Might have ruined the rectifiers? (Resistance check failure help) [solved]

[senpai3330]

Second time crack builder, somehow managed to not mess up the first time only to do it the second time.

I did resistance checks with one lead clipped to terminal 12 as recommended. Every resistance tested fine until I had the other lead clipped to terminal 20. The resistance was supposed to read 0, but it looked like it was climbing toward 270K similar to what it should've read had I clipped the lead to terminal 13. I thought it was weird everything else was fine and only this one terminal check point was off, so I went ahead and tried to do voltage checks anyway. Heaters were working, tubes glowed fine, but the first couple of check points all read ~1.0V instead of 90V, 170V, etc. I decided I should stop pushing my luck and turned it off.

I went back step by step, did continuity checks for each point to point connection and reflowed any of the duller looking solder points. Unfortunately, while I was doing this, I think I discharged one of the 220 uF capacitors (big spark) because my soldering iron shorted two of the rectifiers together (picture below).

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FLDvtHT6.png&hash=de6f3e82363df1d6dfc5fe179358b7b7deed02b2)

Is that enough to ruin the rectifiers?

Note: I did make the 5/6/10 revision, transformer terminal 4 was connected to 14U instead of 22L.

[senpai3330]

The resistance on 20 still doesn't read zero and continues to rise.

[Grainger49]

I assume that that is a grab from the manual not your second Crack in the picture.  Did you solder with power on?  Did you unplug?  If you wait 30s the charge is very low on the capacitors.  So unless you didn't wait or soldered with the amp hot (always a bad idea) you only zapped a little voltage.

Try testing the diodes using your meter.  They should show a low resistance with the black lead on the striped end and high with the red lead on the striped end.

If one or more diodes test bad the UF4007 diode(s) could be blown.  They are cheap, maybe 17 cents?

If you can source them locally you are back to work fast, or call/email Bottlehead for replacement parts.

[fullheadofnothing]

Something is very wrong if you got a spark on an amp that is not connected to power. Please post pictures of your actual build. Do you have the black wire connecting 20U to 14U?

Additionally, one resistance being wrong is a sign something is wrong with the wiring. It is not safe to power on an amplifier that does not pass resistance checks.

[senpai3330]

Try testing the diodes using your meter.  They should show a low resistance with the black lead on the striped end and high with the red lead on the striped end.

If one or more diodes test bad the UF4007 diode(s) could be blown.  They are cheap, maybe 17 cents?

If you can source them locally you are back to work fast, or call/email Bottlehead for replacement parts.

I tested the diodes, they're reading ~560 ohms each, I don't know if that qualifies as low or if you're talking about ~0 as low. I'd probably just get them from mouser since I was going to get some other supplies as well. I'll call Bottlehead to verify, but I'm assuming it's just these UF4007 from Fairchild. http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/UF4007/?qs=sGAEpiMZZMtbRapU8LlZDz3zbYnpxBJB5elDBydTRZE%3d

Something is very wrong if you got a spark on an amp that is not connected to power. Please post pictures of your actual build. Do you have the black wire connecting 20U to 14U?

Additionally, one resistance being wrong is a sign something is wrong with the wiring. It is not safe to power on an amplifier that does not pass resistance checks.

Again, I think the spark was from a capacitor that hadn't fully discharged yet. I'll post photos when I'm home later. I do have the black wire connecting 20U and 14U.

Thanks for the help!

[senpai3330]

Update: I took the nuclear option and deconstructed a lot of the parts around the transformer area, especially anything to do with terminal 14U (overcrowding) and 20 (weird resistance).

I moved some of the wiring to 14L and installed the capacitor that goes across terminal 12 and terminal 13 on some short leads for now to free up some working space around there.

Resistances all check fine now! But I'm not sure if that's a valid statement with the rectifiers potentially dead. The tubes no longer glow when I turn the amp on now, I'm assuming dead rectifiers would explain that?

Small photo gallery: http://imgur.com/a/9FaIW

The resistors and output caps toward the front are on messy long leads since I'm removing them and installing speedball/new caps after this troubleshooting is solved, but they haven't given me any issues.

[Doc B.]

Have you checked the fuse?

[senpai3330]

Have you checked the fuse?

Hi Doc,

I did pop out the fuse holder just out of curiosity earlier. The little filament inside looks the same as when I installed it so I'm assuming it didn't blow (melt?).

On a more general note, could someone confirm whether the 560 ohm resistance reading across the rectifiers is indicative of them failing?

[Grainger49]

I would need the answer to these questions to tell you how bad the spark was.

Did you solder with power on?

Did you unplug?

Did you wait the required 30s?

[senpai3330]

The amp was off and unplugged. I feel like I definitely spent more than 30 seconds looking over the amp before the short happened, but the spark still seemed kinda strong with a loud pop/crack noise.

[Grainger49]

Then, if you have the discharge resistor in your power supply, a 270k IIRC, the voltage involved would be minimal.  That would have been downstream of the diodes.  Even if they were shorted when you were soldering the voltage, again, would be minimal.  It should cause no damage.

Is it possible you had the meter on VDC instead of VAC?  That will read a very low voltage regardless of the DC present.  We did that last weekend checking out the newly built Paramours.  A D'oh moment!

[fullheadofnothing]

You got a spark because your the the first cap in the supply was not connected to ground. The resistance check showed this. With the end of the cap not connected to ground, it held a charge until it had the ability to discharge it, which it did through the tip of your iron.

Looking at your build, I see a lot of untrimmed leads, which are a shorting hazard. I also see a lot of unnecessarily long leads, which also pose shorting risks. The general decisions to not follow the manual (non-stock wiring and parts, not orienting parts as shown) also make it hard to visually tell where there may be problems.

To answer your question, no the tubes not glowing is not a sign of dead rectifiers. The glow is from the heater supply (the twisted wires connecting the power transformer to the tubes); the rectifiers are in the high voltage supply.

Please post your voltages to determine the source of your problems.

[senpai3330]

@Grainger

My multimeter was definitely on DC, but yeah that would've been embarrassing haha.

@fullheadofnothing

It's a messy build, but a lot of it is just for ease of parts removal later. I'm very confident the leads sticking out aren't an immediate issue. Sorry the non-stock parts add to the confusion.

I think I figured out why the heaters aren't working.

Each of the transformer's soldering tabs are wired to the transformer itself with some fairly thin wires that peek out from plastic molding. I think I severed the transformer terminal 5 wire when I was trying to remove a solder bridge. Crossing my fingers and hoping this is it. I'll post results/voltages when I get a chance in a few hours.

[senpai3330]

Yeah it was that wire for the transformer terminal, the voltages checked out fine (all within 10%).

For the dc offset on startup, I peaked at around 11.3V. Is that too high?

1    88 (90)
2    171 (170)
3    0 
4    171 (170)
5    85 (90)
6    0
7    109 (100)
8    0
9    107 (100)
10  0
11  0
12  0
13  170 (170)
14  0
15  192 (185)
20  0
21  212 (206)

A1  83 (90)
A2  0
A3  1.6 (1.5)
A4  0
A5  0
A6  86 (90)
A7  0
A8  1.6 (1.5)
A9  0

B1  86 (90)
B2  171 (170)
B3  108 (100)
B4  83 (90)
B5  171 (170)
B6  107 (100)
B7  0
B8  0

[Doc B.]

Those numbers are fine, and the 11.3 is OK too. Do take some care in making sure the excess wires poking through terminals are trimmed short and that no leads from neighboring terminals are close to touching. This is an artform where neatness counts.