Question on hum/buzz in stock ST-70

[joeriz]

One of my ST-70s is a stock unit in which I recently replaced the driver board. The new board is simply the stock circuit with new/improved materials.

The unit is dead quiet and sounds great. However, prior to adding the new board - and even now with the new board installed - after about an hour of warmup I get a rather loud hum/buzz (if pressed I'd have to say 'buzz' rather than 'hum') through both speakers. This noise starts out intermittent and then becomes constant. The odd thing is that the noise stops when I touch the volume control on my preamp (a Van Alstine Super PAS-3i). It is also worse when I have the volume on the preamp set to its minimum position. Touching the amp itself lowers the level of the noise only somewhat. The other odd thing is that - while I initially suspected the preamp - the noise does not manifest itself when the preamp is connected to my DIYTube ST-35 amp.

Now, obviously, there is some sort of ground problem like a poor connection that only makes itself known as the amp heats up. Given that it is in both channels, I'm suspecting the quad cap (replaced with an 80-40-30-20 about a year ago) or the main ground point next to the quad cap.

Any thoughts/advice? Sound like I'm on the right track?

Thanks,
Joe

[Slartibartfast]

Can you plug another source into the ST-70 and see if that noise returns, like an iPod or something with a volume control?

[kheper]

Try to isolate the cause of the buzz.

Short the inputs of the ST70, then turn it on
and wait. If it is silent, the cause is further
upstream. If there is noise, redo your
grounding scheme and look for bad wiring.

[joeriz]

OK -- ran it last night for about an hour and a half with one of my Squeezeboxes run directly into it. Not so much as a hiccup so I guess I can assume it's in my preamp. Odd, though, that it does not do this with my ST-35... Yellow_Light_Colorz_PDT_21

[Slartibartfast]

OK -- ran it last night for about an hour and a half with one of my Squeezeboxes run directly into it. Not so much as a hiccup so I guess I can assume it's in my preamp. Odd, though, that it does not do this with my ST-35... Yellow_Light_Colorz_PDT_21

I had a slight hum(buzz) when ever I would touch the volume control. A new volume pot and no hum. You could try reflowing all the solder joints leading up to and including the volume pot.

[joeriz]

Just to follow up on this...

Oddly enough, the problem seems to have been caused by a weak 7199 tube (or perhaps the pair...I replaced them both). I found that after full warmup I could trigger the problem by touching one of the 7199s. I replaced both of them and the problem has yet to return. I have no idea how old they were as I bought the amp about 10 years ago but they were mismatched. One was an RCA and one was a GE; I'm guessing the RCA may have been original to the amp. The GE was in really bad shape as I could hear distortion in the channel with it (which followed the tube), particularly during tracks with heavy bass.

Why this apparent grounding problem didn't manifest itself with the Squeezebox plugged into the amp is a mystery to me...

Joe