Hi Octane Fault Finding

[Flyingsparks]

After many years of great service my Hi Octane amp seems to have developed a fault, and I am struggling to fathom out what is causing it. I noticed a fizzing distorted sound when playing a bottom E string, which got worse as I increased the volume. I connected a function generator to the output, and inputted a sine wave at 100 Hz, then monitored the output on my ocscilloscope. When I turn up either the master volume or gain, I get a repeated noise from the speaker, which shows up as possibly two notches half way down on the positive side of the sine wave, which appear to get larger as i turn up the volume . At 200hz it didn't seem as bad , but seems to be worse at 80hz . So far I have replaced the coupling caps, and the main filter caps were replaced last year. I have replaced all the tubes and had a good poke round with a chopstick, but can't find any loose connections. I am now suspecting the Hammond 125ESE output transformer may possibly be going faulty. Would any of you knowledgeable people, be able to help me and offer any suggestions as to what the fault may be, as I don't really want to purchase a new transformer , until I am certain it is actually faulty ?

[Stevem]

You need to find out with your O-scope at what point in the signal chain those notches you post about on the signwave first show up.

Start at the primary side ( input ) to the output transformer and work your way back upstream towards the first gain stage.

[sluckey]

Be careful with that scope. If you connect a probe to the primary of the OT you may end up with a dead scope.

[Flyingsparks]

Thanks for the replies. I thought that I may have to start working back from the output with the scope, but as I have never used one on high voltages before, I am not confident about how to do this without causing damage to the scope.

[Stevem]

How much dc is on the OT?

To block that and just get the sine way Clip a 630 or better yet 800 vdc rated coupling cap to your probes tip and use the other end of the cap as your new probe.

Be sure to shrink tube insulate this end so that only a short end of the lead is left exposed to make contact with the circuit your probing .

A .5 to 1 uf cap should do it .

[nuke]

The symptom kind of points at filter capacitor, or other power supply issue.

Perhaps revisit the previously replaced main filter caps, particularly inspecting the work. Also, just because they were new doesn't always mean they haven't failed or faltered. It happens.

[Stevem]

Ditto!

I too maybe twice have had to deal with new yet bad filter caps .