Crack blowing fuses? (solved, THANKS!)

[smog]

Hi, brand new builder here.  No real electronics knowledge but can follow directions and solder.  I bought a Crack kit back in 2013, life got in the way but after nearly four years I finally got around to building it over the past couple days.  After completing the build and visually inspecting it for quite awhile I was confident that everything was where it is was supposed to be in the correct orientation.  I moved onto the testing.

Resistance checks were close to perfect.  Terminals with * were fluctuating similarly. Terminals expecting 2.4K read 2.48K.  Terminals expecting 2.9K read 2.94.  RCA jack grounds 0.  RCA Jack Center Pin were both between 90K-100K.

So, I went to the next stage and powered it up.  Tube filaments were glowing.  I took Voltage measurements...but didn't write these down as everything was reading quite close to the expected voltages.  Certainly within 10%. 

Except for B1.  Manual says this should read 90 but it was reading 145.  I made a note of that.

I went ahead and performed the Final Check at the TRS jack and it climbed up to 9V and then dropped to zero.

I retraced the wiring and checked the orientation again and it all checked out.  I trimmed up a couple spots where wire was sticking out a bit.  I found one wire that wasn't properly soldered on the TRS jack.  I re soldered B1 along with where the wire from it attaches at the Terminal.  I re soldered at the IEC and a couple spots on the transformer.   

Went through the resistance measurements again and they were once again almost perfect.  Powered it up to take voltage measurements but the tubes didn't light up.  Powered it off and checked the fuse, it was blown?   Today I picked up a couple replacement 500ma fuses.  Each time I powered it on, the fuse immediately blew.   

I've been searching through this site and read that blown fuses might indicate blown rectifier diode.  I don't really know enough about testing them, but read the manual of my DMM and it has a Diode mode.  I took readings of them, from one orientation of the probes it would read .45V and with the probes reversed it would read 0L.  This was the same with all four of the rectifier diodes.  I don't know what that means.

I don't know what else to do at this point?  I've read most problems are the result of a bad solder joint.  But would bad solder joints cause the fuses to blow?  I've also read on the forum that the crack uses 1 amp fuses now. My kit came with 500ma fuse.  Should I try 1 amp fuses? 

Any help would be greatly appreciated.     

Many thanks! 

[Doc B.]

Yeah, try a 1A fuse. If it runs, go back to voltage measurements and make sure you have the issue at B1 worked out. Sounds like the rectifers are OK, so If it blows you will need to look for a backwards capactor or a miswire.

[smog]

Hi, thanks for the quick reply.  Will pick up some 1A fuses tomorrow.  Here's some pics of the orientation.   I'll retrace the wiring again.  If it powers up with a 1amp fuse and the B1 reading is still way off what steps should I take?  I have no knowledge about circuits.  In the one photo you can see that i touched the iron to the jacket of that red wire and it melted a bit.  I was going to replace that length, it is attached to the terminal B1 is attached to...any chance that could be related to the reading being off?   

Thanks for the help!     

[Paul Birkeland]

B1 is connected to terminal 1 with a piece of wire.  Consequently, if you've measured your voltages, T1 should also be off.  B3 is partially controlled by the voltage at B1, so this voltage should also be much higher than the manual allows for.

Do both LED's on the 9 pin socket glow? (if one doesn't light up, don't focus on it.  It is likely not the issue)
If not, do both sides of the 12AU7 light up when you power up the amp?

Having 145V at terminal 1 and one LED out no the socket would mean that half of the tube isn't being heated, which is usually caused by the wire going to A4/A5 not connecting those two socket pins together well enough.

Having 145V at terminal 1 and both LED's lit on the socket can either mean that there is a short between terminal 1 and terminal 2 (or potentially a short in the 6080, you can pull the 6080 and just run the amp with the 12AU7 to see if that voltage drops down), or this can be caused by accidentally not installing the jumper between the ground lugs on each level of the volume pot.

-PB

[smog]

Hi, Thanks so much for the help.

I was only able to find more 500ma fuses local so I bought a hand full.  Will have to wait till friday to run into town to find some 1 amp fuses.  I went through and resoldered all the joints on the IEC, ground lug, and to the Power Transformer.  Unfortunately the 500ma just keep blowing on power on.

