resistance test problem

[sdes crack]

Hi there so I've  just completed my first build of a crack and running the first resistance tests but getting some odd readings.
Its set up to be 240 volt uk power...

terminal 6  - 2.46
terminal 7  - 2.99

terminal 9  - 2.93
terminal 10  - 2.48

both rca terminal centres are reading default 1 rather than the target figures...

All other figures are correct...

Of course I've not powered it up yet but i have had a thorough check of all the connections and can't find anything obvious? although its sure to be.

I'm really not sure what I'm missing and any friendly guidance would be very much appreciated.

Sean

edit typos

 

[Paul Birkeland]

Hi there so I've  just completed my first build of a crack and running the first voltage tests but getting some odd readings.
Of course I've not powered it up yet but i have had a thorough check of all the connections and can't find anything obvious? although its sure to be.

Hello Sean,

The first tests are resistance tests that should be done with the amplifier turned off.  Terminals 7 and 9 will show about 3000 ohms, is this consistent with the reading on your meter? 2.93K would be 2930 ohms.

terminal 6  - 2.46
terminal 10  - 2.48

Terminals 6 and 10 are grounded by the headphone jack when no headphones are plugged in.  I would guess that the junction of the black wires on the headphone jack isn't all the way soldered, so terminals 6 and 10 are grounding through the 2.49K resistors instead.

 

[sdes crack]

Thanks so much for the quick reply!

Your mention of the headphone jack reminded me that I'd placed an adaptor jack in the socket!!!! All figures are normal now that I've removed it ...

Thank you so much!

Onto the voltage checks now

Sean

 

[fromnowon]

Wow, that's very nice wiring!  I love how you straightened the runs and made such careful bends.
I'm going to refer back to your picture when doing mine. 

 

[sdes crack]

Very kind - it helps me see whats what but I must have errors some where, as after the weekend I've come back to the build and I've blown a fuse and now have climbing resistance figures!...

 

[sdes crack]

So I thought I had solved the earlier problem by removing the headphone adaptor (doh) but it was late and I figured I'd resume the build after the weekend. 1 blown fuse later...

... I did the resistance test again and now I have slowly climbing resistance values ( i.e. the value on the meter is slowly getting larger and larger over a few seconds when I touch the following terminals - 1, 2, 4, 5...

Any Ideas what I have done wrong?

Many thanks

Sean

 

[Paul Birkeland]

Your resistance measurements of terminals 1, 2, 4, and 5 are consistent with what's written in the manual.

If the green wire going to A4/5 touches terminal A6, then you'll have some issues.  I can't tell if this is happening or not, but I'd move those apart a bit and do a little trimming on those socket lugs.

It looks like either you are using lead free solder, or not enough heat (maybe both). When I look at terminals 3U and 3L, I would cook those more until the solder flows smoothly.

The UF4007 diodes that are on the inside of the 6 lug strip (above the power transformer) are not mounted in a manner that's consistent with the instructions.  If two leads on two different diodes are touching, and those leads go to different terminals, then you'll blow fuses until your power transformer dies.  There's a big warning with red letters on the top of page 38 warning about this exact situation.  The body of the second UF4007 diode needs to sit against the lead of the first, and they need to be mounted just like the manual shows.

It's totally possible that none of this solves the issue you're having.  Does the fuse blow immediately when you turn on the amp?  If so, then I'd be looking for component leads that are touching things they aren't already connected to.  If the fuse doesn't blow immediately, voltage measurements will be extremely helpful.

 

[sdes crack]

Excellent!

Im going to troubleshoot all of those points - thanks for the feedback...
It's not lead free solder its 60-40 Tin Lead Rosin Core Solder Wire (0.6mm) and I'm using a Hakko iron set to 450C, I'm certainly not very experienced at soldering though and I'll do exactly as you say and reflow those joints. I guess I'm not heating the wires / joints up enough before I apply the solder?

I'll re-do those UF4007 diodes too and hopefully I'll report back with success!

Only blown one fuse (before I bought some more today) - Before it blew, the valves all glowed for about 3-5 seconds...

Thanks again

Sean

 

[Paul Birkeland]

With leaded solder and that hot of an iron tip, you'll get good joints.  You'll see when you heat the joint enough that the solder turns into a runny liquid and wicks into the gaps between the terminals and your component leads.

 

[ALL212]

Also check to insure you didn't solder the insulation instead of the metal.  I've done this a few times getting the wires exactly the right length and ended up with an open connection.  I like the look and it appears you're on the same track but it can give you issues.

Nice wiring layout!!

 

[sdes crack]

Thanks every one for great fast and clear advice! I desoldered those diodes with some wicking copper and trimmed and bent them to spec before re mounting them and ta da! all working!...

Sounds very lush, smooth and warm with the Massdrop 6XX's I bought this for.
I'll live with it for a while before adding the speedball kit I bought at the same time I think, as that was enough soldering for the time being and I want to just enjoy it...

Out of interest though why does trimming the legs to certain lengths and making the diodes touch make a difference? I had the same circuit "on paper" before as far as I can figure out?

I finished it in walnut stain and black spray Hammerite and I'm very happy with this as my first build.
Thanks again

Sean

 

[Paul Birkeland]

It's quite possible for leads of adjacent diodes to touch, and if the wrong two leads are squished together, that shorts out the HV supply.

When you put one diode close to the terminal strip and then the body of the other up against the DC side of the first diode, you greatly reduce the chances of this happening. 

 

[fullheadofnothing]

Is there a particular reason everyone is ignoring the fact that the diode 21L->18L diode was installed backwards, the error was corrected, and THAT is why the amplifier is no longer blowing fuses?

 

[Paul Birkeland]

Ha, I didn't spot that, good catch!