HELP! Screeching from SEX

[denti alligator]

Turned on my SEX today, which I use every day without problems and I got a high pitched screeching that sounded like a teapot that was really, really ready. Turning the volume down did nothing. Power off did.

I unplugged the speakers and turned it on and I can hear a slight sound coming from the amp, like the screeching only much quieter.

I'll be testing resistance and voltage now. What could this be?

[Jim R.]

Sam, could be a microphonic tube or maybe just a dirty pin/contact -- just try resetting the tubes in the sockeets a couple of times as a first pass.   Did you bring any new cell phones, or other electrical goodies -- wireless networking, etc. into your listening room lately?

Also, dod you have any shorting plugs -- just to make sure it's not coming from the source?

Sorry can't be more help,

Jim

[chrisby]

am I the only one snorting my coffee when reading the subject lines of some of these threads


I mean, usually the above costs extra

[denti alligator]

OK, guys. It's not the tubes. Switched them out and used de-oxit on pins.

Resistance readings were funny at 39 and B4, so  resoldered and they're fine now. Yeah. But, instead of screeching I get white noise/whooshing in the left channel. Right channel is fine.

Voltage readings are funny now: B5 is 99.3 and B6 is 3.186. These are too high. Shawn troubleshooted this with me on the phone and we couldn't come up with anything. I'm glad it's no longer screeching, but the sound in the left channel is horrible.

Help?

What could be causing this?

[Jim R.]

Sam,  Sorry, other than what I suggested, I don't have any more to offer right now.

Chris, and yes, I almost said something like I had a girlfriend like that once... but as you can see, I was being cautious.  The operative word here being "was" :-).

-- Jim

[Paul Joppa]

...

Voltage readings are funny now: B5 is 99.3 and B6 is 3.186. These are too high. Shawn troubleshooted this with me on the phone and we couldn't come up with anything. I'm glad it's no longer screeching, but the sound in the left channel is horrible.

Help?

What could be causing this?

B6 too high (the cathode; should be 2.5v) indicates too much current through the driver section. B5 too high (the plate; should be 70v) indicates not enough current through the driver section. No wonder you and Shawn had a hard time!

Make sure the grid stopper Rstop, (B4 to T39) is 220 ohms - it is somewhat fragile so look for any damage.

If it failed or had a bad connection as you suggested, that might have led to some damage to the cathode and/or plate resistors So ... check those two resistors. Cathode resistor Rkdriver (B6 to T37 ground) should be 1270 ohms, and RL should be 150K (B5 to T36? B+driver). Better check the resistor from T31 to T36 as well.

[denti alligator]

These test out ok, Paul. Except between B5 and 36, which I think is altered with the C4S.

So what else could be causing this?  Note that the white noise does not drown out music, and does not change in volume when the volume pot is turned.

[Paul Joppa]

Oh - didn't know you had the C4S. I assume this is the first stereo SEX version? Sorry for the confusion, we really do need to get clear on the version identifications!

The voltages seem to indicate the C4S is providing a bit too much current. First check is of course whether the LEDs glow properly on both boards. You could swap the C4S feed (just the wires that go to B5 and A5, if they are long enough) and see if the noise moves to the other channel - if it does then it's definitely the C4S board. The bad grid connections could conceivably have resulted in some damage to the MJE350 - but I would just get a whole new board rather than try to figure out which parts are bad.

If swapping the C4S outputs does not swap the noise, then it's somewhere else. Does the volume control have any effect on the noise?

[denti alligator]

Yes, when I switch the C4S boards the sound switches channels. I'm removing the C4S for now and putting the old resistors in until I can get replacement parts from Bottlehead.

Thanks, Paul!

[denti alligator]

C4S boards removed and old resistors in place. Works fine!

I think the screeching may have come from an improperly soldered cap at 34 (to the tube socket). This must have then damaged the C4S board...?

Anyway, thanks for your help everyone.

[denti alligator]

So would BH be willing to send me replacement parts? I left a message at BH HQ. If yes, that would be great. Just let me know if I need to pay for these...

[denti alligator]

After months of no problems and all-day listening yesterday, this happened again this morning.

But I've misplaced my manual, so I can't check resistance and voltage. Anyone willing to give me those figures?

[denti alligator]

Contact cleaner to the tube pins did the trick. I might have spotted a dog hair in there. Damn dog!

Catastrophe averted. For now.

[denti alligator]

And, it's acting up again. Every few days when I start it up in the morning I get feedback from both speakers. Cleaning the tube sockets and pins with contact cleaner works for a few days. This morning it did not work. I've tried cleaning and recleaning. It's oscillating so bad that I don't even need to have the speakers of 'phone plugged in to hear it.

Any clue where to start locating the source of this problem? Why is it always late December when it starts acting up? Weather? Nothing else has changed. I haven't moved the amp one inch.

The only thing I can think of that's I've done differently recently is that I've been using the amp 3 to 4 times more each day. Last night it was on from 6 to 11 or so. Maybe it's gotten heated up with this long usage and this has somehow had an effect ...?

EDIT: I forgot about the volume pot. In each case it was turned all the way down. However, I had forgotten to use the contact cleaner on it. So I did that. Not sure how to clean the inside, so I just drop some of the cleaner into it and moved the pot several times back and forth. Turned on the amp and it made a brief squeak (remember, I don't have speakers or phones plugged in!) and then was silent. I've moved the pot around a bit, turned it off, and will return later. Maybe this was the source of the problem?

[denti alligator]

So, it seems like every morning I wake up and the SEX starts oscillating when I turn it on. I then go on to clean the tube pin contacts and volume pot, and then it's fine until the next day. This just doesn't seem right. What could be going on here?