Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: TexFlood on February 28, 2016, 11:05:38 AM
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First build blowing fuse at voltage check. Fuse blows immediately when plugged in with no tube glow, no pops, no smoke, nothing. Fuse does not blow with power switch off and plugged in. Resistance checks all on point other than a few * values which could be questionable. Terminals 47 and 48 read 0.L or open according to my meter (had initially thought this meant out of range and thus proceeded to voltage check). Terminals 7 and 14 read 4.69 and 4.7 M ohm, and do not fluctuate. A few other terminals with * values fluctuate in the M ohms. I have checked and rechecked all wiring, solder joints (reflowed any remotely questionable), part orientation and values, and checked for shorts with flashlight and magnifying glass. Any assistance is greatly appreciated.
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Perhaps some pics will aid in the investigation.
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A few more.
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Hey Dan,
Thanks so much for all the help. Disconnected the red wires and fuse still blows. Does this mean we've ruled out issues with this section and issue lies in the filter cap section? I'll wait to hear from you as to how I should proceed.
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There's one more step, to disconnect the black wires going to the plate chokes next. If the fuse still blows after that, the 2A3 and 12AT7 circuits are pretty much ruled out as the cause and the problem will most likely be somewhere in the power supply. I apologize that I haven't had the chance to look at the photos you posted yet, but I will try to give them a look this evening.
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Black wires from PCs disconnected and fuse blows.
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OK. Just to verify, the transformer passed the voltage checks without any weird readings when you completed that step, yes?
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Yes
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I don't think this is necessarily the problem since the fuse only blows with the switch on, but I notice that the foil on the shielded twisted pair cable coming from the power switch back to the power entry connector looks very close to the metal of the live connection on the connector, at least from the angle in your photo. Might want to make sure it isn't touching. Also just to cover all the bases, what size fuse have you been testing with?
Next I will suggest inspecting the rectifiers. Amp should be unplugged. Does your meter have a diode test setting? The setting will have a little triangle and straight line symbol if it does. If not, just set the meter to the K ohms range.
Touch the black test lead to one end of one of the UF4007 rectifier diodes at power trans terminal 19 and the red lead to the other end of that same diode. You should see either an OL reading or something roughly around 1.5K ohms. Swap the leads the other way around and check again. If the numbers aren't exactly those it's OK. If you see a very low reading like under 100 ohms, that diode is bad. Check both diodes the same way.
I will also suggest carefully rechecking the connections all of the resistors that are connected under the filter capacitors. A miswire anywhere in there could definitely cause a blown fuse.
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Hey Doc,
Checked foil and it looks ok - Its not touching.
Fuses are 1.5A-250V fast blow.
Rectifiers checked on diode setting:
0.L - .519 V
0.L - .522 V
Checked connections of resistors under filter caps and they look good. I did touch up a solder joint at 29L while in there.
Thanks again for all the help.
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Try a slow blow fuse at the same rating as the stock fuse. If there is a short somewhere it won't help. But its worth a try.
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Fuse shows catastrophic failure. Is use of slow blow still advisable?
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Could you measure the resistance of each of the 130 ohm resistors that are connected to the 100uF 450V filter caps? That would be measuring resistance between T44 and T31, T44 and T30, T31 and T34, and T30 and T27. Check that there are no excess leads hanging off at the same time.
Also make sure that the 270K ohm resistors at between T41 and T42, and T44 and T45 are connected to the proper terminals and no excess leads are touching the chassis.
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Doc,
Between:
44&31 = 128.9
44&30 = 128.9
31&34 = 129.6
30&27 = 129.1
Checked resistors between terminals 41 and 42, 44 and 45 - they are connected to proper terminals and no excess leads are touching the chassis.