When it was originally powering on and I was able to take the Voltage readings, it looked like only half of the 6080 was lighting up.  I think both sides of the 12au7 were lighting up.  I didn't think to look at the LED's, so not sure what either was doing.  I realize now I should have been writing down the Voltage readings but at the time only B1 was way off.

After reading your message I went and just resoldered the wire at B1 and at Terminal 1.  Also resoldered the upper Terminal 1, and Terminal 2.  There is nothing touching between Terminal 1 and  Terminal 2 other than the resistor.  I resoldered the A4/A5 joints.  The black wire from the rca jacks was soldered between the lower and upper lug, I just resoldered both.  I hope that is what you mean by the jumper?   

I just powered it on with only the 12u7 installed, but the fuse blew immediately.  I tried using a different 12au7 and same result? 

I included some more pictures.

Thanks again for all the help   

[Paul Birkeland]

I would post photos of the power supply and the rest of the amp.  There's probably something we can spot if fuses are blowing that readily.

-PB

[smog]

ok.  tried to get some good shots. 

thanks again for helping!  i'm going to pick up 1amp fuses today.

[Paul Birkeland]

The two UF4007 diodes on the inboard side of the 6 lug strip would appear to be touching and may have shorted out the power supply. (Just bend them away from each other so the leads on the capacitor side don't touch)

-PB

[smog]

Ok, Thanks, I'll double check that. 

I picked up 1amp fuses, did resistance check and it was near to perfect.  Powered on the amp and the tubes came to life!  Both Filaments in the power and pre tubes lit up.  The LED closest to the front of the amp lit but not the back LED.  I took two sets of Voltage Readings, first with both tubes in the amp.  Then I pulled the power tube, and ran a second set of measurements again in case the power tube might be the problem.

With both tubes installed Terminal 1, and Terminal 7 are both over range.
A1 is close to 15% under voltage
A6 and A8 are both way over range.
B1 and B3 are both way over range.

I don't know what to make of the reading with the power tube pulled?

T     Expected   Both tubes...........with only 12au7
1                90             147..........232.7
2              170             166..........233
3                  0                 0.............0
4              170             166..........232.6
5                90               79.5.........96.8
6                  0                 0.............0
7              100             146.2..........0
8                  0                 0............ 0
9              100               99.9..........0
10                0                 0............ 0
11                0                 0............ 0
12                0                 0.............0
13            170             165.5.......232.7
14                0                 0.............0
15            185             189.6.......235.2
20                0                 0.............0
21            206             212.6.......236.2


A1              90              78.9.........96.6
A2                0                0.............0
A3               1.5             1.6...........1.5
A4                0                0.............0
A5                0                0.............0
A6              90            147..........233
A7                0                0.............0
A8              1.5            12.4.........19.3
A9                0                0.............0

B1              90            147..........232
B2            170            165.3.......232.8
B3            100            146.4..........0
B4              90              78.9........96.6
B5            170            165.7.......233.2
B6            100              99.6..........0
B7                0                0.2..........0.2
B8                0                0.............0


I don't know what to do next.  So happy it powered on without blowing a fuse, but don't know what steps to take next.  I assume I need to resolder at the the joints where readings are off?

Thanks for all the help!

[Paul Birkeland]

The 12.4V at A8 does actually indicate an issue with the LED. Sometimes if they are stored for long periods of time, they can absorb some moisture and when you solder them, they crack as the moisture boils out.

Luckily, I think you will be just fine if you solder a wire from A3 to A8.

[smog]

I checked and those diodes were not touching, but I moved them further apart anyways. 

Just soldered in a jumper between A3/A8 and here are the Voltage Readings.  Took reading with both tubes and then with only the 12au7.

