Sundown tuneup...

[ic-racer]

Ok, so I got it all packaged in the cabinet again and I was hearing a faint crackle! Before it was a loud crackle (which it had done many times in the past, and it was associated with moving the tube in the loose socket).

So, not the V3 tube socket is all nice an snug, I was getting a very faint crackle. I narrowed it down to 7 components just by noting that the Gain had NO effect on the loudness of the crackle, and the Master caused it to get louder or quieter.

After lifting some components off the board I narrowed it down to either the 100k plate resistor or the .022 coupling cap of V3b. I even removed V3 and it still crackled, so I knew it was not the tube. I could not tell which of the two components was bad because lifting one, or the other caused loss of all signal. So I replaced them both and .... Still it crackled!

Then, on a whim I connected the two lifted ends of the 100k and the .022 coupler (ie bypassing the circuit board and the connection to the tube socket) and NO crackle, so it was either in the socket or the PC board.

Here is a look at the PC board. I had just cleaned the connections of V3 in the middle of the picture. You can see V4 has yet to be cleaned. There was lots of old hardened residual flux all over the board. I cleaned it with pure isopropyl alcohol.

WOW!! No more crackling. So this grunge on the PC board really can affect the signal. So I went ahead and cleaned the whole board the best I could. I'm also glad I didn't have to replace the PC tube socket.

So, now thinking back to when I got the amp in 1996 I have a probable cause for it's behavior. I think it started the low volume crackling and the prior owner or a tech kept swapping out tubes on V3. Thus eventually causing very loose tube pin holders. They got so loose, that when I got the amp, it was non-functional because V3 was not making useful connection in the socket. But I think the reason they kept swapping tubes was the underlying crackling from the crust on the board.

Anyway, now it sounds great. Better then I have ever heard it since I have owned it.

[Wayne]

I remember being taught that "solder guck" absorbs water and can lead to problems down the road. I've never had something fail because of it, despite the fact that 95% of the time I'm too lazy to clean up after a small re-soldering job.

When I do clean up after soldering my weapon of choice is balls of coffee filter paper held in a set of forceps/small needle nose pliers, and contact cleaner. These are things I always have on hand.

W

[ic-racer]

After cleaning this thing up and being amazed at the sound of the amp, I wonder how many other Sundowns have a similar problem. This amp had always been a somewhat mediocre sounding amp, but I kept it in my collection for historical reasons, and because it had some cool circuitry designed by Dennis.

[Bear]

Sundowns are famous for problems, and I had particularly heard of hard-to-track crackle issues. I don't know how much of it has to do with compromising tone. Makes you wonder if a non-PCB clone might fare better on the reliability and tone fronts. I imagine the new problem is working out the lead dress to cope with the high gain.

Bear

[ic-racer]

Sundowns are famous for problems, and I had particularly heard of hard-to-track crackle issues. I don't know how much of it has to do with compromising tone. Makes you wonder if a non-PCB clone might fare better on the reliability and tone fronts. I imagine the new problem is working out the lead dress to cope with the high gain.

Bear

Agree. Doing a clone of this amp circuit might have some promise. I'll admit I have not seen everything, but Dennis' OD circuit looks unique to me. Might appeal to the person tired of doing the same old circuits.

[Structo]

First time I have heard of flux casuing problems.
Of course I am not that experienced in tube amps.
I wonder if that stuff conducts small amounts of current?

[Bear]

Flux is much more likely to be an issue on a PCB amp than any PTP/lug/turret/eyelet arrangement because the traces are so dang close to each other. The symptoms kind of sound like those of "tweed disease" where on old fiber circuit board has absorbed too much moisture and the thing becomes capacitative and screws with things.

Bear

[ic-racer]

First time I have heard of flux casuing problems.
Of course I am not that experienced in tube amps.
I wonder if that stuff conducts small amounts of current?

The flux was hard and crispy, almost crystalized. Also, there was dirt and dust covering the entire board. Trapping of moisture or other pollutant may have been the ultimate cause.

I did have an experience working on a motor power supply with an oscillating circuit. The resistor controlling the oscillations seemed to go out of spec, changing the frequency of the motor-controlling oscillations. Well, when I measured everything the resistor was 'in-spec' and I was getting like 400k ohms from the resistors PC trace to ground. The rest of the board's traces gave over 1M resistance to ground. In that case cleaning the board did not seem to have much of an effect, so I wound up just putting in a larger resistor and it worked fine. But I never could see anything physical on either side of the board that could possibly be conducting, so perhaps it was the board substance itself.

I had a crazy theory about the crackling in that it was due to some 'semiconducting' property of the flux caused by a pollutant on the flux, causing a random building up and releasing of minute charge. Like a nanoscopic version of lightning.

[ic-racer]

What's a Sundown test without a Scofield style track...

http://soundclick.com/share?songid=6684297

Unlike John, I used the Red channel and no 'Rat.'

[Structo]

I don't know how I missed this clip.

That sounds really good!

How loud was it in the room? (volume)