I've got a hum that has developed on my modified ST-70 that I just can't figure out and could use some help.

This ST-70 has the Kara Chaffee driver mod (for the MkIII) that was described AudioXpress 3/01 adapted for 6L6GAs rather than KT88s. The driver is one 12AT7 and two 6CG7/6FQ7 configured as a long-tailed pair (Schmidt) phase splitter. I got if from an Audiogon'er a few months ago and it sounded good and was very quiet.

After a couple of months, it developed an intermittent scratch (or spitting) through the right speaker. I got it on the bench and started looking at the point-to-point arrangement, disconnect the feedback and started trying to trace down the source of the spitting. Nothing really obvious jumped out. I got the freeze spray on one a resistor that I thought might be bad but that didn't fix the problem.

Over bench time though, the spitting became more infrequent making it more difficult to test. Now the spitting has stopped and a hum in that same channel.

With my scope, I can see that there is some +/-40mV, 120Hz ripple at the speaker terminal. This does not exixt on the other side. Working backwards, I see LESS junk on the offending side than on the than on the good side, making me think it's the transformer. I also am hearing some mechanical vibration coming out of the right transformer: not a hum but the kind of vibration when two edges of sheet metal vibrate together.

The bias control range is now much far different than before.

So could it be the transformer? If so, why 120Hz? [That made me look to the rectifier first but the problem is only on one side.] Can it be fixed? Can the spitting and the hum be related? And the bias? Is there a ghost in my machine?

Any help would be appreciated.

Output transformer could have a shorted winding. Arcing inside transformer could be the sputtering.



Dave

So could it be the transformer? If so, why 120Hz?

120Hz is what your would get if you had a power transformer issue. However it bothers me that you are only getting it on one side.

Stupid question... When you are on the bench, are you testing it with the inputs shorted and the outputs with an 8 or 10 ohm load? Make sure. If one of the shorting plugs on the inputs is not shorting, you can scope it all you want and be just getting random stray voltages. If the output load is bad or not quite connected completely, this would certainly cause a transformer to hum. Pretty loudly too.

Also, take a look at the bias pot. I've had 'em go bad and had lots of odd issues. It may also be worth your time to simply retouch the solder joints on that side.

It couldn't be as simple as wire dress could it?


Please don't be offended by the primer here. These are all screwups I've made. Wrecked an expensive output x-former. :tears:



Wish I could help more. I just don't know the ST-70 very well.

Yes, there is some ripple on the primary side. It's a sloppy sine (120Hz) as the the secondary output is but much bigger, ~1V pp. It appears, almost identically on both sides on all 8 pins of the power tubes. The red lead has a less sloppy sine of ~1.4V pp.

In verifying these measurements, I found that I could feel the physical vibration while probing the tubes on the right side.

I really appreciate the thoughts. I feel like I'm losing it troubleshooting this problem.

I'm making these measurements with 6ohm speakers attached to the outputs. I've swapped the speakers and they are identical. I've also done the measurements with and without shorting plugs.

As additional background, I've swapped the tubes and he problem does not follow them.

I took a quick look at the bias pots. Things have definitely changed here but strangely, they have changed more or less symmetrically. I'll look more closely at the connections.

As an aside, related to the bias pots, is that I've verified the schematic and respective component values by slogging through the point-to-point layout -- very time-consuming but kinda fun. It differed from the schematic the seller gave me which made me question the bias he recommended: 1.2V over 15ohms. Operating in AB1 mode, it should be 1.35V, I think. However I can just barely get to 1.1V at the moment (on both sides!). Baffling but maybe not related to the ripple problem.

It may be useful to note that the C-354 choke is mounted directly under the right channel OPT. Any possibility of coupling here?

Aaaargh!

I guess I should be happy but now I'm at a loss as to what to do next. I swapped the OPTs and the problem did not move with them. While I was at it, I measured impedances and they look symmetrical and very close to one another.

I could really use some guidance on where to look next ideas on where this 120Hz junk (and the other mysteries) may be coming from. Here is the schematic that I've sorted out in the process along with a (http://www.bauer.org/~kurt/ak/st70Schematic.jpg)and a photograph (http://192.168.0.2/~kurt/ak/ST-70%20001.jpg)of the underside, albeit slightly off center. Note that the orange wire that terminates in a black blob just to the left of the PT is the global feeback wire that I disconnected from the 8ohm speaker terminal.

Go talk to Shannon at diytube (http://www.diytube.com/phpBB2/index.php). His name there is 'separks'. He is a freek'en genius when it comes to St-70s . So, I believe is Tom McNally.

If anyone can, they can get you straightened out. :thmbsp: