Right channel dead after working for years

[Pharane0]

Hello I would like to ask for your help please.

My right channel died spontaneously after having the amp working perfectly for a year or two.

I did the volume mod to the volume pot with 75k / 33k resistors two weeks ago. Worked flawlessly again. But then one morning my right channel was dead. I unplugged / replugged everything, checked that it wasn't my dac with another amp (it's not my dac). The Crack worked perfectly again for a day, with both channels working again but then today it seem to have died this morning AGAIN for good this time.

I swapped the two tubes for other ones, same problem, the right channel is dead.

I did the resistance check from the manual, before plugging it in to check to voltages, and I saw that 4 measurements were faulty:
-7
-9
-B3
-B6
These four are supposed to read 2.9kOhm but read either super super high MOhm or even "O.L". I'm not an electronics expert and I'm quite lost with this amp. Do you guys have experience with these and know what it could mean please ? Since the resistance were off, I didn't perform the voltage check. The red leds are all lighting up when the amp is ON and the tubes are glowing.

Thank you for your help.
Alexis

 

[Paul Birkeland]

You have a Speedball upgrade installed, so 7, 9, B3, and B6 are not going to be 3K.

You will need to post your DC voltages in order for us to understand what's going on.

 

[Pharane0]

Performed the voltage check today. Everything is within the spec written in the manual. Did the measurements for the speedball and the crack. The only thing that slightly differed was OA / OB on the big board at 110V instead of the spec [75-100] and on the crack terminal 2 and 4 are at 190V instead of the spec [170]. My AC voltage is 220V btw. All leds and tubes are glowing. I don't know what's wrong with my amp. Sometimes, after measurements and me hooking it back up to my dac it's working again and then, the next morning when I turn it up it's dead again. I don't understand.

 

[Paul Birkeland]

That would tend to indicate a solder joint or two that aren't 100% solid. You can poke around the circuit while it's running with a wooden chopstick to see if you can find a spot where pressing on it causes the circuit to stop working properly. It can help to have music playing and a cheap pair of headphones plugged in to be able to pick up on this more clearly.

 

[2wo]

You say you recently added the 75k/33k resistors, that may be a good place to revisit.

 

[Pharane0]

Did the poking around. I pushed everywhere multiple times. I of course insisted on the latest area I touched, the newly added the 75k/33k resistors like you said. Unfortunately couldn't find the source of the problem. What are the next steps to debug this ? I'm sorry but it seems stuck. Could my problem be temperature related ? Because sometimes it works again and the next morning you can be sure the right channel will be dead. I'm not talking about the first minute when you let the amp start up and the tubes start to glow, I mean leaving the amp OFF in a room that slowly goes from fairly cold to mild in 5 hours and then SOMETIMES the amp works again. Sometimes not. Super weird.

 

[Jameson]

Could you post some pictures? I'd focus on the areas that you touched when installing the Speedball, including the board itself.

These sorts of issues are very commonly cold solder joints or flaky connections of some sort.

 

[Doc B]

This is either a matter of a bad connection somewhere where the current flows thru the right channel half of the tubes or a bad connection where the signal goes in our comes out of the right channel tubes.

With an intermittent problem you have to be sure you are getting the amp into the failure mode before taking measurements. Since you didn't say whether the right channel was out or working when you took the voltage measurements, I'd suggest that you go back and verify if all LEDS are lit when the right channel is not working, or whether the right channel just happened to be working the way it was supposed to when you measured voltages with everything lit properly. If the right channel is out but the LEDs are lit and the voltages measure in spec then the tubes are working properly and the problem is somewhere in the signal path, like a bad solder joint in the pad you installed or the RCA input or volume pot.

I'd probably reflow the 75K and 33K padding resistor connections on the right channel for starters and carefully inspect the right RCA jack to be sure it's in good shape - not loose or melted and with solid connections to it. Might be worth checking that the problem is not at the headphone jack as well - loose contacts or solder connections. The RCA and headphone jacks get some mechanical stress whenever cables are plugged in or pulled out so this can be a common point for joints to fracture and fail if the jacks aren't snugly secured to the chassis.