Old Crack - new problem [resolved]

[loweart]

Hi Peeps,

It's been a while since I've used my Crack Amp and I recently connected it up and fired it up to check it was working correctly before loaning it to a friend to listen to and it was working fine for the 30mins that I had it running. I got a call from my friend saying that after he had it on and was listening to it for about an hour, he thought he heard a slight pop and then no sound in one channel ( he didn't mention which channel ). I picked the amp up and confirmed this and I have swapped both the the 12AU7 and the rear 6080 tubes out with other ones and the same thing, no sound from the left channel, no hiss, no hum, no crackles, just dead silent.   

I've opened up the case and visually inspected the 2x 100uf 160volt capacitors and could not see any signs of leakage or brown/burn out or bulging. I've also checked the solder joins at the headphone socket and volume pot and also around the tube sockets and nothing loose and I have even touched up the solder joins in these areas.

HOPING someone here can provide some advice/help on how to troubleshoot this further ?  Could it be those caps or maybe those 2.49k ohm resistors in headphone socket that's gone bad ?

My Crack is the first edition I believe and the build manual is dated 5/28/10 and I think I bought this kit in 2011 ?  It has worked reliably since it was first assembled by me until this recent incidence and frankly it's had an easy life with only about 800 to 900 hrs of use over the years on a Sennheiser HD600 as it's not my main rig..

Thanks in advance for any help provided in regards to what to look at or what the issue could be ?

[Paul Birkeland]

HOPING someone here can provide some advice/help on how to troubleshoot this further ?  Could it be those caps or maybe those 2.49k ohm resistors in headphone socket that's gone bad ?

You would need to start with the voltage check in the manual.  Can you post a photo of the entire bottom of the amp?

[loweart]

Thanks for coming to the rescue Paul.

Do I need to go through the resistance check as well as the voltage check as per the manual ?

Also, the voltage check instructions mentions that it was conducted using 119VAC. Here in Australia we use 240VAC.  Can I assume that the readings quoted
in the manual would be the same seeing it's DC voltages ?

Please see additional photos of the insides of my Crack amp and yes, this is a custom top and base that was made at the time of building this amp kit.

Thank you,

[Paul Birkeland]

You have a 240V power transformer, so your DC voltages will be the same as what's in the manual.  Since you don't have a Speedball, you can also check the resistances and that may illuminate what's going on.

Another thing that I notice is that you've soldered wires right to the Alps Blue volume pot, which often causes the pins on the pot to break off.  If that happens, it will often manifest in weird voltages.  Don't feel too badly about that though, I just opened up a very high end Japanese preamp in a beautiful pair of green boxes today with an Alps blue with wires soldered to the PC mount pins.

[loweart]

Thanks for the advice regarding checking the resistance as well as the voltages Paul.

I guess I have been lucky to this point in regards to soldering the hookup wire to the pins on the ALPs pot. You've made a good point and I never thought of it that way and at the time I was
just begrudging that ALPS don't have this type of volume pot with solder lugs 

It's meant to be raining here in Brisbane, Australia for this weekend so hopefully I can go through the checks soon and report back on what I find.

Thanks in advance.

[loweart]

Hi Paul,

I managed to steal some time during my lunch hour to perform the resistance check and all came back as per what is quoted on the build manual.

[loweart]

Hello again Paul,

So, the voltage check revealed some interesting things of which the first thing I noticed before even testing one point was that the RED LED soldered to A3 is NOT lit whilst the other RED LED soldered to A8 is lit up as expected.  See the below test results :

1 =  76.8 VDC  ( manual quotes 90 )
2 =  160.9 VDC  ( manual quotes 170 )
3 =  0 VDC  ( manual quotes 0 )
4 =  160.9 VDC  ( manual quotes 170 )
5 =  145 VDC  ( manual quotes 90 )     
6 =  0 VDC  ( manual quotes 0 )
7 =  100 VDC  ( manual quotes 100 )
8 =  0 VDC  ( manual quotes 0 )
9 =  144.5 VDC  ( manual quotes 100 )
10 = 0 VDC  ( manual quotes 0 )
11 = 0 VDC  ( manual quotes 0 )
12 = 0 VDC  ( manual quotes 0 )
13 = 0 VDC  ( manual quotes 170 )
14 = 0 VDC  ( manual quotes 0 )
15 = 0 VDC  ( manual quotes 185 )
20 = 0 VDC  ( manual quotes 0 )
21 = 0 VDC  ( manual quotes 206 )