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OK, we're running out of things to try without detaching a connection or two. Let's take the power supply filter completely out of the equation and just run the high voltage power transformer secondary unloaded. Sorry to do this as it's kind of PITA, but detach the pair of UF4007 rectifier diodes from power trans terminal 19. Also detach the white wire from T20. This will completely disconnect the secondary from any potential shorts in the PS. With the tubes out of the amp try powering up and see if the fuse holds.
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Terminals 19 and 20 free of connections and fuse intact. I do hope the poor diodes survived.
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OK there must be something pulling excess current in the power supply. Do any filter capacitors look swollen on the end cap?
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No. Perhaps I should have tested once more before disconnecting 19 an 20 to determine if reflowing 29L made any difference.
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Don't reconnect them yet. Measure resistance from T41 to T44. You may see cap charging/discharging behavior, where the meter climbs up or down. That's OK. We're looking for a really low ohms reading somewhere, that shouldn't be low.
It's really stormy here at the moment and my monitor just flickered like the power may fail. If I don't answer right back it's not because I am ignoring you.
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Between 41 and 44 is high 2 M ohms and dropping.
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OK, now measure resistance between T41 and T42, and T44 and T45.
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Both start in the 2 M ohms and drop.
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I am looking at one of your photos that shows the resistor connected from T27 to T30. In the manual it is attached to the lower terminal holes. In your picture it looks like it might be attached to the upper holes (difficult to tell) and the lead of the resistor connected to T30 might be touching a capacitor lead at T29 that it should not touch. This may all be wrong and it's all connected correctly, it's hard to tell from the angle of the shot.
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Resistors are both lower. There is just the reflection off the black wire from 29L to 41L going under the resistor.
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One more thing - While doing the last resistance checks, I noticed that as I put pressure on the terminals with the leads, the terminal strip would move and as it did I would get 0.L reading then back to dropping M ohm reading. This would happen with pressure and movement of the terminal strip.
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The measurements across 41 and 42 and across 44 and 45 should not go to zero. Sounds like something is shorting out as the strip is moving, and that could definitely take out a fuse. An very thorough visual inspection around that T strip seems like the next step.
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Reflowed 30L and 31L where resistors attach and cross over to 44L . Readings are now M ohm and dropping.
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Ok, it sounds like it might be time to reattach the rectifiers at 19 and the white wire at 20 and try it to see if the fuse holds.
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Fuse blows.
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OK, unplug amp. Clip black meter lead to 8U and remeasure resistance of T21 thru T35 and T41 thru T45, looking for discrepancies from the numbers in the manual.
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No discrepancies.
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Hey Doc,
1.6A slow intact. Time to reassemble?
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Excellent! I think it's time to put it back together and check some voltages. Congratulations, I admire your sticking with it! Isn't it funny how it's always the last thing you try that solves the problem? How the heck does that work?
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Voltages are on/within tolerances except:
T4 - 19 mv (fluctuated as high as 32.4 mv)
T7 - 244
T16 - 7.4 mv
T17 - 54.5 mv (fluctuated up and down like T4)
T29 - .9 mv
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Those are all just fine. Sounds like you need to hook that puppy up and listen! Well done!
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Hey Doc,
1.6A slow intact. Time to reassemble?
So what was the fix?
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It seems the stock 1.5A fast blow fuse was just a bit to quick to blow. The 1.6A slow blow was enough to get past the initial current surge. It just needed more slow and less blow.
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We spec fuses with the idea that we want to avoid any damage due to the relatively high possibility (compared to a factory built product) that there might be an error in construction. Thus we spec them really close to the maximum startup current inrush. Sounds like what has happened in this case is that small batch variations in the charging current of the filter capacitors just stacks up such that the occasional amp needs a tiny bit higher rated fuse. I think this is the first time we have seen that with Stereomours. I do recall having a similar situation occur a few years ago some Crack amps, IIRC. Ofttimes that fuse blowing will just happen the first time, and after the caps have formed you can get away with the stock value. But for a .1 amp difference like we see here I wouldn't even worry about that, just leave that 1.6A slo-blo in there.