Gnarly noise problem

[donzoid]

Wreck Express:
PPIMV when built and cut control added later...Edcor PT and OT.

Not convinced this is a "wreck-specific" issue but if moderators want to relocate to the wreck section....that's ok too since it is an Express clone!

This (for me) is an advanced (read: intermittent and I can't pin it down) problem so I'm going to ask the really smart folks "how/what" to do to troubleshoot it.

Started a few months ago, nowhere near this severe as a sort of odd distortion/saturation/loss of punch almost like clockwork during the 4th set after she was good and warm and had been run for hours.

I thought I prolly had a bad tube, so I swapped the 6v6 pair back to el34's, then swapped the pre and PI tubes, chopstuck around, inspected solder points, cleaned / checked my input jack and thought since it wasn't misbehaving after leaving her on for 6 hours, then playing through it for an additional hour, it didn't make any weird noises. THEN...

Last gig, on the last set, we were about to go back on, and flipped the standby switch - crazy ugly noise (and loud). so I shut her down and just ran the other amp.

Now its on the bench, with nothing plugged into the input, and the sound clip is what happens when I flip the standby switch.
So far I have just checked voltages on the plates, screens, grids and bias of the power tubes and nothing appears to be out of whack.

Any suggestions on what to check/test and what methodology will get me to the failing component? Does this noise indicate anything specific? I appreciate your help and input.

EDIT: I should point out that the noise kicking on and off is not me messing with anything. In the midst of that clip I flipped the bypass switch for the cut control, but it still just randomly starts and stops.

[tele_player]

I have no theory or suggestions, but I'd be curious to see if it does this with preamp tubes removed. This might help to isolate the problem. Or not.

[donzoid]

Me too...I have suspected the V1 socket before when it started, cleaned it then with some DeOxit. Just wondered if, before that I should check voltages on pre section but yeah since its amplified so much, seems like it must be early in the chain...

[M Fowler]

The reservoir filter cap ground wire loose from ground?

[matt h]

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[donzoid]

Mark,

Thanks - I just checked the reservoir cap(s) and they are good n tight, with the solder joints intact on both the pos. and neg. sides. And the bleeder resistors too. Of course that doesn't mean it isn't an "early life failure" as we call that in computer manufacture...but they don't show any signs of bulge or expansion etc. I guess I could measure their ohmage to see what that is.

Matt - thanks also, I will put a battery in my old analog meter and hook it up to see if it swings (hard?) during the blessed events. It did occur to me that when switching the standby on, I'm draining those caps or at least putting them to work. Yes I left the amp powered on, and then threw the standby switch.

I mean, in reality this thing never gets "hot hot" the transformers even after three hours aren't so toasty that I couldn't put my hand on them...I expect them to get good n warm though! It seems like it must be a component instead of a tube because now it has an entirely different set in there...thus the reason for this post. Dang now I'm gonna have to learn how to run this oscilloscope...

[donzoid]

Well, as expected...heheh I just hooked up the old Micronta analog and because it hasn't been sitting there powered on it won't do it. I'll power it up first thing tomorrow am, and let it sit for a while, then try it again to see "how much" of a swing the meter gets - this jumps from a 250v DC max to 1000v setting so we shall see if it shows anything.

[donzoid]

Hahah! Maddening, in a sick, humorous way.

It had been sitting there for a good ten hrs. I flipped on the ol' standby with the old analog meter hooked up to B+. And there went the noise....

No real change on the meter. But, I'm not necessarily trusting the meter...but suffice to say it didn't move significantly when the noise kicks in and out.

Not leaving well enough alone (tongue in cheek) I started chopsticking around on the power board and reservoir cap solder joints/connects. Now, this is really crazy of course because I can't tell if my poking/prodding is causing the noise to stop, OR it's just randomly doing it...

I moved on to the bias pot connects, and the power tube socket, and when I got near the IMPEDANCE selector, again, it started and stopped, (and again), so I reached back and flipped the switch from 8 ohm, to 16, and back to 8.

All of that nasty noise stopped for about 5 min. Unfortunately it has resumed...probably just coincidence. I'm still (gently) chopsticking around to try to find the source. Clearly a female, she leaves me not knowing where or what to touch, and when...

I have left off at the V1b cathode bypass cap...the triode connected to the volume control... it seemed to be noise responsive but now it is silent so I'll have to wait again until it starts making noise to keep checking it.

[Stevem]

Have you jumped out the standby switch yet?

[donzoid]

Stevem, not yet. Good idea!

I have had this thing on all morning, and wish I could bill myself for shop hourly rate...



So far the most activity and re activity is chopsticking right at the .0022 coupler and the cathode bypass cap for V1b...but I am resisting the urge yet to start swapping out components until I'm sure.

(I will try bypassing the standby switch too).

[Colossal]

So far the most activity and re activity is chopsticking right at the .0022 coupler and the cathode bypass cap for V1b...but I am resisting the urge yet to start swapping out components until I'm sure.

(I will try bypassing the standby switch too).

Hi Don,

You might visually inspect that V1b cathode to ground connection. That joint might have gone cold and may need to be reflowed. I agree, best not to resort to changing components without a little more hair pulling and swearing.

[Stevem]

I have had one or two new caps over the passed 40 years that had one of its leads making a poor connection internally, so you should move them around real good to make sure tgey are solid, this seems to happen a lot to Ciramic caps in Sears amps for some reason!

[donzoid]

Yes Dave, life isn't worth living unless one has enough hair pulling and swearing! Although I normally save THOSE for automotive jobs...lol. I will check the solder joints...all of them. For about the 12th time in 3 months. I must be missing just one yeah? And of course that's the "one that's bad".

[donzoid]

OK I think maybe I am making progress. Today it's doing it STEADILY after replacing the .0022 cap and letting it warm up. I chopstuck all round the area of V1 and no change. So I pulled V1 and its STILL doing it.

So, now what? Pull V2 and see if it still doing it?

[matt h]

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