!
best, b
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The 1uF cap - the one with the hole in the picture - is the parafeed cap. The coupling cap is 0.1uF; 0.2uF is an acceptable deviation for coupling in the SEX amp, but not for parafeed.
Otherwise, yes - no - yes - maybe. Mounting non-stock parts is often an issue, and always an issue you have to resolve yourself. That's part of the fun, right?!
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Paul - thanks again (also for straightening me out about the parafeed and coupling caps - I have a 2 uF/600V cap I can use to replace the parafeed cap)!
I am almost certain to now have another problem, having replaced the blown Shottky diodes and replaced the parafeed caps. When I run the resistance checks it is perfect, and when I power it up the tubes glow and the thing plays music fine BUT...there is still a low level buzz coming out my transformer (same as before the diodes were blown), and the right tube is way hotter than the left. When I switch tubes the right tube is again way hotter (like too hot to touch within 30 seconds, whereas the left tube is just warm), meaning this is something about the circuit and not the the tube. I am taking voltages now, and will post them, but any preliminary thoughts would be appreciated....
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It sounds like the right side tube is not properly biased. That could be due to a bad cathode bypass capacitor. Start by remeasuring your voltages.
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Total disaster.
I was measuring voltages, which seemed totally off anyways (f/e terminals 1 and 4 were in the millivolt range). When measuring 24 I shorted across my meter lead to 25 (I think - I thought that my hands were pretty steady); I heard a pop, and then most of the voltages went to zero. Fuse was intact, but now terminals 1 and 4 and pretty much all of the other terminals 1-40 read zero, and my tubes do not glow. My wife pulled me away from the rig before I could get any additional voltages (she thought I would be better at this with a night off), so I don't know the numbers for the transformer or C1-C5. Any ideas as to where to start figuring out what went wrong? As you can tell I am totally new at this, but I would love it if I could fix this thing myself. I will recheck resistances tomorrow.
Back in med school I learned a word: iatrogenesis. It is when, in an attempt to fix things, we make them worse :(
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Start with transformer AC voltages. They are measured by putting your Red/Black probes on the pairs of power transformer terminals.
http://www.youtube.com/watch?v=_ZRkGL0eISE (http://www.youtube.com/watch?v=_ZRkGL0eISE)
Let us know if the values are about where they should be, though you may see some deviation for other reasons.
-PB
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Thanks! Will do this and get right back!
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So ignore most of the last post: the short-circuit so scared me that I (a) had my meter on VAC the whole time and (b) put the fuse in wrong. Fuse in right, both tubes now glow with (L) tube relatively cold and (R) tube relatively hot.
Took voltages, transformer VACs were dead on, as were all the A side readings (which corresponds to the Right tube).
The B side (the one with the relatively cold tube) had lots of errors (if not indicated, reading was normal):
31 -6.8 VDC
36 -.7 VDC
B2 -.8 VDC
B5 -.8 VDC
and the plate choke on that side reads:
-.9 VDC for all terminals (1,4,5,8).
The C1-C5 is also normal (which, I hope, means I installed the new diodes correctly).
Any ideas? I assume I should be getting current through the plate choke so something must be wrong on that side :)
Thanks, again, to all of your for your patience and help - I'm hoping to level up as I learn to go through all of this.
best to all - b
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I would look for inconsistencies between the power supplies on each side.
-PB
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Where do I look for/how do I identify those inconsistencies? Thx again - b
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Visually - since you have the older SEX amp, both power supplies should look roughly the same.
No voltage on any of the PC-2 terminals means that a ground connection is loose or missing, or another part to that power supply is awry.
The dysfunctional power supply is on terminals 21-30, the working one on the A side is 1-10. You can compare voltages from side to side as well.
You have roughly 0V at 31/36, you'll need to see those bump up to 300+ for order to be restored.
-PB
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Yes replace the cap... I didnt read any of what is written in this thread, but it doesnt matter. For us wacko's upgrading caps is like taking a dump. You dont think much about it. It comes natural.
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Both sides look normal (nothing burned, loose or bulging), so I rechecked resistances, and they are normal throughout (would this be the case with a missing ground?). I also checked voltages to compare 1-10 and 21-30, and all are normal EXCEPT 26 is 466 VDC and 19 is Zero volts. 21 (233V) and 22 (233V) and 24 (220V) are also on the high side. So definitely something wrong with the power supply on that side - any suggestions as to how to proceed?
