Hum in right channel, developed a few months in

[eudisld15]

Hello,

So it seems like my crackatwo developed a hum on the right channel. Audio volume in the right is also significantly lower as well. When I first built this kit, it passed all voltage and resistance tests at stock. I then added in some mundorf caps (100uf and 0,1uf) and it passed all tests too. About 2ish months in right channel seemingly developed a hum literally a few hours after powering it down for the night and then powering it on in the morning.

I re-ran all the tests and got significantly off results on the High Current C4S board closest to power outlet.

I went and checked all the cabling and solder, reflowed some traces/joints and replaced seemingly loose/bad cables. Returned the caps back to stock and still the voltage tests fails.
I also swapped around some tubes with 1 to 1 equivalents I had and swapped high current c4s boards. Still fails voltage test on the side closest to power inlet. Swapped 100uf cap sides. Still fails on the same side. Also checked all the traces of the low current C4S and I cannot see any bridges.

I put back the mundorf caps back on and went back to square one to mimic the day the right channel went bad and remeasured voltages, below are the results:

Low Current C4S

IA 95
OA 69
KRegA 0.344
bRegA 97.7

IB 199.4
OB 66.3
KregB 17.4
bRegB 199.4

High current C4S (closest to power inlet)

IA 201
OA 199
bA 0
IB 0
OB 110
bB 199


High Current C4S (furthest from power inlet)

IA 196
OA 97
bA 0
IB 0
OB 74
bB 96

Does anyone have any Idea on where to start checking for defects again?

[eudisld15]

For some reason when I try to upload images the site times out, here's a Google drive folder with pictures

https://drive.google.com/folderview?id=1qOeSiA1fVzfX006OrQuOrESk5sGNLn6J

[Paul Birkeland]

Neither channel is working properly. Both have unique issues.  I would suspect loose connections or broken wires that got disturbed when you parts swapped.

[Paul Birkeland]

I hate to say this, but you make a huge gamble putting in sockets like that.  I used to do a ton of kit repairs where I would pull out fancy sockets from people's kits and put our sockets in to get them working properly. It got so common and tedious that we disqualified kits with different sockets from our repair services entirely. 

[eudisld15]

Only the two center tubes have the fancy sockets. Do you recommend I swap it back to the stock ones first. I don't mind doing it tonight.

[Paul Birkeland]

To start with, what are your IA, OA, and OB voltages on the big PC boards with no 6AQ5s plugged in?  How about OA and OB on the small board with no 6AQ5s?

[eudisld15]

Here are the voltages:

No 6AQ5s

Low Current C4S

IA 207
OA 69
IB 207
OB 65

High Current C4S (closest to power inlet)

IA 210
OA 208
OB 113
bB 208

High Current (furthest from power inlet)

IA 208
OA 206
OB 116
bB 206

[Paul Birkeland]

OK, this is somewhat of a good sign.  Can you listen to the amp under these conditions and let me know if everything appears to be working OK without the 6AQ5s?  This will reduce the amp to something a lot closer to the basic Crack. 

[eudisld15]

With out the 6AQ5s I get humming on both side. Volume control doesn't work and only audio out of my left channel.

I restocked both center tubes and I get sound from both sides now. But still hums.

[Paul Birkeland]

With out the 6AQ5s I get humming on both side. Volume control doesn't work and only audio out of my left channel.

I restocked both center tubes and I get sound from both sides now.

This suggests that you have socket issues and operational issues with the amp.  The operational issues could have originated from socket issues, so I would suggest pulling out the aftermarket sockets and replacing them with the stock ones to start.

[eudisld15]

So I swapped 8 pin and 9 pin sockets to stock.

The 7 pins are already stock.

I took out the mundorf caps and put on the stock caps

I tripled checked all the components and wiring for a loose wire and touched up more solder joints. All the voltages measure essentially the same as before.

Could the issue be related to the low power C4S? If I were to flip this board around and mirror the wiring connecting to the rest of the amp would the operation of the amp still be the same? I want to see if flipping the board carries the A and B side Kreg voltages with it.

[eudisld15]

So I didn't flip the board around. Instead I measured the resistance of the traces on the parts on the low power C4S board. Seems like something died on side A. All the resistors give me at least 0.0 ohm resistance or higher. So does the 0.1uf cap and the DME-F250V CDEC - 225k part. On the A side the LM431A, PN2907A and MJE350 resistors when trying to measure resistance on the outer pins I get OL, but on the B side I get some resistance measurements on the same outer pins.

What do you think of this?

Also I have not mentioned this before since it states LEDs are not meant to be used to troubleshoot but when powering on the amp on the side closest to IEC inlet the A half on the High power C4S the LEDs quickly light up and then dim. However on the side furthest the IEC Inlet the A half LEDs power and stay bright. This phenomenon happens even after swapping both High power C4S. The A half of the High power C4S closest to IEC inlet gets bright then dims.

I triple checked the wiring for all tube sockets and measured resistance on the wires. It all appears well.

So, perhaps it's one of or all three of these resistors that are problematic?

I placed an order on mouser for a bunch of the above resistors to test.

[Paul Birkeland]

The 431 regulators are the most sensitive parts on the boards, and you could certainly try replacing them.  Since the amp has at least correct DC voltages with no 6AQ5s, that tends to point to the 431 regulators being problematic.

Still, the amp should work properly (perhaps with a little additional noise) with no 6AQ5s installed, and since yours doesn't that would suggest that there are additional issues beyond the high voltage regulator.  With the replaced sockets, will the amp work properly with no 6AQ5s?  (Be sure to confirm that the voltages are OK first, with OA on the big boards and IA/IB on the smaller C4S board being more like 200V rather than 150V)

[eudisld15]

Update:

Replaced Low power C4S PN2907a, mje350 and lm431. Issue still persists with 6AQ5s. When removed I still get a hum on both channels, but I get sound from both channels now and have volume control.

Post resistor/regulators replacement voltages.
-----

No 6AQ5s

Low Current C4S

IA 206
OA 45
KregA 0
bRegA 207

IB 206
OB 46
KRegB 0
bRegB 206

High Current C4S (closest to power inlet)

IA 208
OA 206
bA 0
IB 0
OB 101
bB 205

High Current (furthest from power inlet)

IA 208
OA 206
bA 0
IB 0
OB 102
bB 206

-----

With 6AQ5s

Low Current C4S

IA 95
OA 44
KregA 0.34
bRegA 95

IB 200
OB 46
KRegB 16.89
bRegB 200

High Current C4S (closest to power inlet)

IA 203
OA 200
bA 0
IB 0
OB 99
bB 200

High Current (furthest from power inlet)


IA 196
OA 95
bA 0
IB 0
OB 58
bB 96

----

-Resistance (no tubes):   

1 0
2 OL
3 0
4 15k
5 2.15k
6 36.7k
7 -
8 0 
9 -
10 OL
11 0
12 15.5k
13 0
14 OL
15 2.12k
16 40k
17 -
18 0
19 -
20 OL
21 0
22 OL
23 OL
24 345k
25 0
26 665k
27 624k
28 0
29 0
30 2.63m
31 0
32 1.24m
33 0
34 0

[Paul Birkeland]

Update:

Replaced Low power C4S PN2907a, mje350 and lm431.

Low Current C4S
OA 45
KregA 0
bRegA 207
KRegB 0
bRegB 206

You've made things worse by replacing more than what was recommended.  The MJE350 and PN2907 on the center low current board were working properly and giving the correct voltages, now they are not. 

This is about the point where sending the kit in for repair starts to make a lot more sense than continuing here.