A1 = 0 VDC  ( manual quotes 90 )
A2 = 0 VDC  ( manual quotes 0 )
A3 = 0.5 VDC  ( manual quotes 1.5 )
A4 = 0 VDC  ( manual quotes 0 )
A5 = 0 VDC  ( manual quotes 0 )
A6 = 76.6 VDC  ( manual quotes 90 )
A7 = 0 VDC  ( manual quotes 0 )
A8 = 1.5 VDC  ( manual quotes 1.5 )
A9 = 0 VDC  ( manual quotes 0 )

B1 = 0 VDC  ( manual quotes 90 )
B2 = 0 VDC  ( manual quotes 170 )
B3 = 0 VDC  ( manual quotes 100 )
B4 = 0 VDC  ( manual quotes 90 )
B5 = 0 VDC  ( manual quotes 170 )
B6 = 144 VDC  ( manual quotes 100 )
B7 = 0 VDC  ( manual quotes 0 )
B8 = 0 VDC  ( manual quotes 0 )

I'm not an expert, but I'm guessing that RED LED soldered to A3 not being lit has affected the input tube voltage biasing ?

Appreciate your expert advice in due course Paul.

Thanks in advance.       

[Paul Birkeland]

If the wire connected to A4 is not well soldered, then only half of the 12AU7 will glow and one LED will go out.  This will also be indicated by 0V on A3.


That wire does not look well soldered at A4, nor does it look all that well soldered at A5. 

[loweart]

If the wire connected to A4 is not well soldered, then only half of the 12AU7 will glow and one LED will go out.  This will also be indicated by 0V on A3.


That wire does not look well soldered at A4, nor does it look all that well soldered at A5.

Thanks for that suggestion Paul and I have re-tinned/soldered up A4 & A5 and took the opportunity to re-do A3 whilst I was at it as that looked more "suspect" than A4 & A5.  No change, A3 LED is still not lighting up at all.

I decided to perform the voltage check again and this time around the ones that were previously way off from the expected voltages read the following :-

PIN 13  - now reads 170 VDC immediately upon contact with + probe tip and then counts down to 0 
PIN 15  - now reads 122 VDC immediately upon contact with + probe tip and then counts down to 0
PIN 21  - now reads 208 VDC

PIN B1  - now reads 77 VDC
PIN B2  - now reads 162 VDC
PIN B3  - now reads 100 VDC
PIN B4  - now reads 147 VDC
PIN B5  - now reads 162.5 VDC

The other PINS read the same as originally tested.

Curious...........

Many thanks,

[Paul Birkeland]

took the opportunity to re-do A3 whilst I was at it

I would not have done that.  These LEDs are hygroscopic and slowly absorb moisture over time.  When they have sat out for a few years, as soon as you heat them with a solder iron the moisture blows out of them and it can destroy the LED.  This is part of why we state in the manual not to mess with the LEDs if they don't light up.

What DC voltage do you get at A3? Do both halves of the 12AU7 glow?

[loweart]

I would not have done that.  These LEDs are hygroscopic and slowly absorb moisture over time.  When they have sat out for a few years, as soon as you heat them with a solder iron the moisture blows out of them and it can destroy the LED.  This is part of why we state in the manual not to mess with the LEDs if they don't light up.

What DC voltage do you get at A3? Do both halves of the 12AU7 glow?

Ooops , sorry about that.

A3 voltage hasn't changed since my initial test prior to resoldering A4 & A5 and it currently reads as before at 0.5 VDC

[Paul Birkeland]

Do both halves of the 12AU7 glow?

[loweart]

Sorry Paul, forgot to mention previously that both halves of the 12AU7 tube glows/lit

[Paul Birkeland]

Just to double check, I would pull the 6080 out and recheck terminals 1-5.

Since you have 0V at A3 and ~150 at terminal 5, the tube is not conducting.  The most common cause for this is that A4/A5 solder joint, which I will emphasize again, and also double check that by both halves, I don't mean a glowing dot on the top and bottom of the tube, but a glowing dot on both of the internal plate structures inside the tube. 

If it's not that, then the wire going to A1 could be broken or just not well enough connected to conduct.

[loweart]

I've tried 3x different 12AU7 tubes and al of them only glow at the top and bottom tips, can't seem to see any glowing dot coming from the internal plates on any input tubes I've tried.

Just to be clear, you're suggesting that turn off the amp, then remove the 6080 tube and the power up the amp without it and check the voltages at points 1,2,3,4 & 5 ?

Seeing I've already re-soldered and flowed a considerable amount of of solder earlier on A4 & A5, what else can I do there to be sure it's conducting ?

I'm guessing that if all else fails, you'd suggest to replace the wire at A1 ?