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There's a resistor between 26 and 31. You have high voltage on one side, and 0V on the other.
Either you have a short after the resistor, and you are drawing all the current through that resistor (this is 99% unlikely since the voltage at 26 is so high). The other possibility is that this resistor is loose or has a funky solder joint that is preventing it from passing voltage to the rest of the circuit (and your plate choke for that matter).
-PB
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great! I'll start by reheating those joints and will let you know how it goes!
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Definitely onto something.
When I measure resistance between 6-11 I get the expected 680 ohms. Between 26-31 my ohmmeter varies, like I am charging a capacitor - it slowly ramps from the kiloOhm to megaOhm range. If I come back later the resistance is back in the kiloOhm range. I measured capacitances across all of the caps on both power supplies and they seemed comparable, and I couldn't identify any obvious weak joints or shorts. Any ideas for where to head next?
Also, a related question: this all came about after I replaced the shottky diodes and the 1 uF parafeed caps that run from A2-20 and B2-40 with Auricaps. The serial numbers on the Auricaps and their dimensions are consistent with them being 2uF/600V caps (what I ordered), but when I measure their capacitance in the context of the circuit (installed) they measure about 20 uF. All of the other caps in the power supply measure what they are rated, so I am not misreading the meter (at least this time). Does the parafeed caps reading 20 instead of 2 make sense?
As always, thanks in advance for your collective advice - I can feel myself getting closer to fixing this thing!
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Definitely onto something.
When I measure resistance between 6-11 I get the expected 680 ohms. Between 26-31 my ohmmeter varies, like I am charging a capacitor
Is the 680 Ohm resistor present between 26 and 31? If so, can you measure it?
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Yup, the 680 Ohm resistor is present (and seems perfectly fine from the outside), but when I measure across it I get this very strange behavior where is looks like a capacitor (where the resistance slowly ramps from kOhms to mOhms). I could even measure a capacitance on my meter across this junction (about 30 uF). I get this result whether I measure the top or the bottom tabs, which makes sense (680 Ohms on the working side, the changing resistances on the broken side). I reheated the joints at 26 and 31 and added a bit of solder, which did not change this property. Comparing the capacitors on both sides of the power supply give essentially the same capacitance readings, and when I use the ohm-meter to measure resistance of each capacitor, all of the caps charge and discharge as one might expect, and so from my naive perspective the caps seem fine.
Is the right move to replace the resistor? The fact that the 26-31 junction looks like a capacitor makes me think I must have a short somewhere else, but I don't understand (although I wish I did!) the underlying circuit.
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If the other 680 is reading fine that 680 is probably open. One thing that is important is to measure with the probe tips touching the actual leads of the resistor, not the terminals it is soldered to. That way the measurement can't be affected by a bad connection to the terminal. If that doesn't bring any new info to light, the most sure way to verify the resistor's value is to disconnect one end from the circuit so it can't be affected by caps charging or other components' resistance.
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OK, I will remove one end of that 680, remeasure, and report back!
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So I removed that 680 Ohm resistor between 26 and 31, and sure enough, it was open!
When I now measure resistance across terminals 26 and 31 (in the absence of the resistor) it charges like a cap, just like before when the broken resistor was in place, so I assume the capacitance is some sort of circuit property that reveals itself when the resistor is blown or removed. I will call the Queen today for a replacement. If there is anything you think I should check before replacing the resistor, please let me know. Will report back when I install the new 680. Thanks so much Paul and Doc - you guy are amazing!
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Yes if the relatively low value 680 ohm resistor is functional it will effectively shunt any charge buildup across a large value cap that is in parallel in the circuit, and thus you won't see much charging behavior in the ohm meter. Once that resistor opens up, the cap will exhibit that typical "rising ohms" characteristic when the meter battery tries to charge it.
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I just wanted to thank everyone again for their help with this problem - I replaced the resistor, and the diagnosis was correct! Voltages all dead on, and really great music from both channels. Thanks so much for the help - you guys really are the best and make this hobby a ton of fun.