Weird LA-2a issues? Thoughts?

[matta]

Hi Guys,

I'm working on my LA-2a, it was built on Drip's original V1 board and has been working everyday in the studio for over 5 years without issue, then out of nowhere it started being intermittent giving hum, distorted signals and the like. I've uploaded a clip HERE

It's compressing, the meter is showing GR and I'm hearing it, in between the noise.

I've troubleshot it by checking all the voltages, all seem to look good, checked the grounds, all ohm out even replaced all the tubes, one by one, still the problem persists  :-\

I'm not a wizard with my scope, but know the basics, threw it on and the input seems fine across R6 into the GR circuit, also good on the Threshold and Make-Up gain pots, but tested on the output at C5 and there is what looks like a lot of ripple and the occasional spike that goes off the chart and pegs the VU meter.

I've uploaded a short clip of me taking reading with my scope, HERE. The noise you can see is at the junction of C2 + R13, as you can see the gain knobs works as does the gain reduction. At the end I move to the gain knob, and it looks clean.

Any ideas where to look next? As shared not the tubes, pulled and replaced each one, so the noise/ripple is coming from somewhere else.

Thanks in advance

Matt

 

[CJ]

maybe the pwr supply?
try bridging the last filter cap with another cap if you have one,

check you tube sockets to make sure the pins are held tight,

this is just a visual inspection,

 

[matta]

maybe the pwr supply?
try bridging the last filter cap with another cap if you have one,

check you tube sockets to make sure the pins are held tight,

this is just a visual inspection,

Hi CJ, by last filter cap you mean the 22uF/350V at the junction of R16? Bridged it and re-flowed the joints, same problem  :-[ As shared the voltages are looking good and well with spec. Am I looking at this all wrong? Is the problem down the line? Any thoughts on where to go from here?

I appreciate the helps and words of wisdom.

Thanks

Matt

 

[SSLtech]

Pull the 12BH7. Unplug it. (careful, it gets hot!)

Now see if the noise is gone. -Be mindful that the output won't drive anywhere near as hot, but you should be able to get signal to about -8VU.

If the noise is gone, replace the 12BH7. There are a LOT of bad ones out there, and I've seen dozens of failed EH versions recently....

 

[mjrippe]

Pull the 12BH7. Unplug it. (careful, it gets hot!)

Now see if the noise is gone. -Be mindful that the output won't drive anywhere near as hot, but you should be able to get signal to about -8VU.

If the noise is gone, replace the 12BH7. There are a LOT of bad ones out there, and I've seen dozens of failed EH versions recently....

^ Yes, this ^

Had the identical problem with an LA-2A and it was a 12BH7 with an intermittent short.

 

[matta]

You both called it! The weird thing I DID replace the tubes, one by one, with new tubes, but the NEW EHX 12BH7 was also bad, so had me running around in circles. Pulled it as SSLTech suggested, scoped it, beautiful, so it brought it back to the tube, got ANOTHER new tube and it now works. EHX you SUCK!

Cheers

Matt

 

[mjrippe]

Glad you got it sorted!  Personally I'd use a NOS American tube - RCA, GE, etc.  Much more reliable.

 

[SSLtech]

When I rock, I rock...

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