High frequency buzz in left channel ST-70

Hello,

I rebuilt my ST-70 about 4 months ago, all new caps, resistors, rectifier, etc... It has been great, but has recently developed a high frequency buzz in the left channel. I have unplugged my preamp from the input and it still is there. I know it could be many things, but do you guys have any thoughts on an obvious place to start?

Thanks!

Geoff

For a start I suggest swapping the tubes over from one channel to the other to see if the buzz travels with them. Then check all your solder connections for dry or loose joints, particularly on the tube pins.

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Will do. Thanks!

If switching tubes did not reveal the problem, keep in mind that OEM 70s quite often suffered from internal oscillation issues. Causes were several, but primarily from mis-routed wiring. Obtain a copy of the original assembly manual and refer to the pictoral diagram inside. If the wiring inside your amp does not closely resemble what is in the picture, try rerouting what you can and see if that helps.

If neither of these expedients work then:
a) Under very good light and with a magnifier, examine the circuit board for cracked or damaged traces. If you find anything, bridge it with a strand of copper wire in addition to the solder. Also check for bad connection, quite common on the vintage phenolic boards.

b) As a last resort, and with the unit unplugged and tubes pulled, check every connection to-ground and across each component, carefully comparing Channel A to Channel B. Where you find a discrepancy will be the place to start your diagnosis. DO complete the process even if you find something obviously bad. The issue is the possibility of a cascade problem....

Best of luck with it!

I moved the tubes from left to right, one at a time, rebiased, and it seems fine. Thanks for the suggestions!

Do you have access to a scope? I would be curious if there were any ultrasonic artifacts remaining.

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PeterCapo wrote:Curious that it went away completely.  Would have at least expected it to switch channels...  intermittent solder or foil trace?

 ...dirty connection cleaned by friction, different level of tube drift to component drift...

Many mysteries, and why I suggest a scope to make sure that the issue has not simply moved out of the immediately audible range.