T    expected    both tubes.........only 12au7

1    90                   83.4..........236
2    170                171............237
3    0                        0...............0
4    170                171............237
5    90                    82............236
6    0                        0...............0
7    100                103...............0
8    0                        0...............0
9    100                103...............0
10  0                        0...............0
11  0                        0...............0
12  0                        0...............0       
13  170                171............238
14  0                        0...............0
15  185                193............238
20  0                        0...............0
21  206                213............237

A1  90                   81.4..........237
A2  0                       0................0
A3  1.5                   1.6............20
A4  0                       0................0
A5  0                       0................0
A6  90                    83.3.........237
A7  0                       0................0
A8  1.5                   1.5............20
A9  0                       0.2.............0.2

B1  90                     141............237
B2  170                   159............238
B3  100                   140...............0
B4  90                     140............238
B5  170                   159............238
B6  100                   139...............0
B7  0                           0.2............0
B8  0                           0...............0


Everything looked good and close to expected until I got to the B readings.  Could that indicated a bad tube?

Thanks again for taking the time to help me out

[Paul Birkeland]

You have some pretty serious measuring issues going on here.  For instance, you quoted terminals 7 and 9 as both being 103V, but B3 and B6 are more like 140.  There are sold wire connections that connect these points together, they can't differ by 50V.  Another example is B2/B5/T2/T3.  All 4 of those are wired together, and your readings indicate very different voltages.

Just out of curiosity, can you inspect the solder tab on the headphone jack where all the black wires meet.  Are they all well soldered?  Can one of them be moved by chance?

Your readings for both tubes of terminals 1-21 are what I would expect for a working amplifier, but the inconsistencies tell a different story and would have me looking for a soldering issue wherever two or more black wires meet. 

[smog]

Yeah, I can't understand why I'm getting these readings??  I understand what you're saying, but don't have a clue.  Every time I've taken measurements I've used the exact same method, attaching the DMM ground with an alligator clip to Terminal 12.  Been extra careful when touching a terminal with the probe to take measurements. 

Regarding that headphone jack where all of the black wires meet, YES!  After my initial testing was off I inspected every joint and found that one of the black wires on the inboard side wasn't soldered.  I re soldered there, but I initially powered on the amp that first time with that wire not properly soldered.  From there on the 500ma fuses kept blowing.  Since using a 1A fuse, it has not blown.  I'll be sure to re-check all of the black wire joints.     

Today was much better.

After originally finishing the build and getting the high B1 Voltage Reading I searched through the forum and found out about those LED being fragile.  Saw that Bottlehead sold replacements and that they should be used within a year.  Well, considering I bought this kit in 2013 I went ahead and ordered the LED pack.  Those arrived today, that was pretty quick! 

At some point after/while taking those last posted Voltage readings with a jumper wire between A3/A8 the A3 LED also went out. 

I have been worried that the 6080 might be the cause of some of these troubles, though looks like the LED could also have been suspect.  So, today I used a 7236 that I know works.

First I installed 2 new LED at A3 and A8.  Also tidied up joints and replaced a couple lengths of wire.  Everything looked visually good when I powered on, voltage readings were looking great but as soon as I touched the probe to the A8 terminal that LED went out and the voltage reading was 12V.  I guess I must have made a bad joint?  So, I replaced that LED with another new one and made sure I had a good joint without overheating.  Powered it back up, both LED lit up.  Everything was looking good and ALL of the Voltage Checks are within 10% of the expected values now!!

I'm relieved that all the readings are finally in spec.  But kinda hesitant to hook it up and listen, guess I wanna savor the small victory for tonight in case there are still other issues to face tomorrow, haha. 

I really appreciate all of your help, many thanks

[Paul Birkeland]

Yeah, the bad solder joint at that black wire junction on the headphone jack will lead to weird voltage readings and a terrible sounding amplifier.

Regarding soldering the LED's, they are really tiny, and everything you're soldering them to is really tiny, so they don't need much solder or heat.

It sounds like you're ready to give the amp a listen.

-PB

[smog]

I took all new measurements today and voltages were close to expected, no more than 10% off so I fired it up.

It sounds incredible!  I listened for a couple of hours and only beautiful music was coming through my cans, no weird noises. 

Thanks again for taking your time to help me out.  I already ordered the Speedball upgrade kit, the pics of good/bad solder joints in its manual were very helpful for me...there were some joints I made on the Crack with too much solder.  Hopefully it won't take me another three years to find the time to build/install the Speedball

All the best